SCCmec Typing in Methicillin-Resistant Staphylococcus aureus Strains of Animal Origin

To the Editor: Van Loo et al. described the presence of staphylococcal cassette chromosome mec (SCCmec) type III in some methicillin-resistant Staphylococcus aureus sequence type (ST) 398 isolates related to pig farming (1). SCCmec types are based on the allotype of ccr genes and the mec gene complex. Class A mec has intact mecI/R regulator genes. Type III SCCmec has type 3 ccr genes and class A mec complex, whereas type V SCCmec contains ccrC and class C mec (2,3). The authors typed SCCmec of the isolates by the method of Zhang et al. (4), in which type III is defi ned by amplifi cation of a 280-bp fragment located in the junkyard region. This fragment is found in SCCmer that is associated with SCCmec type III. We have typed SCCmec of the same 4 isolates that were reported to be SCCmec type III positive by using the primer sets defi ned by Ito et al. (2,3) and Lim et al. (5) for ccr types 1–3 and ccrC and 4 additional primers developed at our institute (Table) in single PCRs. All ST398 isolates were PCR negative when primers specifi c for SCCmec type III were used, but PCR positive with the ccrC-specifi c primers. DNA sequencing confi rmed the product as ccrC. Further, the isolates did not have a class A mec complex, a requisite for SCCmec type III, because a mecI-specifi c PCR was negative for these isolates. In addition, Southern hybridizations with digoxigenin-dUTP–labeled PCR fragments obtained with our primer pair specifi c for ccr3 and primers for ccrC (3) showed no hybridization with the ccrA/B3 probe (except for the positive control). All of the ST398 isolates hybridized with the ccrC-specifi c probe. We conclude that on the basis of generally accepted defi nitions SCCmec type V is present in these ST398 pigfarming–related isolates, not SCCmec type III. Therefore, researchers should be aware that some typing methods may lead to inadequate results.