Rapid development of Acinetobacter baumannii resistance to tigecycline

Phenotypical and Molecular Characterization of Acinetobacter baumannii Isolated from Hospitalized Patients During the COVID-19 Pandemic in Brazil

The goal of the present study was to analyze Acinetobacter baumannii strains isolated from hospitalized patients in the period of the COVID-19 pandemic by phenotypic and molecular methods and evaluate their antimicrobial resistance patterns and biofilm production. Forty-seven strains were isolated in 2021-2022 from a hospital in Brazil, and were identified by VITEK®2, MALDI-TOF/MS (VITEK MS® and MALDI Biotyper®), and 16S rRNA sequencing. Fourier-transform infrared (FTIR) spectroscopy was applied for typing and antimicrobial susceptibility testing (AST). In addition, biofilm formation and disinfectant tolerance tests were used. All methods accurately identified all the A. baumannii strains. FTIR typing identified 23 different profiles and 11 clusters, as well as differentiated between the strains from patients with and without COVID-19. Most strains exhibited resistance to the drugs tested, 91.5% of the strains were classified as XDR, 6.4% of the strains were MDR and only 1 strain was classified as non-MDR. Over half of the strains (n = 27, 57.4%) produced biofilms on polystyrene. Sodium hypochlorite (1.0%/15 min) was the best option for effective disinfection. Overall, this study will lay the foundation for further research on effective cleaning protocols for the eradication of A. baumannii biofilms, as well as the use of FTIR for pathogen surveillance in healthcare settings.

Inside-out, antimicrobial resistance mediated by efflux pumps in clinical strains of Acinetobacter baumannii isolated from burn wound infections

Acinetobacter baumannii belongs to the ESKAPE group. It is classified as a critical priority group by the World Health Organization and a global concern on account of its capacity to acquire and develop resistance mechanisms to multiple antibiotics. Data from the United States indicates 500 deaths annually. Resistance mechanisms of this bacterium include enzymatic pathways such as ß-lactamases, carbapenemases, and aminoglycoside-modifying enzymes, decreased permeability, and overexpression of efflux pumps. A. baumannii has been demonstrated to possess efflux pumps, which are classified as members of the MATE family, RND and MFS superfamilies, and SMR transporters. The aim of our work was to assess the distribution of efflux pumps and their regulatory gene expression in clinical strains of A. baumannii isolated from burned patients. METHODS: From the Clinical Microbiology Laboratory at the Instituto Nacional de Rehabilitación Luis Guillermo Ibarra Ibarra collection in Mexico, 199 strains were selected. Antibiotics susceptibilities were performed by broth microdilutions to determine minimal inhibitory concentrations. Phenotypic assays with efflux pump inhibitors were conducted using carbonyl cyanide 3-chlorophenylhydrazone (CCCP) and phenylalanine-arginine ß-naphthylamide (PAßN) in conjunction with amikacin, ceftazidime, imipenem, meropenem and levofloxacin. A search was conducted for structural genes that are linked to efflux pumps, and the relative expression of the adeR, adeS, and adeL genes was analyzed. RESULTS: Among a total of 199 strains, 186 exhibited multidrug resistance (MDR). Fluoroquinolones demonstrated the highest resistance rates, while minocycline and amikacin displayed comparatively reduced resistance rates (1.5 and 28.1, respectively). The efflux activity of fluorquinolones exhibited the highest phenotypic detection (from 85 to 100%), while IMP demonstrated the lowest activity of 27% with PAßN and 43.3% with CCCP. Overexpression was observed in adeS and adeL, with adeR exhibiting overexpression. Concluding that clinical strains of A. baumannii from our institution exhibited efflux pumps as one of the resistance mechanisms.

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.

Isolation of extensively drug resistant Acinetobacter baumannii from environmental surfaces inside intensive care units

BACKGROUND

Acinetobacter baumannii is a nosocomial pathogen that has emerged as a major threat in the health-care settings, particularly intensive care units (ICUs). The aim of this study was to investigate the prevalence of A. baumannii in the environment of intensive care and emergency units in 4 hospitals in Jordan.

METHODS

A total of 311 surface and 26 air samples were collected from 6 different ICUs and 2 emergency units. Examined high-touch surfaces included bed rails, sinks, food tables, trolley handles, ventilator inlets, blankets, sheets, door handles, light switches, bedside tables and drawers, curtains, normal saline stands and neonatal incubators. A. baumannii isolates were identified by CHROMagar and confirmed using 2 different PCR assays. All obtained isolates were characterized for their antibiotic resistance phenotypes, biofilm formation capacities and were typed by multi-locus sequence typing.

RESULTS

Of the 337 samples, 24 A. baumannii isolates were recovered, mostly from surfaces in the internal medicine ICUs. Among the 24 isolates, 10 isolates were classified as extensively-resistant (XDR), harbored the bla_OXA-23 like gene and able to form biofilms with varying capacities. ST2 was the most frequent sequence type, with all ST2 isolates classified as XDRs.

CONCLUSIONS

Our results showed that high-touch surfaces of adult and pediatric ICUs were contaminated with XDR A. baumannii isolates. Therefore, the cleaning practices of the surfaces and equipment surrounding ICU patients should be optimized, and health-care workers should continuously wash their hands and change their gloves constantly to control the spread of this pathogen.

