Antimicrobial Combinations against Pan-Resistant Acinetobacter baumannii Isolates with Different Resistance Mechanisms

Genomic Insights into and In Vitro Evaluation of Antimicrobial Combination Therapies for Carbapenem-Resistant Acinetobacter baumannii

Background and Objectives: Acinetobacter baumannii (A. baumannii), particularly carbapenem-resistant A. baumannii (CRAB), represents a grave concern in healthcare settings and is associated with high mortality. This study aimed to conduct molecular, mutational, and phylogenetic analyses of specific genes in CRAB and evaluate the synergistic effects of selected antimicrobial combinations. Materials and Methods: Phenotypic characterization was performed on six CRAB strains by using the Modified Hodge Test (MHT) and IMP-EDTA Double-Disc Synergy Test (IMP-EDTA DDST). Carbapenemase- and metallo-beta-lactamase-encoding genes were amplified by using Polymerase Chain Reaction. Phylogenetic analysis using the MEGA 11 tool was used to determine the evolutionary relatedness of these genes. Mutational analysis was performed by using I-Mutant, MUPro, and PHD-SNP bioinformatics tools to predict mutations in the carbapenemase-encoding genes. Microdilution checkerboard titration assessed the synergistic effects of antimicrobial combinations (azithromycin-meropenem, rifampicin-meropenem, meropenem-colistin, and azithromycin-colistin) on these CRAB isolates. Results: The phenotypic characterization of six CRAB isolates revealed positive results for MHT and IMP-EDTA DDST. The molecular characterization revealed that carbapenemase- and MBL-encoding genes were present in all isolates with varying frequencies, including blaOXA-51 (100%) and blaIMP (0%). The sequence analysis revealed high evolutionary relatedness to sequences in the NCBI database. The mutational analysis identified 16 mutations, of which 1 mutation (P116L) in the blaOXA-58 gene predicted a change in the protein product, potentially contributing to carbapenem resistance. The checkerboard titration method did not reveal any synergism among the tested antimicrobial combinations against CRAB. Conclusion: This study's findings underscore the significant challenges posed by CRAB isolates harboring multiple resistant genes in treatment. This highlights the urgent need for novel antimicrobial agents, a crucial step towards reducing mortality rates not only in Pakistan but also globally.

Genotypic and phenotypic mechanisms underlying antimicrobial resistance and synergistic efficacy of rifampicin-based combinations against carbapenem-resistant Acinetobacter baumannii

Purpose

Carbapenem-resistant Acinetobacter baumannii (CRAB) is an urgent concern to public health. This study focuses on exploring the resistance mechanisms and the in vitro results of using rifampicin in combination with conventional antibiotics for the management of CRAB.

Methods

The synergistic and bactericidal effects of rifampicin with conventional antibiotics were evaluated using chequerboard assay and time-kill assay, while the phenotypic and genotypic characteristics of resistant determinants were performed by efflux pump detection and whole genome sequencing on 29 isolates from ICU patients with underlying health diseases.

Results

The isolates showed multidrug resistance, with over 60% showing addictive responses to rifampicin-based combinations at FICI ranging from 0.6 to 0.8. The time-kill assay revealed 99 % killing for rifampicin and minocycline combination in one isolate at 1/4 MIC rifampicin plus 1/4 MIC minocycline, while a bacteriostatic effect was observed at 1/2 MIC rifampici plus 1/2 MIC for a second isolate. Combination with tigecycline resulted in a 99% killing in two out of three isolates with a 2.5-3 log reduction in CFU at 1/4 MIC rifampicin plus 1/4 MIC tigecycline. Rifampicin plus colistin exhibited bactericidal activity against three out of four isolates. The combinations of rifampicin with ciprofloxacin, chloramphenicol, and trimethoprim-sulfamethoxazole were ineffective against the isolates. In addition, a 4-fold reduction in rifampicin MIC was observed in 2 out of 14 isolates in the presence of an efflux pump inhibitor. The pan-genome study demonstrated a progressive evolution with an accessory genome estimated to cover 58% of the matrix. Seven of the ten sequenced isolates belong to sequence type 2 (ST2), while one isolate each was assigned to ST164, ST16, and ST25. Furthermore, 11 plasmids, 34 antimicrobial resistance (AMR) genes, and 65 virulence-associated genes were predicted from the whole genome data. The blaOXA-23blaADC-25, blaOXA-66, blaPER-7, aph(6)-Id, armA, and arr-3 were prevalent among the isolates. Sequence alignment of the bacteria genome to the reference strain revealed a deleterious mutation in the rpoB gene of 4 isolates.

Conclusion

The study suggests that rifampicin in combination with either minocycline, tigecycline, or colistin might be a treatment option for CRAB clinical isolates. In addition, genotypic analysis of the bacteria isolates may inform the clinician of the suitable drug regimen for the management of specific bacteria variants.

Exploring Synergistic Combinations in Extended and Pan-Drug Resistant (XDR and PDR) Whole Genome Sequenced Acinetobacter baumannii

Background: The diminishing antimicrobial options for the treatment of XDR and PDR Acinetobacter baumannii is an increasing concern. In this study, we assessed the in vitro synergy of the fosfomycin (FOS) with meropenem (MEM), amikacin (AK), tigecycline (TGC), and colistin (CL) in whole genome sequenced isolates. Methods: Non-replicate whole genome sequenced (illumina next-generation sequencing platform, Clevergene, India), A. baumanii (7 XDR, 1PDR) were subjected to in vitro synergy testing by checkerboard (CB) and time kill assay (TKA) after MIC determination, with glucose-6-phosphate being incorporated in all runs. FOS was used as a cornerstone drug in four combinations and colistin in one. ResFinder, MLST, PlasmidFinder, and CSIPhylogeny tools were used. Results: Mortality occurred in three patients. Diverse MLST were observed, ST-1962 (3 isolates) and one each of ST2062, ST2063, ST1816, ST1806, ST234. FOS MICs ranged from 32 to 128 mg/L, MEM MIC: 16-64 mg/L, TGC MIC: ≤2-≤4 mg/L and AK MIC: >512 mg/L. CL: MIC range, 0.25-≤2 mg/L, PDR MIC > 16 mg/L. Synergy results by CB: FOS-MEM: synergy in ⅞ (90%) isolates. Synergy lowered MEM MICs to susceptibility breakpoints in 6/8 cases.

CL-MEM

Excellent synergy (3/3) isolates.

FOS-AK

Indifference in ⅞, antagonism ⅛ (AK-susceptible isolate).

FOS-TGC

Partial synergy (PS) in 8/8 (TGC MIC dropped to ≤0.25 mg/L in 3/8). In the PDR isolate, synergy was seen in FOS-MEM, CL-MEM, PS in FOS-CL, FOS-TGC, indifference in FOS-AK. TKA: Excellent synergy was observed with FOS-MEM from 4 h, while FOS-AK and FOS-TGC demonstrated synergy at 24 h. Synergy was achieved despite presence of widespread resistance markers against aminoglycosides (AacAad, AadA, AadB, Aph3″Ia, ArmA, Arr, StrA, StrB), beta-lactams (ADC, BlaA1, BlaA2, Zn-dependent_hydrolase, OXA-23, OXA-51, PER-1,TEM-1D, CARB-5, Mbl), sulphonamides (SulII, SulI), phenicols (CatBx, CmlA), macrolides (MphE, MsrE) and tetracycline (TetB) were widespread. Carbapenemase, CARB-5 was present in one isolate. Beta-lactamase genes OXA-23, OXA-51, BlaA2, Zn-dependent_hydrolase, ADC, Mbl and macrolide resistance genes MphE, MsrE were present in all 8 isolates. Conclusions: FOS-MEM and CL-MEM are promising combinations against A. baumannii. Synergy of FOS-MEM in intrinsically resistant A. baumannii shows that this antibiotic combination might be useful in treating such XDR and PDR pathogens.

Carbapenem-Resistant Acinetobacter baumannii: Biofilm-Associated Genes, Biofilm-Eradication Potential of Disinfectants, and Biofilm-Inhibitory Effects of Selenium Nanoparticles

This study aimed to investigate the biofilm-production ability of carbapenem-resistant Acinetobacter baumannii (CRAB), the biofilm-eradication potential of 70% ethanol and 0.5% sodium hypochlorite, the effects of selenium nanoparticles (SeNPs) against planktonic and biofilm-embedded CRAB, and the relationship between biofilm production and bacterial genotypes. A total of 111 CRAB isolates were tested for antimicrobial susceptibility, biofilm formation, presence of the genes encoding carbapenemases, and biofilm-associated virulence factors. The antibiofilm effects of disinfectants and SeNPs against CRAB isolates were also tested. The vast majority of the tested isolates were biofilm producers (91.9%). The bap, ompA, and csuE genes were found in 57%, 70%, and 76% of the CRAB isolates, with the csuE being significantly more common among biofilm producers (78.6%) compared to non-biofilm-producing CRAB (25%). The tested disinfectants showed a better antibiofilm effect on moderate and strong biofilm producers than on weak producers (p < 0.01). The SeNPs showed an inhibitory effect against all tested planktonic (MIC range: 0.00015 to >1.25 mg/mL) and biofilm-embedded CRAB, with a minimum biofilm inhibitory concentration of less than 0.15 mg/mL for 90% of biofilm producers. In conclusion, SeNPs might be used as promising therapeutic and medical device coating agents, thus serving as an alternative approach for the prevention of biofilm-related infections.

