Our PFGE data show the presence of several MRSA major and minor clones circulating in the rural communities of northern and central Wisconsin. The representative isolates from MCGs 1, 7, and 18 had the same ST, ST5, suggesting those strains may have originated from a single clone. Additional evidence that supports this conclusion was based on the presence of a similar antibiogram, the same SCCmec, and the lack of pvl genes. The MLST allelic profiles of the representative isolates of MCG-10 were represented by STs 225 and 373 due to differences in alleles of gmk and tpi genes. A single base change (in WI-MRSA 280) at nucleotide position 329 (T→A) in the gmk gene created the new ST373 (Table ).
MLST data also suggest the presence of at least two international clones, ST5 and ST45, described as the New York/Japan clone and the Berlin clone, respectively, in Wisconsin (13
). ST45 has been reported from several European countries and in a CA-MRSA from western Australia (http://saureus.mlst.net/
). The ST225, which is related to ST5, has been reported in MRSA isolates from the United States (13
). Our ST373 is a new ST represented in the midwestern United States. When we combine the MLST and SCCmec
data to define clonality, we could identify five clonal groups: ST1-IV, ST5-II, ST45-II, ST225-II, and ST373-II. Of these five, ST1-IV (CA-MRSA), ST5-II (New York/Japan clone), and ST45-11 have been described before (2
). To the best of our knowledge, the remaining two combinations have not been reported yet. Therefore, they may well represent some of the HA-MRSA clones still limited to the midwestern United States.
One of the significant features that emerged from the molecular characterization of MRSAs from rural Wisconsin was the presence of a single clonal group, MCG-2, that had very distinct genotypic features of CA-MRSA. The majority of these isolates came from clinics primarily visited by Native American communities. This fact strongly suggests the presence of predominantly a single CA-MRSA clone circulating in the Native American communities of Wisconsin. Our molecular data show that almost all CA-MRSA present in Wisconsin are identical or clonally related to the hypervirulent strain, MW2, the strain that caused fatal septicemia and septic arthritis in a 16-month-old girl in North Dakota, in 1998 (6
). The presence of CA-MRSA in the late 1990s in a rural Native American community in the midwestern United States has been described previously based on the patient record (16
). However, our data suggest the presence of the MW2 related clone in a Native American population in the midwestern United States since the early 1990s. Another marker for the Wisconsin CA-MRSA is the presence of the class B1 mec
complex in mec
DNA. About 25% of these isolates had this deleted version of the mec
The fact that SCCmec
IV was present in several PFGE types suggests the possibility of multiple and independent acquisition of this mec
genotype in different MRSA genetic backgrounds. However, a high percentage of the SCCmec
IV had the same mecA
promoter mutation and the same type of deletion in the mec
complex suggests that source(s) of SCCmec
IV in Wisconsin MRSA isolates may be limited (40
). It is likely to be from a single or a very small number of source organism(s).
In conclusion, several important observations were made from the present study of a collection of temporal isolates from a predominantly rural environment where the dynamics of nosocomial and community-based spread of infectious diseases is expected to be different from the coastal U.S. cities and hospitals. First, the molecular evidence that CA-MRSA coming out of the Native American clinics in northern and central Wisconsin were clones of the virulent strain MW2. Second, it seems that there are three closely related clones of HA-MRSA circulating in hospitals and long-term care institutions in Wisconsin. Third, almost all CA-MRSA isolates contain SCCmec
IV and pvl
genes, but not all SCCmec
IV harboring MRSA would be CA-MRSA. It appears that the pvl
genes and SCCmec
IV together offers some selective advantage for CA-MRSA (43
). In addition, SCCmec
type IV was integrated into some isolates of other m
CGs representing multidrug-sensitive nosocomially related isolates. Whether the acquisition of SCCmec
IV by nosocomial isolates provides them any advantage in colonization remains to be seen. Genetically, it should be more favorable for an S. aureus
strain to acquire an approximately 21- to 24-kb genetic element with two functional recombinase genes than for an approximately 67-kb element to gain mecA
based methicillin resistance. Smaller genetic elements, such as SCCmec
IV, can be packaged in phage more efficiently than larger genetic elements.