Major variation in MICs of tigecycline in Gram-negative bacilli as a function of testing method

Emergence of eravacycline heteroresistance in carbapenem-resistant Acinetobacter baumannii isolates in China

Carbapenem-resistant Acinetobacter baumannii (CRAB) is resistant to almost all antibiotics. Eravacycline, a newer treatment option, has the potential to treat CRAB infections, however, the mechanism by which CRAB isolates develop resistance to eravacycline has yet to be clarified. This study sought to investigate the features and mechanisms of eravacycline heteroresistance among CRAB clinical isolates. A total of 287 isolates were collected in China from 2020 to 2022. The minimum inhibitory concentration (MIC) of eravacycline and other clinically available agents against A. baumannii were determined using broth microdilution. The frequency of eravacycline heteroresistance was determined by population analysis profiling (PAP). Mutations and expression levels of resistance genes in heteroresistant isolates were determined by polymerase chain reaction (PCR) and quantitative real-time PCR (qRT-PCR), respectively. Antisense RNA silencing was used to validate the function of eravacycline heteroresistant candidate genes. Twenty-five eravacycline heteroresistant isolates (17.36%) were detected among 144 CRAB isolates with eravacycline MIC values ≤4 mg/L while no eravacycline heteroresistant strains were detected in carbapenem-susceptible A. baumannii (CSAB) isolates. All eravacycline heteroresistant strains contained OXA-23 carbapenemase and the predominant multilocus sequence typing (MLST) was ST208 (72%). Cross-resistance was observed between eravacycline, tigecycline, and levofloxacin in the resistant subpopulations. The addition of efflux pump inhibitors significantly reduced the eravacycline MIC in resistant subpopulations and weakened the formation of eravacycline heteroresistance in CRAB isolates. The expression levels of adeABC and adeRS were significantly higher in resistant subpopulations than in eravacycline heteroresistant parental strains (P < 0.05). An ISAba1 insertion in the adeS gene was identified in 40% (10/25) of the resistant subpopulations. Decreasing the expression of adeABC or adeRS by antisense RNA silencing significantly inhibited eravacycline heteroresistance. In conclusion, this study identified the emergence of eravacycline heteroresistance in CRAB isolates in China, which is associated with high expression of AdeABC and AdeRS.

Phenotypic and Genotypic Characterization of Pan-Drug-Resistant Klebsiella pneumoniae Isolated in Qatar

In secondary healthcare, carbapenem-resistant Enterobacterales (CREs), such as those observed in Klebsiella pneumoniae, are a global public health priority with significant clinical outcomes. In this study, we described the clinical, phenotypic, and genotypic characteristics of three pan-drug-resistant (PDR) isolates that demonstrated extended resistance to conventional and novel antimicrobials. All patients had risk factors for the acquisition of multidrug-resistant organisms, while microbiological susceptibility testing showed resistance to all conventional antimicrobials. Advanced susceptibility testing demonstrated resistance to broad agents, such as ceftazidime-avibactam, ceftolozane-tazobactam, and meropenem-vaborbactam. Nevertheless, all isolates were susceptible to cefiderocol, suggested as one of the novel antimicrobials that demonstrated potent in vitro activity against resistant Gram-negative bacteria, including CREs, pointing toward its potential therapeutic role for PDR pathogens. Expanded genomic studies revealed multiple antimicrobial-resistant genes (ARGs), including blaNMD-5 and blaOXA derivative types, as well as a mutated outer membrane porin protein (OmpK37).

Intermethod comparability analyses of gepotidacin antimicrobial susceptibility tests using a large collection of globally collected Escherichia coli and Staphylococcus saprophyticus clinical isolates

Gepotidacin (GSK2140944) is a novel, bactericidal, first in class triazaacenaphthylene bacterial type II topoisomerase inhibitor in development for the treatment of uncomplicated urinary tract infections and gonorrhea. The performance of several antimicrobial susceptibility methods (broth microdilution, gradient diffusion, and disk diffusion) for gepotidacin were evaluated using over 5800 recent Escherichia coli and Staphylococcus saprophyticus clinical isolates. Reference broth microdilution gepotidacin MICs showed an essential agreement of 95.9 % and 98.1 % with MICs by gradient diffusion for E. coli and S. saprophyticus isolates, respectively. Gepotidacin susceptibility using disks produced by 2 manufacturers had good agreement with an R2 values of 0.95 and 99.2 % of overall zone diameters agreeing within 3 mm. A correlation with an overall R2 value of 0.72 between MICs by broth microdilution and zone diameters by disk diffusion was observed. This data should assist in the clinical development of gepotidacin and provide reliable susceptibility methods to evaluate its activity.

Significant Impact of AcrB Amino Acid Polymorphism at Residue 716 on Susceptibility to Tigecycline and Other Antibiotics in Klebsiella pneumoniae

AcrAB-TolC is a multidrug RND-type efflux pump that is widespread in Gram-negative bacteria. As the substrate-binding subunit, AcrB was shown to modulate antimicrobial resistance in Escherichia coli, but the influence of AcrB mutation on Klebsiella pneumoniae, a major clinical pathogen, has not been well-studied. The finding of an R716L mutation in AcrB in a clinical tigecycline-nonsusceptible K. pneumoniae S1 strain inspired us to probe the role of AcrB residue 716 in antimicrobial resistance. This residue was subsequently subjected to saturation mutagenesis, followed by antibiotic susceptibility tests, survival assays, and antibiotic accumulation assays, showing strong influences of AcrB mutation on antimicrobial resistance. In particular, resistance levels to azithromycin, tetracycline, tigecycline, and cefoxitin were significantly changed by AcrB mutation at residue 716. Mutations to charged residues, polar residues, and residues that disrupt secondary structures have particularly reduced the antimicrobial susceptibility of bacteria, except for azithromycin, and the impact is not due to the abolishment of the efflux function of the pump. Therefore, it is concluded that residue 716 is an important residue that significantly influences antimicrobial resistance in K. pneumoniae, adding to our understanding of antimicrobial resistance mechanisms in this key clinical pathogen.

A widespread single amino acid mutation in AcrA reduces tigecycline susceptibility in Klebsiella pneumoniae

IMPORTANCE

Tigecycline, a glycecycline antibiotic with broad-spectrum activity against almost all Gram-positive and Gram-negative bacteria, is a highly concerned "last-resort" antibiotic. In addition to plasmid-hosted mobile tet(X) conferring high-level resistance to tigecycline, there are many reports suggesting increased expression of AcrAB-TolC efflux pump leads to tigecycline non-susceptibility. However, the role of mutations in AcrAB-TolC on tigecycline resistance has not been identified. This study reports a novel T188A mutation of the AcrA subunit of AcrAB-TolC complex in a clinical tigecycline-resistant Klebsiella pneumoniae strain and reveals the role of AcrA mutation on tigecycline resistance in K. pneumoniae. High prevalence of A188 type AcrA in hypervirulent multidrug-resistant K. pneumoniae indicates that mutations of the AcrAB-TolC complex may play a larger role in determining bacterial pathogenesis and antibiotic susceptibility than previously expected.

Carbapenem-resistant Citrobacter freundii harboring blaKPC-2 and blaNDM-1: a study on their transferability and potential dissemination via generating a transferrable hybrid plasmid mediated by IS6100

Introduction

The increase in clinical Enterobacteriaceae with dual carbapenemase has become a serious healthcare concern. It is essential to characterize the transferability and potential dissemination of blaKPC-2- and blaNDM-1-coharboring carbapenem-resistant Citrobacter freundii (CRCF).

Methods

Four blaKPC-2- and blaNDM-1-coharboring CRCF strains were collected from our surveillance of the prevalence of carbapenem-resistant Enterobacteriaceae. The isolates were assessed using species identification, antimicrobial susceptibility testing, conjugation assays, whole-genome sequencing, plasmid stability, and fitness costs. Clonality, genome, plasmidome, and phylogeny were analyzed to reveal potential dissemination.

Results

Three ST523 blaKPC-2- and blaNDM-1-coharboring CRCF strains, collected from the same hospital within 1 month, exhibited high homology (both identity and coverage >99%), implying clonal dissemination and a small-scale outbreak. Moreover, the blaKPC-2 and blaNDM-1 genes were coharbored on an IncR plasmid, probably generated by a blaKPC-2-harboring plasmid acquiring blaNDM-1, in these three strains. Importantly, the IncR plasmid may form a transferable hybrid plasmid, mediated by IS6100 via transposition, with another IncFII plasmid included in the same C. freundii strain. Furthermore, the blaKPC-2 and blaNDM-1 of the fourth CRCF strain are located on two different non-transferable plasmids lacking complete transfer elements. Additionally, throughout the course of the 10-day continuous passage, the genetic surroundings of blaNDM-1 in four CRCF strains were gradually excised from their plasmids after the 8th day, whereas they maintained 100% retention for blaKPC-2. Genome and plasmidome analyses revealed that blaKPC-2- or blaNDM-1-harboring C. freundii were divergent, and these plasmids have high homology to plasmids of other Enterobacteriaceae.

Conclusion

Clonal dissemination of ST523 blaKPC-2- and blaNDM-1-coharboring CRCF strains was detected, and we first reported blaKPC-2 and blaNDM-1 concomitantly located on one plasmid, which could be transferred with mediation by IS6100 via transposition. Continued surveillance should urgently be implemented.

Population genomic analysis reveals the emergence of high-risk carbapenem-resistant Escherichia coli among ICU patients in China

OBJECTIVES

The increasing incidence of carbapenem-resistant Enterobacterales (CRE) mediated nosocomial infections has caused a significant public health burden globally. Currently, the prevalence and genomic characteristics of carbapenem-resistant Escherichia coli (CREC) in patients admitted to the intensive care unit (ICU) are unknown.

