[The correlation between expression level of femB and resistance phenotype of methicillin-resistant Staphylococcus aureus (MRSA)].

OBJECTIVE: To investigate the correlation between the phenotype and expression level of femB of Staphylococcus aureus (MRSA), to discuss the mechanism of different phenotypes in methicillin-resistant Staphylococcus aureus. METHODS: The minimal inhibitory concentrations (MICs) of oxacillin against 71 clinical isolates of Staphylococcus aureus were determined by agar dilution method according to NCCLS. The production of beta-lactamase was identified by Cefinase paper strip method. The isolation rate of beta-lactamase-producing strains was counted and the correlation between the resistance phenotype and isolation rate of beta-lactamase was analysed by statistics. Real time fluorescent quantitative PCR was used to quantify the mRNA expression of femB of non-beta-lactamase-producing strains. RESULTS: The resistance rate of 71 Staphylococcus aureus to oxacillin was 66.20% (47/71), the isolation rate of beta-lactamase-producing MSSA strains was 58.3%,and that of strains of high- and low-level resistance to oxacillin were 63.15% and 55.56%. The standard curve was performed by series dilution of the heterogeneous resistant strain BB270, and the amount of femB-specific mRNA in strain BB270 was set to be 1. The calculated femB amounts in MSSA strains were from 0.4830-3.3636, while the amounts were from 0.4204-3.3636 in low-level MRSA strains, and 0.0718-16.0000 in high-level MRSA strains. There were no difference in the level of femB among MSSA, high-level MRSA and low-level MRSA. CONCLUSION: The expression level of femB may not be related to the resistance of non-beta-lactamase-producting Staphylococcus aureus to methicillin.

[Study of detection and homology analysis of the 16S rRNA methylase in 374 enterobacteriaceae clinical isolates].

OBJECTIVE: To investigate the prevalence of genes encoding 16S rRNA methylase and the spreading path of these genes in 374 clinical isolate of enterobacteriaceae. METHODS: The genes encoding 16S rRNA methylase in 374 clinical isolate of enterobacteriaceae was detected with PCR. The sequence homogeny analyses were carried out with the NCBL BLAST program and enterbacterial repetitive intergenic consensus PCR (ERIC-PCR). The conjugation experiments were used to determine the transference of 16S rRNA methylase in vitro. RESULTS: In 374 clinical isolates, methylase genes were detected in 24 strains (6.41%), including 10 armA gene positive strains and 10 rmtB gene positive strains, and 4 strains with both genes positive. Highly homogenous strains were confirmed by ERIC-PCR. 45.8% (11/24) of the conjunction experiments for these strains were positive. CONCLUSION: The aminoglycoside resistance in enterobacteriaceae was concerned related to 16S rRNA methylase. The 16S rRNA methylase transferred either by clone or by plasmids horizontal spreading.

[Association of genotype of OXA-carbapenemases of Acinetobacter baumannii and antibiotic resistance].

OBJECTIVE: To determine the association of the antibiotic resistance of clinical isolates of Acinetobacter baumannii and the genotype of OXA-carbapenemases. METHODS: One hundred and twenty Acinetobacter baumannii strains were isolated from clinical specimens in the West China Hospital from January 2005 to June 2006. The MICs of 12 common antimicrobial agents were determined by 2-fold agar dilution method followed by NCCLS recommendations. The bla(OXA-23) and bla(OXA-24) of those with resistance to IMP were amplified by PCR. RESULTS: The resistant rates of the 120 isolates to 11 common antimicrobial agents exceeded 50% except for IMP (44.4%). One hundred and ten strains were resistant to more than three common antimicrobial agents, with a resistant rate of 91.67%. Of the 50 strains resistant to IMP, 13 stains carried bla(OXA-23). No bla(OXA-24) was found in the resistant isolates. CONCLUSION: Multiple antibiotic resistances are common in Acinetobacters baumannii isolated in the West China Hospital. OXA-23-type carbapenemase is the major carbapenemase that contributes to the nosocomial infection of Acinetobacters baumannii.

[Study on the correlation between resistance phenotype and expression level of femA of Staphylococcus aureus].