Evaluation of Fosfomycin-Sulbactam Combination Therapy against Carbapenem-Resistant Acinetobacter baumannii Isolates in a Hollow-Fibre Infection Model

Static concentration in vitro studies have demonstrated that fosfomycin- or sulbactam-based combinations may be efficacious against carbapenem-resistant Acinetobacter baumannii (CRAB). In the present study, we aimed to evaluate the bacterial killing and resistance suppression potential of fosfomycin-sulbactam combination therapies against CRAB isolates in a dynamic infection model. We simulated clinically relevant dosing regimens of fosfomycin (8 g every 8 h, 1 h infusion) and sulbactam (12 g continuous infusion or 4 g every 8 h, 4 h infusion) alone and in combination for 7 days in a hollow-fibre infection model (HFIM) against three clinical isolates of CRAB. The simulated pharmacokinetic profiles in the HFIM were based on fosfomycin and sulbactam data from critically ill patients. Fosfomycin monotherapy resulted in limited bacterial killing. Sulbactam monotherapies resulted in ~ 3 to 4 log10 kill within the first 8 to 32 h followed by regrowth of up to 8 to 10 log10 CFU/mL. A combination of fosfomycin and continuous infusion of sulbactam led to a ~2 to 4 log10 reduction in bacterial burden within the first 24 h, which was sustained throughout the duration of the experiments. A combination of fosfomycin and extended infusion of sulbactam produced a ~4 log10 reduction in colony count within 24 h. This study demonstrated that fosfomycin in combination with sulbactam is a promising option for the treatment of MDR A. baumannii. Further studies are needed to further assess the potential clinical utility of this combination.

Genetic Configuration of Genomic Resistance Islands in Acinetobacter baumannii Clinical Isolates From Egypt

In Acinetobacter baumannii (A. baumannii), a wide repertoire of resistance genes is often carried within genomic resistance islands (RIs), particularly in high-risk global clones (GCs). As the first in Egypt, the current study aimed at exploring the diversity and genetic configuration of RIs in the clinical isolates of A. baumannii. For this purpose, draft genomes of 18 isolates were generated by Illumina sequencing. Disk diffusion susceptibility profiling revealed multidrug resistance (MDR) and extensive drug resistance (XDR) phenotypes in 27.7 and 72.2%, respectively. The highest susceptibility was noted for tigecycline (100.0%) followed by colistin (94.4%), for which an MIC₅₀ of 0.25 μg/ml was recorded by the broth microdilution assay. Sequence typing (ST) showed that the majority of the isolates belonged to high-risk global clones (GC1, GC2, and GC9). A novel Oxford sequence type (ST2329) that also formed a novel clonal complex was submitted to the PubMLST database. A novel bla_ADC variant (bla_ADC−258) was also identified in strain M18 (ST85^Pas/1089^Oxf). In addition to a wide array of resistance determinants, whole-genome sequencing (WGS) disclosed at least nine configurations of genomic RIs distributed over 16/18 isolates. GC2 isolates accumulated the largest number of RIs (three RIs/isolate) followed by those that belong to GC1 (two RIs/isolate). In addition to Tn6022 (44.4%), the comM gene was interrupted by AbaR4 (5.5%) and three variants of A. baumanniigenomic resistance island 1(AbGRI)-type RIs (44.4%), including AbaR4b (16.6%) and two novel configurations of AbGRI1-like RIs (22.2%). Three of which (AbaR4, AbaR4b, and AbGRI1-like-2) carried bla_OXA−23 within Tn2006. With less abundance (38.8%), IS26-bound RIs were detected exclusively in GC2 isolates. These included a short version of AbGRI2 (AbGRI2-15) carrying the genes bla_TEM−1 and aphA1 and two variants of AbGRI3 RIs carrying up to seven resistance genes [mphE-msrE-armA-sul1-aadA1-catB8-aacA4]. Confined to GC1 (22.2%), sulfonamide resistance was acquired by an ISAba1 bracketed GIsul2 RI. An additional RI (RI-PER-7) was also identified on a plasmid carried by strain M03. Among others, RI-PER-7 carried the resistance genes armA and bla_PER−7. Here, we provided a closer view of the diversity and genetic organization of RIs carried by a previously unexplored population of A. baumannii.

Selection and Identification of a DNA Aptamer for Multidrug-Resistant Acinetobacter baumannii Using an In-House Cell-SELEX Methodology

Infections caused by multidrug-resistant A. baumannii are a worldwide health concern with high mortality rates. Rapid identification of this infectious agent is critical as it can easily spread with difficult or no options for treatment. In this context, the development of reliable and economically viable detection and therapeutic methodologies are still challenging. One of the promising solutions is the development of nucleic acid aptamers capable of interacting with bacteria. These aptamers can be used for specific recognition of infectious agents as well as for blocking their functions. Cell-SELEX technology currently allows the selection and identification of aptamers and is flexible enough to target molecules present in an entire bacterial cell without their prior knowledge. However, the aptamer technology is still facing many challenges, such as the complexity of the screening process. Here, we describe the selection and identification of a new aptamer A01, using an in-house whole-cell SELEX-based methodology, against multi-resistant Acinetobacter baumannii, with rapid execution and low cost. In addition, this protocol allowed the identification of the aptamer A01 with the whole A. baumannii cell as a target. The aptamer A01 demonstrated a binding preference to A. baumannii when compared to K. pneumoniae, C. albicans, and S. aureus in fluorescence assays. Although the time-kill assay did not show an effect on bacterial growth, the potential bactericidal or bacteriostatic cannot be totally discarded. The new categorized aptamer (A01) displayed a significant binding affinity to MDR A. baumannii.

A C∧S-Cyclometallated Gold(III) Complex as a Novel Antibacterial Candidate Against Drug-Resistant Bacteria

The worldwide emergence and spread of infections caused by multidrug-resistant bacteria endangers the efficacy of current antibiotics in the clinical setting. The lack of new antibiotics in the pipeline points to the need of developing new strategies. Recently, gold-based drugs are being repurposed for antibacterial applications. Among them, gold(III) complexes have received increasing attention as metal-based anticancer agents. However, reports on their antibacterial activity are scarce due to stability issues. The present work demonstrates the antibacterial activity of the gold(III) complex 2 stabilized as C∧S-cycloaurated containing a diphenylphosphinothioic amide moiety, showing minimum inhibitory concentration (MIC) values that ranged from 4 to 8 and from 16 to 32 mg/L among Gram-positive and Gram-negative multidrug-resistant (MDR) pathogens, respectively. Complex 2 has a biofilm inhibitory activity of only two to four times than its MIC. We also describe for the first time a potent antibacterial synergistic effect of a gold(III) complex combined with colistin, showing a bactericidal effect in less than 2 h; confirming the role of the outer membrane as a permeability barrier. Complex 2 shows a low rate of internalization in Staphylococcus aureus and Acinetobacter baumannii; it does not interact with replication enzymes or efflux pumps, causes ultrastructural damages in both membrane and cytoplasmic levels, and permeabilizes the bacterial membrane. Unlike control antibiotics, complex 2 did not generate resistant mutants in 30-day sequential cultures. We detected lower cytotoxicity in a non-tumoral THLE-2 cell line (IC50 = 25.5 μM) and no acute toxicity signs in vivo after an i.v. 1-mg/kg dose. The characterization presented here reassures the potential of complex 2 as a new chemical class of antimicrobial agents.

In search for a synergistic combination against pandrug-resistant A. baumannii; methodological considerations

PURPOSE

Pending approval of new antimicrobials, synergistic combinations are the only treatment option against pandrug-resistant A. baumannii (PDRAB). Considering the lack of a standardized methodology, the aim of this manuscript is to systematically review the methodology and discuss unique considerations for assessing antimicrobial combinations against PDRAB.

METHODS

Post-hoc analysis of a systematic review (conducted in PubMed and Scopus from inception to April 2021) of studies evaluating antimicrobial combination against A. baumannii, based on antimicrobials that are inactive in vitro alone.

RESULTS

Eighty-four publications were reviewed, using a variety of synergy testing methods, including; gradient-based methods (n = 11), disk-based methods (n = 6), agar dilution (n = 2), checkerboard assay (n = 44), time-kill assay (n = 50), dynamic in vitro PK/PD models (n = 6), semi-mechanistic PK/PD models (n = 5), and in vivo animal models (n = 11). Several variations in definitions of synergy and interpretation of each method were observed and are discussed. Challenges related to testing combinations of antimicrobials that are inactive alone (with regards to concentrations at which the combinations are assessed), as well as other considerations (assessment of stasis vs killing, clinical relevance of re-growth in vitro after initial killing, role of in vitro vs in vivo conditions, challenges of clinical testing of antimicrobial combinations against PDRAB infections) are discussed.

CONCLUSION

This review demonstrates the need for consensus on a standardized methodology and clinically relevant definitions for synergy. Modifications in the methodology and definitions of synergy as well as a roadmap for further development of antimicrobial combinations against PDRAB are proposed.

Prevalence of High-Risk Antibiotic Resistant Acinetobacter baumannii in the Holy Cities of Makkah and Al-Madinah

Background:

Acinetobacter baumannii strains resistant to carbapenems are a global public health problem.

Objectives:

The aim of the present study is to evaluate the prevalence of genetic fingerprints associated with Metallo β-lactamases in A. baumannii in addition to the clonal diversity of A. baumannii in Makkah and Al-Madinah regions of Saudi Arabia, which receive a high number of international visitors.

Methods:

Multi-antibiotic resistant A. baumannii isolates were investigated. Bacterial isolation was conducted employing a basic bacteriological technique after confirming the ID of isolates. The antimicrobial susceptibility test was carried out using the Vitek 2 compact system. The molecular clonal diversity of the isolates was determined by Pulse Field Gel Electrophoresis (PFGE). Clusters were analyzed with BioNumerics software version 6.5. Dice coefficient was used for calculating the similarities.