METHODS

Herein, we present a nationwide genomic investigation of CREC isolates among ICU patients in China in 2018 and 2020. In total, 113 CREC isolates were identified from 1105 samples in 25 hospitals, and investigated with phenotyping and genomics approaches.

RESULTS

Carbapenemases were produced in 94.69% (107/113) of CREC isolates, which comprise KPC-2 (n = 53, 49.53%), NDM (n = 51, 47.66%), IMP-4 (n = 2, 1.87%), and OXA-181 (n = 1, 0.93%). Notably, CREC isolates co-carrying mcr-9 and bla_NDM-5 or tet(X4) and bla_NDM-5 were first identified in clinical settings. The carbapenemase genes of most isolates were located on the plasmids. The bla_KPC gene was mainly mediated by IncFII plasmids (n = 37, 69.81%), and bla_NDM was located on the IncX3 plasmid (n = 36, 70.59%). CREC isolates belonged to diverse sequence types (STs) of which ST131 was the most prevalent bla_KPC-positive CREC isolates (34/113, 30.09%), while bla_NDM was associated with ST617 and ST410 isolates, thereby indicating that multiple CREC clones spread in Chinese ICU patients.

CONCLUSIONS

This study highlights the emerging threat of high-risk CREC isolates such as ST131 circulating in the ICU in China. Hence, stringent monitoring of such high-risk clones should be performed.

Molecular Characteristics of an NDM-4 and OXA-181 Co-Producing K51-ST16 Carbapenem-Resistant Klebsiella pneumoniae: Study of Its Potential Dissemination Mediated by Conjugative Plasmids and Insertion Sequences

The carbapenem-resistant Klebsiella pneumoniae (CRKP) strain GX34 was recovered from the respiratory tract of an elderly male with severe pneumonia, and only susceptible to amikacin, tigecycline, and colistin. Complete genome suggested that it belonged to K51-ST16 and harbored plasmid-encoded NDM-4 and OXA-181, located on IncFIB plasmid GX34p1_NDM-4 and ColKP3/IncX3 plasmid GX34p4_OXA-181, respectively. A series of transconjugants generated in the plasmid conjugation assays, including Escherichia coli J53-N1 (harboring a self-transmissible and bla_NDM-1-producing plasmid Eco-N-1-p), J53-N2 (harboring a bla_NDM-4-producing plasmid and a helper plasmid GX34p5), and J53-O (harboring a bla_OXA-181-producing plasmid), could be stably inherited after 10 days of serial passage and no significant biological fitness costs were detected. Furthermore, we first reported the bla_NDM-1 gene, derived from bla_NDM-4 mutation (460C>A) under meropenem pressure, could be in vitro transferred into a self-conjugative, recombined plasmid Eco-N-1-p of J53-N1. Eco-N-1-p was mainly recombined by GX34p4_OXA-181 (40,449 bp, 75.16%) and GX34p1_NDM-4 (8,553 bp, 15.89%), in which IS26 and IS5-like probably played a major role. Eco-N-1-p could be transferred into the conjugation recipient K. pneumoniae KP54 and make the latter sacrifice fitness. The retention rates of bla_NDM-1 remained high stability (>80% after 200 generations). The comparative genomic analysis of GX34 and those carrying bla_NDM-4 or bla_OXA-181 genes retrieved from the NCBI RefSeq database showed all bla_NDM-4 (26/26, 100.00%) and bla_OXA-181 (13/13, 100.00%) were surrounded by IS26. The immediate environment of bla_NDM-4 and bla_OXA-181 in GX34 and some retrieved strains shared identical features, hinting at their possible dissemination. Effective measures should be taken to monitor the spread of this clone.

Susceptibility profile of bla OXA-23 and metallo-β-lactamases co-harbouring isolates of carbapenem resistant Acinetobacter baumannii (CRAB) against standard drugs and combinations

Background

The rapid emergence of carbapenem resistant Acinetobacter baumannii (CRAB) has resulted in an alarming situation worldwide. Realizing the dearth of literature on susceptibility of CRAB in genetic context in the developing region, this study was performed to determine the susceptibility profile against standard drugs/combinations and the association of in-vitro drug synergy with the prevalent molecular determinants.

Methods and findings

A total of 356 clinical isolates of A. baumannii were studied. Confirmation of the isolates was done by amplifying recA and ITS region genes. Susceptibility against standard drugs was tested by Kirby Bauer disc diffusion. Minimum inhibitory concentration (MIC), MIC50 and MIC90 values against imipenem, meropenem, doripenem, ampicillin/sulbactam, minocycline, amikacin, polymyxin B, colistin and tigecycline was tested as per guidelines. Genes encoding enzymes classes A (bla GES, bla IMI/NMC-A, bla SME, bla KPC), B (bla IMP, bla VIM, bla NDM) and D (bla OXA-51, bla OXA-23 and bla OXA-58) were detected by multiplex polymerase chain reaction. Synergy against meropenem-sulbactam and meropenem-colistin combinations was done by checkerboard MIC method. Correlation of drug synergy and carbapenemase encoding genes was statistically analyzed.

Results

Of the total, resistance above 90% was noted against gentamicin, ciprofloxacin, levofloxacin, ceftazidime, cefepime, ceftriaxone, cotrimoxazole and piperacillin/tazobactam. By MIC, resistance rates from highest to lowest was seen against imipenem 89.04% (n=317), amikacin 80.33% (n=286), meropenem 79.49% (n=283), doripenem 77.80% (n=277), ampicillin/sulbactam 71.62% (n=255), tigecycline 55.61% (n=198), minocycline 14.04% (n=50), polymyxin B 10.11% (n=36), and colistin 2.52% (n=9). CRAB was 317 (89.04%), 81.46% (n=290) were multidrug resistant and 13.48% (n=48) were extensively drug resistant. All the CRAB isolates harboured bla OXA-51 gene (100%) and 94% (n=298) bla OXA-23 gene. The bla IMP gene was most prevalent 70.03% (n=222) followed by bla NDM, 59.62% (n=189). Majority (87.69%, 278) were co-producers of classes D and B carbapenemases, bla OXA-23 with bla IMP and bla NDM being the commonest. Synergy with meropenem-sulbactam and meropenem-colistin was 47% and 57% respectively. Reduced synergy (p= <0.0001) was noted for those harbouring bla OXA-51+blaOXA-23with bla NDM gene alone or co-producers.

Conclusion

Presence of bla NDM gene was a significant cause of synergy loss in meropenem-sulbactam and meropenem-colistin. In bla NDM endemic regions, tigecycline, minocycline and polymyxins could be viable options against CRAB isolates with more than one carbapenemase encoding genes.

Increasing trends of colistin resistance in patients at high-risk of carbapenem-resistant Enterobacteriaceae

INTRODUCTION

Occurrence of colistin-resistant Enterobacteriaceae in response to the unregulated use of this antibiotic has been documented. This study reports an investigation of colistin resistance rates among carbapenem-resistant enterobacterial clinical isolates.

METHODS

A total of 196 multidrug-resistant Enterobacteriaceae isolates (Klebsiella pneumoniae (n = 100), Escherichia coli (n = 89) and Enterobacter cloacae (n = 7) were selected from Gram-negative isolates over one year. Susceptibility to antimicrobials was determined using Vitek2. Broth microdilution method was used to detect colistin antimicrobial susceptibility. Identification of ESBL and carbapenemases were both done phenotypically and by PCR.

RESULTS

All the studied isolates showed multidrug-resistant phenotypes with 51.5% resistance to carbapenems (meropenem, imipenem). Very low resistance rates towards tigecycline (n = 9) 4.6% were found. Thirty-nine isolates (19.9%) showed reduced susceptibility to colistin among the MDR isolates. Sixty-four isolates (32.7%) were ESBL producers. Hundred isolates (51%) were carbapenemase producers using Carba NP test. The PCR amplification results revealed that 40 isolates (20%) harboured NDM-1 and 40 isolates contained OXA-48-like gene. Coexistence of both (NDM-1 and OXA-48-like) was observed in nine (4.59%) isolates. A Statistically significant relationship was observed between carbapenem resistance and each of the followings; OXA-48 producers (p= .009), amikacin resistance (p = .000), gentamicin resistance (p = .032), tobramycin resistance (p = .000), and tigecycline resistance (p-value ≤ .001). A statistical significance was detected between ESBL-producing isolates and carbapenem susceptible isolates ESBL producers with p = 0.000.

CONCLUSION

An alarming sign is the increasing colistin resistance rates among carbapenem-resistant isolates. Aminoglycosides are still a therapeutic option to decrease the use of colistin and avoid further development of resistance.KEY MESSAGESHigh rates of colistin resistance among carbapenem-resistant Enterobacteriaceae.The choice of antibiotic is significantly associated with the clinical site of infection.Aminoglycosides are offered choices for treating multiple drug-resistant Enterobacteriaceae to preserve the colistin and carbapenems.

Clinical Efficacy, Antibiotic Resistance Genes, Virulence Factors and Outcome of Hospital-Acquired Pneumonia Induced by Klebsiella pneumoniae Carbapenemase 2-Producing with Tigecycline Treatment in the ICU

Purpose

Tigecycline is an agent for carbapenemase-producing Klebsiella pneumonia (KPC-KP), given its penetration into lung tissues. Our study focused on the molecular and clinical efficacy of tigecycline for hospital-acquired pneumonia (HAP) in the ICU.

Patients and Methods

A retrospective cohort study of 52 adult KPC-KP HAP patients by searching hospital medical records from January 2018 to December 2020 was established to investigate the epidemiology of KPC-KP infections for tigecycline treatment and the associated clinical efficacy of tigecycline. The KPC-KP isolates underwent multilocus sequence typing. Molecular typing, antimicrobial resistance, and virulence profiling were also analyzed by whole-genome sequencing of KPC-KP.