OBJECTIVE: To detect the expression level of femA of Staphylococcus aureus strains with different phenotype. METHODS: 15 strains of the non-beta-lactamase-producing clinical isolates with different phenotype by agar dilution and by nitrocephin paper strip method were chosen as the object of test, in addition to 4 donative strains (BB270, BB308, BB586, COL). Total RNA were extracted and analysed by agarose gel electrophoresis. Real time fluorescent quantitative PCR was performed to quantify the expression of femA gene. The expression level of femA of BB270 was set to be standard(100%). RESULTS: The expressions of femA were observed in all the tested strains. The amount of femA-specific mRNA in the mutant strain BB308 was approximately 37.82% and that of stain BB586 was 240.50%, homogeneous resistant strain COL was 862.61%. The amounts in MSSA strains were from 0.00353% to 29.92%, that in low-level MRSA strains were from 0.00554% to 310%, otherwise that in high-level MRSA strains were from 13.88% to 55000%, which were different among these groups. There was no significant difference in amount of femA-mRNA between MSSA and low-level MRSA strains (P1 = 0.83) but marked between high-level MRSA and low-level MRSA/MSSA strains (P2 = 0.006, P3 = 0.01)). CONCLUSION: Expression level of femA in high-level MRSA was significant higher than that in low-level MRSA and MSSA. femA was essential for the expression of high-level methicillin resistance in MRSA.

[A study on the mechanism of drug resistance in Staphylococcus aureus].

OBJECTIVE: To inquire into the mechanism of drug resistance in Staphylococcus aureus. METHODS: A total of 198 strains of Staphylococcus aureus were isolated from the samples sent to the Clinical Laboratory of Microbiology,West China Hospital. The resistance of Staphylococcus aureus to Methicillin was assayed with agar dilution. Staphylococcus aureus mecA gene was measured by PCR assay and beta-lactamase was detected by Nitrocephin. RESULTS: The rate of resistance to methicillin was 64.65% in 198 strains of Staphylococcus aureus; 118 strains of methicillin-resistant staphylococcus aureus(MRSA) were found to have high level resistance in 128 MRSA;10 strains of MRSA were found to have low level resistance; 41(58.57%) strains of methicillin-sensitive Staphylococcus aureus (MSSA) expressed beta-lactamase; 2 Staphylococcus aureus had mecA among them; 67 Staphylococcus aureus expressed beta-lactamase in high level resistance, 63(53.39%)Staphylococcus aureus expressed beta-lactamase in high level resistance, among them, 5 Staphylococcus aureus had mecA; 40.00% MRSA expressed beta-lactamase in low level resistance, 55 MRSA did not express beta-lactamase in high level resistance, which had all mecA; 9 Staphylococcus aureus did not express beta-lactamase in low level resistance, among them, 5 Staphylococcus aureus had mecA. The difference in expression of beta-lactamase was statistically significant between MSSA and MRSA; MRSA(53.39%) was lower than MSSA (58.57%); the other differences were not significant. The difference in having mecA was statistically significant between MRSA(having high resistant level and no expression of beta-lactamase) and the others; MRSA had higher mecA than did the others. CONCLUSION: The resistance in Staphylococcus aureus mainly involved two mechanisms: the expression of beta-lactamase and the expression of mecA.

[Surveillance on antibiotic resistance of Stenotrophomonas maltophilia in Chengdu and Chongqing area].

OBJECTIVE: To survey the antibiotic resistance of Stenotrophomonas maltophilia in Chengdu and Chongqing area and guide the rational antibiotics usage in the treatment of Stenotrophomonas maltophilia infection. METHODS: Minimal inhibitory concentrations (MICs) of 9 antibiotics against Stenotrophomonas maltophilia were measured using two-fold agar dilution method. RESULTS: A total of 154 strains of Stenotrophomonas maltophilia are multi-drug resistant. But the resistant ratios of trimethoprim-sulfamethoxazole, ticarcillin-clavavulanic acid and fluoroquinolones are lower; especially, new fluoroquinolones have stronger antimicrobial activities. CONCLUSION: The rate of isolating Stenotrophomonas maltophilia strains from clinical samples has been rising. In the therapy of Stenotrophomonas maltophilia infection, trimethoprim-sulfamethoxazole or fluoroquinolones is empirically the medicine of choice. For the treatment of serious infection, the administration of trimethoprim-sulfamethoxazole combined with ticarcillin-clavavulanic acid or new fluoroquinolones is rational.