Results:

The results indicated resistance in 82.5% of A. baumannii isolates against the carbapenems. All the isolates were found to be sensitive to colistin, while 5% of isolates were resistant to tigecycline. The screening of carbapenem-resistant A. baumannii isolates showed that the dissemination of imipenem and meropenem resistance was 81 and 84%, respectively, while the majority of the strains were susceptible to tigecycline and colistin. The blaOXA and blaVIM were the most encountered genes in A. baumannii isolates, while ISAba1 was the prominent insertion sequence. The genetic fingerprinting results (PFGE) revealed two types of epidemic clones: monoclonal and polyclonal models of 17 clusters.

Conclusion:

The current investigation indicates the diversity in genetic fingerprints of carbapenem-resistant A. baumannii in Makkah and Al-Madinah region of Saudi Arabia, and that two types of epidemic clones are present. It has also been demonstrated that such clones create serious infection dissemination to other parts of the world as heavy pilgrimage traffic is received throughout the year in Makkah and Al-Madinah, especially in the Haj season.

Systematic Review of Antimicrobial Combination Options for Pandrug-Resistant Acinetobacter baumannii

Antimicrobial combinations are at the moment the only potential treatment option for pandrug-resistant A. baumannii. A systematic review was conducted in PubMed and Scopus for studies reporting the activity of antimicrobial combinations against A. baumannii resistant to all components of the combination. The clinical relevance of synergistic combinations was assessed based on concentrations achieving synergy and PK/PD models. Eighty-four studies were retrieved including 818 eligible isolates. A variety of combinations (n = 141 double, n = 9 triple) were tested, with a variety of methods. Polymyxin-based combinations were the most studied, either as double or triple combinations with cell-wall acting agents (including sulbactam, carbapenems, glycopeptides), rifamycins and fosfomycin. Non-polymyxin combinations were predominantly based on rifampicin, fosfomycin, sulbactam and avibactam. Several combinations were synergistic at clinically relevant concentrations, while triple combinations appeared more active than the double ones. However, no combination was consistently synergistic against all strains tested. Notably, several studies reported synergy but at concentrations unlikely to be clinically relevant, or the concentration that synergy was observed was unclear. Selecting the most appropriate combinations is likely strain-specific and should be guided by in vitro synergy evaluation. Furthermore, there is an urgent need for clinical studies on the efficacy and safety of such combinations.

Recent chemical syntheses of bacteria related oligosaccharides using modern expeditious approaches

Apart from some essential and crucial roles in life processes carbohydrates also are involved in a few detrimental courses of action related to human health, like infections by pathogenic microbes, cancer metastasis, transplanted tissue rejection, etc. Regarding management of pathogenesis by microbes, keeping in mind of multi drug-resistant bacteria and epidemic or endemic incidents, preventive measure by vaccination is the best pathway as also recommended by the WHO; by vaccination, eradication of bacterial diseases is also possible. Although some valid vaccines based on attenuated bacterial cells or isolated pure polysaccharide-antigens or the corresponding conjugates thereof are available in the market for prevention of several bacterial diseases, but these are not devoid of some disadvantages also. In order to develop improved conjugate T-cell dependent vaccines oligosaccharides related to bacterial antigens are synthesized and converted to the corresponding carrier protein conjugates. Marketed Cuban Quimi-Hib is such a vaccine being used since 2004 to resist Haemophilus influenza b infections. During nearly the past two decades research is going on worldwide for improved synthesis of bacteria related oligosaccharides or polysaccharides towards development of such semisynthetic or synthetic glycoconjugate vaccines. The present dissertation is an endeavour to encompass the recent syntheses of several pathogenic bacterial oligosaccharides or polysaccharides, made during the past ten-eleven years with special reference to modern expeditious syntheses.

Pandrug-resistant Gram-negative bacteria: a systematic review of current epidemiology, prognosis and treatment options

BACKGROUND

The literature on the epidemiology, mortality and treatment of pandrug-resistant (PDR) Gram-negative bacteria (GNB) is scarce, scattered and controversial.

OBJECTIVES

To consolidate the relevant literature and identify treatment options for PDR GNB infections.

METHODS

A systematic search in MEDLINE, Scopus and clinical trial registries was conducted. Studies reporting PDR clinical isolates were eligible for review if susceptibility testing for all major antimicrobials had been performed. Characteristics and findings of retrieved studies were qualitatively synthesized.

RESULTS

Of 81 studies reviewed, 47 (58%) were published in the last 5 years. The reports reflected a worldwide dissemination of PDR GNB in 25 countries in 5 continents. Of 526 PDR isolates reported, Pseudomonas aeruginosa (n=175), Acinetobacter baumannii (n=172) and Klebsiella pneumoniae (n=125) were most common. PDR GNB were typically isolated in ICUs, but several studies demonstrated wider outbreak potential, including dissemination to long-term care facilities and international spread. All-cause mortality was high (range 20%-71%), but appeared to be substantially reduced in studies reporting treatment regimens active in vitro. No controlled trial has been performed to date, but several case reports and series noted successful use of various regimens, predominantly synergistic combinations, and in selected patients increased exposure regimens and newer antibiotics.

CONCLUSIONS

PDR GNB are increasingly being reported worldwide and are associated with high mortality. Several treatment regimens have been successfully used, of which synergistic combinations appear to be most promising and often the only available option. More pharmacokinetic/pharmacodynamic and outcome studies are needed to guide the use of synergistic combinations.

Identification of a bacteria-produced benzisoxazole with antibiotic activity against multi-drug resistant Acinetobacter baumannii

The emergence of multi-drug resistant pathogenic bacteria represents a serious and growing threat to national healthcare systems. Most pressing is an immediate need for the development of novel antibacterial agents to treat Gram-negative multi-drug resistant infections, including the opportunistic, hospital-derived pathogen, Acinetobacter baumannii. Herein we report a naturally occurring 1,2-benzisoxazole with minimum inhibitory concentrations as low as 6.25 μg ml-1 against clinical strains of multi-drug resistant A. baumannii and investigate its possible mechanisms of action. This molecule represents a new chemotype for antibacterial agents against A. baumannii and is easily accessed in two steps via de novo synthesis. In vitro testing of structural analogs suggest that the natural compound may already be optimized for activity against this pathogen. Our results demonstrate that supplementation of 4-hydroxybenzoate in minimal media was able to reverse 1,2-benzisoxazole's antibacterial effects in A. baumannii. A search of metabolic pathways involving 4-hydroxybenzoate coupled with molecular modeling studies implicates two enzymes, chorismate pyruvate-lyase and 4-hydroxybenzoate octaprenyltransferase, as promising leads for the target of 3,6-dihydroxy-1,2-benzisoxazole.

Genetic factors involved in fosfomycin resistance of multidrug-resistant Acinetobacter baumannii

The treatment of infections caused by A. baumannii is a challenge and fosfomycin has been used as a combination therapy. Moreover, data regarding the fosfomycin resistance mechanism is scarce. The goals of this study were to evaluate fosfomycin susceptibility in polyclonal multi-resistant A. baumannii isolates and characterize the fosfomycin resistance. We analyzed 32 A. baumannii isolates from a Brazilian bacterial collection, followed by their minimum inhibitory concentration (MIC), and whole-genome sequence to detect fosfomycin resistance genes. The isolates showed a fosfomycin MIC ranging from 32 to ≥256 mg/L. All isolates were negative for fosA and fosB genes, and four isolates carried the fosX gene. Two different metabolic pathways that form peptidoglycan precursors were identified. Mutations were observed in the adenylate cyclase gene. All A. baumannii isolates studied showed Val132Ala substitutions in MurA. The analysis showed different ways that may lead to the intrinsic fosfomycin-resistance of A. baumannii, such as alterations on the glycerol-3-phosphate transporter system caused by adenylate cyclase mutations; and a possible connection of cell wall recycling by different metabolic pathways.

Systematic review and meta-analysis of in vitro efficacy of antibiotic combination therapy against carbapenem-resistant Gram-negative bacilli

The superiority of combination therapy for carbapenem-resistant Gram-negative bacilli (CR-GNB) infections remains controversial. In vitro models may predict the efficacy of antibiotic regimens against CR-GNB. A systematic review and meta-analysis was performed including pharmacokinetic/pharmacodynamic (PK/PD) and time-kill (TK) studies examining the in vitro efficacy of antibiotic combinations against CR-GNB [PROSPERO registration no. CRD42019128104]. The primary outcome was in vitro synergy based on the effect size (ES): high, ES ≥ 0.75, moderate, 0.35 < ES < 0.75; low, ES ≤ 0.35; and absent, ES = 0). A network meta-analysis assessed the bactericidal effect and re-growth rate (secondary outcomes). An adapted version of the ToxRTool was used for risk-of-bias assessment. Over 180 combination regimens from 136 studies were included. The most frequently analysed classes were polymyxins and carbapenems. Limited data were available for ceftazidime/avibactam, ceftolozane/tazobactam and imipenem/relebactam. High or moderate synergism was shown for polymyxin/rifampicin against Acinetobacter baumannii [ES = 0.91, 95% confidence interval (CI) 0.44-1.00], polymyxin/fosfomycin against Klebsiella pneumoniae (ES = 1.00, 95% CI 0.66-1.00) and imipenem/amikacin against Pseudomonas aeruginosa (ES = 1.00, 95% CI 0.21-1.00). Compared with monotherapy, increased bactericidal activity and lower re-growth rates were reported for colistin/fosfomycin and polymyxin/rifampicin in K. pneumoniae and for imipenem/amikacin or imipenem/tobramycin against P. aeruginosa. High quality was documented for 65% and 53% of PK/PD and TK studies, respectively. Well-designed in vitro studies should be encouraged to guide the selection of combination therapies in clinical trials and to improve the armamentarium against carbapenem-resistant bacteria.