Results

Among 52 patients with KPC-KP, the ICU mortality rate was 14/52 (27%), and there was no significant statistical difference in mortality between the effective group and failure group (p = 0.754). However, the duration of tigecycline was statistically different between the two groups of patients (14.4 vs 10 days, p=0.046). The total bacterial clearance rate was 6/52 (11.5%). There was no significant statistical difference in both groups (p=0.416). Antibiotic resistance genes (aac3iia) and virulence gene (AREO-iutA, Capsule-wzc) were negatively correlated with clinical efficacy (p = 0.011, OR = 1.237).

Conclusions

Bla_kpc was the main carbapenemase in all K. pneumoniae strains. ST11-KL64 KPC-KP was the most common virulence factors in KPC-KP isolates. This study suggested that antibiotic resistance genes (aac3iia) and virulence gene (AREO-iutA, Capsule-wzc) were independent mortality risk factors for patients with Klebsiella pneumoniae carbapenemase-2 producing K. pneumoniae infections, when during the tigecycline treatment. Molecular analysis of K. pneumoniae may provide an option when choosing the antimicrobial treatment.

Replacement of the Dominant ST191 Clone by ST369 Among Carbapenem-Resistant Acinetobacter baumannii Bloodstream Isolates at a Tertiary Care Hospital in South Korea

The clonal dissemination of carbapenem-resistant Acinetobacter baumannii (CRAB) bacteremia is a serious clinical problem worldwide. However, the factors related to the emergence and replacement of predominant CRAB clones in nosocomial settings are unclear. By multilocus sequence typing (MLST), we evaluated the genetic relatedness of CRAB bloodstream isolates at a tertiary care hospital over a 3.5-year period and investigated the clinical and microbiologic characteristics of the predominant sequence types (STs). One hundred and seventy-nine CRAB bloodstream isolates were collected from June 2016 to December 2019, and their MLSTs according to Oxford scheme and clinical data were obtained. The predominant STs were assessed for in vitro growth, competitive growth, and virulence in a mouse model of intraperitoneal infection. Two dominant clones—ST369 (n = 98) and ST191 (n = 48)—belonging to international clone 2 (IC2) were recovered from patients admitted to intensive care units (ICUs) or wards. ST191 predominated (61%, 27/43) from June 2016 to July 2017, whereas ST369 (72%, 98/136), which was first isolated from a patient admitted to the emergency room, replaced ST191 (15%, 21/136) after August 2017. In a multivariate analysis, leukopenia (OR = 3.62, 95% CI 1.04–12.6, p = 0.04) and ST191 or 369 (OR = 5.32, 95% CI 1.25–22.65, p = 0.02) were independent risk factors for 7-day mortality. Compared with non-ST369, ST369 was associated with a shorter time to bacteremia from ICU admission (7 vs. 11 days, p = 0.01), pneumonia as an origin of bacteremia (67 vs. 52%, p = 0.04), leukopenia (28 vs. 11%, p &lt; 0.01), and a lower 7-day survival rate (41 vs. 70%, p &lt; 0.01). In vitro, ST 369 isolates had significantly higher growth rates and enhanced competitive growth compared to ST191. Finally, ST369 had greater virulence and a higher mortality rate than other STs in a mouse infection model. We report almost-complete replacement of the predominant ST191 clone by ST369 within an 8-month period at our hospital. ST369 had a high incidence density rate of CRAB bacteremia, a short time to bacteremia after ICU admission, and a high early mortality rate, which may be in part explained by its faster competitive growth rate and higher virulence than ST191.

Changes in molecular epidemiology of carbapenem-resistant Klebsiella pneumoniae in the intensive care units of a Greek hospital, 2018-2021

The spread of multi-drug resistant (MDR) Gram-negative bacteria, including Klebsiella pneumoniae, constitutes a global threat. The most frequent mechanism of acquired carbapenem resistance is the production of carbapenemases, especially KPC, NDM, VIM, IMP and OXA-48. We analyzed the epidemiological trend of carbapenem resistance genes of carbapenem-resistant K. pneumoniae (CRKP) strains isolated from critically ill patients in a Greek tertiary hospital. The study included 150 CRKP isolates collected from 116 (77.4%) patients hospitalized in the adult ICU and 17 (11.3%) each in the pediatric and the two neonatal ICUs between March 2018 and March 2021. Identification and antimicrobial susceptibility testing were performed using VITEK-2. A multiplex lateral flow immunoassay was used for the detection of carbapenemases, while the detection of bla VIM, bla KPC, bla NDM, bla IMP and bla OXA-48-like genes was achieved by multiplex PCR. The bla NDM was mainly detected in adults (54/116, 46.9%), while in children the most often detected gene was bla KPC (24/34, 70.6%). The predominant carbapenem resistance gene during 2018-2019 was bla KPC alone or in combination with bla VIM, reaching 44.4% in 2019, while during 2020-2021 the detection of bla NDM prevailed significantly, reaching 45.5 and 60.7% for 2020 and 2021, respectively. A shift in the molecular epidemiology of CRKP was seen during 2018-2021, which is probably associated with the recent excessive empiric use of newer antimicrobials. Surveillance studies and proper and strict implementation of infection control measures are highly needed to decrease the spread of MDR bacteria, including CRKP.

Clinical and Epidemiological Characteristics of Stenotrophomonas maltophilia Associated Lower Respiratory Tract Infections in Qatar: A Retrospective Study

Background

Stenotrophomonas maltophilia is a rapidly emerging nosocomial pathogen with intrinsic or acquired resistance mechanisms to several antibiotic classes. It can cause life-threatening opportunistic pneumonia, particularly among hospitalized patients. Incidence of infections by S. maltophilia has been reported as 0.07-0.4% of hospital discharges, but its mortality is 20 -60%. This is the first study from Qatar indexing the clinical and epidemiological characteristics and antibiotic susceptibility of S. maltophilia.

Materials and methods

This retrospective descriptive epidemiological study was conducted in 6 tertiary care hospitals under Hamad Medical Corporation in Doha, Qatar, analyzing inpatient respiratory isolates of S. maltophilia during 2016-17. Out-patients, children below 14 years, and non-respiratory samples except blood cultures in patients with pneumonia were excluded. Clinical records were reviewed to identify possible risk factors. Infection and colonization were identified using the Centers for Disease Control and Prevention (CDC) algorithm for clinically defined pneumonia and statistically analyzed using the chi-square test and Pearson's correlation.

Results

S. maltophilia was isolated from 2.07% (317/15312) of all respiratory samples received in the microbiology lab during our study period. Three hundred seventeen patients studied had a mean age of 60 ± 20 years, and 68% were men. Most of the isolates were from sputum (179), followed by tracheal aspirate (82) and bronchoscopy (42). Fourteen blood culture samples from patients diagnosed with pneumonia were also included. 67% were hospitalized for more than two weeks, 39.1% were on mechanical ventilators, and 88% had received a broad-spectrum antibiotic before the event. 29.1% were deemed to have an infection and 70.9% colonization. Incidence of infection in those with Charlson’s Co-morbidity Index (CCI) ≥ 3 was 36.5% compared to 24.2% in those with CCI < 3 (Relative Risk (RR)=1.52; 95% CI: 1.04,2.18; p=0.01). Patients with recent chemotherapy, immunosuppressant, or steroid use had a significantly higher infection risk than those without (69.2% v/s 23.3% RR=2.96; 95% CI:2.2,3.9; p<0.005). The most common symptoms in patients with infection were fever (96%) and expectoration (61.9%). The most common radiological finding was lobar consolidation (71.6%). Mean CRP and procalcitonin were 106.5±15.5 mg/l and 12.3 ± 14 ng/ml. Overall mortality was 16.3%. Patients on mechanical ventilator with IBMP-10 score ≥ 2 had 22.8% mortality compared to 5.7% in those with score < 2 (RR=3.9;95%CI:0.9,16.6; p<0.015). As per The US Clinical and Laboratory Standards Institute (CSLI) breakpoint values, Trimethoprim-Sulfamethoxazole (TMP-SMX) showed the highest sensitivity (97.8%), followed by levofloxacin (71.6%). 0.3% of samples were pan-drug resistant.

Conclusions

S. maltophilia is a frequent nosocomial colonizer, but it can cause nosocomial pneumonia in almost one-third of cases, specifically in immunocompromised and patients with CCI ≥ 3 with a high risk of mortality due to ventilator-associated pneumonia (VAP) in those with IBMP-10 ≥ 2. Prolonged hospital stay is a risk factor for colonization by S. maltophilia, while recent chemotherapy, immunosuppressant, or steroid use are risk factors for hospital-acquired pneumonia due to S. maltophilia. TMP-SMX and levofloxacin are the only reliable agents for monotherapy of respiratory infections due to S. maltophilia in Qatar.

Emergence of ST39 carbapenem-resistant Klebsiella pneumoniae producing VIM-1 and KPC-2

INTRODUCTION

Carbapenem-resistant Klebsiella pneumoniae (CRKP) causes life-threatening hospital-acquired infections. KPC and VIM carbapenemase production is the main molecular mechanism for carbapenem resistance. The aim of the current study was the genetic characterization of four ST39 CRKP isolates simultaneously producing VIM-1 and KPC-2, obtained in a Greek tertiary hospital.

METHODS

Identification and antimicrobial susceptibility testing were performed through VITEK 2. Multiplex PCR, multiplex lateral flow immunoassay, phenotypic tests and next generation sequencing were applied. The sequence reads were de novo assembled and annotated, while antimicrobial resistance genes and plasmids were identified using bioinformatics software. Genomic comparison and core genome single-nucleotide polymorphism-based phylogenetic analysis were also performed.

