Methicillin-resistant Staphylococcus aureus (MRSA) strains have become prevalent in health care facilities and in the community worldwide (3, 4). MRSA strains produce penicillin binding protein 2Ј or 2a, which is poorly acylated by ␤-lactam antibiotics (5,22,25). The mecA gene, encoding PBP2a, is carried on a peculiar type of mobile genetic element inserted into the staphylococcal chromosome, designated staphylococcal cassette chromosome mec (SCCmec) elements (12,14,24).SCCmec elements typically share four characteristics: first, they carry the mec gene complex (mec) consisting of the methicillin resistance determinant mecA and its regulatory genes and insertion sequences; second, they carry the ccr gene complex (ccr) consisting of ccr genes that are responsible for the mobility of the element and its surrounding sequences; third, they have characteristic directly repeated nucleotide sequences and inverted complementary sequences at both ends; and last, they integrate into the 3Ј end of an open reading frame (ORF), orfX.Despite these similarities, the structures of SCCmec elements are rather divergent. Allotypic differences that are used for SCCmec type definitions have been identified in both ccr and mec. Five types of ccr and four classes of mec have been reported. ccr types 1 to 4 carry the ccrA and ccrB genes, which share approximately 80% identity with each other, and the type 5 ccr carries the ccrC gene (10,11,17,19). Four classes of the mec gene complexes have been identified among methicillinresistant staphylococcal strains of various species: class A mec, consisting of IS431mec-mecA-mecR1-mecI; class B mec, consisting of IS431mec-mecA-⌬mecR1-IS1272; class C mec, consisting of IS431mec-mecA-⌬mecR1-IS431; and class D mec, consisting of IS431mec-mecA-⌬mecR1 with no insertion sequences downstream of ⌬mecR1 identified by PCR as of yet (13). In S. aureus strains, mec classes A, B, and C have been identified. Insertion sequences have sometimes been found to be integrated in or around the class A mec. A class A mec carrying IS431 downstream of mecI was found in Staphylococcus haemolyticus (13). Recently, Shore et al. identified MRSA strains carrying class A mec with an insertion of IS1182 in and around the mecI gene and designated them classes A3 and A4 (23).The SCCmec element type has been defined by the combination of ccr type and mec class. In MRSA strains, six types of SCCmec elements, that is, six combinations of ccr and mec, have been reported (Table 1). These six SCCmec elements have been further classified by differences in regions other than ccr and mec, which are designated junkyard (J) regions. The J regions comprise three parts: J1 (the region between ccr and the right-flanking chromosomal region), J2 (the region between mec and ccr), and J3 (the region between orfX and mec). The J regions are not always specific to each SCCmec type, but certain J regions are commonly shared among certain types of SCCmec elements. Of the three regions, we regard J1 as being the most fundamental, because we presume t...

Staphylococcal cassette chromosome mec (SCCmec) is a mobile genetic element composed of the mec gene complex, which encodes methicillin resistance, and the ccr gene complex, which encodes the recombinases responsible for its mobility. The mec gene complex has been classified into four classes, and the ccr gene complex has been classified into three allotypes. Different combinations of mec gene complex classes and ccr gene complex types have so far defined four types of SCCmec elements. Now we introduce the fifth allotype of SCCmec, which was found on the chromosome of a community-acquired methicillin-resistant Staphylococcus aureus strain (strain WIS [WBG8318]) isolated in Australia. The element shared the same chromosomal integration site with the four extant types of SCCmec and the characteristic nucleotide sequences at the chromosome-SCCmec junction regions. The novel SCCmec carried mecA bracketed by IS431 (IS431-mecA⌬mecR1-IS431), which is designated the class C2 mec gene complex; and instead of ccrA and ccrB genes, it carried a single copy of a gene homologue that encoded cassette chromosome recombinase. Since the open reading frame (ORF) was found to encode an enzyme which catalyzes the precise excision as well as site-and orientation-specific integration of the element, we designated the ORF cassette chromosome recombinase C (ccrC), and we designated the element type V SCCmec. Type V SCCmec is a small SCCmec element (28 kb) and does not carry any antibiotic resistance genes besides mecA. Unlike the extant SCCmec types, it carries a set of foreign genes encoding a restriction-modification system that might play a role in the stabilization of the element on the chromosome.

