Effect of vancomycin on the cytoplasmic membrane fatty acid profile of vancomycin-resistant and -susceptible isolates of Staphylococcus aureus.

This study was designed to analyze the effect of vancomycin on the cytoplasmic membrane fatty acid (FA) composition of vancomycin-resistant Staphylococcus aureus (VRSA), vancomycin-intermediate resistant S. aureus (VISA), and vancomycin-susceptible S. aureus. One low-level vancomycin-resistant isolate (LLR-VRSA) termed CP2, along with two vancomycin intermediate-resistant S. aureus isolates (VISA-CP1) and Mu50 (ATCC #700699), were studied. The LLR-VRSA isolate CP2, recovered from the blood sample of a postoperative cardiac patient, exhibited vanA type vancomycin resistance [minimum inhibitory concentration (MIC) 16 μg/ml], and the vanA cassette was located on a plasmid. CP1, isolated from the pus sample of the same patient, exhibited vancomycin intermediate resistance (MIC 8 μg/ml) in the absence of the vanA, vanB, or vanC gene. As susceptible controls, we used PSA (vancomycin MIC 2 μg/ml), which was isolated from the pus sample of a neonate, and S. aureus (ATCC# 29213). Membrane FA analysis was carried out using gas chromatography coupled with mass spectrometry. For this purpose, CP1, CP2, Mu50, and the susceptible control isolates were grown in the presence and absence of vancomycin. Comparative analysis showed an increase in the relative proportion of unsaturated FAs during growth under vancomycin stress. The isolate CP2 (LLR-VRSA) exhibited a higher MIC to vancomycin than the other isolates used in present study (16 μg/ml) and under vancomycin stress conditions, quantitatively, it showed a high rate of conversion of saturated to unsaturated membrane FAs than CP1, Mu50 (VISA isolate) and the susceptible control PSA. The rate of saturated-to-unsaturated FA conversion increased as the concentration of vancomycin in the growth media was increased. Therefore, it is concluded that S. aureus tend to modify their membrane lipid chemistry from saturated to unsaturated in order to survive in a vancomycin stress environment.