Isolation and Characterization of a Novel Autographiviridae Phage and Its Combined Effect with Tigecycline in Controlling Multidrug-Resistant Acinetobacter baumannii-Associated Skin and Soft Tissue Infections

Multidrug-resistant Acinetobacter baumannii (MDR A. baumannii) is one of the ESKAPE pathogens that restricts available treatment options. MDR A. baumannii is responsible for a dramatic increase in case numbers of a wide variety of infections, including skin and soft tissue infections (SSTIs), resulting in pyoderma, surgical debridement, and necrotizing fasciitis. To investigate an alternative medical treatment for SSTIs, a broad range lytic Acinetobacter phage, vB _AbP_ABWU2101 (phage vABWU2101), for lysing MDR A. baumannii in associated SSTIs was isolated and the biological aspects of this phage were investigated. Morphological characterization and genomic analysis revealed that phage vABWU2101 was a new species in the Friunavirus, Beijerinckvirinae, family Autographiviridae, and order Caudovirales. Antibiofilm activity of phage vABWU2101 demonstrated good activity against both preformed biofilms and biofilm formation. The combination of phage vABWU2101 and tigecycline showed synergistic antimicrobial activities against planktonic and biofilm cells. Scanning electron microscopy confirmed that the antibacterial efficacy of the combination of phage vABWU2101 and tigecycline was more effective than the phage or antibiotic alone. Hence, our findings could potentially be used to develop a therapeutic option for the treatment of SSTIs caused by MDR A. baumannii.

Tigecycline antibacterial activity, clinical effectiveness, and mechanisms and epidemiology of resistance: narrative review

Tigecycline is unique glycylcycline class of semisynthetic antimicrobial agents developed for the treatment of polymicrobial infections caused by multidrug-resistant Gram-positive and Gram-negative pathogens. Tigecycline evades the main tetracycline resistance genetic mechanisms, such as tetracycline-specific efflux pump acquisition and ribosomal protection, via the addition of a glycyclamide moiety to the 9-position of minocycline. The use of the parenteral form of tigecycline is approved for complicated skin and skin structure infections (excluding diabetes foot infection), complicated intra-abdominal infections, and community-acquired bacterial pneumonia in adults. New evidence also suggests the effectiveness of tigecycline for the treatment of severe Clostridioides difficile infections. Tigecycline showed in vitro susceptibility to Coxiella spp., Rickettsia spp., and multidrug-resistant Neisseria gonnorrhoeae strains which indicate the possible use of tigecycline in the treatment of infections caused by these pathogens. Except for intrinsic, or often reported resistance in some Gram-negatives, tigecycline is effective against a wide range of multidrug-resistant nosocomial pathogens. Herein, we summarize the currently available data on tigecycline pharmacokinetics and pharmacodynamics, its mechanism of action, the epidemiology of tigecycline resistance, and its clinical effectiveness.

Efficacy and safety of tigecycline for complicated urinary tract infection: a systematic review

BACKGROUND

Facing the global threat of emerging resistance to antibiotics, tigecycline, a novel glycylcycline antibiotic, is developed to against multidrug-resistant pathogens, but not recommended for the treatment of complicated urinary tract infection (cUTI). We performed a summary of the literatures to characterize and evaluate the efficacy and safety of tigecycline in patients with cUTI.

METHODS

We searched PubMed, EMBASE, Cochrane and Clinical Trials using appropriate syntax to retrieve potential articles up to Jan 2020. General information, pathogen, medication regimen, comorbidities of patients from eligible literatures were recorded. Univariate logistic regression analysis was used to detect the potential factors associated with clinical cure.

RESULTS

Nineteen articles comprising 31 cases were included. The subpopulation with transplantation (25.8% of the patients) was the most common comorbidity, and cUTIs were mainly caused by Klebsiella pneumoniae (K. pneumoniae) (48.28%) in our research. Tigecycline 100 mg per day as monotherapy was most common. Clinical cure was reported as majority (77.4%), and microbiological eradication cases accounted for the most (65.2%) among the clinical cure cases. Univariate analysis showed that K. pneumoniae caused cUTI and tigecycline as a single treatment have significant meaning to clinical outcomes (P=0.044 and P=0.034, respectively).

CONCLUSIONS

Clinical and microbiological outcomes of tigecycline treatment revealed high rate of successful response. Tigecycline monotherapy may have a role in the treatment of cUTI except that caused by the pathogen K. pneumoniae. Further randomized controlled trials was still needed to evaluate tigecycline monotherapy for cUTI.

A systematic review of implications, mechanisms, and stability of in vivo emergent resistance to colistin and tigecycline in Acinetobacter baumannii

The potential of A. baumannii for acquired resistance to last resort antibiotics (colistin and tigecycline) during treatment has important clinical implications, especially when dealing with patients failing to improve despite treatment with an active antimicrobial. However, the relevant literature remains scattered. Therefore, a systematic search was conducted in PubMed and Scopus. Several studies reported emergence of resistance to colistin or tigecycline during treatment, in most cases (86%) resulting in persistent or recurrent infections, especially in cases of emergent resistance without fitness cost. Lipopolysaccharide modification in the case of colistin and overexpression of efflux pumps in the case of tigecycline were the main mechanisms of resistance. Emergent colistin resistance is often associated with fitness cost which may result in re-emergence of the fitter and more virulent colistin susceptible strain after cessation of antibiotic pressure. Prospective studies are needed to determine the frequency of emergent resistance during treatment and its impact on patient outcomes.