RESULTS

Three isolates were pandrug-resistant, and one was extensively drug-resistant; they all carried bla_VIM-1 and bla_KPC-2 genes and were assigned to ST39. Bla_VIM-1 was integrated in a class 1 integron. They all harboured many antimicrobial resistance genes and various plasmids. The mgrB gene of all isolates was disrupted by an insertion sequence (ISKpn14). Genome comparison and phylogenetic analysis revealed that the isolates were closely related.

CONCLUSION

To our knowledge this is the first report on detection of CRKP ST39 isolates simultaneously producing VIM-1 and KPC-2 in addition to colistin resistance. The knowledge of the clonal relatedness of the isolates can lead to the implementation of strict infection control measures absolutely needed to eliminate their spread.

Whole-genome epidemiology and characterisation of mcr-1-encoding Escherichia coli in aquatic bird farms from the Pearl River Delta, China, 2019-2020

Due to their unique breeding pattern, aquatic bird farms are increasingly considered as hotspots in the development and spread of antimicrobial resistance. However, comprehensive studies addressing the whole-genomic features of colistin-resistant bacteria in aquatic bird farms are scarce. Over a 2-year period, we conducted surveillance to determine the whole-genome epidemiology and characterisation of mcr-1-positive Escherichia coli in aquatic bird farms in southeastern coastal China. A total of 100 mcr-1-producing isolates among 654 E. coli strains were recovered from 781 samples collected in 11 aquatic bird farms and 1 veterinary clinic in the Pearl River Delta area. Higher resistance phenotypes to 17 antibiotics were found in mcr-1-positive isolates compared with other isolates. Subsequently, 20 mcr-1-carrying isolates were sequenced to analyse the whole-genomic features. Molecular typing as well as antimicrobial resistance gene and virulence factor profiles of the isolates showed considerable diversity. Three types of genetic backbones of mcr-1 in the isolates were assembled and were identified in diverse broad-host-range plasmids and bacterial species. Pangenome analyses revealed a large genetic pool composed of the isolates. Furthermore, phylogenetic trees both of the isolates in this study and a global data set were built, indicating the spread of the three mcr-1 backbones and the mcr-1-positive isolates among different habitats, farms and even countries. This study highlights that aquatic bird farms may act as an important reservoir for mcr-1-producing E. coli, from which colistin resistance may be spread to diverse habitats, different geographical locations and even across bacterial species.

Overexpression of the adeB Efflux Pump Gene in Tigecycline-Resistant Acinetobacter baumannii Clinical Isolates and Its Inhibition by (+)Usnic Acid as an Adjuvant

Acinetobacter species are among the most life-threatening Gram-negative bacilli, causing hospital-acquired infections, and they are associated with high morbidity and mortality. They show multidrug resistance that acts via various mechanisms. In Acinetobacter baumannii, efflux pump-mediated resistance to many antimicrobial compounds, including tigecycline, has been widely reported. Natural compounds have been used for their various pharmacological properties, including anti-efflux pump activity. The present study aimed to evaluate the efflux pump-mediated resistance mechanism of Acinetobacter baumannii and the effect of (+)Usnic acid as an efflux pump inhibitor with tigecycline. For detecting the efflux pump activity of tigecycline-resistant Acinetobacter baumannii isolates, microbroth dilution method and real-time quantitative reverse transcription–polymerase chain reaction was used. (+)Usnic acid was added to tigecycline and tested by the checkerboard method to evaluate its efficacy as an efflux pump inhibitor. qRT-PCR analysis was carried out to show the downregulation of the efflux pump in the isolates. Out of 42 tigecycline-resistant Acinetobacter baumannii isolates, 19 showed efflux pump activity. All 19 strains expressed the adeB gene. (+)Usnic acid as an adjuvant showed better efficacy in lowering the minimum inhibitory concentration compared with the conventional efflux pump inhibitor, carbonyl cyanide phenylhydrazone.

In Vitro Synergy of Colistin in Combination with Meropenem or Tigecycline against Carbapenem-Resistant Acinetobacter baumannii

Acinetobacter baumannii is currently classified as one of six pathogens that contribute to increased patient mortality. Thus, exploratory studies navigating alternative treatment strategies are of supreme interest. Herein, we completed minimum inhibitory concentration (MIC) testing, and time-kill analyses (TKA) on 50 carbapenem-resistant Acinetobacterbaumannii isolates including 28 colistin-resistant isolates. Upon testing of MEM or TGC in the presence of sub-inhibitory COL against the 50 isolates, there was a median 2-fold reduction in MEM and TGC MICs. In the TKAs, the COL+MEM combination was synergistic in 45 (90%) isolates and bactericidal in 43 (86%) isolates at 24 hours, whereas the COL+TGC combination TKAs demonstrated synergy in 32 (64%) isolates and bactericidal activity was shown in 28 (56%) isolates. Additionally, sulbactam (SUL) and TGC were added to the COL+MEM dual therapy regimen to assess the possible utility of a triple therapy regimen against five non-responsive isolates. The COL+MEM+SUL and COL+MEM+TGC regimens effectively restored synergy in (5/5) 100% of the isolates. The results of this study demonstrate the potential utility of COL combinations in the treatment of carbapenem-resistant isolates.

Preliminary view of the global distribution and spread of the tet(X) family of tigecycline resistance genes

BACKGROUND

The emergence of plasmid-mediated tet(X3)/tet(X4) genes is threatening the role of tigecycline as a last-resort antibiotic to treat clinical infections caused by XDR bacteria. Considering the possible public health threat posed by tet(X) and its variants [which we collectively call 'tet(X) genes' in this study], global monitoring and surveillance are urgently required.

OBJECTIVES

Here we conducted a worldwide survey of the global distribution and spread of tet(X) genes.

METHODS

We analysed a comprehensive dataset of bacterial genomes in conjunction with surveillance data from our laboratory and the NCBI database, as well as sufficient metadata to characterize the results.

RESULTS

The global distribution features of tet(X) genes were revealed. We clustered three types of genetic backbones of tet(X) genes embedded or transferred in bacterial genomes. Our pan-genome analyses revealed a large genetic pool composed of tet(X)-carrying sequences. Moreover, phylogenetic trees of tet(X) genes and tet(X)-like proteins were built.

CONCLUSIONS

To the best of our knowledge, our results provide the first view of the global distribution of tet(X) genes, demonstrate the features of tet(X)-carrying fragments and highlight the possible evolution of tigecycline-inactivation enzymes in diverse bacterial species and habitats.

Comparative Evaluation of Seven Tigecycline Susceptibility Testing Methods for Carbapenem-Resistant Enterobacteriaceae

Purpose

Carbapenem-resistant Enterobacteriaceae (CRE) strains are extensively resistant to most antibiotics. Tigecycline is one of the few effective drugs that can be used to treat infections caused by CRE. The aim of this study was to evaluate the accuracy of different methods for detecting the susceptibility of CRE to tigecycline.

Methods

Seven commonly used drug susceptibility testing methods were compared and evaluated for the ability to determine CRE tigecycline susceptibility: broth microdilution (BMD), agar dilution method (ADM), disk diffusion method, Etest, MicroScan, Vitek2 COMPACT, and BD Phoenix 100.

Results

The minimum inhibitory concentration (MIC) of tigecycline to inhibit 50% and 90% of CRE growth (MIC₅₀ and MIC₉₀, respectively) assessed by ADM and BD Phoenix 100 was the same as that determined by the reference method, BMD. The MIC₅₀ was 2 µg/mL, and the MIC₉₀ was 4 µg/mL. The highest number of susceptible strains was detected by MicroScan, followed by BMD, Etest, ADM, BD Phoenix 100, Vitek2 COMPACT, and disk diffusion method, in descending order. No significant differences were observed among the tigecycline susceptibility results (P > 0.05) obtained from MicroScan, Etest, BD Phoenix 100, and BMD. BMD confirmed that 82.0% of strains were susceptible to tigecycline. ADM, MicroScan, and BD Phoenix 100 yielded the categorical agreement of 96%, 92%, and 93%, respectively. No method was found to present any very major errors (VMEs), and only the Vitek2 COMPACT yielded major errors (MEs) greater than 3%.

Conclusion

Among the seven methods tested, the ADM, MicroScan, and BD Phoenix 100 methods were accurate for determining the tigecycline susceptibility of CRE. MicroScan was acceptable with better performance than other methods.

A Clinical Extensively-Drug Resistant (XDR) Escherichia coli and Role of Its β-Lactamase Genes

An extensively-drug resistant (XDR) Escherichia coli W60 was isolated from the urine sample of a patient. The genetic basis for its XDR phenotype was investigated, particularly the basis for its resistance toward β-lactam/BLI (β-Lactamase Inhibitor) combinations. Following determination of the XDR phenotype, third generation genomic sequencing was performed to identify genetic structures in E. coli W60. Further cloning analysis was performed to identify determinants of β-lactam/BLI combination resistance. It was found that E. coli W60 is resistant to nearly all of the tested antibiotics including all commonly used β-lactam/BLI combinations. Analysis of the genomic structures in E. coli W60 showed two novel transferable plasmids are responsible for the resistance phenotypes. Further genetic analysis showed bla NDM-5 leads to high resistance to β-lactam/BLI combinations, which was enhanced by co-expressing ble MBL. pECW602 harbors a truncated bla TEM that is not functional due to the loss of the N-terminal signal peptide coding region. Research performed in this work leads to several significant conclusions: the XDR phenotype of E. coli W60 can be attributed to the presence of transferable multidrug resistance plasmids; NDM-5 confers high resistance to β-lactam/BLI combinations; co-expression of ble MBL enhances resistance caused by NDM-5; the signal peptides of TEM type β-lactamases are essential for their secretion and function. Findings of this work show the danger of transferable multidrug resistance plasmids and metallo-β-lactamases, both of which should be given more attention in the analysis and treatment of multidrug resistant pathogens.

