Yu-lin Zhu

Surveillance of antimicrobial susceptibility patterns among Shigella species isolated in China during the 7-year period of 2005-2011.

Background Shigella is a frequent cause of bacterial dysentery in the developing world. Treatment with antibiotics is recommended for shigellosis, but the options are limited due to globally emerging resistance. This study was conducted to determine the frequency and pattern of antimicrobial susceptibility of Shigella in China. Methods We studied the antimicrobial resistance profiles of 308 Shigella spp. strains (260 S. flexneri, 40 S. sonnei, 5 S. boydii, and 3 S. dysenteriae) isolated from fecal samples of patients (age, from 3 months to 92 yr) presenting with diarrhea in different districts of Anhui, China. The antimicrobial resistance of strains was determined by the agar dilution method according to the CSLI guidelines. Results The most common serogroup in the Shigella isolates was S. flexneri (n=260, 84.4%), followed by S. sonnei (n=40, 13.0%). The highest resistance rate was found for nalidixic acid (96.4%), followed by ampicillin (93.2%), tetracycline (90.9%), and trimethoprim/sulfamethoxazole (80.8%). Among the isolates tested, 280 (91.0%) were multidrug resistant (resistant to ≥2 agents). The most common resistance pattern was the combination of ampicillin, tetracycline, and trimethoprim/sulfamethoxazole (70.8%). Resistance to ampicillin and tetracycline were more common among S. flexneri than among S. sonnei isolates. Conclusions S. flexneri is predominant in Anhui, China, and its higher antimicrobial resistance rate compared with that of S. sonnei is a cause for concern. Continuous monitoring of resistance patterns is necessary to control the spread of resistance in Shigella. The recommendations for antimicrobial treatment must be updated regularly based on surveillance results.

Detection of plasmid-mediated quinolone resistance determinants and the emergence of multidrug resistance in clinical isolates of Shigella in SiXian area, China

A total of 123 Shigella isolates were collected from SiXian area in Anhui, China. Screening was carried out by polymerase chain reaction (PCR) amplification of plasmid-mediated quinolone resistance (PMQR) determinants. Different β-blactamases genes, plasmid-borne bla(AmpC), 16S rRNA methylase genes, integrons, and mutations in quinolone resistance-determining regions were analysed by PCR for the PMQR-positive isolates.

Comparative study of the mutant prevention concentrations of vancomycin alone and in combination with levofloxacin, rifampicin and fosfomycin against methicillin-resistant Staphylococcus epidermidis

No mutant-prevention concentration (MPC) with methicillin-resistant Staphylococcus epidermidis (MRSE) has been reported. The study aimed to evaluate the propensity of vancomycin individually and in combination to prevent MRSE from mutation. A total of 10 MRSE clinical isolates were included in the study. Susceptibility testing demonstrated that the susceptibility rates to vancomycin, rifampicin, levofloxacin and fosfomycin were 100, 100, 50 and 90%, respectively. The fractional inhibition concentration indices (FICI) for vancomycin combined with rifampicin, levofloxacin or fosfomycin were ⩾1.5 but ⩽2, ⩾1.5 but ⩽2, and >0.5 but ⩽1.5, respectively, implying indifferent interactivity. The MPC with susceptible strains was determined to be the lowest antibiotic concentration inhibiting visible growth among 1010 CFU on four agar plates (9 cm in diameter) after a 72-h incubation at 37 °C. The MPCs were 16∼32, >64, ⩾64 and 4∼16 μg ml−1 for vancomycin, rifampicin, fosfomycin and levofloxacin, respectively. The vancomycin MPCs of combinations with fosfomycin (32 μg ml−1), levofloxacin (2 μg ml−1) and rifampicin (2 or 4 μg ml−1) were 1∼4, 16∼32 and 16∼32 μg ml−1, respectively. Against mutants selected by vancomycin, rifampicin, levofloxacin and fosfomycin individually, antibiotics had standard MICs of 2∼4 μg ml−1 for vancomycin, >64 μg ml−1 for rifampicin, 4∼8 μg ml−1 for levofloxacin and ⩾64 μg ml−1 for fosfomycin. Thus single-step mutation can lead to resistance of MRSE to rifampicin, levofloxacin and fosfomycin, rather than non-susceptibility to vancomycin. Vancomycin–fosfomycin combination might be a superior alternative to vancomycin in blocking the growth of MRSE mutants, especially for high-organism-burden infections.

Prevalence of plasmid-mediated quinolone resistance determinants in association with β-lactamases, 16S rRNA methylase genes and integrons amongst clinical isolates of Shigella flexneri.

Shigellosis remains a public health concern throughout the world (Kotloff et al., 1999). However, the emergence of multidrug resistance (MDR) amongst clinical isolates has made the selection of effective antimicrobial therapy more difficult (Niyogi, 2007). Quinolones are among the most important antibacterial agents used extensively for the treatment of shigellosis. Recently, quinolone resistance has been rising among Shigella isolates (Xiong et al., 2010). Quinolone resistance mainly results from chromosomal mutations in the quinolone resistance-determining regions (QRDRs) of DNA gyrase and topoisomerase IV (Dutta et al., 2005). However, since 1998, three kinds of plasmid-mediated quinolone resistance (PMQR) determinants have been described: Qnr, Aac(69)-Ib-cr and QepA. These mechanisms are prevalent amongst common clinical isolates and have been detected conferring low-level resistance to quinolones (Cattoir & Nordmann, 2009). Co-existence of resistance genes, such as blactamase genes, 16S rRNA methylase genes and integrons, on the same plasmid could, in part, explain the appearance of MDR strains. These responsible genes are primarily located on transferable plasmids and could enhance the acquisition and dissemination of antimicrobial resistance genes by horizontal transfer (Luo et al., 2011; Hu et al., 2011). We investigated the prevalence of PMQR determinants in clinical isolates of Shigella flexneri, and assessed the relatedness of b-lactamase genes, 16S rRNA methylase genes and integrons with PMQR determinants within the same strain.

