Diversity in Adaptive Evolution of Methicillin-Resistant Staphylococcus aureus Clinical Isolates Under Exposure to Continuous Linezolid Stress in vitro

Background

Linezolid resistance in methicillin-resistant Staphylococcus aureus (MRSA) was reported frequently in recent years, but the mechanism underlying this process was less reported, especially for clinical isolates with different genetic background. Thus, this study aims to explore the adaptive evolution characteristics underlying linezolid resistance in MRSA clinical isolates exposed to continuous induction stress of linezolid in vitro.

Methods

The in vitro susceptibility of 1032 MRSA clinical isolates to linezolid was detected using commercial VITEK-2 equipment via broth microdilution. MRSA isolates with different minimum inhibitory concentration (MIC) values for linezolid were randomly selected to perform the assay of adaptive laboratory evolution with sub-inhibitory concentrations of linezolid. Polymerase chain reaction assays and sequencing techniques were performed to detect well-known molecular determinants related to linezolid resistance, including the expression of optrA and cfr, mutations of 23S rRNA gene and ribosomal protein (L3, L4, L22) encoding genes (rplC, rplD, rplV).

Results

After induction with sequentially increasing concentrations of linezolid, all four MRSA strains (L914, L860, L1096, and L2875) evolved into linezolid-resistant strains over various induction times (480, 384, 288, and 240 h) and universally formed small colony variants. A new mutation in the domain V region of 23S rRNA gene (C2404T) and one mutation in amino acid sequences of ribosomal protein (Met208Thr) were firstly identified among linezolid-resistant strains. Except G2576T mutations in 23S rRNA gene, the distribution of other mutations (A2451T, T2504A, C2404T, T2500A, G2447T) exhibited obvious strain heterogeneity. Furthermore, as the MIC to linezolid increased, the copy numbers of point mutations in the V region of 23S rRNA gene increased correspondingly.

Conclusion

Strain-specific evolution of resistance to linezolid among MRSA clinical isolates was firstly identified in this study. MRSA isolates with higher MICs for linezolid evolved more easily into resistant ones, which calls for precise monitoring of linezolid resistance levels in patients receiving treatment for MRSA infections with linezolid.

Pharmacodynamics of Linezolid Plus Fosfomycin Against Vancomycin-Resistant Enterococcus faecium in a Hollow Fiber Infection Model

The optimal therapy for severe infections caused by vancomycin-resistant Enterococcus faecium (VREfm) remains unclear, but the combination of linezolid and fosfomycin may be a good choice. The 24-h static-concentration time-kill study (SCTK) was used to preliminarily explore the pharmacodynamics of linezolid combined with fosfomycin against three clinical isolates. Subsequently, a hollow-fibre infection model (HFIM) was used for the first time to further investigate the pharmacodynamic activity of the co-administration regimen against selected isolates over 72 h. To further quantify the relationship between fosfomycin resistance and bacterial virulence in VREfm, the Galleria mellonella infection model and virulence genes expression experiments were also performed. The results of SCTK showed that the combination of linezolid and fosfomycin had additive effect on all strains. In the HFIM, the dosage regimen of linezolid (12 mg/L, steady-state concentration) combined with fosfomycin (8 g administered intravenously every 8 h as a 1 h infusion) not only produced a sustained bactericidal effect of 3∼4 log₁₀ CFU/mL over 72 h, but also completely eradicated the resistant subpopulations. The expression of virulence genes was down-regulated to at least 0.222-fold in fosfomycin-resistant strains compared with baseline isolate, while survival rates of G. mellonella was increased (G. mellonella survival ≥45% at 72 h). For severe infections caused by VREfm, neither linezolid nor fosfomycin monotherapy regimens inhibited amplification of the resistant subpopulations, and the development of fosfomycin resistance was at the expense of the virulence of VREfm. The combination of linezolid with fosfomycin produced a sustained bactericidal effect and completely eradicated the resistant subpopulations. Linezolid plus Fosfomycin is a promising combination for therapy of severe infections caused by VREfm.

