Expert Consensus on Polymyxin Antimicrobial Susceptibility Testing and Clinical Interpretation.

The polymyxins are important antimicrobial agents against antibiotic-resistant gram-negative bacilli. In 2020, the Clinical and Laboratory Standards Institute modified the clinical breakpoints for polymyxin susceptibility test by eliminating the "susceptible" interpretive category, only reporting intermediate (≤2 mg/L) and resistant (≥4 mg/L). However, the European Committee on Antimicrobial Susceptibility Testing recommended the use of clinical breakpoints of ≤2 mg/L as susceptible and >2 mg/L as resistant. The first-line laboratorians and clinicians in China have been perplexed by the inconsistence of international polymyxin clinical breakpoints and discouraged by the difficulty of conducting polymyxin susceptibility testing. Therefore, it is urgently needed to make it clear for the laboratorians in China to know how to accurately carry out polymyxin susceptibility testing and standardize the interpretation of susceptibility testing results. To this end, the experts from relevant fields were convened to formulate this consensus statement on the testing and clinical interpretation of polymyxin susceptibility. Relevant recommendations are proposed accordingly for laboratorians and clinicians to streamline their daily work.

Prevalence of clinical meticillin-resistant Staphylococcus aureus (MRSA) with high-level mupirocin resistance in Shanghai and Wenzhou, China.

A total of 803 clinical meticillin-resistant Staphylococcus aureus (MRSA) isolates obtained from Shanghai and Wenzhou in China were subjected to a screening test by disk diffusion for detection of mupirocin resistance. Among the 803 strains, 53 (6.6%) were mupirocin-resistant. Of these 53 strains, all were discovered by the agar dilution method and polymerase chain reaction (PCR) to be high-level mupirocin-resistant and to harbour the mupA gene. Plasmid DNA hybridisation and curing experiments disclosed that mupA was located on a large plasmid varying in size between 23.0kb and 52.4kb in all strains. Susceptibility testing of 10 antibiotics revealed that resistance rates between the Shanghai isolates and the Wenzhou isolates to trimethoprim/sulfamethoxazole and rifampicin differed significantly. Molecular typing by pulsed-field gel electrophoresis (PFGE), staphylococcal chromosomal cassette mec (SCCmec) and staphylococcal protein A (spa) revealed that PFGE A-SCCmec IIIA-spa t030 and PFGE B-SCCmec IIIA-spa t030 represented all of the Wenzhou strains, whereas PFGE N-SCCmec I-spa t318, PFGE P-SCCmec III-spa t037, PFGE I-SCCmec III-spa t037 and PFGE M-SCCmec IIIA-spa t002 were the predominant profiles among Shanghai isolates. These findings indicated that high-level mupirocin resistance mediated by plasmids prevailed in the clinical mupirocin-resistant MRSA from Shanghai and Wenzhou and was mainly related to the transmission of clones.

Coexistence of qnrB4 and qnrS1 in a clinical strain of Klebsiella pneumoniae.

AIM: To identify the location and the relationship, and to analyze the genetic background of 2 plasmid-mediated quinolone resistance genes, qnrB4 and qnrS1, carried by a clinical strain of Klebsiella pneumoniae (K pneumoniae). METHODS: The plasmids carrying qnrB4 or qnrS1 were identified by Southern blotting. A HindIII fragment containing qnrB4 or qnrS1 was cloned into plasmid puc18 and sequenced. RESULTS: qnrB4 and qnrS1 were located on 2 different plasmids, pHS7 and pHS8, and were 180 and 45 kb in size, respectively. A transconjugant carrying plasmid pHS7 bearing qnrB4 and another transconjugant carrying pHS9 bearing qnrB4 and qnrS1 were obtained by conjugation. Plasmid pHS8 bearing qnrS1 was also transferred to J53 by transformation. The ciprofloxacin minimal inhibitory concentrations (MIC) for J53 transconjugants or the transformant carrying qnrB4 only, qnrS1 only, and both qnrB4 and qnrS1 were 0.19, 0.25, and 0.25 mg/L, respectively, while the parent clinical strain of K pneumoniae had a MIC of 0.75 mg/L. qnrB4 was located in a sul1-type integron with blaDHA-1, ampR and psp genes in upstream and insertion sequence IS26, and sap genes in downstream of qnrB4. qnrS1 was not located in an integron, but IS26 was found both upstream and downstream, and IS2 was found directly upstream of qnrS1. CONCLUSION: qnrB and qnrS can be harbored simultaneously by a single clinical strain of K pneumoniae. These 2 genes are carried by 2 different plasmids and have different genetic environments in plasmid DNA structure.

[Changes of antimicrobial resistance among clinical isolates of Escherichia coil in Shanghai 1990-2004].

OBJECTIVE: To investigate the trend of resistance to antimicrobial agents among clinical isolates of Escherichia coli 1990-2004. METHODS: Agar diffusion test was used to analyze the changes of drug susceptibility of 33,495 strains of E. coli isolated from 11 hospitals in Shanghai to 21 antimicrobial agents 1990-2004. RESULTS: The resistance rates of 33,495 E.coli isolates to 21 antimicrobial agents mostly increased 1990-2004. The resistance rates to ampicillin and piperacillin increased from 69% and 30% to 85% and 71.4% respectively. The resistance rates to cephalosporins, except ceftazidime and cefepime, all increased, e. g., the resistance rates to cefazolin (24.0%-->48.3%), cefuroxime (18.0%-->45.7%), and cefaclor (33.3%-->46.8%), especially that to cefotaxime (6.0%-->35.2%). The resistance rate to fluoroquinolones increased from 11.0% to 55.4%. The resistance rate to gentamicin increased from 44.0% to 54.0%. The resistance rates to tetracycline, chloramphenicol, SMZ/TMP remained at high levels. However, ceftazidime, cefepime, imipenem, amikacin, beta-lactams/beta-lactamase inhibitors, and nitrofurantoin remained active against the E.coli isolates. The detectable rate of extended-spectrum beta-lactamase-producing strains in E. coli increased from 14.7% to 36.5%. CONCLUSION: The trend of resistance of E. coli to commonly used antimicrobials was upward 1990-2004.

In vitro activities of tigecycline against clinical isolates from Shanghai, China.

To evaluate the in vitro activity of tigecycline, the minimum inhibitory concentrations (MICs) of tigecycline against 1,201 strains of recent clinical isolates from 10 hospitals in Shanghai, China were determined and compared with selected comparators. Results showed that tigecycline had broad-spectrum antimicrobial activity. It was highly active against Gram-positive cocci, including methicillin-resistant Staphylococcus spp., penicillin-intermediate Streptococcus pneumoniae, Enterococcus faecalis and E. faecium. Tigecycline also had good activity against most strains of Enterobacteriaceae, Haemophilus influenzae, Neisseria gonorrhoeae, and Moraxella catarrhalis. However, it was poorly active against Acinetobacter baumannii and Pseudomonas aeruginosa. Tigecycline was highly active against anaerobic Gram-positive cocci such as Peptococcus spp. The in vitro activity of tigecycline was significantly better than that of minocycline and tetracycline. It was as active as or slightly more active than vancomycin and teicoplanin in the activity against resistant aerobic Gram-positive cocci. Tigecycline was bactericidal against all Gram-positive cocci tested except Enterococcus spp. Inoculum size but not pH of medium or concentration of human serum in broth had significant effect on the in vitro activity of tigecycline. Aged media (48-72 hours after preparation) used in the test and specific resistance problem in China may have some effects on MIC values of tigecycline.