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

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Abstract
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 MLSB phenotype (72%), 25% had the M phenotype and 3% the MS phenotype. The MLSB and M phenotypes increased in the same proportion during the study period. All the MLSB 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.

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