In vitro activity of tigecycline in combination with various antimicrobials against multidrug resistant Acinetobacter baumannii

Antibacterial Interactions of Ethanol-Dispersed Multiwalled Carbon Nanotubes with Staphylococcus aureus and Pseudomonas aeruginosa

Infectious diseases are acknowledged as one of the leading causes of death worldwide. Statistics show that the annual death toll caused by bacterial infections has reached 14 million, most of which are caused by drug-resistant strains. Bacterial antibiotic resistance is currently regarded as a compelling problem with dire consequences, which motivates the urgent identification of alternative ways of fighting bacteria. Various types of nanomaterials have been reported to date as efficient antibacterial solutions. Among these, carbon-based nanomaterials, such as carbon nanodots, carbon graphene oxide, and carbon nanotubes (CNTs), have been shown to be effective in killing a wide panel of pathogenic bacteria. With this study, we aim to provide additional insights into this topic of research by investigating the antibacterial activity of a specific type of multiwalled CNTs, with diameters from 50 to 150 nm, against two representative opportunistic pathogens, i.e., the Gram-positive bacterium Staphylococcus aureus and the Gram-negative bacterium Pseudomonas aeruginosa, both included among the top antibiotic-resistant pathogens. We also test the synergistic effect of CNTs with different antibiotics commonly used in the treatment of infections caused by S. aureus and/or P. aeruginosa. Additionally, a novel approach for quantitatively analyzing bacterial aggregation in brightfield microscopy images was implemented. This method was utilized to assess the effectiveness of CNTs, either alone or in combination with antibiotics, in dispersing bacterial aggregates. Finally, atomic force microscopy coupled with a newly devised image analysis pipeline was used to examine any potential morphological changes in bacterial cells following exposure to CNTs and antibiotics.

Antimicrobial combination effects against multidrug-resistant Acinetobacter baumannii and Pseudomonas aeruginosa strains: A cross-sectional study

Background and Aims

Emergence of multidrug resistance in non-fermenting Gram-negative bacilli is a threat to public health. Combination therapy is a strategy for the treatment of antibiotic-resistant infections.

Methods

In this cross-sectional study, a total of 63 nonduplicate clinical isolates of Acinetobacter baumannii and Pseudomonas aeruginosa were collected from various specimens. Identification of bacterial isolates was performed by phenotypic and molecular tests. Antibiotic susceptibility patterns and detection of β-lactamase genes were determined using the broth microdilution and polymerase chain reaction (PCR) techniques, respectively. Then, the combined effects analysis was determined by the checkerboard method. Based on the status of resistance to carbapenems (imipenem and meropenem), 25 isolates of each genus were selected for further investigation.

Results

For A. baumannii, bla OXA-23, bla OXA-58, and bla OXA-48 genes were positive in 21 (84%), 17 (68%), and 11 (44%) of isolates, respectively. In P. aeruginosa isolates, bla VIM was the most common gene (44%) and other genes including bla IMP, bla NDM, and bla OXA-23 were found in nine (36%), six (24%), and three (12%) isolates, respectively. Meropenem (MER)-tigecycline (TIG) had a significant synergistic effect against 20 (80%) A. baumannii (p value < 0.001). This combination was also efficient against 5 (20%) P. aeruginosa isolates. Moreover, the other combination, tigecycline-amikacin (TIG-AMK) was effective against 10 (40%) A. baumannii isolates. The combination of colistin (COL) and MER showed a significant synergistic effect against 21 (84%) A. baumannii (p value < 0.001) and 17 (68%) P. aeruginosa isolates (p value < 0.001).

Conclusion

The MER-TIG and COL-MER combinations are promising options against resistant bacteria. Our study could be helpful for the development of a new treatment recommendation.

Clonal evolution and antimicrobial resistance of Acinetobacter baumannii isolates from Korean hospitals over the last decade

The wide-spread of drug-resistant Acinetobacter baumannii is a global health problem. This study investigated the clonal distribution and antimicrobial resistance of 167 A. baumannii isolates from two Korean university hospitals from 2009 to 2019 by analyzing the sequence types (STs), antimicrobial resistance, and resistance determinants of carbapenems and aminoglycosides. Twenty STs, including 16 pre-existing STs and four unassigned STs, were identified in A. baumannii isolates using the Oxford multilocus sequence typing scheme. Two STs, ST191 (n = 77) and ST451 (n = 40), were prevalent, and majority (n = 153) of the isolates belonged to clonal complex 208. The ST191 isolates were detected during the study period, whereas ST451 isolates were detected after 2016. One hundred forty-seven (87%) of 167 A. baumannii isolates were non-susceptible to carbapenems. The ST191 and ST451 isolates exhibited higher resistance to antimicrobial agents than that of the sporadic ST isolates. Interestingly, ST451 isolates exhibited lower susceptibility to minocycline and tigecycline than the other ST isolates. All carbapenem-non-susceptible A. baumannii isolates, except four, carried the ISAbaI-bla_OXA-23 structure. armA was detected in all amikacin-non-susceptible isolates (n = 128) except for one isolate. Five aminoglycoside-modifying enzyme (AME) genes were detected, but their carriage varied between STs; ant(3″)-Ia and aac(6')-Ib were more common in ST191 than in ST451, while aph(3')-Ia was more common in ST451 than in ST191. This study demonstrated the clonal evolution related to antimicrobial resistance in A. baumannii.

Thymol Increases Sensitivity of Clinical Col-R Gram-Negative Bacteria to Colistin

Colistin-resistant (Col-R) bacteria are increasing sharply, which poses a serious threat to public health. Thymol is a phenolic compound used for its wide-spectrum antimicrobial activity, while the combination of nontraditional drugs to restore colistin activity is an attractive strategy to treat infections caused by these pathogens. This study showed that thymol could play a synergistic role with colistin against Gram-negative bacteria (GNB), including nonfermenting bacteria and Enterobacteriaceae. According to antimicrobial resistance profiles, most of the colistin-resistant strains we collected showed multidrug-resistant (MDR) phenotypes. The checkerboard method and time-kill curve confirmed the synergistic effect of thymol combined with colistin against Col-R GNB. The synergistic antibiofilm activity of thymol combined with colistin was assessed via crystal violet staining and scanning electron microscopy (SEM) assays. Results showed that compared with a single drug, the combination partially destroyed bacterial cells and inhibit the formation of bacterial biofilms. Mechanismly, the thymol/colistin combination synergistically potentiated the antibacterial activity by accelerating the damage and permeability of the bacterial outer membrane. Preliminary data indicated that the thymol/colistin combination could decrease the number of bacteria ≥2 log10 CFU/mL after 24 h of therapy in a mouse thigh infection model. Our results fully prove that thymol and colistin combination possesses a promising treatment option against colistin-resistant GNB infections. IMPORTANCE Colistin is being considered "the last ditch" treatment in many infections caused by multidrug-resistant GNB clinical isolates, but colistin-resistant (Col-R) strains with different drug resistance mechanisms have appeared worldwide. Hence, it is of great significance to rejuvenate sensitization of clinical Col-R Gram-negative bacteria to colistin. In this study, the thymol/colistin combination showed notable antibacterial activity in vitro and in vivo. These findings suggest that the thymol/colistin combination may have promise as a treatment approach for treating the infections caused by Col-R pathogens.

iTDtest: an Easy-to-Handle and Visual Assay To Detect Tolerant and Persister Cells in an Antibiotic Combination Regimen

In a recent article, Balaban and colleagues developed the iTDtest allowing characterization of the type of interactions between different antibiotics at bactericidal concentrations (J.-F. Liu et al., mBio 13:e00004-22, 2022). This visual and semiquantitative assay is designed to determine how antibiotic cocktails affect tolerance and persistence, two phenomena of major importance for the eradication of difficult-to-treat pathogens. Importantly, Balaban and colleagues identified antibiotic combinations allowing for complete clearance of persister and tolerant cells. This commentary discusses the translation of this assay in clinical settings, where antibiotic combination therapies appear to be applied in specific contexts, such as in acute infections or in the case of multidrug or extensively drug-resistant pathogens.