Calculated parenteral initial treatment of bacterial infections: Microbiology

This is the second chapter of the guideline "Calculated initial parenteral treatment of bacterial infections in adults - update 2018" in the 2nd updated version. The German guideline by the Paul-Ehrlich-Gesellschaft für Chemotherapie e.V. (PEG) has been translated to address an international audience. Preliminary microbiological findings regarding the patient and their immediate environment are crucial for the calculation of treatment with antibiotics in each case, as well as the resistance situation of the ward on which the patient is being cared for. If such data is not available, regional or supra-regional data can be used as a fallback. This chapter describes the methods of susceptibility testing, informs about the resistance situation in Germany and describes the main resistance mechanisms of bacterial pathogens against antibiotics. Further, the chapter informs about collateral damage of antibiotics as well as medical measures against increasing resistance.

Multidrug-resistant Gram-negative bacterial infections in solid organ transplant recipients-Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice

These updated guidelines from the Infectious Diseases Community of Practice of the American Society of Transplantation review the diagnosis, prevention, and management of infections due to multidrug-resistant (MDR) Gram-negative bacilli in the pre- and post-transplant period. MDR Gram-negative bacilli, including carbapenem-resistant Enterobacteriaceae, MDR Pseudomonas aeruginosa, and carbapenem-resistant Acinetobacter baumannii, remain a threat to successful organ transplantation. Clinicians now have access to at least five novel agents with activity against some of these organisms, with others in the advanced stages of clinical development. No agent, however, provides universal and predictable activity against any of these pathogens, and very little is available to treat infections with MDR nonfermenting Gram-negative bacilli including A baumannii. Despite advances, empiric antibiotics should be tailored to local microbiology and targeted regimens should be tailored to susceptibilities. Source control remains an important part of the therapeutic armamentarium. Morbidity and mortality associated with infections due to MDR Gram-negative organisms remain unacceptably high. Heightened infection control and antimicrobial stewardship initiatives are needed to prevent these infections, curtail their transmission, and limit the evolution of MDR Gram-negative pathogens, especially in the setting of organ transplantation.

Clinical Characteristics of Acinetobacter baumannii Infection in Solid-Organ Transplant Recipients

OBJECTIVES

Acinetobacter baumannii, depending on the immune status of the host, may result in one of the most serious hospital infections. Infections involving A. baumannii infection have been recently rising. However, little is known about the clinical features of A. baumannii infection in solid-organ transplant recipients. We aimed to share our clinical experiences with A. baumannii infection in our transplant recipients.

MATERIALS AND METHODS

Between 2011 and 2017, 41 solid-organ transplant patients developed A. baumannii infection at Baskent University Hospital. Medical records were reviewed, and patient demographics, microbiology results, and overall outcome data were noted.

RESULTS

Of 41 solid-organ transplant patients with A. baumannii infection, 29 were male and 12 were female patients with mean age of 47.15 ± 13.24 years. Our infection rate with A. baumannii infection was 6.1%. The most common sites of infection were deep tracheal aspirate (48.8%)and bloodstream (36.6%). Onset of infection 1 year posttransplant was identified in 58.5% of recipients. Risk factors included presence of invasive procedures (56.1%) and administration of high-dose corticosteroids for rejection 1 year before infection (68.3%). Thirty-day mortality rate was 41.5% (17/41 patients) and was not associated with the infection site, microbiological cure, clinical cure, and drug resistance in our study group.

CONCLUSIONS

Acinetobacter baumannii is an important cause of hospital-acquired infection and mortality worldwide. A major problem with A. baumannii infection is delayed initiation of appropriate antibiotic treatment and the rising numbers of extensively drug-resistant organisms. Predicting the potential risk factors, especially in the already at-risk solid-organ transplant population, has an important role in patient outcomes.

Whole-Genome Analysis of an Extensive Drug-Resistant Acinetobacter Baumannii ST195 Isolate from a Recipient After DCD Renal Transplantation in China

BACKGROUND/AIMS

Infection with Acinetobacter baumannii was emerging as one of the leading causes of mortality after donation after cardiac death transpalantion.

METHODS

We reported a case of a recipient who underwent DCD renal transplantation and later got infected by A.baumannii. Etests were done to verify the susceptibility test results in clinic. Whole-genome analysis was applied to investigate the resistant mechanism at gene level.

RESULTS

The pathogen was isolated from his draining liquid the day after the surgery, and susceptibility test reavealed that it was sensitive to tigecycline. However, the isolate obtained from the draining liquid became tigecycline-resistant after fifteen-day administration of tigecycline. The Susceptibility tests showed that the pathogen recovered from tigecycline resistance and became intermediated to tigecycline. Whole-Genome analysis revealed the genetic level change leading to tigecycline resistance and we identified the location of mutation by comparing the whole genome sequence of the isolates. Three loci were figured out which may contribute to drug resistance, including genes encoding HTH domain protein, MFS transporter and AdeS.

CONCLUSION

Understanding the genetic characteristics associated with drug resistance mechanism and antimicrobial profiles of pathogen is important in controlling infection outbreak and preventing serious complications and gives a new insight into the development of antimicrobial agents.

