Frequency, clonal heterogeneity and antibiotic resistance of methicillin-resistant Staphylococcus aureus (MRSA) isolated in 1992-1994.

Since 1992, the proportion of methicillin-resistant Staphylococcus aureus (MRSA) isolated from patients of the University Hospital of Frankfurt/Main and six community hospitals increased to a level of 11% and has remained constant during the following two years. MRSA and methicillin-sensitive Staphylococcus aureus (MSSA) were distributed equally among almost all specimens except blood. There was evidence of a diminished potency of MRSA to cause bacteremia. All MRSA strains were susceptible to glycopeptides and mupirocin. Resistance rates to other non-beta-lactam antibiotics were low for fusidic acid (7.1%), fosfocin (8.3%), amikacin (11.4%) and cotrimoxazole (18.3%) and high for gentamicin (90.7%), ofloxacin (94.3%) and erythromycin (95.5%). Among 378 MRSA strains originating from 180 individuals, macrorestriction analysis of chromosomal DNA revealed 39 different genotypes. These could be divided into 14 epidemic strains isolated from 155 patients and 25 sporadic strains isolated from single patients. As most of the sporadic strains emerged in close local proximity to epidemic strains, we suppose a horizontal genetic transfer from MRSA to MSSA leading to the appearance of novel MRSA genotypes. Upon repeated isolation of MRSA strains from the same individuals, resistance rates and genotypes remained stable. Resistance patterns of the non-beta-lactams correlated poorly with macrorestriction analysis, whereas several MRSA genotypes could be distinguished by particular MICs of methicillin.