Genetic diversity and characteristics of high-level tigecycline resistance Tet(X) in Acinetobacter species

BACKGROUND

The recent emergence and dissemination of high-level mobile tigecycline resistance Tet(X) challenge the clinical effectiveness of tigecycline, one of the last-resort therapeutic options for complicated infections caused by multidrug-resistant Gram-negative and Gram-positive pathogens. Although tet(X) has been found in various bacterial species, less is known about phylogeographic distribution and phenotypic variance of different genetic variants.

METHODS

Herein, we conducted a multiregional whole-genome sequencing study of tet(X)-positive Acinetobacter isolates from human, animal, and their surrounding environmental sources in China. The molecular and enzymatic features of tet(X) variants were characterized by clonal expression, microbial degradation, reverse transcription, and gene transfer experiments, while the tet(X) genetic diversity and molecular evolution were explored by comparative genomic and Bayesian evolutionary analyses.

RESULTS

We identified 193 tet(X)-positive isolates from 3846 samples, with the prevalence ranging from 2.3 to 25.3% in nine provinces in China. The tet(X) was broadly distributed in 12 Acinetobacter species, including six novel species firstly described here. Besides tet(X3) (n = 188) and tet(X4) (n = 5), two tet(X5) variants, tet(X5.2) (n = 36) and tet(X5.3) (n = 4), were also found together with tet(X3) or tet(X4) but without additive effects on tetracyclines. These tet(X)-positive Acinetobacter spp. isolates exhibited 100% resistance rates to tigecycline and tetracycline, as well as high minimum inhibitory concentrations to eravacycline (2-8 μg/mL) and omadacycline (8-16 μg/mL). Genetic analysis revealed that different tet(X) variants shared an analogous ISCR2-mediated transposon structure. The molecular evolutionary analysis indicated that Tet(X) variants likely shared the same common ancestor with the chromosomal monooxygenases that are found in environmental Flavobacteriaceae bacteria, but sequence divergence suggested separation ~ 9900 years ago (7887 BC), presumably associated with the mobilization of tet(X)-like genes through horizontal transfer.

CONCLUSIONS

Four tet(X) variants were identified in this study, and they were widely distributed in multiple Acinetobacter spp. strains from various ecological niches across China. Our research also highlighted the crucial role of ISCR2 in mobilizing tet(X)-like genes between different Acinetobacter species and explored the evolutionary history of Tet(X)-like monooxygenases. Further studies are needed to evaluate the clinical impact of these mobile tigecycline resistance genes.

Mutations at Novel Sites in pmrA/B and lpxA/D Genes and Absence of Reduced Fitness in Colistin-Resistant Acinetobacter baumannii from a Tertiary Care Hospital, India

Background: Colistin resistance in Acinetobacter baumannii, the last resort drug for serious infections, is emerging worldwide. There has been paucity of data on this aspect from India, which is one of the largest producers of colistin. We studied colistin resistance in A. baumannii and characterized the isolates with respect to resistance mechanisms and virulence. Methods: A total of 365 A. baumannii isolates were studied. Antimicrobial susceptibility testing was performed as per standards. Colistin resistance mechanisms were studied by mutation detection in pmrA/B and lpxA/C/D genes, phenotypic loss of lipopolysaccharide, presence of mcr1-5 genes, and carbonyl cyanide 3-chlorophenylhydrazone (CCCP) effects. Biofilm formation, desiccation survival, and growth kinetics were studied and statistically analyzed for colistin-resistant and colistin-susceptible isolates. Results: All the colistin-resistant isolates (9, 2.5%) showed multiple mutations at novel sites in pmrA/B and/or lpxA/D genes with reversion of resistance with CCCP. Majority of these isolates (6, 66.6%) were from patients without prior colistin therapy. All received prior carbapenems. The resistant isolates demonstrated no significant difference in biofilm formation and desiccation survival but were slow growers. Conclusion: Mutations in pmrA/B and/or lpxA/D genes were the main resistance mechanism in these colistin-resistant isolates that showed no reduction in fitness.

In vitro activity of biofilm inhibitors in combination with antibacterial drugs against extensively drug-resistant Acinetobacter baumannii

Acinetobacter baumannii is a common pathogen of nosocomial infection, and its ability to form biofilms further contributes to its virulence and multidrug resistance, posing a great threat to global public health. In this study, we investigated the inhibitory effects of five biofilm inhibitors (BFIs) (zinc lactate, stannous fluoride, furanone, azithromycin, and rifampicin) on biofilm formation of nine extensively drug-resistant A. baumannii (XDRAB), and assessed the synergistic antibacterial effects of these BFIs when combined with one of four conventional anti-A. baumannii antibiotics (imipenem, meropenem, tigecycline, and polymyxin B). Each of the five BFIs tested was found to be able to significantly inhibit biofilm formation of all the clinical isolates tested under sub-minimal inhibitory concentrations. Then, we observed synergistic effects (in 22%, 56% and 11% of the isolates) and additive effects (56%, 44% and 44%) when zinc lactate, stannous fluoride and furanone were combined with tigecycline, respectively. When zinc lactate and stannous fluoride were each used with a carbapenem (imipenem or meropenem), in 33% and 56-67% of the isolates, they showed synergistic and additive effects, respectively. Additivity in > 50% of the isolates was detected when rifampicin was combined with imipenem, meropenem, tigecycline, or polymyxin B; and a 100% additivity was noted with azithromycin-polymyxin B combination. However, antagonism and indifference were noted for polymyxin B in its combination with zinc lactate and stannous fluoride, respectively. In conclusion, five BFIs in combination with four antibacterial drugs showed different degrees of in vitro synergistic and additive antibacterial effects against XDRAB.

Spectrum of Bacterial Colonization in Patients Hospitalized for Treatment of Multidrug-Resistant Tuberculosis

This study investigated the bacterial colonization in patients admitted for treatment of drug-resistant tuberculosis in a specialized TB hospital. Identification and antimicrobial susceptibility testing of bacterial isolates (n = 62) from nasal, groin, and rectal swabs [patient cohort (n = 37)] were determined by the VITEK-MS system. Resistance gene analysis was by PCR and DNA sequencing. Molecular typing of Klebsiella pneumoniae isolates was by Multilocus Sequencing Typing (MLST). Patients (n = 13/37; 35%) were colonized by multidrug-resistant (MDR) bacteria (ESBL and MRSA) on admission. Of the 24 patients who were not colonized by MDR bacteria on admission, 46% (17/37) became colonized by MDR bacteria within 1 month of admission, mostly with ESBL-producing Enterobacteriales and resistance to aminoglycosides and fluoroquinolones. ESBL Escherichia coli (41/62; 66%) and K. pneumoniae (14/62; 23%) predominated. Genes encoding for ESBLs (bla_CTX-M-14, bla_CTX-M-15, bla_SHV-28, bla_OXA-1, and bla_OXY-2) and plasmid-mediated quinolone resistant genes (qnrB1, qnrB4, and qnrB10) were detected. MLST revealed genetic diversity among the K. pneumoniae isolates from hospitalized patients. This study provides insight into bacterial pathogen colonization in hospitalized TB patients with the first occurrence of the qnrB4 and qnrB10 genes and co-expression of genes: qnrB4+aac(6')-lb-cr, qnrB10+aac(6')-lb-cr, qnrB4+qnrS1, and qnrB10+qnrS1 in fluoroquinolone-resistant E. coli isolates within South Africa. However, the source and colonization routes of these isolates could not be determined.

Monte Carlo simulation evaluation of tigecycline dosing for bacteria with raised minimum inhibitory concentrations in non-critically ill adults

PURPOSE

Tigecycline is one of few antibiotics active against multidrug-resistant bacteria; however, the assessment of dosing strategies to optimize its activity is needed. The purpose was to use Monte Carlo Simulation (MCS) to determine if safe tigecycline dosing options attaining breakpoints for pharmacokinetic/pharmacodynamic (PK-PD) targets in non-critically ill adults could be identified.

METHODS

Publications that evaluated tigecycline dosing regimens and provided mean PK variables of interest (minimum 2 of: elimination rate constant or half-life and volume of distribution or clearance), with SDs, were included. Weighted mean (±SDs) for each PK parameter were determined. Food and Drug Administration minimum inhibitory concentration (MIC) tigecycline breakpoints for susceptible (MIC ≤ 2 μg/mL), intermediate (MIC 4 μg/mL), and resistant (MIC ≥ 8 μg/mL) Enterobacteriaceae were used. MCS probability distributions for PK-PD target attainment of AUC for total tigecycline plasma concentration from 0 to 24 h following an intravenous dose (AUC_{total, 0-24h}) to MIC ratios of ≥ 18, 7, and 4.5 were generated, with success defined as ≥ 80% probability of target attainment at a given MIC.

RESULTS

Ten studies (n = 442) were eligible. Tigecycline 150 mg IV q12h for ward patients with resistant bacteria up to a MIC of 0.48, 1, and 2 μg/mL for an AUC_{total, 0-24h}/MIC target attainment of 18, 7, and 4.5, respectively, may be appropriate.

CONCLUSION

Bacterial infections with tigecycline MICs ≥ 0.48-2 μg/mL, depending on AUC_{total, 0-24h}/MIC target, may require treatment with alternate antibiotics due to target attainment failure.

