These investigations have been continued since 1981. In this year the isolation frequencies and sensitivities to antibiotics were investigated for 654 bacterial strains isolated from respiratory tract infections in 20 institutions during the period of October 1990 to September 1991. Among Staphylococcus aureus isolated from inpatients, many were methicillin-resistant S. aureus (MRSA), and the frequency of their isolation was 56.3% for inpatients and 4.3% for outpatients. Of the relation to antibiotic administration, the isolation frequency of MRSA before administration of antibiotics was 19.6% (10/51), and after administration was 75.0% (27/36). The sensitivities of S. aureus to imipenem and clindamycin (MIC80) decreased from 0.2 µg/ml to 64 µg/ml and from 0.2 µg/ml to 128 µg/ml, respectively. We investigated year to year changes in the backgrounds of patients with respiratory tract infections. Bacterial pneumonia was 31.6% among respiratory tract infections in the period of the study, this trend has increase from 1989. Frequencies of different etiological bacteria in respiratory tract infections did not change appreciably from year to year, and S. aureus. Streptococcus pneumoniae and Pseudomonas aeruginosa were the most showing some frequent pathogens.
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