Rescuing the Last-Line Polymyxins: Achievements and Challenges

Antibiotic resistance is a major global health challenge and, worryingly, several key Gram negative pathogens can become resistant to most currently available antibiotics. Polymyxins have been revived as a last-line therapeutic option for the treatment of infections caused by multidrug-resistant Gram negative bacteria, in particular Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacterales. Polymyxins were first discovered in the late 1940s but were abandoned soon after their approval in the late 1950s as a result of toxicities (e.g., nephrotoxicity) and the availability of "safer" antibiotics approved at that time. Therefore, knowledge on polymyxins had been scarce until recently, when enormous efforts have been made by several research teams around the world to elucidate the chemical, microbiological, pharmacokinetic/pharmacodynamic, and toxicological properties of polymyxins. One of the major achievements is the development of the first scientifically based dosage regimens for colistin that are crucial to ensure its safe and effective use in patients. Although the guideline has not been developed for polymyxin B, a large clinical trial is currently being conducted to optimize its clinical use. Importantly, several novel, safer polymyxin-like lipopeptides are developed to overcome the nephrotoxicity, poor efficacy against pulmonary infections, and narrow therapeutic windows of the currently used polymyxin B and colistin. This review discusses the latest achievements on polymyxins and highlights the major challenges ahead in optimizing their clinical use and discovering new-generation polymyxins. To save lives from the deadly infections caused by Gram negative "superbugs," every effort must be made to improve the clinical utility of the last-line polymyxins. SIGNIFICANCE STATEMENT: Antimicrobial resistance poses a significant threat to global health. The increasing prevalence of multidrug-resistant (MDR) bacterial infections has been highlighted by leading global health organizations and authorities. Polymyxins are a last-line defense against difficult-to-treat MDR Gram negative pathogens. Unfortunately, the pharmacological information on polymyxins was very limited until recently. This review provides a comprehensive overview on the major achievements and challenges in polymyxin pharmacology and clinical use and how the recent findings have been employed to improve clinical practice worldwide.

Polymyxin Resistance Among XDR ST1 Carbapenem-Resistant Acinetobacter baumannii Clone Expanding in a Teaching Hospital

Acinetobacter baumannii is an opportunistic pathogen primarily associated with multidrug-resistant nosocomial infections, for which polymyxins are the last-resort antibiotics. This study investigated carbapenem-resistant A. baumannii strains exhibiting an extensively drug-resistant (XDR) phenotype, including four isolates considered locally pan drug-resistant (_LPDR), isolated from inpatients during an outbreak at a teaching hospital in Brazil. ApaI DNA macrorestriction followed by PFGE clustered the strains in three pulsotypes, named A to C, among carbapenem-resistant A. baumannii strains. Pulsotypes A and B clustered six polymyxin-resistant A. baumannii strains. MLST analysis of representative strains of pulsotypes A, B, and C showed that they belong, respectively, to sequence types ST1 (clonal complex, CC1), ST79 (CC79), and ST903. Genomic analysis of international clones ST1 and ST79 representative strains predicted a wide resistome for β-lactams, aminoglycosides, fluoroquinolones, and trimethoprim-sulfamethoxazole, with bla_OXA–23 and bla_OXA–72 genes encoding carbapenem resistance. Amino acid substitutions in PmrB (Thr232Ile or Pro170Leu) and PmrC (Arg125His) were responsible for polymyxin resistance. Although colistin MICs were all high (MIC ≥ 128 mg/L), polymyxin B MICs varied; strains with Pro170Leu substitution in PmrB had MICs > 128 mg/L, while those with Thr232Ile had lower MICs (16–64 mg/L), irrespective of the clone. Although the first identified polymyxin-resistant A. baumannii strain belonged to ST79, the ST1 strains were endemic and caused the outbreak most likely due to polymyxin B use. The genome comparison of two ST1 strains from the same patient, but one susceptible and the other resistant to polymyxin, revealed mutations in 28 ORFs in addition to pmrBC. The ORF codifying an acyl-CoA dehydrogenase has gained attention due to its fatty acid breakdown and membrane fluidity involvement. However, the role of these mutations in the polymyxin resistance mechanism remains unknown. To prevent the dissemination of XDR bacteria, the hospital infection control committee implemented the patient bathing practice with a 2% chlorhexidine solution, a higher concentration than all A. baumannii chlorhexidine MICs. In conclusion, we showed the emergence of polymyxin resistance due to mutations in the chromosome of the carbapenem-resistant A. baumannii ST1, a high-risk global clone spreading in this hospital.

Evaluation of the Synergistic Antibacterial Effects of Fosfomycin in Combination with Selected Antibiotics against Carbapenem-Resistant Acinetobacter baumannii

The spread of multi-drug resistant (MDR) pathogens and the lagging pace in the development of novel chemotherapeutic agents warrant the use of combination therapy as a reliable, cost-effective interim option. In this study, the synergistic effects of fosfomycin in combination with other antibiotics were assessed. Of the 193 isolates, 90.6% were non-susceptible to fosfomycin, with minimum inhibitory concentrations (MICs) of ≥128 µg/mL. Antibacterial evaluation of fosfomycin-resistant isolates indicated multi-drug resistance to various antibiotic classes. Combinations of fosfomycin with 12 commonly used antibiotics synergistically inhibited most fosfomycin-resistant isolates. The fractional inhibitory concentration index indicated that combining fosfomycin with either aminoglycosides, glycylcyclines, fluoroquinolones, or colistin resulted in 2- to 16-fold reduction in the MIC of fosfomycin. Time-kill kinetics further confirmed the synergistic bactericidal effects of fosfomycin in combination with either amikacin, gentamicin, tobramycin, minocycline, tigecycline, or colistin, with more than 99.9% reduction in bacterial cells. Fosfomycin-based combination therapy might serve as an alternative option for the treatment of MDR A. baumannii. Further steps including in vivo efficacy and toxicity in experimental models of infection are required prior to clinical applications.

Genetic diversity of colistin resistance Nosocomial Acinetobacter baumannii strains from Iran

Background:

Drug-resistant Acinetobacter baumannii is a global health problem since its ability to acquire new resistance mechanisms. Here, we aimed to determine the association of common types of A. baumannii and assess their drug resistance of A. baumannii and contribution of integrons (Ints) and oxacillinase genes in Zanjan, Iran.

Materials and Methods:

Among 68 isolated Acinetobacters from patients, 48 isolates were A. baumannii strains. Antibiotic susceptibility pattern and colistin resistance were determined by disk diffusion and broth microdilution, respectively. The presence of Int I, II, III, and oxacillinase genes examined using polymerase chain reaction. The clonal relationship of clinical isolates of A. baumannii determined by Pulsed Field Gel Electrophoresis method.

Results:

The results showed the highest antibiotic susceptibility (58%) for colistin. 96% of isolates were considered as multidrug resistant, and 46% as extensively drug resistant, and 16% as pandrug resistant. Frequencies of Int I, II, III resistance genes were 60%, 28%, and 0%, respectively, and 12% of strains had no isoform of Ints. Frequencies of Carbapenem resistance genes were 74%, 24%, 100%, and 4% for blaOXA-23, blaOXA-24, blaOXA-51, and blaOXA-58, respectively. The above samples were group into 26 pulsotypes.

Conclusion:

The studied A. baumannii strains had several resistance genes, and the colistin resistance showed an extraordinary ascending tendency that could be a severe issue in nosocomial infections, and the presence of high genetic diversity indicated a variation in A. baumannii strains and possibly a variety of sources of contamination or infection.

Characterization of carbapenem-resistant hypervirulent Acinetobacter baumannii strains isolated from hospitalized patients in the mid-south region of China

Background

Acinetobacter baumannii has traditionally been considered an opportunistic pathogen with low virulence. In this study, we characterized the carbapenem-resistant hypervirulent A. baumannii (CR-hvAB) stains isolated from our hospital in mid-south region of China.

Results

Blood samples collected between January 2017 and May 2019 were used for virulence experiments and biofilm assays of individual carbapenem-resistant A. baumannii (CR-AB) strains, performed using a Galleria mellonella infection model and crystal violet staining method, respectively. CR-AB isolates that induced high mortality in the G. mellonella infection model were subjected to genotyping, susceptibility testing, and clinical data analysis, and the genetic characterization of these isolates was performed by whole-genome sequencing (WGS). Among the 109 CR-AB clinical strains, the survival rate of G. mellonella larvae infected with 7 (6.4%) CR-AB isolates (number of strains with mortality of 0, 10 and 20% was 4, 1, and 2, respectively), was significantly lower than that of A. baumannii ATCC 19606 (100.0%) and the remaining CR-AB isolates (> 80.0%). Consistent with these results, patients infected with these seven isolates had an average 7-day mortality rate of 42.9%, suggesting that the isolates were CR-hvAB. These seven isolates belonged to four sequence types (STs): ST457, ST195, ST369, and ST2088 (a new ST), and mainly ST457 (n = 4). The results of the biofilm study showed that eight strains had powerful biofilm ability (strong [n = 1] and moderate [n = 7] biofilm producers) including these seven CR-hvAB isolates.

Conclusions

CR-hvAB isolates that induced a high mortality rate were cloned in our hospital, most of which belonged to ST457; thus, monitoring of these strains, particularly ST457, should be strengthened in the future. Meanwhile, A. baumannii, which was isolated from blood specimens and found to powerful biofilm-forming ability, is a probable hvAB isolate.