Use of Monte Carlo simulation to evaluate the development of vancomycin resistance in meticillin-resistant Staphylococcus aureus

Monte Carlo simulations were performed for various vancomycin dosage regimens to evaluate the potential for development of vancomycin resistance in meticillin-resistant Staphylococcus aureus (MRSA). When the target of free AUC(24)/MIC≥200 was considered (where AUC(24) is the area under the drug concentration-time curve in a 24-h interval and MIC is the minimum inhibitory concentration), a standard dose regimen (1000 mg every 12 h) yielded unacceptable simulated outcomes in patients with normal renal function; in particular, the probability of target attainment (PTA) was only 30.5% at an MIC of 1mg/L. For the same dosage regimens and the mutant prevention concentration (MPC)-based pharmacokinetic target (total AUC(24)/MPC>15), the cumulative fraction of response exceeded 80% for all renal function strata; low values of PTA (<80%) were obtained only for isolates with MPCs of ≥22.4 mg/L, which consisted of all 21 strains of heterogeneous vancomycin-intermediate S. aureus (hVISA) and a handful of non-hVISA strains with MICs of 2mg/L (32%; 16/50). Based on the current status of vancomycin resistance, we conclude that total AUC(24)/MPC>15, derived from in vivo experiments, is more suitable to predict the development of vancomycin resistance. In clinical practice, individualised vancomycin therapy should be considered to minimise selection of resistance mutations.

16S rRNA methylase gene, together with quinolone resistance determinant aac(6')-Ib-cr, in a Serratia marcescens isolate from China.

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Mechanisms of antimicrobial resistance in Serratia marcescens

Serratia marcescens, one of the Enterobacteriaceae, was considered originally to be non-pathogenic. But in recent decades, nosocomial infections caused by this organism are often hard to treat because of both the intrinsic resistance of this species and its abilities to acquire further resistance to multiple groups of antimicrobial agents, including β-lactams, aminoglycosides and fluoroquinolones. S. marcescens have their own ways to become resistant in common with other Enterobacteriaceae: production of β-lactamases including extended-spectrum β-lactamases, AmpC-type cephalosporinase and carbapenemases; diminished outer membrane permeability; modification of the target site-PBPs; overexpression of active efflux systems; synthesis of aminoglycoside-modifying enzymes; and structural alterations of the GyrA protein. This review will show a brief history, the virulence factors and the known resistance mechanisms to the most frequently administrated antimicrobial agents: βlactams, aminoglycosides and fluoroquinolones in S. marcescens.

ACT-6, a novel plasmid-encoded class C β-lactamase in a Klebsiella pneumoniae isolate from China

The purpose of this study was to investigate the phenotypic and molecular characterization of a novel plasmid-mediated AmpC-type β-lactamase in Klebsiella pneumoniae E701 isolated from Anhui province in China. In comparison with the ACT-1, sequence analysis revealed that there were 43 point mutations in the coding gene, and 10 of which led to amino-acid substitution. Resistance could be transferred by conjugation or transformation with plasmid DNA into E. coli JM109, which was due to the production of a β-lactamase with an isoelectric point of 8.4 named ACT-6. Cloning, expression, purification and kinetics were carried out to study the characterization of the novel AmpC-type β-lactamase. The results of MIC determinations and substrate profiles showed there was no significant difference in the activities of the novel enzyme and ACT-1. Moreover, the class 1 integron and the whole open reading frame of the novel AmpC-type β-lactamase from K.pneumoniae E701 were detectable in the same size plasmid. This is the first report on the emergence of the novel ACT-6 type β-lactamases in K. pneumoniae.

Vancomycin MICs of the resistant mutants of S. aureus ATCC43300 vary based on the susceptibility test methods used

Clinical failures with vancomycin against meticillin-resistant Staphylococcus aureus (MRSA) infections have challenged vancomycin’s standing as a first-line antimicrobial for these infections. Conventional MIC tests were not predictive of the in vivo therapeutic effect of vancomycin. Thus, we tested the susceptibility for the resistant mutants in the mutant selection window of S. aureus ATCC43300 (a MRSA strain) by three different MIC-testing methods in this paper. The MIC of vancomycin was estimated at 2 μg ml−1 on the Mueller–Hinton agar (MHA) plate only for the resistant mutant that was selected from the plate of vancomycin concentration 12 μg ml−1. The obvious changes of susceptibility testing were found between the resistant mutants and S. aureus ATCC43300 on the Brain–Heart Infusion Agar (BHIA) plates. There were subtle changes in the MIC trend within the susceptible range with the result of Etest for the resistant mutants. The susceptibility for the subcultures of resistant mutants would fall back when the external drug environment disappeared. In comparison with the S. aureus ATCC43300, sequence analysis revealed that there were no mutations in the staphylococcal protein A (spa) sequencing of the resistant mutants. The spa tape is t421 for all isolates.