Emergence of Carbapenem- and Tigecycline-Resistant Proteus cibarius of Animal Origin

The emergence of tet(X) and carbapenemase genes in Enterobacterales pose significant challenges to the treatment of infectious diseases. Convergence of these two categories of genes in an individual pathogen would deteriorate the antimicrobial resistance (AMR) crisis furthermore. Here, tigecycline-resistant Enterobacterales strains were isolated and detected with carbapenemase genes, characterized by antimicrobial susceptibility testing, PCR, conjugation assay, whole genome sequencing, and bioinformatics analysis. Three tigecycline-resistant isolates consisting of one plasmid-mediated tet(X4)-bearing Escherichia fergusonii and two chromosomal tet(X6)-bearing Proteus cibarius were recovered from chicken feces. The tet(X4) was located on a conjugative IncX1 plasmid pHNCF11W-tetX4 encoding the identical structure as reported tet(X4)-bearing IncX1 plasmids in Escherichia coli. Among two P. cibarius strains, tet(X6) was located on two similar chromosomal MDR regions with genetic contexts IS26-aac(3)-IVa-aph(4)-Ia-ISEc59-tnpA-tet(X6)-orf-orf-ISCR2-virD2-floR-ISCR2-glmM-sul2 and IS26-aac(3)-IVa-aph(4)-Ia-ISEc59-tnpA-tet(X6)-orf-orf-ISCR2-glmM-sul2. Apart from tet(X6), P. cibarius HNCF44W harbored a novel transposon Tn6450b positive for bla_NDM–1 on a conjugative plasmid. This study probed the genomic basis of three tet(X)-bearing, tigecycline-resistant strains, one of which coharbored bla_NDM–1 and tet(X6), and identified P. cibarius as the important reservoir of tet(X6) variants. Emergence of P. cibarius encoding both bla_NDM–1 and tet(X6) reveals a potential public health risk.

Biofilm-Formation in Clonally Unrelated Multidrug-Resistant Acinetobacter baumannii Isolates

This study analyzed the genotype, antibiotic resistance, and biofilm formation of Acinetobacter baumannii strains and assessed the correlation between biofilm formation, antibiotic resistance, and biofilm-related risk factors. A total of 207 non-replicate multi-drug-resistant A. baumannii strains were prospectively isolated. Phenotypic identification and antimicrobial susceptibility testing were carried out. Isolate biofilm formation ability was evaluated using the tissue culture plate (TCP), Congo red agar, and tube methods. Clonal relatedness between the strains was assessed by enterobacterial repetitive intergenic consensus-PCR genotyping. Of the 207 isolates, 52.5% originated from an intensive care unit setting, and pan resistance was observed against ceftazidime and cefepime, with elevated resistance (99–94%) to piperacillin/tazobactam, imipenem, levofloxacin, and ciprofloxacin. alongside high susceptibility to tigecycline (97.8%). The Tissue culture plate, Tube method, and Congo red agar methods revealed that 53.6%, 20.8%, and 2.7% of the strains were strong biofilm producers, respectively, while a significant correlation was observed between biofilm formation and device-originating respiratory isolates (p = 0.0009) and between biofilm formation in colonized vs. true infection isolates (p = 0.0001). No correlation was detected between antibiotic resistance and biofilm formation capacity, and the majority of isolates were clonally unrelated. These findings highlight the urgent need for implementing strict infection control measures in clinical settings.

Deciphering the Structural Diversity and Classification of the Mobile Tigecycline Resistance Gene tet(X)-Bearing Plasmidome among Bacteria

Tigecycline is an expanded-spectrum tetracycline used as a last-resort antimicrobial for treating infections caused by superbugs such as carbapenemase-producing or colistin-resistant pathogens. Emergence of the plasmid-mediated mobile tigecycline resistance gene tet(X4) created a great public health concern. However, the diversity of tet(X4)-bearing plasmids and bacteria remains largely uninvestigated. To cover this knowledge gap, we comprehensively identified and characterized the tet(X)-bearing plasmidome in different sources using advanced sequencing technologies for the first time. The huge diversity of tet(X4)-bearing mobile elements demonstrates the high level of transmissibility of the tet(X4) gene among bacteria. It is crucial to enhance stringent surveillance of tet(X) genes in animal and human pathogens globally.

The emergence of novel plasmid-mediated resistance genes constitutes a great public concern. Recently, mobile tet(X) variants were reported in diverse pathogens from different sources. However, the diversity of tet(X)-bearing plasmids remains largely unknown. In this study, the phenotypes and genotypes of all the tet(X)-positive tigecycline-resistant strains isolated from a slaughterhouse in China were characterized by antimicrobial susceptibility testing, conjugation, pulsed-field gel electrophoresis with S1 nuclease (S1-PFGE), and PCR. The diversity and polymorphism of tet(X)-harboring strains and plasmidomes were investigated by whole-genome sequencing (WGS) and single-plasmid-molecule analysis. Seventy-four tet(X4)-harboring Escherichia coli strains and one tet(X6)-bearing Providencia rettgeri strain were identified. The tet(X4)-bearing elements in 27 strains could be transferred to the recipient strain via plasmids. All tet(X4)-bearing plasmids isolated in this study and 15 tet(X4)-bearing plasmids reported online were analyzed. tet(X4)-bearing plasmids ranged from 9 to 294 kb and were categorized as ColE2-like, IncQ, IncX1, IncA/C2, IncFII, IncFIB, and hybrid plasmids with different replicons. The core tet(X4)-bearing genetic contexts were divided into four major groups: ISCR2-tet(X4)-abh, △ISCR2-abh-tet(X4)-ISCR2, ISCR2-abh-tet(X4)-ISCR2-virD2-floR, and abh-tet(X4)-ISCR2-yheS-cat-zitR-ISCR2-virD2-floR. Tandem repeats of tet(X4) were universally mediated by ISCR2. Different tet(X)-bearing strains existed in the same microbiota. Reorganization of tet(X4)-bearing multidrug resistance plasmids was found to be mediated by IS26 and other homologous regions. Finally, single-plasmid-molecule analysis captured the heterogenous state of tet(X4)-bearing plasmids. These findings significantly expand our knowledge of the tet(X)-bearing plasmidome among microbiotas, which establishes a baseline for investigating the structure and diversity of human, animal, and environmental tigecycline resistomes. Characterization of tet(X) genes among different microbiotas should be performed systematically to understand the evolution and ecology.

IMPORTANCE Tigecycline is an expanded-spectrum tetracycline used as a last-resort antimicrobial for treating infections caused by superbugs such as carbapenemase-producing or colistin-resistant pathogens. Emergence of the plasmid-mediated mobile tigecycline resistance gene tet(X4) created a great public health concern. However, the diversity of tet(X4)-bearing plasmids and bacteria remains largely uninvestigated. To cover this knowledge gap, we comprehensively identified and characterized the tet(X)-bearing plasmidome in different sources using advanced sequencing technologies for the first time. The huge diversity of tet(X4)-bearing mobile elements demonstrates the high level of transmissibility of the tet(X4) gene among bacteria. It is crucial to enhance stringent surveillance of tet(X) genes in animal and human pathogens globally.

Genetic Diversity of Imipenem-Resistant Acinetobacter baumannii Infections at an Intensive Care Unit

Introduction. Imipenem-resistant Acinetobacter baumannii (IRAB) represents a major clinical threat. Dissemination in critical care areas necessitates effective action measures including genotyping tools to study the clonality of these strains and trace their origin. The main aim of this study is to assess the genetic relatedness between IRAB isolates in our institution intensive care units (ICU) which are at a particular risk of outbreaks.

Dissemination of blaOXA-23-harbouring carbapenem-resistant Acinetobacter baumannii clones in Pakistan

OBJECTIVES

The rise of carbapenem resistance in Acinetobacter baumannii represents a challenge for the therapeutic management of infections. The present study aimed to investigate the sequence types (STs) and carbapenem resistance in A. baumannii strains collected from various clinical specimens from patients admitted to five tertiary-care hospitals in Pakistan.

METHODS

A total of 156 A. baumannii clinical strains were analysed for antimicrobial susceptibility, followed by genetic screening for carbapenem resistance determinants. All of the strains were typed by multilocus sequence typing (MLST) according to the Pasteur scheme.

RESULTS

Of the 156 A. baumannii isolates, 139 (89.1%) were carbapenem-resistant, of which 136 carried bla_OXA-23-like genes. Interestingly, the most commonly identified ST was ST589 (n = 52), classified as clonal complex 1 (CC1). ST2 was the second most common (n = 38), corresponding to CC2/92 (Pasteur/Oxford scheme), which was distributed in all five hospitals.

CONCLUSION

Diverse clones of carbapenem-resistant A. baumannii, including previously reported STs as well as new STs, carrying bla_OXA-23 are distributed in Pakistan. This is the first study to describe the molecular epidemiology of widely disseminated A. baumannii isolates in Pakistan. The findings will help to improve our knowledge of the predominant STs and will be valuable for a deeper understanding of resistance mechanisms among various STs.

Colistin-sparing approaches with newer antimicrobials to treat carbapenem-resistant organisms: Current evidence and future prospects

Antimicrobial resistance is on the rise across the globe. Increasing incidence of infections due to carbapenem resistance organisms is becoming difficult to treat, due to the limited availability of therapeutic agents. Very few agents such as colistin, fosfomycin, tigecycline and minocycline are widely used, despite its toxicity. However, with the availability of novel antimicrobials, beta-lactam/beta-lactamase inhibitor-based and non-beta-lactam-based agents could be of great relief. This review covers three important aspects which include (i) current management of carbapenem-resistant infections, (ii) determination of specific types of carbapenemases produced by multidrug-resistant and extensively drug-resistant Gram-negative pathogens and (iii) the currently available novel beta-lactam/beta-lactamase inhibitors and non-beta-lactam-based agents' laboratory findings, clinical outcome and implications.