We identified a new type of staphylococcal cassette chromosome mec (SCCmec) from two communityacquired methicillin-resistant Staphylococcus aureus (MRSA) strains. The novel element, designated type IV SCCmec, had a unique combination of the class B mec gene complex and the type 2 ccr gene complex and was much smaller in size (21 to 24 kb) than previously identified SCCmec elements of hospital-acquired MRSA. Consistent with the strains' susceptibilities to various non-␤-lactam antibiotics, the type IV SCCmec was devoid of any antibiotic resistance genes other than the mecA gene.

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We determined the entire nucleotide sequence of Sa2958-carrying Panton-Valentine leukocidin (PVL) gene, which was lysogenized in a sequence type 5 staphylococcal cassette chromosome mec (SCCmec) type II strain of methicillin-resistant Staphylococcus aureus (MRSA). Based on the nucleotide sequences of PVL phages, we developed PCRs to discriminate among five PVL phages, with a preliminary classification into two morphological groups (elongated-head type and icosahedral-head type) with four PCRs, including two PCRs for identifying the gene lineage between lukS-PV and the tail gene. The phages were then classified into five types by four PCRs identifying each phage-specific structure. With these PCRs, we examined the PVL phage types of 67 MRSA strains isolated in Japan from 1979 through 1985 and since 2000 and found that two morphologically distinct phages were predominant in Japan. The icosahedral-head-type phage, represented by the 108PVL type, was identified for 39 of 53 strains isolated from 1979 through 1985. Of 26 other Japanese isolates, 25 belonged either definitively or presumably to elongated-head types as follows: 3 belonged to the Sa2958 type; 8 were determined to belong to an elongated-head type, but a determination of greater specificity was not made; and 14 belonged to a Sa2958-like phage of unknown type. We induced prophages by treatment with mitomycin C from six strains of the Sa2958 type or of Sa2958-like unknown-type phages; five of six strains carried intact PVL-carrying phages, which can infect other S. aureus strains and might generate novel PVL-positive strains of S. aureus. That various SCCmec elements were carried by different strains of the same phage type suggests that S. aureus strains might independently acquire PVL phages before they acquire various SCCmec elements.

We previously reported that the methicillin resistance gene mecA is carried by a novel type of mobile genetic element, SCCmec (staphylococcal cassette chromosome mec), in the chromosome of methicillin-resistant Staphylococcus aureus (MRSA). These elements are precisely excised from the chromosome and integrated into a specific site on the recipient chromosome by a pair of recombinase proteins encoded by the cassette chromosome recombinase genes ccrA and ccrB. In the present work, we detected homologues of the ccr genes in Staphylococcus hominis type strain GIFU12263 (equivalent to ATCC 27844), which is susceptible to methicillin. Sequence determination revealed that the ccr homologues in S. hominis were type 1 ccr genes (ccrA1 and ccrB1) that were localized on a genetic element structurally very similar to SCCmec except for the absence of the methicillin-resistance gene, mecA. This genetic element had mosaic-like patterns of homology with extant SCCmec elements, and we designated it SCC 12263 and considered it a type I staphylococcal cassette chromosome (SCC). The ccrB1 gene identified in the S. hominis strain is the first type 1 ccrB gene discovered to retain its function through the excision process as judged by two criteria: (i) SCC 12263 was spontaneously excised during cultivation of the strain and (ii) introduction of the S. hominis ccrB1 into an MRSA strain carrying a type I SCCmec whose ccrB1 gene is inactive generated SCCmec excisants at a high frequency. The existence of an SCC without a mec determinant is indicative of a staphylococcal site-specific mobile genetic element that serves as a vehicle of transfer for various genetic markers between staphylococcal species.

A novel, methicillin-resistant Staphylococcus aureus clone (Uruguay clone) with a non–multidrug-resistant phenotype caused a large outbreak, including 7 deaths, in Montevideo, Uruguay. The clone was distinct from the highly virulent community clone represented by strain MW2, although both clones carried Panton-Valentine leukocidin gene and cna gene.