Effectiveness and Nephrotoxicity of Loading Dose Colistin-Meropenem versus Loading Dose Colistin-Imipenem in the Treatment of Carbapenem-Resistant Acinetobacter baumannii Infection

Carbapenem-resistant Acinetobacter baumannii (CRAB) is becoming more widely recognized as a serious cause of nosocomial infections, and colistin has been reintroduced in recent years for the treatment of CRAB infection. Combinations of colistin and meropenem or imipenem have been found to be effective against CRAB isolates, whereas clinical investigations have not definitively demonstrated the theoretical benefits of colistin combined therapy in patients with CRAB infections. The objective of this study was to compare the primary outcome (30-day survival rate) and secondary outcomes (clinical response, microbiological response and nephrotoxicity) between patients who received loading dose (LD) colistin−meropenem and LD colistin−imipenem for the treatment of CRAB infection. A retrospective cohort analysis was performed at Chiang Mai University Hospital in patients with CRAB infection who received LD colistin−meropenem or LD colistin−imipenem between 2011 and 2017, and 379 patients fulfilled the requirements for the inclusion criteria. The results of this study showed that patients who received LD colistin−imipenem had a lower 30-day survival rate (adjusted HR = 0.57, 95% CI: 0.37−0.90; p = 0.015) and a lower clinical response (aHR = 0.56, 95% CI: 0.35−0.90; p = 0.017) compared with those who received LD colistin−meropenem. The microbiological response in patients with LD colistin−imipenem was 0.52 times (aHR) lower than that in those who received colistin−meropenem (95% CI: 0.34−0.81; p = 0.004); however, there was no significant difference in nephrotoxicity (aHR = 1.03, 95% CI: 0.67−1.57; p = 0.897) between the two combination regimens. In conclusion, when comparing the combination of LD colistin with imipenem or meropenem, the combination of LD colistin and meropenem provides a better survival rate for treating CRAB. Thus, we suggest that combinations of LD colistin and meropenem should be considered when treating CRAB infections.

Discovery of the Lead Molecules Targeting the First Step of the Histidine Biosynthesis Pathway of Acinetobacter baumannii

Acinetobacter baumannii is a multidrug-resistant, opportunistic, nosocomial pathogen for which a new line of treatments is desperately needed. We have targeted the enzyme of the first step of the histidine biosynthesis pathway, viz., ATP-phosphoribosyltransferase (ATP-PRT). The three-dimensional structure of ATP-PRT was predicted on the template of the known three-dimensional structure of ATP-PRT from Psychrobacter arcticus (PaATPPRT) using a homology modeling approach. High-throughput virtual screening (HTVS) of the antibacterial library of Life Chemicals Inc., Ontario, Canada was carried out followed by molecular dynamics simulations of the top hit compounds. In silico results were then biochemically validated using surface plasmon resonance spectroscopy. We found that two compounds, namely, F0843-0019 and F0608-0626, were binding with micromolar affinities to the ATP-phosphoribosyltransferase from Acinetobacter baumannii (AbATPPRT). Both of these compounds were binding in the same way as AMP in PaATPPRT, and the important residues of the active site, viz., Val4, Ser72, Thr76, Tyr77, Glu95, Lys134, Val136, and Tyr156, were also interacting via hydrogen bonds. The calculated binding energies of these compounds were -10.5 kcal/mol and -11.1 kcal/mol, respectively. These two compounds can be used as the potential lead molecules for designing antibacterial compounds in the future, and this information will help in drug discovery programs against Acinetobacter worldwide.

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.

In vitro and in vivo comparison of eravacycline- and tigecycline-based combination therapies for tigecycline-resistant Acinetobacter baumannii

Several antimicrobial combination therapies are used to treat multiple drug resistant (MDR) and extensively drug resistant (XDR) Acinetobacter baumannii infections. A novel antibiotic, eravacycline, shows a higher potency than tigecycline. The efficacies of eravacycline-based therapies have not yet been evaluated. We demonstrated the effectiveness of eravacycline- and tigecycline-based combination therapies in XDR and especially tigecycline resistant A. baumannii. Thirteen eligible isolates were selected from 642 non-duplicate Acinetobacter blood isolates from four medical centres in 2010-2014. Tigecycline/imipenem and eravacycline/imipenem combinations were simultaneously effective against some isolates in vitro with fractional inhibitory concentration index of 0.5. In contrast, eravacycline- and tigecycline-based combination therapies provided no additional benefits in mouse survival compared to those for monotherapy. In summary, colistin is still the final resort for XDR-A. baumannii treatment according to the sensitivities. Owning to rapid development of resistance in A. baumannii, novel antibiotics are urgently needed.

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.

Effects of colistin and tigecycline on multidrug-resistant Acinetobacter baumannii biofilms: advantages and disadvantages of their combination

We investigated the antimicrobial effects of colistin (CST) and tigecycline (TGC), either alone or in combination, on biofilm-dispersed and biofilm-embedded multidrug-resistant Acinetobacter baumannii (MDRAB) strains R1 and R2. The bacterial growth of biofilm-dispersed MDRAB was inhibited by CST or TGC. However, the inhibitory effects were attenuated by a combination of CST and low concentrations of TGC. The bactericidal effects of CST, but not TGC, were observed on biofilm-dispersed MDRAB. Notably, the bactericidal effects increased with a combination of CST and high concentrations of TGC, whereas they were attenuated with the combination of CST and low concentrations of TGC. Although biofilm formation by MDRAB decreased with increasing concentrations of CST or TGC, there was no complete disruption of the biofilms. Additionally, the biofilms increased with a combination of 1–2 μg/mL CST and TGC at 2 μg/mL and 2–4 μg/mL for strains R1 and R2, respectively. Biofilm-embedded MDRAB was eradicated with CST, but not TGC. Notably, the eradication effects increased with a combination of CST and high concentrations of TGC, whereas attenuation happened with the combination of CST and low concentrations of TGC. These results provide information on the combined effects of CST and TGC in the treatment of biofilm-associated MDRAB infection.

Dose optimisation based on pharmacokinetic/pharmacodynamic target of tigecycline

Tigecycline, a new first-in-class glycylcycline antibiotic, has shown promising efficacy against a broad range of micro-organisms. It is widely prescribed for various infections, with most prescriptions being considered for off-label use. However, only a few years after its approval by the US Food and Drug Administration (FDA), tigecycline is suspected of increasing all-cause mortality. Some clinicians have suggested such unfavourable outcomes correlate with inadequate drug exposure at the infection site. The pharmacokinetic/pharmacodynamic (PK/PD) profile of a drug plays an important role in predicting its antibiotic effect, which for tigecycline is determined as the ratio of area under the concentration-time curve (AUC) to minimum inhibitory concentration (MIC). In this study, PK/PD targets based on infection sites, bacterial isolates and patient populations are discussed. Generally, a higher dosage of tigecycline for the treatment of serious infections has been recommended in previous reports. However, the latest finding of tigecycline's atypical protein binding property requires consideration when recommending further use. In addition, combination therapy with other antibiotics provides another option by potentially lowering the MICs of multidrug-resistant bacteria.

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.