Bad bug, no test: Tigecycline susceptibility testing challenges and way forward

Tigecycline is a reserve antibiotic increasingly used for the treatment of multidrug-resistant bacteria, especially Klebsiella pneumoniae and Acinetobacter baumannii. At present, there are concerns regarding the testing and interpretation of tigecycline susceptibility to bugs such as K. pneumoniae and A. baumannii, which limit clinicians in appropriate usage. Use of appropriate method for testing such as broth microdilution is essential. In addition, tigecycline susceptibility testing is a challenge due to inconsistent results from various antimicrobial susceptibility testing automated platforms. There is a great need to define a suitable methodology along with interpretive criteria, especially for K. pneumoniae and A. baumannii. The European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Food and Drug Administration (FDA) breakpoints show wide variation and are defined for different set of organisms. Non-species-related pharmacokinetic/pharmacodynamic (PK/PD) breakpoints defined by the EUCAST can be used for organisms such as K. pneumoniae and A. baumannii.

Emergence of plasmid-mediated high-level tigecycline resistance genes in animals and humans

Tigecycline is a last-resort antibiotic that is used to treat severe infections caused by extensively drug-resistant bacteria. tet(X) has been shown to encode a flavin-dependent monooxygenase that modifies tigecycline^1,2. Here, we report two unique mobile tigecycline-resistance genes, tet(X3) and tet(X4), in numerous Enterobacteriaceae and Acinetobacter that were isolated from animals, meat for consumption and humans. Tet(X3) and Tet(X4) inactivate all tetracyclines, including tigecycline and the newly FDA-approved eravacycline and omadacycline. Both tet(X3) and tet(X4) increase (by 64-128-fold) the tigecycline minimal inhibitory concentration values for Escherichia coli, Klebsiella pneumoniae and Acinetobacter baumannii. In addition, both Tet(X3) (A. baumannii) and Tet(X4) (E. coli) significantly compromise tigecycline in in vivo infection models. Both tet(X3) and tet(X4) are adjacent to insertion sequence ISVsa3 on their respective conjugative plasmids and confer a mild fitness cost (relative fitness of >0.704). Database mining and retrospective screening analyses confirm that tet(X3) and tet(X4) are globally present in clinical bacteria-even in the same bacteria as bla_NDM-1, resulting in resistance to both tigecycline and carbapenems. Our findings suggest that both the surveillance of tet(X) variants in clinical and animal sectors and the use of tetracyclines in food production require urgent global attention.

Risk factors with the development of infection with tigecycline- and carbapenem-resistant Enterobacter cloacae

Background

Tigecycline is regarded as a last resort treatment for carbapenem-resistant Enterobacter cloacae (CREC) infections, and increasing numbers of tigecycline- and carbapenem-resistant E. cloacae (TCREC) isolates have been reported in recent years. However, risk factors and clinical impacts of these isolates are poorly characterized.

Patients and methods

We conducted a retrospective case-case-control study of hospitalized patients with TCREC infection during the period 2012-2016 in Chongqing, China. Case patients with TCREC and those with CREC were compared to a control group with no E. cloacae infection. Multivariate logistic regression models were used to identify independent risk factors for acquiring TCREC and CREC.

Results

A total of 36 TCREC cases, 36 CREC cases, and 100 controls were enrolled in our study. Multivariable analysis indicated that nasal catheter (OR: 8.9; 95% CI: 1.1-75.2), exposure to penicillin (OR: 95.9; 95% CI: 8.9-1038.3), aminoglycosides (OR: 42.1; 95% CI: 2.1-830.6), and fluoroquinolones (OR: 18.6; 95% CI: 1.9-185.6) were independent predictors for acquiring TCREC. In addition, venous catheterization (OR: 12.2; 95% CI: 2.5-58.5), penicillin (OR: 30.8; 95% CI: 7.9-120.0), and broad-spectrum cephalosporin (OR: 5.0; 95% CI: 1.5-17.3) were independently associated with CREC acquisition.

Conclusion

Reasonable antibiotic stewardship programs and surveillance are necessary to control the tigecycline resistance among high-risk patients.

Hidden Carbapenem Resistance in OXA-48 and Extended-Spectrum β-Lactamase-Positive Escherichia coli

The purpose of this study was to report the identification OXA-48 carbapenemase in seven extended-spectrum β-lactamase (ESBL)-positive Escherichia coli clinical isolates, fully susceptible to all carbapenems by disk diffusion and E-test methods, but with borderline minimal inhibitory concentration (MIC) values of ertapenem. This report points to the necessity for determination of carbapenem MICs in ESBL-positive E. coli isolates and additional phenotypic testing for carbapenemases in all isolates with borderline ertapenem MIC defined by EUCAST. The isolates showed a high level of resistance to expanded-spectrum cephalosporins because of the production of an additional ESBL belonging to CTX-M family. All isolates and their respective tranconjugants were found to possess L plasmid. Pulsed-field gel electrophoresis analysis revealed two clusters containing highly related isolates. The global spread of multidrug-resistant E. coli should be monitored closely because of the ability of isolates to rapidly obtain additional antibiotic resistance traits such as plasmid-mediated OXA-48 genes.

COMPARISON OF TWO DIFFERENT METHODS FOR TIGECYCLINE SUSCEPTIBILITY TESTING IN ACINETOBACTER BAUMANNII

SUMMARY – Tigecycline susceptibility testing (TST) presents a tremendous challenge for clinical microbiologists. Previous studies have shown that the Epsilometer test (E-test) and Vitek 2 automated system significantly overestimate the minimum inhibitory concentrations for tigecycline resistance compared to the broth microdilution method (BMM). This leads to very major errors or false susceptibility (i.e. the isolate is called susceptible when it is actually resistant). The aim of this study was to compare E-test against BMM for TST in carbapenem-resistant and carbapenem-susceptible Acinetobacter (A.) baumannii and to analyze changes in tigecycline susceptibility between two time periods (2009-2012 and 2013-2014), with BMM as the gold standard. Using the EUCAST criteria, the rate of resistance to tigecycline for the OXA-23 MBL-positive, OXA-23 MBL-negative and carbapenemase-negative strains for BMM was 54.5% (6/11), 29.4% (5/17) and 2.7% (1/37), respectively; the OXA-24/40 and OXA-58 producing organisms did not exhibit any resistance. With E-test, all OXA-23 MBL-positive organisms (11/11), 23.5% (4/17) of OXA-23 MBL-negative, and 4.1% of OXA-24/40 (3/74) strains displayed tigecycline resistance; there were no resistant strains among the OXA-58 and carbapenemase-negative isolates. Resistance emerged in the bacterial isolates from 2013 to 2014. Although tigecycline does not display cross-resistance, the highest rates of resistant A. baumannii isolates were observed among those producing VIM MBL, regardless of the testing method. These findings suggest that the commercial E-test does not provide reliable results for TST of A. baumannii. Further confirmation with the dilution method should be recommended, particularly in cases of serious infections.

Overexpression of OqxAB and MacAB efflux pumps contributes to eravacycline resistance and heteroresistance in clinical isolates of Klebsiella pneumoniae

This study investigated the characteristics and mechanisms of eravacycline resistance and heteroresistance in clinical Klebsiella pneumoniae isolates. A total of 393 clinical K. pneumoniae isolates were collected and subjected to eravacycline and tigecycline MIC determinations using the agar dilution method. Eravacycline heteroresistance was assessed by a population analysis profile (PAP). The expression levels of efflux pumps and their regulators were determined by quantitative reverse-transcription PCR (qRT-PCR). This study identified 67 eravacycline-nonsusceptible isolates; among the extended-spectrum β-lactamase (ESBL)-positive isolates, eravacycline-nonsusceptible isolates were detected more frequently than tigecycline-nonsusceptible isolates (21.7% vs. 9.4%, p = 0.001). The study sample was observed to include 20 K. pneumoniae isolates with eravacycline heteroresistance. Compared to the reference strain, oqxA or oqxB overexpression was observed in nine eravacycline-nonsusceptible isolates (range, 35.64–309.02-fold) and 13 eravacycline-heteroresistant isolates (8.42–296.34-fold). The overexpression of macA or macB was detected in 12 eravacycline-heteroresistant isolates (3.23–28.35-fold). Overexpression of the efflux pump regulator gene ramA was observed in 11 eravacycline-nonsusceptible isolates (3.33–94.05-fold) and 18 eravacycline-heteroresistant isolates (3.89–571.70-fold). The eravacycline MICs were increased by one–fourfold by overexpression of oqxAB or macAB in three eravacycline-sensitive isolates. In conclusion, the overexpression of OqxAB and MacAB efflux pumps and the transcriptional regulator RamA were suggested to be involved in K. pneumoniae eravacycline resistance and heteroresistance.

Bacteremia and meningitis caused by OXA-23-producing Acinetobacter baumannii - molecular characterization and susceptibility testing for alternative antibiotics

Background

Carbapenem-resistant Acinetobacter baumannii infection is a concern in developing countries due to high incidence, few therapeutic options, and increasing costs.

Objective

Characterize and analyze the antibiotic susceptibility patterns of carbapenem-resistant A. baumannii isolates and evaluate clinical data of meningitis and bacteremia caused by this microorganism.

Methods

Twenty-six A. baumannii isolates from 23 patients were identified by MALDI-TOF and automated methods and genotyped using pulsed field genotyping electrophoresis. Clinical data and outcomes were evaluated. Susceptibility of isolates to colistin, tigecycline, meropenem, imipenem, and doxycycline was determined.