Efficacy of intravenous tigecycline in patients with Acinetobacter complex infections: results from 14 Phase III and Phase IV clinical trials

Background

Acinetobacter infections, especially multidrug-resistant (MDR) Acinetobacter infections, are a global health problem. This study aimed to describe clinical outcomes in patients with confirmed Acinetobacter spp. isolates who were treated with tigecycline in randomized clinical trials.

Materials and methods

Data from 14 multinational, randomized (open-label or double-blind), and active-controlled (except one) Phase III and IV studies were analyzed using descriptive statistics.

Results

A total of 174 microbiologically evaluable patients with Acinetobacter spp. infections (including MDR infections) were identified, and 95 received tigecycline to treat community-acquired pneumonia (CAP), diabetic foot infections (DFIs), hospital-acquired pneumonia (HAP), complicated intra-abdominal infections (cIAIs), infections with resistant pathogens (RPs), or complicated skin and skin-structure infections. The rate of cure of tigecycline for most indications was 70%–80%, with the highest (88.2%) in cIAIs. The rate of cure of the comparators was generally higher than tigecycline, but within each indication the 95% CIs for clinical cure for each treatment group overlapped. For most Acinetobacter isolates, the minimum inhibitory concentration of tigecycline was 0.12–2 μg/mL, with seven at 4 μg/mL and one at 8 μg/mL. The cure rate by tigecycline was 50% (95% CI 12.5%–87.5% in CAP) to 88.2% (95% CI 66.2%–97.1% in cIAIs) for all Acinetobacter, and 72.7% (95% CI 54.5%–93.2% in HAP) to 100% (95% CI 25%–100.0% in cIAIs) for MDR Acinetobacter. For the comparators, it was 83.8% (95% CI 62.8%–95.9% in HAP) to 100% (95% CI 75%–100% in cIAIs and 25%–100.0% in RPs) and 88% (95% CI 66%–97% in HAP) to 100% (95% CI 25%–100% in cIAIs and 75%–100% in DFIs), respectively.

Conclusion

These findings suggest that with appropriate monitoring, tigecycline may be a useful consideration for Acinetobacter infections alone or in combination with other anti-infective agents when other therapies are not suitable.

Management of multidrug resistant Gram-negative bacilli infections in solid organ transplant recipients: SET/GESITRA-SEIMC/REIPI recommendations

Solid organ transplant (SOT) recipients are especially at risk of developing infections by multidrug resistant (MDR) Gram-negative bacilli (GNB), as they are frequently exposed to antibiotics and the healthcare setting, and are regulary subject to invasive procedures. Nevertheless, no recommendations concerning prevention and treatment are available. A panel of experts revised the available evidence; this document summarizes their recommendations: (1) it is important to characterize the isolate's phenotypic and genotypic resistance profile; (2) overall, donor colonization should not constitute a contraindication to transplantation, although active infected kidney and lung grafts should be avoided; (3) recipient colonization is associated with an increased risk of infection, but is not a contraindication to transplantation; (4) different surgical prophylaxis regimens are not recommended for patients colonized with carbapenem-resistant GNB; (5) timely detection of carriers, contact isolation precautions, hand hygiene compliance and antibiotic control policies are important preventive measures; (6) there is not sufficient data to recommend intestinal decolonization; (7) colonized lung transplant recipients could benefit from prophylactic inhaled antibiotics, specially for Pseudomonas aeruginosa; (8) colonized SOT recipients should receive an empirical treatment which includes active antibiotics, and directed therapy should be adjusted according to susceptibility study results and the severity of the infection.

Bloodstream infections caused by Acinetobacter species with reduced susceptibility to tigecycline: clinical features and risk factors

INTRODUCTION

During recent decades, the rates of multidrug resistance, including resistance to carbapenems, have increased dramatically among Acinetobacter species. Tigecycline has activity against multidrug-resistant Acinetobacter spp, including carbapenem-resistant isolates. However, reports of tigecycline-resistant Acinetobacter spp are emerging from different parts of the world. The purpose of this study was to evaluate potential risk factors associated with tigecycline non-susceptible Acinetobacter bacteremia.

METHODS

The medical records of 152 patients with Acinetobacter bacteremia attending Samsung Medical Center between January 2010 and December 2014 were reviewed. Non-susceptibility to tigecycline was defined as a minimum inhibitory concentration (MIC) of tigecycline ≥4μg/ml. Cases were patients with tigecycline non-susceptible Acinetobacter bacteremia and controls were those with tigecycline-susceptible Acinetobacter bacteremia.

RESULTS

Of the 152 patients included in the study, 61 (40.1%) had tigecycline non-susceptible Acinetobacter bacteremia (case group). These patients were compared to 91 patients with tigecycline-susceptible Acinetobacter bacteremia (control group). The case group showed high resistance to other antibiotics (>90%) except colistin (6.6%) and minocycline (9.8%) when compared to the control group, which exhibited relatively low resistance to other antibiotics (<50%). Multivariate analysis showed that recent exposure to corticosteroids (minimum 20mg per day for more than 5 days within 2 weeks) (adjusted odds ratio (OR) 2.887, 95% confidence interval (CI) 1.170-7.126) and carbapenems (within 2 weeks) (adjusted OR 4.437, 95% CI 1.970-9.991) were significantly associated with tigecycline non-susceptible Acinetobacter bacteremia. Although prior exposure to tigecycline was more common in the case group than in the control group (9.8%, 6/61 vs. 2.2%, 2/91; p=0.046), this variable was found not to be a significant factor associated with tigecycline non-susceptibility after adjustment for other variables (adjusted OR 1.884, 95% CI 0.298-11.920; p=0.501).