Antimicrobial Effects of Minocycline, Tigecycline, Ciprofloxacin, and Levofloxacin against Elizabethkingia anophelis Using In Vitro Time-Kill Assays and In Vivo Zebrafish Animal Models

Elizabethkingia anophelis is a multidrug-resistant pathogen. This study evaluated the antimicrobial activity of minocycline, tigecycline, ciprofloxacin, and levofloxacin using in vitro time-kill assays and in vivo zebrafish animal models. The E. anophelis strain ED853-49 was arbitrarily selected from a bacterial collection which was concomitantly susceptible to minocycline, tigecycline, ciprofloxacin, and levofloxacin. The antibacterial activities of single agents at 0.5-4 × minimum inhibitory concentration (MIC) and dual-agent combinations at 2 × MIC using time-kill assays were investigated. The therapeutic effects of antibiotics in E. anophelis-infected zebrafish were examined. Both minocycline and tigecycline demonstrated bacteriostatic effects but no bactericidal effect. Minocycline at concentrations ≥2 × MIC and tigecycline at concentrations ≥3 × MIC exhibited a long-standing inhibitory effect for 48 h. Bactericidal effects were observed at ciprofloxacin and levofloxacin concentrations of ≥3 × MIC within 24 h of initial inoculation. Rapid regrowth of E. anophelis occurred after the initial killing phase when ciprofloxacin was used, regardless of the concentration. Levofloxacin treatment at the concentration of ≥2 × MIC consistently resulted in the long-lasting and sustainable inhibition of bacterial growth for 48 h. The addition of minocycline or tigecycline weakened the killing effect of fluoroquinolones during the first 10 h. The minocycline-ciprofloxacin or minocycline-levofloxacin combinations achieved the lowest colony-forming unit counts at 48 h. Zebrafish treated with minocycline or a combination of minocycline and levofloxacin had the highest survival rate (70%). The results of these in vitro and in vivo studies suggest that the combination of minocycline and levofloxacin is the most effective therapy approach for E. anophelis infection.

Polypyridine ligands as potential metallo-β-lactamase inhibitors

Bacteria have developed multiple resistance mechanisms against the most used antibiotics. In particular, zinc-dependent metallo-β-lactamase producing bacteria are a growing threat, and therapeutic options are limited. Zinc chelators have recently been investigated as metallo-β-lactamase inhibitors, as they are often able to restore carbapenem susceptibility. We synthesized polypyridyl ligands, N,N'-bis(2-pyridylmethyl)-ethylenediamine, N,N,N'-tris(2-pyridylmethyl)-ethylenediamine, N,N'-bis(2-pyridylmethyl)-ethylenediamine-N-acetic acid (N,N,N'-tris(2-pyridylmethyl)-ethylenediamine-N'-acetic acid, which can form zinc(II) complexes. We tested their ability to restore the antibiotic activity of meropenem against three clinical strains isolated from blood and metallo-β-lactamase producers (Klebsiella pneumoniae, Enterobacter cloacae, and Stenotrophomonas maltophilia). We functionalized N,N,N'-tris(2-pyridylmethyl)-ethylenediamine with D-alanyl-D-alanyl-D-alanine methyl ester with the aim to increase bacterial uptake. We observed synergistic activity of four polypyridyl ligands with meropenem against all tested isolates, while the combination N,N'-bis(2-pyridylmethyl)-ethylenediamine and meropenem was synergistic only against New Delhi and Verona integron-encoded metallo-β-lactamase-producing bacteria. All synergistic interactions restored the antimicrobial activity of meropenem, providing a significant decrease of minimal inhibitory concentration value (by 8- to 128-fold). We also studied toxicity of the ligands in two normal peripheral blood lymphocytes.

Colistin monotherapy versus colistin-based combination therapy for treatment of bacteremia in burn patients due to carbapenem-resistant gram negative bacteria

Carbapenem-resistant gram negative pathogen (CR-GNP) infection in burn patients is a growing concern since treatment options are limited and resistance to the main line of treatment, colistin, is increasing. The goal of this study was to compare treatment outcomes of colistin monotherapy versus colistin-based combination therapy for CR-GNP bacteremia in burn patients. A retrospective observational study was conducted between 2014 and 2017 in Hangang Sacred Heart Hospital located in Seoul, South Korea. Among the burn patients admitted to the burn intensive care unit with CR-GNP bacteremia due to wound infections, colistin monotherapy or colistin-based combination therapy were investigated. We determined both eradication rate within seven days as well as mortality rate within 30 days. A total of 84 burn patients with CR-GNP bacteremia were analyzed-32 were treated with colistin monotherapy and 52 with colistin-based combination therapy. We found that eradication rate within 7 days and 30-day mortality rate were not significantly different between the two groups (71.9% versus 75.0%, P = 0.752 and 31.2% versus 38.5%, P = 0.503). In the Cox regression analysis, Charlson's comorbidity index, renal replacement therapy before colistin use, and duration of antibiotics were associated with 30-day mortality (HR, 1.23; 95% CI, 1.02-1.49; P = 0.030, HR, 2.28; 95% CI, 1.05-4.94; P = 0.037 and HR, 0.94; 95% CI, 0.89-0.99, P = 0.042, respectively). Colistin-based combination therapy did not show significant differences with regard to microbiologic and clinical outcomes compared with colistin monotherapy.

Zinc Chelators as Carbapenem Adjuvants for Metallo-β-Lactamase-Producing Bacteria: In Vitro and In Vivo Evaluation

Infections caused by metallo-β-lactamase (MBL)-producing bacteria are emerging and carry a significant impact on patients' outcome. MBL producers are spread worldwide, both in community and hospital setting, with increasingly reported epidemic clusters and the search for MBL inhibitors is an important topic for public health. MBLs are zinc-dependent enzymes whose functioning can be hampered by zinc chelators. We evaluated the potential of six zinc chelators (disulfiram, nitroxoline, 5-amino-8-hydroxyquinoline, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid [DOTA], cyclam, and N,N,N',N'-tetrakis (2-pyridymethyl) ethylenediamine [TPEN]) in restoring carbapenem activity against MBL producers. Zinc chelators alone or in combination with meropenem against MBL-producing Klebsiella pneumoniae, Chryseobacterium indologenes, Elizabethkingia meningoseptica, and Stenotrophomonas maltophilia isolates were tested in vitro and in vivo (Galleria mellonella). In vitro experiments showed a synergistic activity between TPEN and meropenem toward all the strains. Nitroxoline alone retained activity against S. maltophilia, C. indologenes, and E. meningoseptica. In vivo experiments showed that TPEN or nitroxoline in combination with meropenem increased survival in larvae infected with E. meningoseptica, S. maltophilia, and K. pneumoniae. Based on our data, zinc chelators are potential carbapenem adjuvants molecules (restoring carbapenem activity) against MBL-sustained infections and could represent an interesting option for infections induced by these microorganisms.

Acinetobacter baumannii Resistance: A Real Challenge for Clinicians

Acinetobacter baumannii (named in honor of the American bacteriologists Paul and Linda Baumann) is a Gram-negative, multidrug-resistant (MDR) pathogen that causes nosocomial infections, especially in intensive care units (ICUs) and immunocompromised patients with central venous catheters. A. baumannii has developed a broad spectrum of antimicrobial resistance, associated with a higher mortality rate among infected patients compared with other non-baumannii species. In terms of clinical impact, resistant strains are associated with increases in both in-hospital length of stay and mortality. A. baumannii can cause a variety of infections; most involve the respiratory tract, especially ventilator-associated pneumonia, but bacteremia and skin wound infections have also been reported, the latter of which has been prominently observed in the context of war-related trauma. Cases of meningitis associated with A. baumannii have been documented. The most common risk factor for the acquisition of MDR A baumannii is previous antibiotic use, following by mechanical ventilation, length of ICU/hospital stay, severity of illness, and use of medical devices. Current efforts focus on addressing all the antimicrobial resistance mechanisms described in A. baumannii, with the objective of identifying the most promising therapeutic scheme. Bacteriophage- and artilysin-based therapeutic approaches have been described as effective, but further research into their clinical use is required.

