Characterization of the mobile genetic element responsible for methicillin resistance in environmental and clinical staphylococcal isolates

The sharp increase in community-acquired methicillin-resistant Staphylococcus aureus (MRSA) infections on Vancouver Island may have been facilitated by the spread of the mobile genetic element SCCmec, which carries a gene responsible for methicillin-resistance. The proliferation in this multidrug resistant pathogen has spurred many investigations to discover the source of the SCCmec element in the community. Earlier research at Malaspina has shown a low recovery of MRSA from the environment, in contrast 70% of coagulase-negative Staphylococcus (CoNS) isolates were methicillin-resistant. These results led to the hypothesis, that if the two groups of staphylococci share the same types of the SCCmec genetic element, then perhaps they are exchanging genetic material horizontally. If so, the origin of community-acquired outbreaks could be susceptible S. aureus clones obtaining resistance from the more resilient and numerous methicillin-resistant coagulase-negative staphylococci. In order to test this hypothesis, the multiplex PCR technique designed by Zhang et al (2005), was utilized to characterize the mobile genetic element, SCCmec, in environmental and clinical isolates. This technique successfully allowed the typing of the majority of MRSA, 95%, but it was only able to type 25% of the methicillin-resistant coagulase-negative staphylococci. In addition, it was observed that since 2004 SCCmec subtype IVa has become predominant among MRSA on Southern Vancouver Island. The multiplex PCR technique did not allow us to type enough CoNS isolates; therefore an insufficient sample size was obtained and no clear conclusions can be drawn on the distribution of the SCCmec types. Consequently, no definitive trends were observed dealing with the comparative distribution of SCCmec types between MRSA and CoNS.