Treatment options for K. pneumoniae, P. aeruginosa and A. baumannii co-resistant to carbapenems, aminoglycosides, polymyxins and tigecycline: an approach based on the mechanisms of resistance to carbapenems

The management of carbapenem-resistant infections is often based on polymyxins, tigecycline, aminoglycosides and their combinations. However, in a recent systematic review, we found that Gram-negative bacteria (GNB) co-resistant to carbapanems, aminoglycosides, polymyxins and tigecycline (CAPT-resistant) are increasingly being reported worldwide. Clinical data to guide the treatment of CAPT-resistant GNB are scarce and based exclusively on few case reports and small case series, but seem to indicate that appropriate (in vitro active) antimicrobial regimens, including newer antibiotics and synergistic combinations, may be associated with lower mortality. In this review, we consolidate the available literature to inform clinicians dealing with CAPT-resistant GNB about treatment options by considering the mechanisms of resistance to carbapenems. In combination with rapid diagnostic methods that allow fast detection of carbapenemase production, the approach proposed in this review may guide a timely and targeted treatment of patients with infections by CAPT-resistant GNB. Specifically, we focus on the three most problematic species, namely Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii. Several treatment options are currently available for CAPT-resistant K. pneumonia. Newer β-lactam-β-lactamase combinations, including the combination of ceftazidime/avibactam with aztreonam against metallo-β-lactamase-producing isolates, appear to be more effective compared to combinations of older agents. Options for P. aeruginosa (especially metallo-β-lactamase-producing strains) and A. baumannii remain limited. Synergistic combination of older agents (e.g., polymyxin- or fosfomycin-based synergistic combinations) may represent a last resort option, but their use against CAPT-resistant GNB requires further study.

Fosfomycin as Partner Drug for Systemic Infection Management. A Systematic Review of Its Synergistic Properties from In Vitro and In Vivo Studies

Fosfomycin is being increasingly prescribed for multidrug-resistant bacterial infections. In patients with systemic involvement, intravenous fosfomycin is usually administered as a partner drug, as part of an antibiotic regimen. Hence, the knowledge of fosfomycin pharmacodynamic interactions (synergistic, additive, indifferent and antagonistic effect) is fundamental for a proper clinical management of severe bacterial infections. We performed a systematic review to point out fosfomycin’s synergistic properties, when administered with other antibiotics, in order to help clinicians to maximize drug efficacy optimizing its use in clinical practice. Interactions were more frequently additive or indifferent (65.4%). Synergism accounted for 33.7% of total interactions, while antagonism occurred sporadically (0.9%). Clinically significant synergistic interactions were mostly distributed in combination with penicillins (51%), carbapenems (43%), chloramphenicol (39%) and cephalosporins (33%) in Enterobactaerales; with linezolid (74%), tetracyclines (72%) and daptomycin (56%) in Staphylococcus aureus; with chloramphenicol (53%), aminoglycosides (43%) and cephalosporins (36%) against Pseudomonas aeruginosa; with daptomycin (97%) in Enterococcus spp. and with sulbactam (75%) and penicillins (60%) and in Acinetobacter spp. fosfomycin-based antibiotic associations benefit from increase in the bactericidal effect and prevention of antimicrobial resistances. Taken together, the presence of synergistic interactions and the nearly total absence of antagonisms, make fosfomycin a good partner drug in clinical practice.

The role of fosfomycin for multidrug-resistant gram-negative infections

PURPOSE OF REVIEW

In the last decade, an increasing interest in using fosfomycin for the treatment of multidrug-resistant gram-negative (MDR-GNB) infections have been registered, especially when none or only a few other active alternatives remained available.

RECENT FINDINGS

Fosfomycin may remain active against a considerable proportion of MDR-GNB. In observational studies, a possible curative effect of oral fosfomycin monotherapy has been described for uncomplicated urinary tract infections (UTI) and bacterial prostatitis caused by MDR-GNB, whereas intravenous fosfomycin has been mostly used in combination with other agents for various type of severe MDR-GNB infections. The ZEUS randomized controlled trial (RCT) has started to provide high-level evidence about the possible use of fosfomycin for complicated UTI caused by extended-spectrum β-lactamase-producing GNB, but no results of large RCT are currently available to firmly guide the use of fosfomycin for carbapenem-resistant GNB.

SUMMARY

Fosfomycin is an important therapeutic option for MDR-GNB infections. Further pharmacokinetic/pharmacodynamic and clinical research is needed to optimize its use.

In Vitro Evaluation of Antimicrobial Peptide Tridecaptin M in Combination with Other Antibiotics against Multidrug Resistant Acinetobacter baumannii

The rapid emergence of antimicrobial resistance in Acinetobacter baumannii coupled with the dried pipeline of novel treatments has driven the search for new therapeutic modalities. Gram-negative bacteria have an extra outer membrane that serves as a permeability barrier for various hydrophobic and/or large compounds. One of the popular approaches to tackle this penetration barrier is use of potentiators or adjuvants in combination with traditional antibiotics. This study reports the in vitro potential of an antimicrobial peptide tridecaptin M in combination with other antibiotics against different strains of A. baumannii. Tridecaptin M sensitized the bacteria to rifampicin, vancomycin, and ceftazidime. Further, we observed that a tridecaptin M and rifampicin combination killed the bacteria completely in 4 h in an ex vivo blood infection model and was superior to rifampicin monotherapy. The study also found that concomitant administration of both compounds is not necessary to achieve the antimicrobial effect. Bacteria pre-treated with tridecaptin M (for 2-4 h) followed by exposure to rifampicin showed similar killing as obtained for combined treatment. Additionally, this combination hampered the survival of persister development in comparison to rifampicin alone. These findings encourage the future investigation of this combination to treat severe infections caused by extremely drug-resistant A. baumannii.

The first nationwide multicenter study of Acinetobacter baumannii recovered in Serbia: emergence of OXA-72, OXA-23 and NDM-1-producing isolates

Background

The worldwide emergence and clonal spread of carbapenem-resistant Acinetobacter baumannii (CRAB) is of great concern. The aim of this nationwide study was to investigate the prevalence of CRAB isolates in Serbia and to characterize underlying resistance mechanisms and their genetic relatedness.

Methods

Non-redundant clinical samples obtained from hospitalized patients throughout Serbia were included in the prospective, observational, multicenter study conducted from January to June 2018. Samples were initially screened for the presence of Acinetobacter baumannii-calcoaceticus (Acb) complex using conventional bacteriological techniques. Acb complexes recovered from clinical samples obtained from inpatients with confirmed bacterial infections were further evaluated for the presence of A. baumannii. Identification to the species level was done by the detection of the bla_OXA-51 gene and rpoB gene sequence analysis. Susceptibility testing was done by disk diffusion and broth microdilution method. CRAB isolates were tested for the presence of acquired carbapenemases (bla_OXA-24-like, bla_OXA-23-like,bla_OXA-58-like, bla_OXA-143-like, bla_IMP, bla_VIM, bla_GIM, bla_SPM, bla_SIM, bla_NDM) by PCR. Clonal relatedness was assessed by pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST).

Results

Acb complex was isolated in 280 out of 2401 clinical samples (11.6%). Overall, A. baumannii was identified in 237 out of 280 Acb complex (84.6%). CRAB prevalence was found to be 93.7% (237/222). The MIC₅₀/MIC₉₀ for imipenem and meropenem were 8/> 32 μg/mL and 16/> 32 μg/mL, respectively. Although susceptibility was high for colistin (95.7%; n = 227) and tigecycline (75.1%; n = 178), ten isolates (4.3%) were classified as pandrug-resistant. The following carbapenemases-encoding genes were found: 98 (44.2%) bla_OXA-24-like, 76 (34.5%) bla_OXA-23-like, and 7 (3.2%) bla_NDM-1. PFGE analysis revealed six different clusters. MLST analysis identified three STs: ST2 (n = 13), ST492 (n = 14), and ST636 (n = 10). Obtained results evaluated that circulating CRAB clones in Serbia were as follows: bla_OXA66/bla_OXA23/ST2 (32.4%), bla_OXA66/bla_OXA23/bla_OXA72/ST2 (2.7%), bla_OXA66/bla_OXA72/ST492 (37.8%), and bla_OXA66/bla_OXA72/ST636 (27.1%).

Conclusion

This study revealed extremely high proportions of carbapenem resistance among A. baumannii clinical isolates due to the emergence of bla_OXA-72, bla_OXA-23, and bla_NDM-1 genes among CRAB isolates in Serbia and their clonal propagation.

A survey of within-host and between-hosts modelling for antibiotic resistance

Antibiotic resistance is a global public health problem which has the attention of many stakeholders including clinicians, the pharmaceutical industry, researchers and policy makers. Despite the existence of many studies, control of resistance transmission has become a rather daunting task as the mechanisms underlying resistance evolution and development are not fully known. Here, we discuss the mechanisms underlying antibiotic resistance development, explore some treatment strategies used in the fight against antibiotic resistance and consider recent findings on collateral susceptibilities amongst antibiotic classes. Mathematical models have proved valuable for unravelling complex mechanisms in biology and such models have been used in the quest of understanding the development and spread of antibiotic resistance. While assessing the importance of such mathematical models, previous systematic reviews were interested in investigating whether these models follow good modelling practice. We focus on theoretical approaches used for resistance modelling considering both within and between host models as well as some pharmacodynamic and pharmakokinetic approaches and further examine the interaction between drugs and host immune response during treatment with antibiotics. Finally, we provide an outlook for future research aimed at modelling approaches for combating antibiotic resistance.

