[The incidence and risk factors for heterogeneous vancomycin intermediate Staphylococcus aureus]

To investigate the prevalence of heterogeneous vancomycin intermediate Staphylococcus aureus (hVISA) and the sensitivity of hVISA to novel antibiotics, and to explore the risk factors and infection attributable mortality associated with hVISA infection.

A total of 456 methicillin resistant Staphylococcus aureus (MRSA) isolates were isolated in Zhongshan Hospital from January, 2008 to November, 2010. All MRSA isolates were investigated for hVISA by two agar screening methods BHIA5T (brain-heart infusion containing teicoplanin 5 mg/L) or BHIA6V (brain-heart infusion containing vancomycin 6 mg/L), as well as macroEtest method (MET). Possible hVISA isolates were tested by modified population analysis profile-area under the curve (PAP-AUC). The minimal inhibitory concentrations (MICs) of vancomycin, teicoplanin and linezolid were determined by microbroth dilution as recommended by Clinical Laboratory Standards Institute (CLSI). The contribution difference between hVISA and vancomycin susceptible Staphylococcus aureus (VSSA) in different MIC range was compared. A retrospective case-control study of the patients with hVISA infection or VSSA infection was carried out and statistical analysis was performed using t test, Mann-Whitney test, χ(2) test and Fisher exact test.

A total of 105 isolates of hVISA were screened by BHIA5T and BHIA6V (23.0%) with other 23 isolates by MET (5.0%) and 21 by PAP-AUC (4.6%). All isolates were 100% sensitive to vancomycin, teicoplanin and linezolid. The vancomycin MIC [(1.76 ± 0.16) mg/L] in hVISA group was significantly higher than that in VSSA group [(1.09 ± 0.07) mg/L, P < 0.01], which was a potential risk factor for hVISA infection. The retrospective study showed chronic obstructive pulmonary disease (COPD) was also a risk factor for hVISA infection of the lower respiratory tract. No significant difference in infection attributable mortality was showed between the hVISA group and the VSSA group.

The overall prevalence of hVISA in Zhongshan Hospital is estimated as 4.6%, while the prevalence of hVISA isolated from blood is as high as 12.5%. All isolates are 100% sensitive to vancomycin and linezolid. COPD is a risk factor for hVISA infection of the lower respiratory tract.