CONCLUSIONS

These data suggest that tigecycline non-susceptible Acinetobacter spp have emerged and disseminated in the hospital in association with a recent exposure to carbapenems and an immunosuppressed state. This indicates that the rational use of antibiotics through a comprehensive antimicrobial stewardship program, especially in immunosuppressed patients, may be essential in limiting the emergence and spread of multidrug-resistant organisms such as tigecycline-resistant Acinetobacter spp, which are difficult to treat.

Epidemiology of antimicrobial resistance in bloodstream infections

Antimicrobial resistance in bacterial pathogens is a worldwide challenge leading high morbidity and mortality in clinical settings. Multidrug resistant patterns in gram-positive and -negative bacteria have resulted in difficult-to-treat or even untreatable infections with conventional antimicrobials. Since the early identification of causative microorganisms and their antimicrobial susceptibility patterns in patients with bacteremia and other serious infections is lacking in many healthcare institutions, broad spectrum antibiotics are liberally and mostly unnecessarily used. Such practice has, in turn, caused dramatic increases in emerging resistance and when coupled with poor practice of infection control, resistant bacteria can easily be disseminated to the other patients and the environment. Thus, availability of updated epidemiological data on antimicrobial resistance in frequently encountered bacterial pathogens will be useful not only for deciding on empirical treatment strategies, but also devising an effective antimicrobial stewardship program in hospitals.

Tetracycline Antibiotics and Resistance

Tetracyclines possess many properties considered ideal for antibiotic drugs, including activity against Gram-positive and -negative pathogens, proven clinical safety, acceptable tolerability, and the availability of intravenous (IV) and oral formulations for most members of the class. As with all antibiotic classes, the antimicrobial activities of tetracyclines are subject to both class-specific and intrinsic antibiotic-resistance mechanisms. Since the discovery of the first tetracyclines more than 60 years ago, ongoing optimization of the core scaffold has produced tetracyclines in clinical use and development that are capable of thwarting many of these resistance mechanisms. New chemistry approaches have enabled the creation of synthetic derivatives with improved in vitro potency and in vivo efficacy, ensuring that the full potential of the class can be explored for use against current and emerging multidrug-resistant (MDR) pathogens, including carbapenem-resistant Enterobacteriaceae, MDR Acinetobacter species, and Pseudomonas aeruginosa.

Treatment options for carbapenem-resistant and extensively drug-resistant Acinetobacter baumannii infections

Acinetobacter baumannii is a leading cause of healthcare-associated infections worldwide. Because of various intrinsic and acquired mechanisms of resistance, most β-lactam agents are not effective against many strains, and carbapenems have played an important role in therapy. Recent trends show many infections are caused by carbapenem-resistant or even extensively drug-resistant (XDR) strains, for which effective therapy is not well established. Evidence to date suggests that colistin constitutes the backbone of therapy, but the unique pharmacokinetic properties of colistin have led many to suggest the use of combination antimicrobial therapy. However, the combination of agents and dosing regimens that delivers the best clinical efficacy while minimizing toxicity is yet to be defined. Carbapenems, sulbactam, rifampin and tigecycline have been the most studied in the context of combination therapy. Most data regarding therapy for invasive, resistant A. baumannii infections come from uncontrolled case series and retrospective analyses, though some clinical trials have been completed and others are underway. Early institution of appropriate antimicrobial therapy is shown to consistently improve survival of patients with carbapenem-resistant and XDR A. baumannii infection, but the choice of empiric therapy in these infections remains an open question. This review summarizes the most current knowledge regarding the epidemiology, mechanisms of resistance, and treatment considerations of carbapenem-resistant and XDR A. baumannii.

Emerging broad-spectrum resistance in Pseudomonas aeruginosa and Acinetobacter baumannii: Mechanisms and epidemiology

Multidrug resistance is quite common among non-fermenting Gram-negative rods, in particular among clinically relevant species including Pseudomonas aeruginosa and Acinetobacter baumannii. These bacterial species, which are mainly nosocomial pathogens, possess a diversity of resistance mechanisms that may lead to multidrug or even pandrug resistance. Extended-spectrum β-lactamases (ESBLs) conferring resistance to broad-spectrum cephalosporins, carbapenemases conferring resistance to carbapenems, and 16S rRNA methylases conferring resistance to all clinically relevant aminoglycosides are the most important causes of concern. Concomitant resistance to fluoroquinolones, polymyxins (colistin) and tigecycline may lead to pandrug resistance. The most important mechanisms of resistance in P. aeruginosa and A. baumannii and their most recent dissemination worldwide are detailed here.

