Trends of β-lactam antibiotic susceptibility in blood-borne methicillin-resistant Staphylococcus aureus (MRSA) and their linkage to the staphylococcal cassette chromosome mec (SCCmec) type

We investigated trends of beta-lactam antibiotic susceptibility in a total of 218 strains of blood-borne methicillin-resistant Staphylococcus aureus (MRSA) isolated from 1978 through 2002 at a middle-size geriatric hospital in Tokyo; the strains were classified by the MRSA marker, staphylococcal cassette chromosome mec (SCCmec). The minimum growth inhibitory concentration (MIC) of cloxacillin at which 50% of the strains were inhibited (MIC50) was 2 µg/ml in the strains isolated in 1978–1984 and 32 to 64 µg/ml in the strains isolated subsequently. Similarly, the MIC50 values of cefazolin and imipenem in the 1978–1984 isolates were 16 and ≤0.25 µg/ml, respectively, and those in the strains isolated subsequently were 128 and 16 to 32 µg/ml, respectively. These results indicated that the MRSA strains isolated after 1985 had gained high-level resistance to beta-lactam antibiotics. The type of SCCmec was investigated by polymerase chain reaction (PCR). We observed an elevated frequency of type II SCCmec, from about 15% in the strains isolated in 1978–1984 to about 50% in the 1985–1989 isolates, and the frequency reached and remained at about 90% subsequently. These results imply that the high-level beta-lactam antibiotic resistance of the MRSA strains isolated after 1985 is most likely due to the increased integration of type II SCCmec.