Comparison of methods for the detection of in vitro synergy in multidrug-resistant gram-negative bacteria

BACKGROUND

The use of combined antibiotic therapy has become an option for infections caused by multidrug-resistant (MDR) bacteria. The time-kill (TK) assay is considered the gold standard method for the evaluation of in vitro synergy, but it is a time-consuming and expensive method. The purpose of this study was to evaluate two methods for testing in vitro antimicrobial combinations: the disk diffusion method through disk approximation (DA) and the agar gradient diffusion method via the MIC:MIC ratio. The TK assay was included as the gold standard. MDR Gram-negative clinical isolates (n = 62; 28 Pseudomonas aeruginosa, 20 Acinetobacter baumannii, and 14 Serratia marcescens) were submitted to TK, DA, and MIC:MIC ratio synergy methods.

RESULTS

Overall, the agreement between the DA and TK assays ranged from 20 to 93%. The isolates of A. baumannii showed variable results of synergism according to TK, and the calculated agreement was statistically significant in this species against fosfomycin with meropenem including colistin-resistant isolates. The MIC:MIC ratiometric agreed from 35 to 71% with TK assays. The kappa test showed good agreement for the combination of colistin with amikacin (K = 0.58; P = 0.04) among the colistin-resistant A. baumannii isolates.

CONCLUSIONS

The DA and MIC:MIC ratiometric methods are easier to perform and might be a more viable tool for clinical microbiology laboratories.

Genomic and Phenotypic Analyses of Acinetobacter baumannii Isolates From Three Tertiary Care Hospitals in Thailand

Antibiotic resistant strains of Acinetobacter baumannii are responsible for a large and increasing burden of nosocomial infections in Thailand and other countries of Southeast Asia. New approaches to their control and treatment are urgently needed and an attractive strategy is to remove the bacterial polysaccharide capsule, and thus the protection from the host's immune system. To examine phylogenetic relationships, distribution of capsule chemotypes, acquired antibiotic resistance determinants, susceptibility to complement and other traits associated with systemic infection, we sequenced 191 isolates from three tertiary referral hospitals in Thailand and used phenotypic assays to characterize key aspects of infectivity. Several distinct lineages were circulating in three hospitals and the majority belonged to global clonal group 2 (GC2). Very high levels of resistance to carbapenems and other front-line antibiotics were found, as were a number of widespread plasmid replicons. A high diversity of capsule genotypes was encountered, with only three of these (KL6, KL10, and KL47) showing more than 10% frequency. Almost 90% of GC2 isolates belonged to the most common capsule genotypes and were fully resistant to the bactericidal action of human serum complement, most likely protected by their polysaccharide capsule, which represents a key determinant of virulence for systemic infection. Our study further highlights the importance to develop therapeutic strategies to remove the polysaccharide capsule from extensively drug-resistant A. baumanii during the course of systemic infection.

Global prevalence of colistin resistance in clinical isolates of Acinetobacter baumannii: A systematic review and meta-analysis

INTRODUCTION

Acinetobacter baumannii antimicrobial resistance is a public health concern in developing and developed countries, especially in the hospital setting. Understanding the antibiotic resistance profile can help to provide better guidelines for the prescription of appropriate antibiotics, reduction of antibiotic resistance, and introducing new and effective treatment options.

METHOD

Using the PRISMA guidelines, databases of PubMed, Embase, and Cochrane Library were searched systematically from January 1, 2000, to January 1, 2018. All statistical analyses were carried out via Comprehensive Meta-Analysis Software Version 2.0 (Biostat, Englewood, NJ). Depending on the heterogeneity test, either random or fix effect models were used for determining the pooled prevalence of drug resistance.

RESULT

A total of 150 studies were included from 41 countries of six different WHO regional offices worldwide. The highest and the lowest rate of resistance were observed for cefotaxime (99%, 95% CI: 95-99.9) in Africa and colistin (1.1%, 95% CI: 0.3-4.5) in Western Pacific, respectively. Lebanon (17.5%, 95% CI: 16-19) and China (12%, 95% CI: 3.5-32.5) had the highest and Germany (0.2%, 95% CI: 0-2.5) had the lowest rate of resistance for colistin.

CONCLUSION

Our analysis showed that prevalence and rate of increased colistin resistance in South-East Asia and Eastern Mediterranean countries are higher than other regions of the world. Therefore, the establishment of appropriate antibiotic usage guidelines should be essential in these countries.

Diaminoquinazoline MMV675968 from Pathogen Box inhibits Acinetobacter baumannii growth through targeting of dihydrofolate reductase

Antibiotic resistance in Acinetobacter baumannii is a major global health threat. New drugs with novel chemical structures are needed to overcome a myriad of resistance mechanisms in A. baumannii. In this study, we screened an open-source Pathogen Box library for anti-A. baumannii compounds. Compound MMV675968 (a diaminoquinazoline analog) was the only non-reference compound found to inhibit the growth of all four A. baumannii test strains with IC₅₀ of 0.6–2.7 μM, IC₉₀ of 0.7–3.9 μM, and MIC of 1.6–10 μM. We showed that MMV675968 targeted A. baumannii dihydrofolate reductase (AbDHFR) as determined by an E. coli surrogate whose growth was dependent on AbDHFR function and by an in vitro DHFR activity assay. Additionally, chemical scaffolds of DHFR inhibitors that are effective as antibiotics against A. baumannii were identified using an in vitro DHFR activity assay and A. baumannii growth inhibition. MMV675968 was the most potent among DHFR inhibitors tested in inhibiting A. baumannii growth. This study shows for the first time that MMV675968 inhibits A. baumannii growth via selective inhibition of AbDHFR and is therefore a promising scaffold for further antibiotic development against A. baumannii.

Dual Role of gnaA in Antibiotic Resistance and Virulence in Acinetobacter baumannii

Acinetobacter baumannii is an important Gram-negative pathogen in hospital-related infections. However, treatment options for A. baumannii infections have become limited due to multidrug resistance. Bacterial virulence is often associated with capsule genes found in the K locus, many of which are essential for biosynthesis of the bacterial envelope. However, the roles of other genes in the K locus remain largely unknown. From an in vitro evolution experiment, we obtained an isolate of the virulent and multidrug-resistant A. baumannii strain MDR-ZJ06, called MDR-ZJ06^M, which has an insertion by the ISAba16 transposon in gnaA (encoding UDP-N-acetylglucosamine C-6 dehydrogenase), a gene found in the K locus. The isolate showed an increased resistance toward tigecycline, whereas the MIC decreased in the case of carbapenems, cephalosporins, colistin, and minocycline. By using knockout and complementation experiments, we demonstrated that gnaA is important for the synthesis of lipooligosaccharide and capsular polysaccharide and that disruption of the gene affects the morphology, drug susceptibility, and virulence of the pathogen.

Colistin resistance in Acinetobacter baumannii isolated from critically ill patients: clinical characteristics, antimicrobial susceptibility and outcome

Background

Acinetobacter baumannii (AB) is increasingly becoming a clinically relevant organism due to the rising number of associated nosocomial infections. The therapeutic options are extremely minimal because of its ability to develop resistance to all available antimicrobials, including colistin (CST). Data on the clinical and microbiological characteristics of colistin-resistant A. baumannii infections remain scarce to date.

Methods

In this prospective study, clinical isolates of colistin resistance among Acinetobacter strain was evaluated from the database of Microbiology Laboratory of King Khalid University Hospital, Saudi Arabia.

Results

In a total of 142 patients with 136 Acinetobacter isolates, Acintobacter baumannii was the predominant serotype 73% of the isolates and Acinetobacter lwoffii constituted 27% of the isolate . There was 8.5% colistin resistant isolates with colistin E-test MIC >4. The clinical characteristics were determined for colistin resistant Acinetobacter baumannii. All patients were critically ill and 64% of them were hositalized in the Intensive Care Unit (ICU). All patients have been previously given antibiotics. Other associated clinical characteristics included; morbid obesity and sleeve gastrectomy (21 %), mechanical ventilation and central venous catheter (50%). High mortality rate was found(28%).

Conclusion

There is an increase of colistin resistance among clinical isolates of Acinetobacter baumannii causing serious infections especially in critically ill patients.

Risk factors and clinical responses of pneumonia patients with colistin-resistant Acinetobacter baumannii-calcoaceticus

BACKGROUND

Nosocomial infections with carbapenem-resistant Acinetobacter baumannii-calcoaceticus complex (ABC) strains are great problem for intensive care units. ABC strains can develop resistance to all the antibiotics available. Carbapenem resistance is common and colistin resistance is rare in our country. Knowing the risk factors for colistin resistance is important since colistin seems to be the only remaining therapeutic option for the patients with pneumonia due to extensively drug resistant ABC for our country.

AIM

To investigate the comparison of clinical responses and outcomes between pneumonia patients with colistin-susceptible and -resistant Acinetobacter sp. Strains.

METHODS

During the study period, 108 patients with pneumonia due to colistin-susceptible strains and 16 patients with colistin-resistant strains were included retrospectively. Continuous variables were compared with the Mann-Whitney U test, and categorical variables were compared using Pearson’s chi-square test or Fisher’s Exact chi-square test for two groups. A binary logistic regression model was developed to identify the potential independent factors associated with colistin resistance in patients with colistin-resistant strains.