Multidrug-resistant Acinetobacter baumannii resists reactive oxygen species and survives in macrophages

We investigated the intracellular survival of multidrug-resistant Acinetobacter baumannii (MDRAB) clinical isolates in macrophages, after phagocytosis, to determine their virulence characteristics. After ATCC 19606 and 5 clinical isolates of MDRAB were phagocytosed by mouse and human macrophages, the bacterial count of MDRAB strains, R4 and R5, increased in the mouse macrophages, 24 hours after phagocytosis. Bacterial count of the strains, R1 and R2, was almost equal 4 and 24 hours after phagocytosis. Intracellular reactive oxygen species was detected in the macrophages after phagocytosis of these bacteria. Further, the strains R1, R2, R4, and R5 showed higher catalase activity than ATCC 19606. Additionally, strains R1, R4, and R5 grew more efficiently than ATCC 19606 in the presence of H₂O₂, whereas growth of strains R2 and R3 was marginally more than that of ATCC 19606 in the presence of H₂O₂. The MDRAB clinical isolates altered the expression of TNF-α, IL-1β, IL-6, and MIP-2 mRNA induced in J774A.1 cells, 24 hours after phagocytosis. These results provide insights into the renewed virulence characteristics of MDRAB clinical isolates. Finally, tigecycline killed MDRAB phagocytosed by the macrophages more effectively than colistin, although colistin and tigecycline are both considered effective antibiotics for the treatment of MDRAB.

Use of Colistin in Critically Ill Patients

Due to lack of better therapeutic options, colistin use for extensively drug-resistant Gram-negative organisms was revived in the past two decades, including in patients in intensive-care units (ICU). There are multiple knowledge gaps pertaining to the clinical use and utility of colistin in critically-ill patients, but due to lack of options, it is used in these high risk patients. In this chapter, we critically review the various topics pertaining to colistin use in critically-ill patients, while highlighting the (lack of) controlled evidence supporting common current practices pertaining to colistin use by clinicians.

Zinc Oxide Spherical-Shaped Nanostructures: Investigation of Surface Reactivity and Interactions with Microbial and Mammalian Cells

Two ZnO materials of spherical hierarchical morphologies, with hollow (ZnO_HS) and solid cores (ZnO_SS), were obtained through the hydrolysis of zinc acetylacetonate in 1,4-butanediol. The nature of the defects and surface reactivity for the two ZnO materials were investigated through photoluminescence, X-ray photoelectron spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy proving the coexistence of shallow and deep defects and, also, the presence of polyol byproducts adsorbed on the outer layers of the ZnO samples. The EPR spectroscopy coupled with the spin-trapping technique showed that the surface of the ZnO samples generates reactive oxygen species (ROS) like hydroxyl (^•OH) and singlet oxygen (¹O₂) as well as carbon-centered radicals. The ZnO materials exhibited a wide spectrum of antimicrobial activity, being active against Gram-positive, Gram-negative, and fungi strains, both in planktonic and, more importantly, adherent growth states. The decrease of antimicrobial efficiency in the presence of a ROS scavenger (mannitol) and the decrease of the cell viability with the ROS level suggest that one of the mechanisms that governs both the antimicrobial and cytotoxic activities on human liver cells is ROS-mediated. However, at active antimicrobial concentrations, the biocompatibility of the tested materials is very good.

Antimicrobial Activity of Gallium Compounds on ESKAPE Pathogens

ESKAPE bacteria are a major cause of multidrug-resistant infections, and new drugs are urgently needed to combat these pathogens. Given the importance of iron in bacterial physiology and pathogenicity, iron uptake and metabolism have become attractive targets for the development of new antibacterial drugs. In this scenario, the FDA-approved iron mimetic metal Gallium [Ga(III)] has been successfully repurposed as an antimicrobial drug. Ga(III) disrupts ferric iron-dependent metabolic pathways, thereby inhibiting microbial growth. This work provides the first comparative assessment of the antibacterial activity of Ga(NO₃)₃ (GaN), Ga(III)-maltolate (GaM), and Ga(III)-protoporphyrin IX (GaPPIX), belonging to the first-, second- and third-generation of Ga(III) formulations, respectively, on ESKAPE species, including reference strains and multidrug-resistant (MDR) clinical isolates. In addition to the standard culture medium Mueller Hinton broth (MHB), iron-depleted MHB (DMHB) and RPMI-1640 supplemented with 10% human serum (HS) (RPMI-HS) were also included in Ga(III)-susceptibility tests, because of their different nutrient and iron contents. All ESKAPE species were resistant to all Ga(III) compounds in MHB and DMHB (MIC > 32 μM), except Staphylococcus aureus and Acinetobacter baumannii, which were susceptible to GaPPIX. Conversely, the antibacterial activity of GaN and GaM was very evident in RPMI-HS, in which the low iron content and the presence of HS better mimic the in vivo environment. In RPMI-HS about 50% of the strains were sensitive (MIC < 32) to GaN and GaM, both compounds showing a similar spectrum of activity, although GaM was more effective than GaN. In contrast, GaPPIX lost its antibacterial activity in RPMI-HS likely due to the presence of albumin, which binds GaPPIX and counteracts its inhibitory effect. We also demonstrated that the presence of multiple heme-uptake systems strongly influences GaPPIX susceptibility in A. baumannii. Interestingly, GaN and GaM showed only a bacteriostatic effect, whereas GaPPIX exerted a bactericidal activity on susceptible strains. Altogether, our findings raise hope for the future development of Ga(III)-based compounds in the treatment of infections caused by multidrug-resistant ESKAPE pathogens.

Treatment outcomes of patients with non-bacteremic pneumonia caused by extensively drug-resistant Acinetobacter calcoaceticus-Acinetobacter baumannii complex isolates: Is there any benefit of adding tigecycline to aerosolized colistimethate sodium?

Few therapeutic options exist for various infections caused by extensively drug-resistant Acinetobacter calcoaceticus-Acinetobacter baumannii (XDR-Acb) complex isolates, including pneumonia. This study investigated the clinical efficacy between aerosolized colistimethate sodium (AS-CMS, 2 million units thrice a day) treatment alone or in combination with standard-dose tigecycline (TGC) in patients with non-bacteremic pneumonia due to XDR-Acb, and explored the factors influencing patients' 30-day mortality.A 1:1 case (n = 106; receiving TGC plus AS-CMS) control (receiving AS-CMS alone with matching scores) observational study was conducted among adult patients with non-bacteremic XDR-Acb complex pneumonia in a Taiwanese medical center from January 2014 through December 2016. The clinically relevant data were retrospectively recorded. The primary endpoint was 30-day case fatality. Secondary endpoints investigated that if the co-morbidities, XDR-A. baumannii as a pneumonic pathogen, therapy-related factors, or airway colonization with colistin-resistant Acb negatively influenced the 14-day clinical condition of enrolled patients.A higher 30-day mortality rate was noted among the group receiving combination therapy (34.0% vs 22.6%; P = .17). The ≥7-day AS-CMS therapy successfully eradicated > 90% of airway XDR-Acb isolates. Nevertheless, follow-up sputum specimens from 10 (6.4% [10/156]) patients were colonized with colistin-resistant Acb isolates. After the conditional factors were adjusted by multivariate logistic analysis, the only factor independently predicting the 30-day case-fatality was the failure of treating XDR-Acb pneumonia at 14 days (adjusted odds ratio [aOR] = 38.2; 95% confidence interval [CI] = 9.96-142.29; P < .001). Cox proportional regression analysis found that chronic obstructive pulmonary disease (COPD) (adjusted hazard ratio [aHR] = 2.08; 95% CI = 1.05-4.10; P = .035), chronic renal failure (aHR = 3.00; 95% CI = 1.52-5.90; P = .002), non-invasive ventilation use (aHR = 2.68; 95% CI = 1.37-5.25; P = .004), and lack of TGC therapy (aHR = 0.52; 95% CI = 0.27-1.00; P = .049) adversely influenced the 14-day clinical outcomes. Conversely, the emergence of colistin-resistant Acb isolates in the follow-up sputum samples was not statistically significantly associated with curing or improving XDR-Acb pneumonia.In conclusion, aggressive pulmonary hygiene care, the addition of TGC, and corticosteroid dose tapering were beneficial in improving the 14-day patients' outcomes.

