Correlations Between The SCCmec-associated Psm-mec Gene, Biofilm Formation Ability,and Antibiotic Resistance Of Blood-derived Staphylococcus Hominis

ObjectiveStaphylococcus hominis is an important causative agent of bloodstream infection with a gradually increased incidence in the last decade. It is difficult to completely cure such infections due to the multiple antimicrobial resistance and biofilm formation. The aim of this study was to characterize biofilm formation ability and antibiotic resistance of Staphylococcus hominis, and to analyze the correlations between psm-mec gene, biofilm formation and antibiotic resistance. This study would provide the basis for prevention and control of nosocomial infections caused by Staphylococcus hominis.Methods1. Staphylococcus hominis were isolated from positive blood culture and identified by automatic microbiological identification system.2. Minimal inhibitory concentrations (MICs) of 16 antibiotics were determined by the agar dilution method. The mecA gene was detected by PCR for differentiation of MRSHo and MSSHo, and differences in antimicrobial susceptibility between MRSHo and MSSHo were analyzed.3. Microtite Plate Assay (TCP) was used to evaluate boifilm formation capacity of Staphylococcus hominis, and the differences in antimicrobial susceptibility between biofilm forming and non-biofilm strains were analyzed.4. psm-mec and fudoh were amplified by PCR, and DNA sequencing offudoh was performed to measure the mutant of psm-mec. Correlations between psm-mec, biofilm formation and antibiotic resistance were analyzed.5. Two pair special PCR primers, mecRl/psm-mec and psm-mec/xylR, were used to analyze the genetic location characteristic of psm-mec in SCCmec.6. SCCmec types of psm-mec positive stains were detected using a special multiplex PCR assay.7. mec and ccr types were measured by two PCR assays to analyze the characteristic of mec and ccr complex.Results1. Fifty-five Staphylococcus hominis strains were isolated from positive blood cultures.2. Antimicrobial susceptibility tests demonstrated that the isolated Staphylococcus hominis strains were very sensitive to linezolid, nitrofurantion, quinupristin/dalfopristin, vancomycin and tigecycline, while the resistance rates for other antibiotics were oxacillin (85.45%), penicillin (92.73%), erythromycin (87.27%), trimethoprim/sulphamethoxazole(65.45%), clindamycin (67.27%), ciprofloxacin (47.27%), lavofloxacin (47.27%), moxifloxacin (34.55%), tetracycline (41.82%), rifampicin (16.36%) and gentamicin (10.91%), respectively.3. Among these strains,46 (83.64%) isolates were MRSHo and 9 isolates were MSSHo. There were significant differences in antimicrobial susceptibility between MRSHo and MSSHo. For ciprofloxacin, lavofloxacin, moxifloxacin, penicillin, tetracycline, trimethoprim/sulphamethoxazole, MRSHo had a higher resistance rates than MSSHo (P<0.05).4. Results of TCP from the 55 Staphylococcus hominis isolates indicated that 35 isolates were biofilm producers (63.6%), and the rest isolates were negative (36.4%).5. There was no significant difference in antimicrobial susceptibility between biofilm forming and non-biofilm strains (P>0.05).6. The detection rate of psm-mec gene was 39.13% (18/46),18 psm-mec positive strains were positive for fudoh gene, and the results of fudoh DNA sequencing were completely matched with psm-mec ORF by Blast.7. Fifteen psm-mec positive strains (83.33%) had the ability of biofilm formation, higher than psm-mec negative strains. Two out of three psm-mec positive non-biofilm strains had a mutation (G>A) located at-12 upstream of psm-mec ORF. psm-mec positive strains were more likely to be resistant to β-lactams, tetracycline and trimethoprim/sulphamethoxazole, the differences were significant (P<0.05).8. All of the 18 psm-mec positive strains were mecRl/psm-mec and psm-mec/xylR positive.9. The results of multiplex PCR from psm-mec positive strains showed that,2 strains belonged to SCCmec Ⅲ,7 strains belonged to SCCmecIII-like, and 9 strains belonged to new SCCmec types.10. Among psm-mec positive isolates,18 (100%) isolates harbored class A mec complex,7 isolates were not positive for any ccr complex,6 isolates contained ccr type 1,2 isolates contained ccr type 1+2,1 isolate contained ccr type 1+3, and 2 isolates contained ccr type 3.Conclusions1. The analyzed Staphylococcus hominis isolates have a high prevalence of methicillin resistance and multiple resistance. MRSHo were likely to resistant to ciprofloxacin, lavofloxacin, moxifloxacin, penicillin, tetracycline, trimethoprim/sulphamethoxazole.2. The Staphylococcus hominis isolates have strong biofilm formation ability, but there was no significant difference in antimicrobial susceptibility between biofilm forming and non-biofilm strains.3. SCCmec-associated psm-mec gene extensively exists in MRSHo isolated from positive blood culture, and mainly distributes in SCCmecIII, SCCmecⅢ-like and new SCCmec types, locates between mecRl and xylR gene.4. All of the psm-mec positive locates are carried by class A mec complex, the genetic diversity of ccr complex may lead the variety of SCCmec types.5. The psm-mec positive strains have a very high possibility of biofilm formation, and the psm-mec positive strains are more likely to be resistant to β-lactams, tetracycline and trimethoprim/sulphamethoxazole. Probably, psm-mec gene has a correlation with biofilm.