First emergence of acrAB and oqxAB mediated tigecycline resistance in clinical isolates of Klebsiella pneumoniae pre-dating the use of tigecycline in a Chinese hospital

Tigecycline is one of the few therapeutic options for treating infections caused by some multi-drug resistant pathogens, such as Klebsiella pneumoniae. However, tigecycline-resistant K. pneumoniae has been discovered recently in China. From 2009 to 2013, nine tigecycline-resistant K. pneumoniae isolates were identified in our hospital. Six of nine strains were identified before using tigecycline. To investigate the efflux-mediated resistance mechanisms of K. pneumoniae, the expression of efflux pump genes (acrA, acrB, tolC, oqxA and oqxB) and pump regulators (acrR, marA, soxS, rarA, rob and ramA) were examined by real-time RT-PCR. Molecular typing of the tigecycline resistant strains was performed. ST11 was the predominant clone of K. pneumoniae strains, while ST1414 and ST1415 were novel STs. Efflux pump inhibitor (EPI)-carbonyl cyanide chlorophenylhydrazone (CCCP) was able to reverse the resistance patterns of 5 resistant K. pneumoniae strains. In comparison with strain A111, a tigecycline-susceptible strain (negative control), we found that the expression levels of efflux pump genes and pump regulators were higher in a majority of resistant strains. Higher expression levels of regulators rarA (2.41-fold, 9.55-fold, 28.44-fold and 18.31-fold, respectively) and pump gene oqxB (3.87-fold, 31.96-fold, 50.61-fold and 29.45-fold, respectively) were observed in four tigecycline resistant strains (A363, A361, A368, A373, respectively). Increased expression of acrB was associated with ramA and marA expression. To our knowledge, studies on tigecycline resistance mechanism in K. pneumoniae are limited especially in China. In our study, we found that both efflux pump AcrAB-TolC and OqxAB contributed to tigecycline resistance in K. pneumoniae isolates.

Role of Tigecycline for the Treatment of Urinary Tract Infections

Objective: To review and critically analyze the literature for the use of tigecycline for the treatment of urinary tract infections (UTIs). Data Sources: A search of the MEDLINE database was performed (2004 to July 2013). Search terms included tigecycline, Tygacil, pyelonephritis, cystitis, and urinary tract infections in addition to a manual search of references from the articles retrieved. Study Selection and Data Exaction: All studies of humans, English-language articles, clinical studies, observational studies, and case reports were evaluated. Data Synthesis: Fourteen cases of tigecycline use for UTIs were identified. No clinical trials were identified via the search of the MEDLINE database. Twelve of the 14 cases described positive clinical outcomes with use of tigecycline for the treatment of UTI. Microbiological clearance was evaluable in 11 patients, of which 9 patients achieved documented microbiological clearance. None of the patients had mortality attributable to the use of tigecycline for the UTI. Two of the 14 cases reported had patients with subsequent cultures growing tigecycline-resistant organisms. Conclusion: Case reports have documented clinical improvement/success with the use of tigecycline for the treatment of UTIs. However, use of tigecycline for the treatment of UTIs remains controversial because of limited data and the lack of randomized control trials demonstrating efficacy. Tigecycline should be avoided for the treatment of UTIs when well-established options such as aminoglycosides and β-lactams are available. When alternative options are nonexistent, tigecycline can be considered.

Tigecycline in treatment of multidrug-resistant Gram-negative bacillus urinary tract infections: a systematic review

OBJECTIVES

To review cases of multidrug-resistant (MDR) Gram-negative bacillus urinary tract infections (UTIs) treated with tigecycline and the literature related to this subject.

METHODS

We performed a systematic review of the literature identifying patients with MDR Gram-negative bacillus UTIs treated with tigecycline.

RESULTS

Fourteen cases describing treatment of UTIs caused by MDR Gram-negative bacilli with tigecycline are reviewed. Favourable clinical outcomes were noted in 11 of 14 cases. An initial favourable microbiological outcome was noted in 12 cases. Post-treatment cultures in two cases were positive for tigecycline-resistant organisms.

CONCLUSIONS

The clinical efficacy of tigecycline for treatment of UTIs has not been extensively evaluated. Based on the available literature, tigecycline appears to have efficacy in some patients with MDR Gram-negative bacillus UTIs. Further research in this area is needed to fully elucidate the role of tigecycline in treating such patients.

Molecular epidemiology and mechanisms of tigecycline resistance in clinical isolates of Acinetobacter baumannii from a Chinese university hospital

Because of its remarkable ability to acquire antibiotic resistance and to survive in nosocomial environments, Acinetobacter baumannii has become a significant nosocomial infectious agent worldwide. Tigecycline is one of the few therapeutic options for treating infections caused by A. baumannii isolates. However, tigecycline resistance has increasingly been reported. Our aim was to assess the prevalence and characteristics of efflux-based tigecycline resistance in clinical isolates of A. baumannii collected from a hospital in China. A total of 74 A. baumannii isolates, including 64 tigecycline-nonsusceptible A. baumannii (TNAB) and 10 tigecycline-susceptible A. baumannii (TSAB) isolates, were analyzed. The majority of them were determined to be positive for adeABC, adeRS, adeIJK, and abeM, while the adeE gene was found in only one TSAB isolate. Compared with the levels in TSAB isolates, the mean expression levels of adeB, adeJ, adeG, and abeM in TNAB isolates were observed to increase 29-, 3-, 0.7-, and 1-fold, respectively. The efflux pump inhibitors (EPIs) phenyl-arginine-β-naphthylamide (PAβN) and carbonyl cyanide 3-chlorophenylhydrazone (CCCP) could partially reverse the resistance pattern of tigecycline. Moreover, the tetX1 gene was detected in 12 (18.8%) TNAB isolates. To our knowledge, this is the first report of the tetX1 gene being detected in A. baumannii isolates. ST208 and ST191, which both clustered into clonal complex 92 (CC92), were the predominant sequence types (STs). This study showed that the active efflux pump AdeABC appeared to play important roles in the tigecycline resistance of A. baumannii. The dissemination of TNAB isolates in our hospital is attributable mainly to the spread of CC92.