Curcumin alleviates persistence of Acinetobacter baumannii against colistin

Persisters are phenotypic variants of normal susceptible bacterial populations that survive prolonged exposure to high doses of antibiotics and are responsible for pertinacious infections and post-treatment relapses. Out of the three antibiotics, Acinetobacter baumannii formed the highest percentage of persister cells against rifampicin followed by amikacin and the least against colistin. Colistin-treated cells formed the high levels of reactive oxygen species (ROS) whose quenching with bipyridyl and thiourea led to an increased persister population. Curcumin, a polyphenolic pro-oxidant, significantly decreased persistence against colistin. The quenching of ROS generated by curcumin-colistin combination and the use of resveratrol, an anti-oxidant, with colistin increased the persister population, supporting the significance of ROS in decreased persistence against this combination. The down-regulation of repair genes by this combination in comparison to colistin alone supported the modulation of gene expression in response to ROS and their importance in decreased persistence. Increased membrane permeability by colistin, facilitating the penetration of curcumin into cells and resulting in increased ROS and compromised repair compounded by the decreased efflux of colistin by the inhibition of efflux pumps, may be responsible for enhanced lethality and low persistence. Hence, the curcumin-colistin combination can be another option with anti-persister potential for the control of chronic A. baumannii infections.

Sub-minimum inhibitory concentrations of colistin and polymyxin B promote Acinetobacter baumannii biofilm formation

We investigated the numbers of planktonic and biofilm cells and the expression levels of genes encoding efflux pumps and biofilm-related proteins in 10 clinical isolates of multi-drug resistant Acinetobacter baumannii (MDRA) as well as in its standard strain ATCC 19606 in the presence of colistin (CST), polymyxin B (PMB), minomycin (MIN), and tigecycline (TGC) at their respective sub-MICs. The number of planktonic and biofilm cells of ATCC 19606 decreased in the presence of all aforementioned antibiotics in a dose-dependent manner. Cell number also decreased in two representative MDRA strains, R2 and R3, in the presence of MIN and TGC in a dose-dependent manner. In contrast, the number of biofilm cells in these two strains increased in the presence of CST, while they increased significantly in the presence of PMB in R2 only. Pearson correlation analysis revealed that the number of biofilm cells was positively and significantly correlated with the mRNA levels of genes encoding efflux pumps (adeB and adeG) and autoinducer synthase (abaI) in strain R2 and adeB, adeG, adeJ, poly-acetyl-glucosamine-porin (pgaA), and abaI in strain R3 in the presence of CST. It was positively and significantly correlated with the mRNA levels of genes encoding adeB in strain R2 and an outer membrane protein A (ompA) and biofilm-associated protein (bap) in strain R3 in the presence of PMB. These results provide valuable insights into the biofilm formation potency of clinical isolates of MDRA that depends on efflux pumps and biofilm-related genes and its regulation by antibiotics.

Genetic characterization and in vitro activity of antimicrobial combinations of multidrug-resistant Acinetobacter baumannii from a general hospital in China

The present study aimed to develop a rational therapy based on the genetic epidemiology, molecular mechanism evaluation and in vitro antibiotic combinations activity in multidrug-resistant Acinetobacter baumannii (MDRAB). MDRAB was screened by the Kirby-Bauer method. The random amplified polymorphic DNA technique was used to establish genetic fingerprinting, and a series of resistance genes were detected by polymerase chain reaction. Antimicrobial agents including amikacin (AK), cefoperazone/sulbactam (SCF I/II), meropenem (MEM), minocycline (MINO) and ciprofloxacin (CIP) were used to determine the minimum inhibitory concentrations (MICs) and interactions between antibiotics by the broth microdilution method and chequerboard assays. In total, 34 MDRAB strains were isolated and classified into 8 phenotypes A-H, according to their general drug susceptibilities. A total of 4 major genotypes (I–IV) were clustered at 60% a genotypic similarity threshold. High positive rates of β-lactamase TEM-1, topoisomerase IV, oxacillinase (OXA)-23, AdeB family multidrug efflux RND transporter adeB, β-lactamase AmpC, class 1 integrons (Int-1), 16S rRNA methylase rmtA, phosphotransferase aph(3), 16S rRNA methyltransferase armA were presented to exceed 90%, acetylyltransferase aac(3)-I, aac(6′-I, ant(3″)-I, 16S rRNA methylase rmtB, oxacillinase OXA-24 and metallo-β-lactamase IMP-5 genes demonstrated positive rates of 29.4–85.29%, while adeRS two-component system was not observed in any strain. MEM+SCF I or SCF II primarily exhibited synergistic effects. AK+SCF I, AK+SCF II, MINO+SCF I, MINO+SCF II, MINO+CIP and MINO+MEM primarily presented additive effects. AK+CIP demonstrated 70.59% antagonism. The antibacterial activity of SCF I was superior compared with that of SCF II. The results indicated the polyclonal genetic epidemiological trend of MDRAB in the Second Xiangya Hospital, and verified the complexity of genetic resistance. In addition, combinations suggested to be efficacious were MEM+SCF I and MEM+SCF II, which were more effective compared with other combinations for the management of MDRAB infection.

Potent β-Lactam Enhancer Activity of Zidebactam and WCK 5153 against Acinetobacter baumannii, Including Carbapenemase-Producing Clinical Isolates

Multidrug-resistant Acinetobacter baumannii has rapidly spread worldwide, resulting in a serious threat to hospitalized patients. Zidebactam and WCK 5153 are novel non-β-lactam bicyclo-acyl hydrazide β-lactam enhancer antibiotics being developed to target multidrug-resistant A. baumannii The objectives of this work were to determine the 50% inhibitory concentrations (IC₅₀s) for penicillin-binding proteins (PBP), the OXA-23 inhibition profiles, and the antimicrobial activities of zidebactam and WCK 5153, alone and in combination with β-lactams, against multidrug-resistant A. baumannii MICs and time-kill kinetics were determined for an A. baumannii clinical strain producing the carbapenemase OXA-23 and belonging to the widespread European clone II of sequence type 2 (ST2). Inhibition of the purified OXA-23 enzyme by zidebactam, WCK 5153, and comparators was assessed. All of the compounds tested displayed apparent K_i values of >100 μM, indicating poor OXA-23 β-lactamase inhibition. The IC₅₀s of zidebactam, WCK 5153, cefepime, ceftazidime, meropenem, and sulbactam (range of concentrations tested, 0.02 to 2 μg/ml) for PBP were also determined. Zidebactam and WCK 5153 demonstrated specific high-affinity binding to PBP2 of A. baumannii (0.01 μg/ml for both of the compounds). The MICs of zidebactam and WCK 5153 were >1,024 μg/ml for wild-type and multidrug-resistant Acinetobacter strains. Importantly, combinations of cefepime with 8 μg/ml of zidebactam or WCK 5153 and sulbactam with 8 μg/ml of zidebactam or WCK 5153 led to 4- and 8-fold reductions of the MICs, respectively, and showed enhanced killing. Notably, several of the combinations resulted in full bacterial eradication at 24 h. We conclude that zidebactam and WCK 5153 are PBP2 inhibitors that show a potent β-lactam enhancer effect against A. baumannii, including a multidrug-resistant OXA-23-producing ST2 international clone.