Results

Mortality due to A. baumannii infections was 73.91%; all patients with meningitis and 7/8 patients with ventilator-associated pneumonia died. All isolates were susceptibility to polymyxin (100%; MIC₅₀, MIC₉₀: 1 μg/mL, 1 μg/mL) and colistin (100%; MIC₅₀, MIC₉₀: 2 μg/mL, 2 μg/mL), and 92% were susceptible to tigecycline (MIC₅₀, MIC₉₀: 1 μg/mL, 1 μg/mL) and doxycycline (MIC₅₀, MIC₉₀: 2 μg/mL, 2 μg/mL). bla_OXA-23 was identified in 24 isolates. Molecular typing showed 8 different patterns: 13 isolates belonged to pattern A (50%).

Conclusion

Carbapenem-resistant A. baumannii infections mortality is high. Alternative antimicrobial therapy (doxycycline) for selected patients with carbapenem-resistant A. baumannii infection should be considered.

Comparison of tigecycline susceptibility testing methods for multidrug-resistant Acinetobacter baumannii

Automated systems Vitek 2, BD Phoenix, MicroScan WalkAway and gradient diffusion assays Etest and MIC Test Strip were compared against broth microdilution for susceptibility testing of tigecycline against Acinetobacter baumannii. BD Phoenix and MIC Test Strip seemed to provide the most accurate determination of tigecycline susceptibility.

Acinetobacter spp. Infections in Malaysia: A Review of Antimicrobial Resistance Trends, Mechanisms and Epidemiology

Acinetobacter spp. are important nosocomial pathogens, in particular the Acinetobacter baumannii-calcoaceticus complex, which have become a global public health threat due to increasing resistance to carbapenems and almost all other antimicrobial compounds. High rates of resistance have been reported among countries in Southeast Asia, including Malaysia. In this review, we examine the antimicrobial resistance profiles of Acinetobacter spp. hospital isolates from Malaysia over a period of nearly three decades (1987–2016) with data obtained from various peer-reviewed publications as well as the Malaysian National Surveillance on Antibiotic Resistance (NSAR). NSAR data indicated that for most antimicrobial compounds, including carbapenems, the peak resistance rates were reached around 2008–2009 and thereafter, rates have remained fairly constant (e.g., 50–60% for carbapenems). Individual reports from various hospitals in Peninsular Malaysia do not always reflect the nationwide resistance rates and often showed higher rates of resistance. We also reviewed the epidemiology and mechanisms of resistance that have been investigated in Malaysian Acinetobacter spp. isolates, particularly carbapenem resistance and found that bla_OXA-23 is the most prevalent acquired carbapenemase-encoding gene. From the very few published reports and whole genome sequences that are available, most of the Acinetobacter spp. isolates from Malaysia belonged to the Global Clone 2 (GC2) CC92 group with ST195 being the predominant sequence type. The quality of data and analysis in the national surveillance reports could be improved and more molecular epidemiology and genomics studies need to be carried out for further in-depth understanding of Malaysian Acinetobacter spp. isolates.

Impact of reduced tigecycline susceptibility on clinical outcomes of Acinetobacter bacteremia

The higher 14-day mortality rate for patients with Acinetobacter bacteremia receiving tigecycline appropriately compared to other appropriate antibiotics (36.4% versus 14.2%, P = 0.028) was due to the poor effect of tigecycline for isolates with a minimum inhibitory concentration of 2 μg/mL (63.6% of 11 versus 14.2% of 127, P = 0.001).

Cluster-distinguishing genotypic and phenotypic diversity of carbapenem-resistant Gram-negative bacteria in solid-organ transplantation patients: a comparative study

Purpose. Solid-organ transplant recipients may display high rates of colonization and/or infection by multidrug-resistant bacteria. We analysed and compared the phenotypic and genotypic diversity of carbapenem-resistant (CR) strains of Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii isolated from patients in the Solid Organ Transplantation department of our hospital.Methodology. Between March 2012 and August 2013, 56 CR strains from various biological fluids underwent antimicrobial susceptibility testing with VITEK 2, molecular analysis by PCR amplification and genotypic analysis with pulsed-field gel electrophoresis (PFGE). They were clustered according to antimicrobial drug susceptibility and genotypic profiles. Diversity analyses were performed by calculating Simpson's diversity index and applying computed rarefaction curves.Results/Key findings. Among K. pneumoniae, KP-producers predominated (57.1 %). VIM and OXA-23 carbapenemases prevailed among P. aeruginosa and A. baumannii (89.4 and 88.9 %, respectively). KPC-producing K. pneumoniae and OXA-23 A. baumannii were assigned in single PFGE pulsotypes. VIM-producing P. aeruginosa generated multiple pulsotypes. CR K. pneumoniae strains displayed phenotypic diversity in tigecycline, colistin (CS), amikacin (AMK), gentamicin (GEN) and co-trimoxazole (SXT) (16 clusters); P. aeruginosa displayed phenotypic diversity in cefepime (FEP), ceftazidime, aztreonam, piperacillin, piperacillin-tazobactam, AMK, GEN and CS (9 clusters); and A. baumannii displayed phenotypic diversity in AMK, GEN, SXT, FEP, tobramycin and rifampicin (8 clusters). The Simpson diversity indices for the interpretative phenotype and PFGE analysis were 0.89 and 0.6, respectively, for K. pneumoniae strains (P<0.001); 0.77 and 0.6 for P. aeruginosa (P=0.22); and 0.86 and 0.19 for A. baumannii (P=0.004).Conclusion. The presence of different antimicrobial susceptibility profiles does not preclude the possibility that two CR K. pneumoniae or A. baumannii isolates are clonally related.

Emergence of carbapenem-resistant Acinetobacter baumannii ST787 in clinical isolates from blood in a tertiary teaching hospital in Northern Taiwan

BACKGROUND/PURPOSE

The purpose of this study is to investigate the predominant clones of carbapenem-resistant Acinetobacter baumannii (CRAB) in our hospital in Taiwan by pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) technique.

METHODS

We collected 108 non-duplicate A. baumannii clinical blood isolates from January 2012 to December 2013 in MacKay Memorial Hospital. PFGE and MLST were used for typing the A. baumannii isolates and for investigation of the predominant clones. Bacteria isolates were screened by polymerase chain reaction for the presence of the carbapenemase-encoding genes.

RESULTS

All 108 isolates were classified as 33 pulsotypes by PFGE. The predominant clones were pulsotype 10 (12.04%) in 2012 and pulsotype 8 (16.67%) in 2013, respectively. The 31 predominant pulsotype isolates were typed by MLST, and ST787 (54.84%) and ST455 (45.16%) were identified. All isolates carried bla_OXA-51-like genes, and bla_OXA-23-like genes was founded in 101 isolates (93.52%). Other identified resistance genes included bla_OXA-24-like and bla_OXA-IMP.

CONCLUSION

To the best of our knowledge, this study is the first to describe the microbiological characteristics of CRAB ST787, which carried high genetic resistance to carbapenem, but remained susceptible to colistin. CRAB ST787 was the predominant clone in our hospital in the study period.

Resistance to Non-glycopeptide Agents in Serious Staphylococcus aureus Infections

The role of vancomycin in the treatment of serious Staphylococcus aureus infections, both methicillin-susceptible and methicillin-resistant, is becoming increasingly ineffective due to increasing MIC and failure. The development of reduced vancomycin susceptibility by S. aureus to glycopeptides highlights the need for clinicians to reexamine the roles of non-glycopeptide therapy. As the use of these alternative non-glycopeptides antimicrobials increases, it will become pertinent to monitor the rates of resistance. Large surveillance programs have provided data for resistance against S. aureus for the non-glycopeptides (daptomycin, ceftaroline, tigecycline, linezolid, and tedizolid). The current published literatures suggest that worldwide resistance rates to these non-glycopeptides for serious MRSA infections are still low. Implementation of antimicrobial stewardship programs will be crucial in prevention of resistance of these antimicrobials against S. aureus.

Comparison of Minocycline Susceptibility Testing Methods for Carbapenem-Resistant Acinetobacter baumannii

Treatment options for infections due to carbapenem-resistant Acinetobacter baumannii are extremely limited. Minocycline is a semisynthetic tetracycline derivative with activity against this pathogen. This study compared susceptibility testing methods that are used in clinical microbiology laboratories (Etest, disk diffusion, and Sensititre broth microdilution methods) for testing of minocycline, tigecycline, and doxycycline against 107 carbapenem-resistant A. baumannii clinical isolates. Susceptibility rates determined with the standard broth microdilution method using cation-adjusted Mueller-Hinton (MH) broth were 77.6% for minocycline and 29% for doxycycline, and 92.5% of isolates had tigecycline MICs of ≤2 μg/ml. Using MH agar from BD and Oxoid, susceptibility rates determined with the Etest method were 67.3% and 52.3% for minocycline, 21.5% and 18.7% for doxycycline, and 71% and 29.9% for tigecycline, respectively. With the disk diffusion method using MH agar from BD and Oxoid, susceptibility rates were 82.2% and 72.9% for minocycline and 34.6% and 34.6% for doxycycline, respectively, and rates of MICs of ≤2 μg/ml were 46.7% and 23.4% for tigecycline. In comparison with the standard broth microdilution results, very major rates were low (∼2.8%) for all three drugs across the methods, but major error rates were higher (∼5.6%), especially with the Etest method. For minocycline, minor error rates ranged from 14% to 37.4%. For tigecycline, minor error rates ranged from 6.5% to 69.2%. The majority of minor errors were due to susceptible results being reported as intermediate. For minocycline susceptibility testing of carbapenem-resistant A. baumannii strains, very major errors are rare, but major and minor errors overcalling strains as intermediate or resistant occur frequently with susceptibility testing methods that are feasible in clinical laboratories.