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 defined by amplification 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 defined 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 specific for SCCmec type III were used, but PCR positive with the ccrC-specific primers. DNA sequencing confirmed the product as ccrC. Further, the isolates did not have a class A mec complex, a requisite for SCCmec type III, because a mecI-specific PCR was negative for these isolates. In addition, Southern hybridizations with digoxigenin-dUTP–labeled PCR fragments obtained with our primer pair specific 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-specific probe.

Table. Primers used to type SCCmec of MRSA ST398 isolates*.

Genes	Primer name	Primer sequence (5′ → 3′)
ccrA/B1	ccr1B-for	CTT TCA CGA TAG ACA CAG
	ccr1B-rev	TAA AAG AAG TTC ATA GCC GTT AAA TTG G
ccrA/B2	ccr2B-for	GCA TTC ATC ATC AAT CAA AAT G
	ccr2B-rev	CTA TAA CCT TCT GTG CTT TGC A
ccrA/B3	ccr3B-for	TCC GTA ATA AGA AGC AAC TTC AC
	ccr3B-rev	ACT ATA GCC TTC AGT ACT TTG GA
ccrA/B4	ccr4B-for	TGA AGA AGC ACA AGA GCG GC
	ccr4B-rev	CTG CAC CAC ATT TTG GGC AC

*SCCmec, staphylococcal cassette chromosome mec; MRSA, methicillin-resistant Staphylococcus aureus; ST, sequence type.

We conclude that on the basis of generally accepted definitions SCCmec type V is present in these ST398 pig-farming–related isolates, not SCCmec type III. Therefore, researchers should be aware that some typing methods may lead to inadequate results.