In Vitro Activity of Various Antibiotics in Combination with Tigecycline Against Acinetobacter baumannii: A Systematic Review and Meta-Analysis

Given that tigecycline-based combination therapy is recognized as a valuable option for the treatment of tigecycline-resistant Acinetobacter baumannii, we conducted this systematic review and meta-analysis to assess the overall evidence of its effectiveness. The synergy rate was defined as the primary outcome that was calculated separately for time-kill, Etest, and checkerboard microdilution methods. The secondary outcomes were bactericidal activity and the efficacy of combination treatment on the development of resistance. In total, 37 published papers and 16 conference proceedings were included. Nine classes consisting of 22 antibiotic types in combination with tigecycline against 1,159 A. baumannii strains were reported in the analysis. For the time-kill studies, combination therapy showed a synergy rate of 37.9% (95% confidence interval [CI], 30.7-46.5); the highest synergy rate was 67.4% (95% CI, 27.3-91.9) for tigecycline in combination with colistin. Moreover, combination with amikacin or colistin could efficiently inhibit the development of tigecycline resistance. Compared with checkerboard microdilution and Etest methods, time-kill studies always showed higher synergy rates. Altogether, these results suggest that the in vitro tigecycline-based combinations resulted in moderate synergy rates and that several combinations could suppress the resistance of A. baumannii to tigecycline, which should be further confirmed in animal models and clinical trials.

Evaluation of the synergistic effect of a combination of colistin and tigecycline against multidrug-resistant Acinetobacterbaumannii

Objective:

Acinetobacter baumannii species cause nosocomial infections and can subsequently develop multidrug resistance (MDR). The objective of this study was to evaluate the susceptibility of A. baumannii to a novel combination of colistin and tigecycline, which may provide a faster and more efficacious treatment via a synergistic effect.

Methods:

We included 50 MDR A. baumannii samples that were isolated in our clinics between 2009 and 2014. We used broth microdilution (BMD) and the E-test to evaluate the effects of colistin and tigecycline, and the E-test to assess the interaction of the colistin-tigecycline combination. The interaction between the two antibiotics was evaluated using the fractional inhibition concentration (FIC) index and was classified as follows: FIC≤0.5, synergistic; 0.5<FIC<1, partially synergistic; FIC=1, additive; 1<FIC<4, indifferent; and FIC≥4, antagonistic.

Results:

No tigecycline and colistin resistance was determined by BMD or E-test. The interaction between colistin and tigecycline, when used in combination, was 2% synergistic, 6% additive, 88% indifferent, and 4% antagonistic.

Conclusion:

Although combination therapy is suggested for MDR A. baumannii infections, our results suggest that the synergistic effect of the colistin-tigecycline combination is insufficient to make it an optimal treatment choice.

Antibiotic Resistance Profiles and Genotypes of Acinetobacter baumannii Isolates and In Vitro Interactions of Various Antibiotics in Combination with Tigecycline and Colistin

Objectives

The aim of this study was to determine the antibiotic resistance profile, clonal relation and efficacy of antibiotic combinations in nosocomial multidrug resistant (MDR) Acinetobacter baumannii.

Materials and Methods

Antibiotic susceptibilities of 84 MDR A. baumannii against tigecycline (TGC), colistin (CL), amikacin (AK), ciprofloxacin (CIP), meropenem (MR), moxifloxacin (MXF), rifampicin (RF) were determined by microdilution method. Clonal relationship was investigated by genotyping using AP-PCR and antibiotyping. Interactions of antibiotic combinations were tested against clonally unrelated strains by the checkerboard (CB) method. The efficacy of the best combinations was also assesed on a selected isolate by the time-kill (TK) method.

Results

CIP, RF, MXF, MR, AK resistance was found as 90.47%; 47.62%; 22.62%; 58.33%; 50% respectively; however; CL and TGC were not ascertained. The isolates were distinguished as 25 different antibiotypes and 15 varied molecular patterns. The best synergistic effect was detected in combinations of CL with RF (100%) and MR (100%), in combinations of TGC with RF (53%) against clonally unrelated 15 MDR A. baumannii isolates by the CB method. While CL-RF and CL-MR showed synergy by TK method like CB, on the other hand TGC-RF indicated additive interactions by TK.

Conclusion

In this study, both synergy tests showed that CL in combination with RF would be a good option in MDR A. baumannii.

A comparison of adeB gene expression levels under conditions of induced resistance by different drugs in vitro in Acinetobacter baumannii

The present study was designed to analyze the difference of the adeB gene expression levels under conditions of induced resistance and to verify the significance of the AdeABC efflux pump in the in vitro induction of Acinetobacter baumannii (A. baumannii). Strains were isolated from blood or drainage samples in the First Hospital of China Medical University. The minimum inhibitory concentration (MIC) was determined by a broth microdilution method. In addition, an in vitro induction of drug-resistance tests was confirmed with the three drugs. The expression level of the adeB was determined by quantitative polymerase chain reaction (qPCR), and there were 19, 17 and 25 strains isolated from this test that were susceptible to amikacin, netilmicin and imipenem, respectively. Furthermore, 11, 15 and 8 stably resistant strains generated against amikacin, netilmicin and imipenem, respectively, by in vitro induction. The MIC values of all the imipenem-sensitive and imipenem-resistant strains decreased by no more than 2-fold under carbonyl cyanide 3-chlorophenylhydrazone (CCCP)-containing conditions. The MIC values of 10 amikacin-resistant and 14 netilmicin-resistant strains decreased 4-fold or more under CCCP-containing conditions. Furthermore, qPCR revealed that none of the imipenem-sensitive or the imipenem-resistant strains expressed the adeB gene, but there were 10 amikacin-resistant and 5 netilmicin-resistant strains that expressed the adeB gene. In conclusion, the present study confirmed that in vitro induction of drug-resistance tests in sensitive A. baumannii strains could produce drug-resistance following long-term exposure to low doses of antibacterial drugs. In addition, amikacin and netilmicin are more prone to producing drug-resistance compared to imipenem in vitro. The results of the efflux phenotype indicate the possible application of CCCP in the inhibition of the efflux system in some drugs. The inconsistency between efflux phenotype and qPCR of adeB indicates that other mechanisms may also be included in the induction of drug-resistance that work with the active mechanism in order to increase drug resistance to common clinically-used antimicrobial drugs.

Biology of Acinetobacter baumannii: Pathogenesis, Antibiotic Resistance Mechanisms, and Prospective Treatment Options

Acinetobacter baumannii is undoubtedly one of the most successful pathogens responsible for hospital-acquired nosocomial infections in the modern healthcare system. Due to the prevalence of infections and outbreaks caused by multi-drug resistant A. baumannii, few antibiotics are effective for treating infections caused by this pathogen. To overcome this problem, knowledge of the pathogenesis and antibiotic resistance mechanisms of A. baumannii is important. In this review, we summarize current studies on the virulence factors that contribute to A. baumannii pathogenesis, including porins, capsular polysaccharides, lipopolysaccharides, phospholipases, outer membrane vesicles, metal acquisition systems, and protein secretion systems. Mechanisms of antibiotic resistance of this organism, including acquirement of β-lactamases, up-regulation of multidrug efflux pumps, modification of aminoglycosides, permeability defects, and alteration of target sites, are also discussed. Lastly, novel prospective treatment options for infections caused by multi-drug resistant A. baumannii are summarized.