Synergistic Combination of Linezolid and Fosfomycin Closing Each Other's Mutant Selection Window to Prevent Enterococcal Resistance

Enterococci, the main pathogens associated with nosocomial infections, are resistant to many common antibacterial drugs including β-lactams, aminoglycosides, etc. Combination therapy is considered an effective way to prevent bacterial resistance. Preliminary studies in our group have shown that linezolid combined with fosfomycin has synergistic or additive antibacterial activity against enterococci, while the ability of the combination to prevent resistance remains unknown. In this study, we determined mutant prevention concentration (MPC) and mutant selection window (MSW) of linezolid, fosfomycin alone and in combination including different proportions for five clinical isolates of Enterococcus and characterized the resistance mechanism for resistant mutants. The results indicated that different proportions of linezolid combined with fosfomycin had presented different MPCs and MSWs. Compared with linezolid or fosfomycin alone, the combination can restrict the enrichment of resistant mutants at a lower concentration. A rough positive correlation between the selection index (SI) of the two agents in combination and the fractional inhibitory concentration index (FICI) of the combination displayed that the smaller FICI of linezolid and fosfomycin, the more probable their MSWs were to close each other. Mutations in ribosomal proteins (L3 and L4) were the mechanisms for linezolid resistant mutants. Among the fosfomycin-resistant mutants, only two strains have detected the MurA gene mutation related to fosfomycin resistance. In conclusion, the synergistic combination of linezolid and fosfomycin closing each other’s MSW could effectively suppress the selection of enterococcus resistant mutants, suggesting that the combination may be an alternative for preventing enterococcal resistance. In this study, the resistance mechanism of fosfomycin remains to be further studied.

Multidrug-Resistant Organisms in the Setting of Periprosthetic Joint Infection-Diagnosis, Prevention, and Treatment

BACKGROUND

Periprosthetic joint infection (PJI) is a rare yet challenging problem in total hip and knee arthroplasties. The management of PJI remains difficult primarily due to the evolution of resistance by the infecting organisms.

METHODS

This review profiles acquired mechanisms of bacterial resistance and summarizes established and emerging techniques in PJI diagnosis, prevention, and treatment.

RESULTS

New techniques in PJI diagnosis and prevention continue to be explored. Antibiotics combined with 1 or 2-stage revision are associated with the higher success rates and remain the mainstay of treatment.

CONCLUSION

With higher prevalence of antibiotic-resistant organisms, novel antibiotic implant and wound care materials, improved methods for organism identification, and well-defined organism-specific treatment algorithms are needed to optimize outcomes of PJI.

Predicting and preventing antimicrobial resistance utilizing pharmacodynamics: Part I gram positive bacteria

INTRODUCTION

Antimicrobial resistance is a potentially inevitable consequence of widespread use of antibiotics in the healthcare system. An enhanced understanding of pharmacodynamic (PD) targets that prevent antimicrobial resistance development will improve currently availably therapies and help to guide future drug development strategies. Current in vitro methods to predict bacterial resistance to antimicrobials consist of serial dilution experiments, determination of the mutant prevention concentration (MPC), mutant selection window (MSW), and human simulated pharmacodynamics studies. Clinical trial data and real -world surveillance studies can help validate or disprove in vitro modeling.

AREAS COVERED

This review will discuss methods of predicting development of resistance and how the use of pharmacodynamics can reduce or eliminate the emergence of resistance among Staphylococcus aureus, Streptococcus pneumoniae, and Enterococcus species.

EXPERT OPINION

Pharmacodynamic targets can be used successfully to guide antimicrobial therapy to prevent resistance development. Currently, PD targets do not take into consideration horizontal resistance gene transfer and various factors may lead to different PD targets based on sites of infection. Further research is necessary to guide future drug development strategies and optimize new drug therapies.

