Rapid increase in macrolide resistance among penicillin non-susceptible pneumococci in Finland, 1996-2000

The aims of this study were to evaluate the resistance patterns and serotypes/groups of penicillin non-susceptible pneumococci (PNSP) in Finland, and to determine phenotypes and resistance mechanisms of the erythromycin-resistant isolates. A total of 1190 PNSP were collected during 1996-2000 in Finland. The MICs of 18 antimicrobials were determined by the agar plate dilution method, and PCR was used to study the resistance mechanisms of the macrolide-resistant isolates. For serotyping, counterimmunoelectrophoresis and latex agglutination were used. Erythromycin resistance increased from 32% in 1996 to 62% in 2000 among PNSP in Finland. Multiresistance (co-resistance to erythromycin, tetracycline and co-trimoxazole) was present in 22% of the isolates in 1996 and in 40% in 2000. The most common macrolide resistance phenotype was the MLS(B) phenotype (72%), 25% had the M phenotype and 3% the MS phenotype. The MLS(B) and M phenotypes increased in the same proportion during the study period. All the MLS(B) isolates had the erm(B) gene, the M isolates the mef(A) gene, and in 11 MS isolates, ribosomal mutations were the cause of resistance. The most common serotypes/groups were 14, 19 and 6. We found a significant increase in multiresistance among PNSP within a short period of time in Finland. Although pneumococcal resistance to erythromycin was 11% in 2000, the same figure was 50% among the PNSP. The rise in erythromycin resistance is worrying, as macrolides are commonly used as first- and second-line drugs in pneumococcal infections.