Sensitivity of levofloxacin in combination with ampicillin-sulbactam and tigecycline against multidrug-resistant Acinetobacter baumannii

BACKGROUND AND OBJECTIVES

The selection of alternative treatment options with antibiotic combinations may be used for successful managing of multidrug-resistant Acinetobacter baumannii. The aim of this study was to determine the synergistic effects of ampicillin-sulbactam combined with either levofloxacin or tigecycline against MDR A. baumannii.

MATERIALS AND METHODS

A total 124 of A.baumannii isolates collected from clinical samples of hospitalized patients which assessed for antibiotic susceptibility using disk diffusion method. E-test was used on 10 MDR A. baumannii isolates to determine the minimum inhibitory concentration (MIC) of ampicillin-sulbactam, levofloxacin and tigecycline. Any synergistic effects were evaluated at their own MIC using E-test assay at 37°C for 24 hours. Synergy was defined as a fractional inhibitory concentration index (FICI) of ≤0.5.

RESULTS

Levofloxacin plus ampicillin-sulbactam combination was found to have synergistic effects (FIC index: ≤0.5) in 90% of the isolates, but there was no synergistic effect for ampicillin-sulbactam/tigecycline and tigecycline/levofloxacin combination. The antagonist effect in 50% of isolates (FIC index: >2) showed in combination of levofloxacin/tigecycline.

CONCLUSION

The emergence of multidrug A. baumannii isolates requires evaluating by combination therapy. The combination of levofloxacin plus a bactericidal antibiotic such as ampicillin-sulbactam is recommended. Results should be confirmed by clinical studies.

In Vitro Interactions of Antibiotic Combinations of Colistin, Tigecycline, and Doripenem Against Extensively Drug-Resistant and Multidrug-Resistant Acinetobacter baumannii

BACKGROUND

Acinetobacter baumannii infections are difficult to treat owing to the emergence of various antibiotic resistant isolates. Because treatment options are limited for multidrug-resistant (MDR) A. baumannii infection, the discovery of new therapies, including combination therapy, is required. We evaluated the synergistic activity of colistin, doripenem, and tigecycline combinations against extensively drug-resistant (XDR) A. baumannii and MDR A. baumannii.

METHODS

Time-kill assays were performed for 41 XDR and 28 MDR clinical isolates of A. baumannii by using colistin, doripenem, and tigecycline combinations. Concentrations representative of clinically achievable levels (colistin 2 μg/mL, doripenem 8 μg/mL) and achievable tissue levels (tigecycline 2 μg/mL) for each antibiotic were used in this study.

RESULTS

The colistin-doripenem combination displayed the highest rate of synergy (53.6%) and bactericidal activity (75.4%) in 69 clinical isolates of A. baumannii. Among them, the-doripenem-tigecycline combination showed the lowest rate of synergy (14.5%) and bactericidal activity (24.6%). The doripenem-tigecycline combination showed a higher antagonistic interaction (5.8%) compared with the colistin-tigecycline (1.4%) combination. No antagonism was observed for the colistin-doripenem combination.

CONCLUSIONS

The colistin-doripenem combination is supported in vitro by the high rate of synergy and bactericidal activity and lack of antagonistic reaction in XDR and MDR A. baumannii. It seems to be necessary to perform synergy tests to determine the appropriate combination therapy considering the antagonistic reaction found in several isolates against the doripenem-tigecycline and colistin-tigecycline combinations. These findings should be further examined in clinical studies.

Extensively drug-resistant Acinetobacter baumannii outbreak cross-transmitted in an intensive care unit and respiratory intensive care unit

BACKGROUND

Extensively drug-resistant Acinetobacter baumannii (XDRAB) is a great threat in intensive care units (ICUs). The aim of this study was to describe an XDRAB outbreak which was cross-transmitted in the ICU and respiratory intensive care unit (RICU) in a tertiary care hospital from January-March 2013.

METHODS

Patient and environmental surveillances were performed. Isolates were tested for antimicrobial susceptibility. Genotypes were analyzed by multilocus sequence typing (MLST). A series of enhanced strategies were implemented to control the outbreak.

RESULTS

A total of 11 patients were infected by XDRAB strains during this outbreak. Three patients in the ICU were found positive for XDRAB at the onset of the outbreak. Thereafter, infections were detected in 6 patients in the RICU, followed by reappearance of this strain in the ICU in 2 patients. All A baumannii strains isolated from patients and the environment were extensively drug resistant. MLST revealed them as ST368. After 3 rounds of environmental screening and cleaning, the laminar flow system connecting the ICU and RICU was found as the source of transmission. Successful control of this outbreak was achieved through multifaceted intervention measures.

CONCLUSIONS

This study suggested the importance of thorough surveillance and disinfection of the environment, including concealed devices, in preventing the transmission of an outbreak.

In Vitro Bactericidal Activity of Trimethoprim-Sulfamethoxazole Alone and in Combination with Colistin against Carbapenem-Resistant Acinetobacter baumannii Clinical Isolates

Trimethoprim-sulfamethoxazole alone and combined with colistin was tested in vitro against six carbapenem-resistant Acinetobacter baumannii (CRAB) clinical strains. After 24 h, at achievable serum concentrations, trimethoprim-sulfamethoxazole effectively killed all strains, while colistin killed only one strain. Trimethoprim-sulfamethoxazole plus colistin rapidly killed all strains after 6 h and for up to 24 h. Trimethoprim-sulfamethoxazole, one of the few remaining antimicrobials that still has a degree of activity, particularly combined with colistin, might represent an effective therapy for severe CRAB infections.

High prevalence of extensively drug-resistant and metallo beta-lactamase-producing clinical Acinetobacter baumannii in Iran

Acinetobacter species particularly Acinetobacter baumannii (A. baumannii) have been widely reported as broad-spectrum antibiotic resistant pathogens. Expression of various types of metallo beta-lactamases (MBL), classified as Ambler class B, has been associated with carbapenem resistance. Here, we attempted to assess the frequency of extensively drug-resistant (XDR) and MBL-producing A. baumannii among clinical isolates. 86 clinical A. baumannii strains were collected from 2014 to 2015 and their susceptibility to meropenem (10 μg), imipenem (10 μg), azteronem (30 μg), pipracillin (100 μg) tazobactam (110 μg), tobramycin (10 μg), fosfomycin (200 μg), rifampicin (5 μg), colistin (10 μg), tigecycline (15 μg), sulbactam/ampicillin (10 μg + 10 μg) and polymixin B (300 U) was evaluated using disk diffusion method. The MBL-producing isolates were screened using combined disc diffusion method. Furthermore, the presence of blaVIM, blaIMP, blaSPM, blaGIM, blaSIM and blaNDM was detected by PCR. 34.9% of isolates were recovered from bronchoalveolar lavage (BAL). 81 (94.2%) and 62 (71.2%) isolates were multidrug resistance (MDR) and XDR, respectively. 44 (51.2%) and 65 (75.6%) isolates were MBL-producing strains with resistance to imipenem and meropenem, respectively. 2 (2.3%), 13 (15.1%), 2 (2.3%), 4 (4.7%) and 2 (2.3%) isolates carried blaVIM, blaIMP, blaSPM, blaGIM and blaSIM genes, respectively. Our data showed that the rate of XDR and MBL A. baumannii is on the rise.