RESULTS

High Acute Physiology and Chronic Health Evaluation II scores (OR = 1.9, 95%CI: 1.4-2.7; P < 0.001) and prior receipt of teicoplanin (OR = 8.1, 95%CI: 1.0-63.3; P = 0.045) were found to be independent risk factors for infection with colistin-resistant Acinetobacter sp. Different combinations of antibiotics including colistin, meropenem, ampicillin/sulbactam, amikacin and trimethoprim/sulfamethoxazole were used for the treatment of patients with colistin-resistant strains. Although the median duration of microbiological cure (P < 0.001) was longer in the colistin-resistant group, clinical (P = 0.703), laboratory (P = 0.277), radiological (P = 0.551), microbiological response (P = 1.000) and infection related mortality rates (P = 0.603) did not differ between the two groups. Among the patients with infections due to colistin-resistant strains, seven were treated with antibiotic combinations that included sulbactam. Clinical (6/7) and microbiological (5/7) response rates were quite high in these patients.

CONCLUSION

The optimal therapy regimen is unclear for colistin-resistant Acinetobacter sp. infections. Although combinations with sulbactam seems to be more effective in our study patients, data supporting the usefulness of combinations with sulbactam is very limited.

Genetic and virulence characterization of colistin-resistant and colistin-sensitive A. baumannii clinical isolates

Treatment of infections caused by A. baumannii is becoming a challenge due to the ability to develop multidrug-resistance, virulence, and high mortality. We described the colistin resistance and virulence genes present in sixA. baumannii clinical isolates using WGS, expression by qPCR, and virulence in the Galleria mellonella model. The colistin-resistant isolates were assigned as ST233 and the colistin-susceptible isolates as ST236 and ST407. The colistin-resistant isolates contained mutations within PmrA/PmrB, and the pmrA showed up-regulation in all of them. Only one colistin-resistant isolate indicating virulence in G. mellonella. This particular isolate belonged to a different clone, and it was the only isolate that presented non-synonymous mutations in pmrB. Colistinresistance in A. baumannii isolates seems to be caused by up-regulation of pmrA gene. Only one isolate appeared to be virulent in the G. mellonella model. This finding indicating low virulence in isolates belonging to emerging clones circulating in our hospital.

Bacteremia by colistin-resistant Acinetobacter baumannii isolate: a case report

BACKGROUND

Carbapenem-resistant Acinetobacter baumannii infections are a major public health problem worldwide, requiring the use of "old" antibiotics such as polymyxin B and E (colistin). However, there is concern regarding the emergence of isolates resistant to these antibiotics.

CASE PRESENTATION

We report a case of a 64-year-old mestizo man hospitalized in an intensive care unit of a health institution in Colombia with identification and clinical and molecular typing of a colistin- and carbapenem-resistant A. baumannii isolate with mechanisms of resistance to colistin not previously reported, causing bacteremia.

CONCLUSIONS

We have identified a strain of A. baumannii with mechanisms of resistance to colistin not previously reported in a patient with bacteremia who required treatment with multiple antibiotic schemes and had an adequate response.

Evaluation of the Bactericidal Activity of Fosfomycin in Combination with Selected Antimicrobial Comparison Agents Tested against Gram-Negative Bacterial Strains by Using Time-Kill Curves

The effects of combining fosfomycin with various antimicrobial agents were evaluated in vitro by broth microdilution checkerboard and time-kill kinetic studies. Checkerboard analyses were used to evaluate the following 30 Gram-negative isolates: 5 Pseudomonas aeruginosa, 5 Acinetobacter baumannii-Acinetobacter calcoaceticus species complex, and 20 Enterobacteriaceae isolates. No isolate exhibited antagonism when fosfomycin was tested in combination, and synergy was observed in more than 25% of the drug combinations tested. The most frequent instances of synergy occurred when testing fosfomycin with β-lactams. Two isolates of Pseudomonas aeruginosa, 2 of Klebsiella pneumoniae, and 1 of the A. baumannii-A. calcoaceticus species complex that exhibited synergy when fosfomycin was tested in combination were subjected to time-kill kinetic analyses for confirmation. Time-kill assays confirmed synergistic activity. These data indicated that combination therapy with fosfomycin may be beneficial.

Synergistic Effect of Ceftazidime-Avibactam with Meropenem against Panresistant, Carbapenemase-Harboring Acinetobacter baumannii and Serratia marcescens Investigated Using Time-Kill and Disk Approximation Assays

Susceptibility of ceftazidime-avibactam and in vitro synergy with meropenem were investigated using disk approximation and time-kill assays against 11 multiresistant Acinetobacter baumannii isolates harboring oxacillinases and 5 Serratia marcescens isolates carrying bla _KPC-2 Ceftazidime-avibactam was very active and synergistic with meropenem against multiresistant S. marcescens isolates. On the other hand, only the A. baumannii isolates coharboring bla _OXA-23 and bla _OXA-117 displayed synergy. The disk approximation technique presented good sensitivity for synergism in S. marcescens infection.

In-Vitro Evaluation of Different Antimicrobial Combinations with and without Colistin Against Carbapenem-Resistant Acinetobacter Baumannii

Carbapenem-resistant Acinetobacter baumannii (CR-Ab) infections are associated with high morbidity and mortality. The aim of the study was to evaluate the in-vitro activity of different antimicrobial combinations (with and without colistin, COL) against clinical isolates of CR-Ab collected from patients with CR-Ab infection, including unconventional combinations such as COL + VANcomycin (VAN) and COL + rifampin (RIF). CR-Ab strains were collected from hospitalized patients at Sapienza University of Rome. Antimicrobial susceptibility patterns were determined throughout MIC50/90s whereas the synergistic activity was evaluated by qualitative (i.e., checkerboard) and quantitative (i.e., killing studies) methods. All the strains were found oxacillinase (OXA) producers and tigecycline (TIG) sensitive whereas 2 strains were resistant to COL. Application of the checkerboard method indicated complete synergism in COL combinations at different extension: 21.4%, 57.1%, 42.8%, 35.7% for COL + meropenem (MEM), COL + RIF, COL + VAN and COL + TIG, respectively, with the non-conventional combinations COL + VAN and COL + RIF exhibiting the highest rate of synergism. Regarding COL-free combination, complete synergism was observed in 35.7% of the strains for MEM + TIG. Killing studies showed that the combinations COL + MEM, COL + TIG and MEM + TIG were bactericidal and synergistic against both colistin-sensitive and low colistin-resistant strains whereas only the combinations COL + VAN and COL + RIF showed an early and durable bactericidal activity against all the tested strains, with absence of growth at 24 h. This study demonstrated that COL-based combinations lead to a high level of synergic and bactericidal activity, especially COL + VAN and COL + RIF, even in the presence of high level of COL resistance.

Synergistic activity of polymyxin B combined with vancomycin against carbapenem-resistant and polymyxin-resistant Acinetobacter baumannii: first in vitro study

PURPOSE

The effect of a combination of polymyxin B (PMB) and vancomycin (VAN) was assessed against six Acinetobacter baumannii clinical isolates belonging to six different clusters (three PMB-susceptible and three PMB-resistant).

METHODOLOGY

The synergistic effect of the PMB-VAN combination was determined with the checkerboard, time-kill, disk-diffusion and M.I.C.Evaluator assays. PMB-resistance was investigated with mcr-1 gene amplification and a mutant frequency assay.

RESULTS

In the checkerboard assay, all PMB-resistant isolates showed a synergistic effect. The time-kill assay demonstrated that the PMB-VAN combination had a bactericidal effect at 24 h against isolates with a high mutant rate for PMB, suggesting that this combination may block the hypermutation of some isolates. No antagonism was detected. All PMB-resistant isolates also showed synergism in the disk-diffusion test, and a significant decrease in VAN MICs in the M.I.C.Evaluator assay.

CONCLUSION

Our findings indicate that the PMB-VAN combination has a synergistic effect on A. baumannii, especially against PMB-resistant isolates.

Clinical relevance of in vitro synergistic activity of antibiotics for multidrug-resistant Gram-negative infections: A systematic review

OBJECTIVES

The aim of this review was to investigate the outcomes of patients infected with multidrug-resistant (MDR) or extensively drug-resistant (XDR) Gram-negative bacteria following synergy-guided antibiotic combination therapy (SGACT).

METHODS

A systematic review of PubMed and Scopus databases was performed. Published studies of any design reporting outcomes of patients with MDR Gram-negative bacteria treated with SGACT were included. Two reviewers independently assessed the relevancy and quality of the retrieved articles and extracted the available data.

RESULTS

Nineteen reports (530 patients) were included. Eleven case reports/series described 26 cases of systemic infection due to MDR Gram-negative bacteria treated with SGACT. Five deaths were reported, two of which were attributed to the infection. Six studies (including one randomised controlled trial) provided comparative data for patients treated with SGACT and those treated with unguided combination therapy (UCT) or active monotherapy. In the pooled analysis of unadjusted data from these studies (504 patients), there was no difference between SGACT and UCT or monotherapy (OR=0.47, 95% CI 0.21-1.04; I²=52%). Analysis of adjusted data showed that SGACT was significantly associated with survival (OR=0.44, 95% CI 0.20-0.98; I²=54%).

CONCLUSION

These limited but promising findings warrant further investigation of SGACT in the outcome of patients with MDR Gram-negative infections in well-designed trials.