Optimal tigecycline dosage regimen is urgently needed: results from a pharmacokinetic/pharmacodynamic analysis of tigecycline by Monte Carlo simulation

BACKGROUND

The number of reported cases of resistance to tigecycline is increasing. The aim of this study was to evaluate the current standard tigecycline dosage regimen from a pharmacokinetic/pharmacodynamic (PK/PD) perspective.

METHODS

Pharmacokinetic parameters and microbiological data were analyzed by Monte Carlo simulation in an evaluation of effectiveness.

RESULTS

Tigecycline exhibits excellent in vitro antimicrobial activity, however the standard tigecycline dosing regimen fails to achieve the best outcome in vivo for the common drug-resistant strains, including Acinetobacter baumannii, Enterobacter spp, and Klebsiella pneumoniae. This may result in a lack of response to tigecycline therapy or to a further increase in the resistance rate.

CONCLUSIONS

In the absence of new drugs on the horizon, rather than using a single fixed dosing regimen, tigecycline dosing needs to be optimized in order to achieve the desired successful clinical response and to prevent an escalation in drug resistance.

In vitro activity of doripenem in combination with various antimicrobials against multidrug-resistant Acinetobacter baumannii: possible options for the treatment of complicated infection

AIMS

The aim of this study was to evaluate the in vitro activity of doripenem (DOR) alone and in combination with a variety of commonly used anti-Acinetobacter chemotherapeutic agents against 22 primary multidrug-resistant (MDR) Acinetobacter baumannii isolates (including 17 isolates that were resistant to DOR) from Intensive Care Unit patients. Antibiotic interactions were evaluated using the chequerboard method and the time-kill assay.

RESULTS

Considering all antimicrobials in combination with DOR, chequerboard analysis showed synergy in 13 A. baumannii strains (54.2%). Seven strains (29.2%) showed ≥2 synergistic interactions. DOR showed synergy in combination with tigecycline (TIG) (eight strains), colistin (COL) (eight strains), amikacin (AMK) (four strains), ampicillin/sulbactam (two strains), and rifampicin (one strain). Remarkably, synergistic effects were detected only in DOR nonsusceptible strains. Time-kill assays confirmed synergy in eight isolates (giving 10 synergistic interactions) for DOR in combination with TIG (n=4), COL (n=5), and AMK (n=1). No antagonistic interactions were observed with both methods.

CONCLUSIONS

This study demonstrates the in vitro synergistic activity of DOR in combination with TIG, COL, and AMK against DOR-resistant A. baumannii strains, opening the way to in vivo assessment of novel combination therapies for treatment of infections caused by MDR A. baumannii.

Intravitreal tigecycline treatment in experimental Acinetobacter baumannii endophthalmitis

PURPOSE

To investigate the clinical and microbiological effectivity of intravitreal tigecycline in an experimental rabbit endophthalmitis model caused by imipenem resistant Acinetobacter baumannii.

MATERIALS AND METHODS

Forty-eight eyes of 24 New Zealand white albino rabbits were divided into six groups (n=8 in each). The right eyes were divided into three groups and defined as infected group; left eyes were divided into three groups and defined as uninfected group. Infected group received 0.1 ml intravitreal A. baumannii suspension. Twenty-four hours after bacterial inoculation, group 1 received 1 mg/0.1 ml tigecycline and group 2 received 0.5 mg/0.1 ml tigecycline. Group 3 eyes received no treatment. In group 4, 0.1 ml of saline solution was injected. Groups 5 and 6 were received intravitreal tigecycline injection of 1 mg/0.1 ml and 0.5 mg/0.1 ml respectively. The eyes were enucleated for histopathological evaluation on the sixth day. Clinical and histological scoring systems were used to evaluate clinical and histological severity of the intraocular infection.

RESULTS

The mean clinical scores of the six groups at the sixth day were 11±1.92, 12.4±6.2, 8.5±2.7, 0, 3±1.3, and 3±1.4 respectively. Mean histopathological scores were 7.8±2.8, 7.0±1.5, 5.6±1.4, 0, 0, and 0 respectively. There was no significant difference in mean clinical and histopathological scores of infected group (groups 1, 2 and 3). There was significant difference in mean clinical scores of groups 5 and 6 compared with group 4. Groups 4, 5 and 6 showed normal histological structure in histopathological evaluation and showed no significant difference. Microbiological cure was achieved in all infected eyes.

CONCLUSIONS

Experimental rabbit endophthalmitis model caused by imipenem resistant A. baumannii was microbiologically cured by intravitreal tigecycline injection. However, a hypersensitivity-like reaction due to intravitreal application of tigecycline limits the use of this antimicrobial agent in A. baumannii endophthalmitis.

In vivo emergence of tigecycline resistance in multidrug-resistant Klebsiella pneumoniae and Escherichia coli

Although resistance to tigecycline has been reported in surveillance studies, very few reports have described the emergence of resistance in vivo. We report two cases of patients with infections due to SHV-12-producing Klebsiella pneumoniae and K. pneumoniae carbapenemase-3 (KPC-3)-producing Escherichia coli, which developed tigecycline resistance in vivo after treatment. The reported limited experience underlines the risk of occurrence of a tigecycline MIC increase under treatment pressure.