Investigating the in vitro synergistic activities of several antibiotic combinationsagainst carbapenem-resistant Acinetobacter baumannii isolates

BACKGROUND/AIM

Acinetobacter baumannii (A. baumannii) is one of the most common healthcare-associated infectious agents worldwide. The aim of this study was to investigate the in vitro synergistic activities of several antibiotic combinations against carbapenem-resistant (CR) A. baumannii isolates.

MATERIALS AND METHODS

Eighteen CR A. baumannii strains were isolated from the patients who were hospitalized in the intensive care unit between June 2012 and August 2012. The in vitro effects of single and binary combinations of meropenem (MEM), colistin (CST), tigecycline (TGC), and sulbactam (SUL) on these isolates were determined using the Epsilometer test (E-test) method.

RESULTS

All 18 isolates were resistant to MEM and SUL and susceptible to CST. TGC was detected as susceptible in two of the isolates and intermediate susceptibility results were observed in the remaining isolates. With MEM-CST and MEM-TGC combinations, synergism was determined against all isolates. The synergistic and/or additive effect ratios were detected in MEM-SUL, CST-SUL, TGC-SUL, and CST-TGC combinations as 16.7%, 38.9%, 16.7%, and 5.6%, respectively.

CONCLUSION

Among the tested antimicrobial combinations, the in vitro combination of MEM with TGC or CST was most effective against the CR A. baumannii strains.

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

The study investigated the effect of antibiotic combinations against 20 clinical isolates of A. baumannii (seven colistin-resistant and 13 colistin-susceptible) with different resistance mechanisms. Clinical data, treatment, and patient mortality were evaluated. The following methods were used: MIC, PCRs, and outer membrane protein (OMP) analysis. Synergy was investigated using the checkerboard and time-kill methods. Clonality was evaluated by PFGE. Based on clonality, the whole genome sequence of six A. baumannii isolates was analyzed. All isolates were resistant to meropenem, rifampicin, and fosfomycin. OXA-23 and OXA-143 were the most frequent carbapenemases found. Four isolates showed loss of a 43kDa OMP_. The colistin-susceptible isolates belonged to different clones and showed the highest synergistic effect with fosfomycin-amikacin. Among colistin-resistant isolates, the highest synergistic effect was observed with the combinations of colistin-rifampicin followed by colistin-vancomycin. All colistin-resistant isolates harbored bla_OXA-23-like and belonged to CC113. Clinical and demographic data were available for 18 of 20 patients. Fourteen received treatment and eight patients died during treatment. The most frequent site of infection was the blood in 13 of 14 patients. Seven patients received vancomycin plus an active drug against A. baumannii; however, mortality did not differ in this group. The synergistic effect was similar for colistin-susceptible isolates of distinct clonal origin presenting with the same resistance mechanism. Overall mortality and death during treatment was high, and despite the high synergism in vitro with vancomycin, death did not differ comparing the use or not of vancomycin plus an active drug against A. baumannii.

Treatment for patients with multidrug resistant Acinetobacter baumannii pulmonary infection

Bacterial infections are common but have become increasingly resistant to drugs. The aim of the present study was to examine the combined treatment of traditional Chinese and Western medicine in 30 cases of pulmonary infection with multidrug resistant Acinetobacter baumannii. Patients were divided into groups A and B according to drug treatments. Cefoperazone or sulbactam and tanreqing were administered in group A, and cefoperazone or sulbactam in group B. The curative effect and prognosis of the two groups were recorded and the remaining treatments were performed routinely in the clinic. For the combined therapy group, which was administered sulperazone and tanreqing, 8 patients were recovered, 6 patients had significant effects, 3 patients exhibited some improvement and 1 patient had no response. One of the patients did not survive after 28 days. By contrast, there were 4 patients that were successfully treated, 3 patients with significant effects, 2 patients with some improvement and 2 patients had no response in the sulperazone group, and 4 patients did not survive after 28 days. In conclusion, the combined therapy of cefoperazone or sulbactam supplemented with tanreqing was identified to be more effective than cefoperazone or sulbactam as monotherapy, for treating multidrug resistant Acinetobacter baumannii.

Holarrhena antidysenterica Extract and Its Steroidal Alkaloid, Conessine, as Resistance-Modifying Agents Against Extensively Drug-Resistant Acinetobacter baumannii

Emergence and spread of antibiotic-resistant Acinetobacter baumannii have become a major public health concern. This study was designed to investigate the efficacy of Holarrhena antidysenterica extract and its major steroidal alkaloid conessine as resistance-modifying agents (RMAs) on the susceptibility of A. baumannii to novobiocin and rifampicin. A significant synergistic activity of both the extract and conessine in combination with either novobiocin or rifampicin with fractional inhibitory concentration index ≤0.5 was demonstrated. Fluorescent dyes and different efflux pump inhibitors were used to further investigate the synergism. Increase in the uptake of 1-N-phenylnaphthylamine in the bacterial cells treated with the extract and conessine was not observed indicating that both substances did not act as permeabilizers. With regard to efflux pump inhibition, no accumulation in ethidium bromide (EtBr) was noticed suggesting that the AdeABC pump was not involved. In contrast, accumulation in Pyronin Y was significantly increased (p < 0.05) demonstrating that the synergism was due to interference with the AdeIJK pump. Study on frequencies of the spontaneous mutational resistance to the extract in combination with antibiotics demonstrated attenuation in drug-resistant organisms. Thus, H. antidysenterica extract and conessine as RMAs may offer a combinatory therapy to restore antibiotic susceptibility in the extensively drug-resistant A. baumannii.

A meta-analysis of in vitro antibiotic synergy against Acinetobacter baumannii

The aim of the work was to describe the different in vitro models for testing synergism of antibiotics and gather the results of antibiotic synergy against multidrug-resistant Acinetobacter baumannii (MDR-Ab). The different original articles were obtained from different web sites. In order to compare the results obtained by the different methods for synergy testing, the Pearson chi-square and the Fischer tests were used. Moreover, non-parametric chi-square test was used in order to compare the frequency distribution in each analysed manuscript. In the current meta-analysis 24 manuscripts, which encompassed 2016 tests of in vitro synergism of different antimicrobials against MDR-Ab, were revised. Checkerboard synergy testing was used in 11 studies, which encompasses 1086 tests (53.9%); time-kill assays were applied in 12 studies, which encompass 359 tests (17.8%); gradient diffusion methods were used in seven studies, encompassing 293 tests (14.5%). And, finally, time-kill plus checkerboard were applied in two studies, encompassing 278 tests (13.8%). By comparing these data, checkerboard and time-kill methods were significantly more used than gradient diffusion methods (p<0.005). Regarding synergy rates obtained on the basis of the applied method, checkerboard provided 227 tests (20.9%) with a synergistic effect; time-kill assays yielded 222 tests (61.8%) with a synergistic effect; gradient diffusion methods only provided 29 tests (9.9%) with a synergistic effect; and, finally, time-kill plus checkerboard yielded just 15 tests (5.4%) with a synergistic effect. When comparing these percentages, synergy rates reported by time-kill methods were significantly higher than that obtained by checkerboard and gradient diffusion methods (p<0.005). On the basis of the revised data, the combinations of a bactericidal antibiotic plus Tigecycline, Vancomycin or Teicoplanin are not recommended. The best combinations of antibiotics are those which include bactericidal antibiotics such as Carbapenems, Fosfomycin, Amikacin, Polymyxins, Rifampicin and Ampicillin/Sulbactam.