Role and antimicrobial resistance of staphylococci involved in prosthetic joint infections

PURPOSE

Staphylococci are responsible for approximately half of all prosthetic joint infections (PJIs) and they are often multi-drug resistant. The main purpose of this study was to evaluate the incidence of PJIs caused by staphylococci in our hospital from March 2010 to February 2012, with particular reference to antibiotic resistance in relation to their classification as contaminant or infecting isolates.

METHODS

We analyzed samples recovered from 124 patients: most of them were male (55.8%) and the mean age was 66 ± 14 years. Prostheses derived from hip (54.8%) or knee (45.2%) replacement and they were processed by sonication. Isolates were identified using conventional biochemical methodologies. Antimicrobial susceptibility testing was carried out using the disk diffusion method as described by the European Committee on Antimicrobial Susceptibility Testing.

RESULTS

A total of 135 staphylococci were isolated: the prevalent species was Staphylococcus aureus, but, on the whole, coagulase-negative staphylococci represented 57% of cases. Fifty-one isolates were recovered from a single sample and were therefore defined as contaminant. Linezolid and glycopeptides showed excellent activity versus all the tested isolates, while penicillin, levofloxacin, and erythromycin offered reduced activity against staphylococci. Interestingly, high resistance rates were observed for coagulase-negative staphylococci other than S. epidermidis classified as contaminant strains.

CONCLUSIONS

We observed a remarkable spread of coagulase-negative staphylococci as causative agents of PJIs; but most of them were classified as contaminants. However, because of their low susceptibility to the antibiotics tested, further studies are necessary to evaluate their role as pathogens or as true contaminants.

Pathogens resistant to antibacterial agents

Resistance to antimicrobial drugs is increasing at an alarming rate among both gram-positive and gram-negative bacteria. Traditionally, bacteria resistant to multiple antimicrobial agents have been restricted to the nosocomial environment. A disturbing trend has been the recent emergence and spread of resistant pathogens in nursing homes, in the community, and in the hospital. This article reviews the epidemiology, molecular mechanisms of resistance, and treatment options for pathogens resistant to antimicrobial drugs.

In vitro selection of resistance in Escherichia coli and Klebsiella spp. at in vivo fluoroquinolone concentrations

BACKGROUND

Fluoroquinolones are potent antimicrobial agents used for the treatment of a wide variety of community- and nosocomial- infections. However, resistance to fluoroquinolones in Enterobacteriaceae is increasingly reported. Studies assessing the ability of fluoroquinolones to select for resistance have often used antimicrobial concentrations quite different from those actually acquired at the site of infection. The present study compared the ability to select for resistance of levofloxacin, ciprofloxacin and prulifloxacin at concentrations observed in vivo in twenty strains of Escherichia coli and Klebsiella spp. isolated from patients with respiratory and urinary infections. The frequencies of spontaneous single-step mutations at plasma peak and trough antibiotic concentrations were calculated. Multi-step selection of resistance was evaluated by performing 10 serial cultures on agar plates containing a linear gradient from trough to peak antimicrobial concentrations, followed by 10 subcultures on antibiotic-free agar. E. coli resistant strains selected after multi-step selection were characterized for DNA mutations by sequencing gyrA, gyrB, parC and parE genes.

RESULTS

Frequencies of mutations for levofloxacin and ciprofloxacin were less than 10-11 at peak concentration, while for prulifloxacin they ranged from <10-11 to 10-5. The lowest number of resistant mutants after multistep selection was selected by levofloxacin followed by ciprofloxacin and prulifloxacin. Both ciprofloxacin- and prulifloxacin-resistant mutants presented mutations in gyrA and parC, while levofloxacin resistance was found associated only to mutations in gyrA.

CONCLUSIONS

Among the tested fluoroquinolones, levofloxacin was the most capable of limiting the occurrence of resistance.