Abstract
Ten vancomycin resistant Staphylococcus aureus (VRSA) isolates were detected in a previous study, five out of them harbored vanA gene. The current study focused on the role of other van genes and mechanisms related to cell wall thickness in the development of vancomycin resistance in these VRSA isolates. The ten VRSA and five vancomycin sensitive Staphylococcus aureus (VSSA) isolates were re-tested by conventional methods in three different laboratories and their identification and sensitivity were confirmed. Multiplex polymerase chain reaction was performed for the detection of vancomycin vanA, vanB, vanC1/2, vanD, vane, and vanG resistance encoding genes as well as S. aureus specific nuc gene. All 15 isolates were molecularly confirmed as S. aureus and five of them were harboring the vanA gene. Other tested vancomycin encoding genes, namely vanB, vanC1/2, vanD, vanE, vanG were not detected in these isolates. Cell wall thickness was measured by transmission electron microscopy in the 15 tested isolates. The mean cell wall thickness was 40.5 ± 2.8, 22.6 ± 4.6, 31.2 ± 10.3, 23.2 ± 5.6 nm for vanA gene positive, vanA negative, VRSA, VSSA isolates, respectively. There was statistically significant difference between the cell wall thickness of vanA positive (+) and vanA negative (-) isolates, while no significant difference between VRSA and VSSA isolates. In conclusion, cell wall thickness was associated with the presence of vanA gene and this thickness was present in 50% of the tested VRSA isolates. Hence, it seems safe to assume that vancomycin resistance can be multifactorial.

Key words: VRSA, multiplex PCR, nuc gen, van genes, cell wall thickness, S. aureus.