A seventeen-year epidemiological survey of antimicrobial resistance in pneumococci in two hospitals.

During a 17 year period (1970-1986), 2753 clinical isolates of Streptococcus pneumoniae isolated in two hospitals were serotyped and tested for antibiotic susceptibility. In the last ten years the number of multiply resistant strains has increased to 60% of the resistant isolates. Resistance to tetracycline was already present in 14% of the isolates in 1970, and was the most frequent resistance encountered during this study (30% of the strains). Resistance to chloramphenicol was first detected in 1972, but this resistance has remained infrequent (3%). Resistance to penicillin is extremely rare and since 1978, only six strains with relative penicillin resistance (MIC 0.1-1.0 mg/l) have been isolated. Resistance to macrolides, lincosamides and streptogramin B (MLSB resistant phenotype) was first detected in 1976. From 1983 to 1986, 131 isolates were MLSB resistant strains. These strains belonged to 20 different serotypes but 75% of the MLSB resistant pneumococci belonged to serotypes 6, 23, 19 and 14 which were among the most frequently isolated serotypes. In contrast serotypes 3 and 9 were epidemic but not resistant during the same period. Resistance markers in S. pneumoniae are often related to particular serotypes. The large monthly fluctuation in the isolation of resistant strains might explain the variable clinical results of empirical treatment of respiratory infections with macrolides.