Glycopeptide susceptibility profiles of nosocomial multiresistant Staphylococcus haemolyticus isolates

Abstract

We investigated 48 Staphylococcus haemolyticus isolates from patients and medical staff in terms of susceptibility to and in-vitro selection for vancomycin and teicoplanin in regard to their antibiotypes. On comparison of multiresistant S. haemolyticus isolates with non-multiresistant isolates, the geometric mean minimum inhibitory concentration (MIC) of vancomycin for multiresistant S. haemolyticus was 2.9 microg/ml, and that of teicoplanin was 18.0 microg/ml, both of which values were significantly greater than the corresponding mean MICs of vancomycin (2.0 microg/ml) and teicoplanin (4.7 microg/ml) for nonmultiresistant isolates. After agar selection, the mean of the highest teicoplanin concentration of selected plates for multiresistant S. haemolyticus was 97.1 microg/ml, which was significantly higher than that for nonmultiresistant isolates (57.8 microg/ml). However, the means' of the highest vancomycin concentrations after agar selection for multiresistant and nonmulti-resistant isolates were the same, at 7.4 microg/ml, with no colonies capable of growing in 32 microg/ml of vancomycin. There was no significant difference in glycopeptide susceptibility between oxacillin-resistant and oxacillin-susceptible isolates among nonmultiresistant S. haemolyticus. The geometric mean MICs of vancomycin for oxacillin-resistant and oxacillin-susceptible isolates were 2.1 microg/ml and 1.6 microg/ml, and those of teicoplanin were 4.4 microg/ml and 5.6 microg/ml, while the means of the highest concentrations of the selected plates of vancomycin were 8.6 microg/ml and 3.3 microg/ml, and those of teicoplanin were 52.8 microg/ml and 74.7 microg/ml, respectively. Multiresistant isolates showed significantly greater mean MICs of vancomycin and teicoplanin and higher teicoplanin concentration of the selected plates than nonmultiresistant isolates, irrespective of oxacillin resistance. These results indicate that methicillin resistance may not be related to reduced susceptibility to glycopeptide in S. haemolyticus, and that a multiresistant profile is associated more with a decreasing susceptibility to glycopeptides then with resistance to oxacillin. In this study, antibiotypes showed good concordance with pulsed-field gel electrophoresis typing results, with a sufficiently high discriminatory ability index, of 0.912. We consider that primary screening with antimicrobial susceptibility testing and antibiotyping, with attention to the multiresistant profile, would be useful for monitoring nosocomial S. haemolyticus colonization and infection.