Tigecycline treatment for urinary tract infections: case report and literature review

Antimicrobial resistance among Gram-negatives is increasing; treatment options are limited. Although tigecycline is used infrequently for urinary tract infection (UTI), greater use is likely as resistance increases. We report successful treatment of an episode of febrile UTI and probable prostatitis with tigecycline, and summarize the relevant literature.

Comparative effectiveness of aminoglycosides, polymyxin B, and tigecycline for clearance of carbapenem-resistant Klebsiella pneumoniae from urine

Carbapenem-resistant Klebsiella pneumoniae (CRKP) is an increasingly common cause of health care-associated urinary tract infections. Antimicrobials with in vitro activity against CRKP are typically limited to polymyxins, tigecycline, and often, aminoglycosides. We conducted a retrospective cohort study of cases of CRKP bacteriuria at New York-Presbyterian Hospital from January 2005 through June 2010 to compare microbiologic clearance rates based on the use of polymyxin B, tigecycline, or an aminoglycoside. We constructed three active antimicrobial cohorts based on the active agent used and an untreated cohort of cases that did not receive antimicrobial therapy with Gram-negative activity. Microbiologic clearance was defined as having a follow-up urine culture that did not yield CRKP. Cases without an appropriate follow-up culture or that received multiple active agents or less than 3 days of the active agent were excluded. Eighty-seven cases were included in the active antimicrobial cohorts, and 69 were included in the untreated cohort. The microbiologic clearance rate was 88% in the aminoglycoside cohort (n = 41), compared to 64% in the polymyxin B (P = 0.02; n = 25), 43% in the tigecycline (P < 0.001; n = 21), and 36% in the untreated (P < 0.001; n = 69) cohorts. Using multivariate analysis, the odds of clearance were lower for the polymyxin B (odds ratio [OR], 0.10; P = 0.003), tigecycline (OR, 0.08; P = 0.001), and untreated (OR, 0.14; P = 0.003) cohorts than for the aminoglycoside cohort. Treatment with an aminoglycoside, when active in vitro, was associated with a significantly higher rate of microbiologic clearance of CRKP bacteriuria than treatment with either polymyxin B or tigecycline.

Molecular epidemiology of carbapenem-nonsusceptible Acinetobacter baumannii in the United States

Acinetobacter baumannii is emerging as an important nosocomial pathogen worldwide. We report molecular epidemiology of 65 carbapenem-nonsusceptible A. baumannii isolates identified from hospitals in New York, Pennsylvania, Florida, Missouri, Nevada, and California between 2008 and 2009. All isolates were subjected to pulsed-field gel electrophoresis (PFGE). Select isolates then underwent multilocus sequence typing (MLST). While the PFGE patterns tended to cluster within each hospital, sequence types (STs) belonging to the clonal complex 92 (CC92) and the pan-European clonal lineage II (EUII; worldwide clonal lineage 2) were predominant in all hospitals. Of them, ST122 and ST208 were the most common and were found in four of the six hospitals. Isolates belonging to the pan-European clonal lineages I and III were identified in one hospital each. Carbapenemase-encoding genes bla(OXA-23) and/or ISAba1-bla(OXA-51-like) were present among the majority of isolates. These findings suggest that carbapenem-nonsusceptible A. baumannii isolates found in U.S. hospitals constitute part of the global epidemic driven by CC92, but have unique STs other than ST92, which may be spreading by means of patient transfer between health care facilities within the United States.

Efficacy and safety of tigecycline for the treatment of infectious diseases: a meta-analysis

BACKGROUND

Multidrug resistance among bacteria increases the need for new antimicrobial drugs with high potency and stability. Tigecycline is one candidate drug, and a previous meta-analysis of only published randomised controlled trials suggested that it might as effective as comparator treatments; we did a meta-analysis to include new and unpublished trials to assess its efficacy for the treatment of adult patients with serious bacterial infection.

METHODS

We searched PubMed, Cochrane Central Register, and Embase up to March 30, 2011, to identify published studies, and we searched clinical trial registries to identify completed unpublished studies, the results of which were obtained through the manufacturer. Eligible studies were randomised trials assessing the clinical efficacy, safety, and eradication efficiency of tigecycline versus other antimicrobial agents for any bacterial infection. The primary outcome was treatment success in patients who received at least one dose of the study drug, had clinical evidence of disease, and had complete follow-up (the clinically assessable population). Meta-analysis was done with random-effects models because of heterogeneity across the trials.

FINDINGS

14 randomised trials, comprising about 7400 patients, were included. Treatment success was lower with tigecycline than with control antibiotic agents, but the difference was not significant (odds ratio 0·87, 95% CI 0·74-1·02). Adverse events were more frequent in the tigecycline group than in the control groups (1·45, 1·11-1·88), with significantly more vomiting and nausea. All-cause mortality was higher in the tigecycline group than in the comparator groups, but the difference was not significant (1·28, 0·97-1·69). Eradication efficiency did not differ between tigecycline and control regimens, but the sample size for these comparisons was small.

INTERPRETATION

Tigecycline is not better than standard antimicrobial agents for the treatment of serious infections. Our findings show that assessment with unpublished studies is needed to make appropriate decisions about new agents.

FUNDING

None.