The prevalence of nasal carriers of community-acquired MRSA is becoming a clinical phenomenon of note in recent years, and, as if to reflect the social environmental context of the phenomenon, the number of reports on MRSA-related contagious impetigo and staphylococcal scalded skin syndrome (SSSS) has been increasing. Because the antibiotic susceptibility patterns of MRSA strains isolated from outpatients with acute infections, such as contagious impetigo and SSSS, often differ from those of MRSA strains isolated in nosocomial infections, we attempted to classify the MRSA strains isolated from outpatients by the in susceptibility pattern. We also assessed the isolated strains by the effects of antibiotics used in combination with fosfomycin (FOM). The MICs (μg/ml) against the MRSA isolates were determined for FOM alone and for CEZ, CTM, CMB, CFPM, SBT/ABPC, LVFX, MINO, ABK, IPM/CS, TEIC and VCM both individually and in combination with FOM. The 28 isolated strains of MRSA were classified into the following three groups by their patterns of susceptibility to MINO and FOM: Type A, strains highly susceptible to both MINO and FOM; Type B, strains highly susceptible to MINO but intermediately susceptible or resistant to FOM; and Type C, strains intermediately susceptible or resistant to both MINO and FOM. Type A and Type B strains were isolated in 6 and 10 cases, respectively. They were susceptible to all antibiotics tested, except for cephem and SBT/ABPC, and the number of cases by disease was largest for acute infection, such as contagious impetigo. When used in combination with FOM, the cephem and SBT/ABPC exhibited high susceptibility levels of MIC against Type A and Type B strains. Type C strains were isolated in 12 cases, and 8 of the 12 cases were observed in prolonged infections such as stasis dermatitis with secondary infection. The isolated strains were susceptible only to ABK, TEIC and VCM; however, LVFX exhibited susceptibility levels of MIC when used in combination with FOM. It was suggested that a long-term administration of antibiotics resulted in resolution of infection by bacteria except for Type C strains of high drug resistance or resulted in letting Type C strains select the high drug resistance, or Type C strains isolated as nosocomial infections during the long-term therapy at outpatient. On the other hand, the acute infections due to Type C strains observed in 4 cases were considered to be community-acquired infections, as were possibly the cases with Type A strain- and Type B strain-related acute infections. These results concern about the prevalence of nasal carriers of Type C strains of high drug resistance in community. In the present study, we demonstrated that the level of synergistic effect in combination with FOM can be predicted for each antibiotic agent based on the pattern of susceptibility of the isolated MRSA strains to each agent, and the predict was thought to be applied to the patients with MRSA infection not only of the skin or the upper respiratory tract but also of the internal organs such as pneumonia. The results of the present study also suggested that infection with Type B strains can be sufficiently treated by the combined use of either the first generation or second generation cephem and FOM without the use of broad spectrum antibiotics or antibiotics fraught with risks of serious adverse reactions, while more aggressive antibiotic therapy using FOM with ABK, TEIC or VCM was considered necessary for relatively more severe cases (even in cases of SSSS) of infection with Type C strains. Since LVFX, which is not indicated for pediatric use, became effective against Type C strains when used in combination with FOM, the combination therapy with FOM and new quinolone was considered useful in adult patients.
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