Rational Combinations of Polymyxins with Other Antibiotics

Combinations of antimicrobial agents are often used in the management of infectious diseases. Antimicrobial agents used as part of combination therapy are often selected empirically. As regrowth and the emergence of polymyxin (either colistin or polymyxin B) resistance has been observed with polymyxin monotherapy, polymyxin combination therapy has been suggested as a possible means by which to increase antimicrobial activity and reduce the development of resistance. This chapter provides an overview of preclinical and clinical investigations of CMS/colistin and polymyxin B combination therapy. In vitro data and animal model data suggests a potential clinical benefit with many drug combinations containing clinically achievable concentrations of polymyxins, even when resistance to one or more of the drugs in combination is present and including antibiotics normally inactive against Gram-negative organisms. The growing body of data on the emergence of polymyxin resistance with monotherapy lends theoretical support to a benefit with combination therapy. Benefits include enhanced bacterial killing and a suppression of polymyxin resistant subpopulations. However, the complexity of the critically ill patient population, and high rates of treatment failure and death irrespective of infection-related outcome make demonstrating a potential benefit for polymyxin combinations extremely challenging. Polymyxin combination therapy in the clinic remains a heavily debated and controversial topic. When combinations are selected, optimizing the dosage regimens for the polymyxin and the combinatorial agent is critical to ensure that the benefits outweigh the risk of the development of toxicity. Importantly, patient characteristics, pharmacokinetics, the site of infection, pathogen and resistance mechanism must be taken into account to define optimal and rational polymyxin combination regimens in the clinic.

AdeR-AdeS mutations & overexpression of the AdeABC efflux system in ciprofloxacin-resistant Acinetobacter baumannii clinical isolates

Background & objectives:

Overexpression of efflux pumps is a cause of acquired resistance to fluoroquinolones in Acinetobacter baumannii. The present study was done to investigate the presence and overexpression of AdeABC efflux system and to analyze the sequences of AdeR-AdeS regulatory system in ciprofloxacin-resistant A. baumannii isolates.

Methods:

Susceptibility of 50 clinical A. baumannii isolates to ciprofloxacin, imipenem, ceftazidime, cefepime and gentamicin antimicrobials was evaluated by agar dilution method. Isolates were screened for the evidence of active efflux pump. Isolates were also examined for adeR-adeS and adeB efflux genes by polymerase chain reaction (PCR). The adeR and adeS regulatory genes were sequenced to detect amino acid substitutions. Expression of adeB was evaluated by quantitative reverse-transcriptase PCR.

Results:

There were high rates of resistance to ciprofloxacin (88%), ceftazidime (88%), cefepime (74%) and imipenem (72%) and less resistance rate to gentamicin (64%). Phenotypic assay showed involvement of active efflux in decreased susceptibility to ciprofloxacin among 16 isolates. The 12.27-fold increase and 4.25-fold increase were found in adeB expression in ciprofloxacin-full-resistant and ciprofloxacin-intermediate-resistant isolates, respectively. Several effective mutations, including A91V, A136V, L192R, A94V, G103D and G186V, were detected in some domains of AdeR-AdeS regulators in the overexpressed ciprofloxacin-resistant isolates.

Interpretation & conclusions:

The results of this study indicated that overexpression of the AdeABC efflux pump was important to reduce susceptibility to ciprofloxacin and cefepime in A. baumannii that, in turn, could be triggered by alterations in the AdeR-AdeS two-component system. However, gene expression alone does not seem adequate to explain multidrug resistance phenomenon. These results could help plan improved active efflux pump inhibitors.

Colistin monotherapy versus colistin/rifampicin combination therapy in pneumonia caused by colistin-resistant Acinetobacter baumannii: A randomised controlled trial

OBJECTIVES

The aim of this study was to confirm the synergistic effect of colistin/rifampicin combination therapy compared with colistin monotherapy in pneumonia caused by colistin-resistant Acinetobacter baumannii (CoRAB). The utility of the Etest was also assessed.

METHODS

Nine subjects with pneumonia caused by CoRAB were enrolled from 20 July 2016 to 21 June 2018. Subjects were randomised to colistin/rifampicin combination therapy or colistin monotherapy. After exclusion of one patient who dropped out, the microbiological response (MR) and clinical response (CR) on Day 14 and mortality on Day 30 were assessed. Etest was conducted using CoRAB isolated at study enrolment.

RESULTS

The MR rate in the colistin/rifampicin combination group (100.0%) was better than that in the colistin group (40.0%), however the difference was not statistically significant (P=0.196). The CR rate was not significantly different between the two groups. The MR (100.0%) and CR (100.0%) rates in subjects with 'partial synergy' as shown by Etest were higher than those (25.0% and 50.0%, respectively) in subjects with 'indifferent' results (i.e. no synergistic effect), however the difference was not statistically significant (P=0.143 and 0.429, respectively). Mortality occurred in two subjects with 'indifferent' results by Etest.

CONCLUSIONS

Colistin/rifampicin combination therapy may have potential to achieve MR in pneumonia caused by CoRAB; however, achieving CR with this treatment is doubtful. 'Partial synergy' of colistin and rifampicin, as shown by Etest, may be a good prognostic factor [ClinicalTrial.gov ID: NCT03622918].

Synergy effect of meropenem-based combinations against Acinetobacter baumannii: a systematic review and meta-analysis

PURPOSE

The main objective of our meta-analysis was to examine the in vitro synergistic effect of meropenem-based combination therapies against Acinetobacter baumannii through a systematic review of the existing literature.

METHODS

An extensive search was performed with no restrictions on date of publication, language, and publication type. Our study evaluated the main conclusions drawn from various studies describing the synergistic activity of combination therapies in vitro.

RESULTS

In this review, 56 published studies were included. Our report included data on 20 types of antibiotics combined with meropenem in 1,228 Acinetobacter baumannii isolates. In time-kill studies, meropenem combined with polymyxin B and rifampicin showed synergy rates of 98.3% (95% CI, 83.7%-100.0%) and 89.4% (95% CI, 57.2%-100.0%), respectively, for Acinetobacter baumannii, modest synergy rates were found for meropenem combined with several antibiotics such as colistin and sulbactam, and no synergy effect was displayed in the combination of meropenem and ciprofloxacin, whereas in checkerboard method, the synergy rates of polymyxin B and rifampicin were 37.0% (95% CI, 0.00%-100.0%) and 56.3% (95% CI, 8.7%-97.8%), respectively.

CONCLUSION

We found that time-kill studies generally identified the greatest synergy, while checkerboard and Etest methods yielded relatively poor synergy rates. Further well-designed in vivo studies should be carried out to confirm these findings.

Molecular characterization and clonal dynamics of nosocomial blaOXA-23 producing XDR Acinetobacter baumannii

The emergence of infections associated to new antimicrobial resistance in Acinetobacter baumannii (Ab) genotypes represents a major challenge. In this context, this study aimed to determine the diversity of resistance mechanisms and investigate clonal dissemination and predominant sequence types (STs) in multidrug-resistant Ab strains of clinical (tracheal aspirate, n = 17) and environmental (surface, n = 6) origins. Additionally, the major clones found in clinical (A) and environmental (H) strains had their complete genomes sequenced. All strains were submitted to polymerase chain reactions (PCR) for the detection of the ISAba1/blaOXA-51-like and ISAba1/blaOXA-23-like genes, while the expression of genes encoding the carO porin, AdeABC (adeB), AdeFGH (adeG), and AdeIJK (adeJ) efflux pumps was determined by real time PCR (qPCR). Most of the strains were characterized as extensively drug-resistant (XDR) with high minimal inhibitory concentrations (MICs) detected for tigecycline and carbapenems. Associations between ISAba1/OXA-51 and ISAba1/OXA-23 were observed in 91.3% and 52.2% of the strains, respectively. Only the adeB gene was considered hyper-expressed. Furthermore, most of the strains analyzed by the MuLtilocus Sequence-Typing (MLST) were found to belong to the clonal complex 113 (CC113). In addition, a new ST, ST1399, belonging to CC229, was also discovered herein. Strains analyzed by whole genome sequencing presented resistance genes linked to multidrug-resistance phenotypes and confirmed the presence of Tn2008, which provides high levels carbapenem-resistance.

Transcriptional regulators: valuable targets for novel antibacterial strategies

Antibiotics have saved millions of lives over the past decades. However, the accumulation of so many antibiotic resistance genes by some clinically relevant pathogens has begun to lead to untreatable infections worldwide. The current antibiotic resistance crisis will require greater efforts by governments and the scientific community to increase the research and development of new antibacterial drugs with new mechanisms of action. A major challenge is the identification of novel microbial targets, essential for in vivo growth or pathogenicity, whose inhibitors can overcome the currently circulating resistome of human pathogens. In this article, we focus on the potential high value of bacterial transcriptional regulators as targets for the development of new antibiotics, discussing in depth the molecular role of these regulatory proteins in bacterial physiology and pathogenesis. Recent advances in the search for novel compounds that inhibit the biological activity of relevant transcriptional regulators in pathogenic bacteria are reviewed.

In vitro activities of carbapenems in combination with amikacin, colistin, or fosfomycin against carbapenem-resistant Acinetobacter baumannii clinical isolates

Carbapenem-resistant Acinetobacter baumannii clinical isolates (n=23) were investigated for carbapenem resistance mechanisms and in vitro activities of carbapenems in combination with amikacin, colistin, or fosfomycin. Major carbapenem resistance mechanism was OXA-23 production. The vast majority of these isolates were OXA-23-producing A. baumannii ST195 and ST542, followed by novel STs, ST1417, and ST1423. The interuption of carO by a novel insertion sequence, ISAba40, was found in two isolates. The combinations of imipenem and fosfomycin, meropenem and amikacin, imipenem and amikacin, and imipenem and colistin were synergistic against carbapenem-resistant A. baumannii by 65.2%, 46.2%, 30.8%, and 17.4%, respectively. Surprisingly, the combination of imipenem and fosfomycin was the most effective in this study against A. baumannii, which is intrinsically resistant to fosfomycin. Imipenem and fosfomycin inhibit cell wall synthesis; therefore, fosfomycin may be an adjuvant and enhance the inhibition of cell wall synthesis of carbapenem-resistant A. baumannii when combined with imipenem.