Influence of decreased penicillin susceptibility on growth rate of beta haemolytic streptococci.

BACKGROUND & OBJECTIVES: Beta haemolytic streptococci (BHS), especially group A are still highly susceptible to penicillin. One possible explanation for this could be reduced growth capability in penicillin resistant BHS mutants. The present study was therefore undertaken to analyze the growth rates of BHS with decreased susceptibility to penicillin. METHODS: Serial passages in the medium with subinhibitory concentration of penicillin were done to induce resistance to this antibiotic in 12 clinical isolates of BHS serogroups A, B, C, and G. Both penicillin susceptible (parental) and variants with decreased susceptibility to penicillin (laboratory strains) were grown in three different media and their growth rates were determined by counting the number of bacterial colonies and by measuring optical density of bacterial culture. RESULTS: The lowest increase in minimal inhibitory concentration (MIC) value for penicillin (8-16 times) was obtained in BHS group A isolates, while the increase in MIC values of BHS groups B, C and G strains was higher (64-128 times) and they reached the level of complete resistance. Laboratory variants differed significantly from parental in their morphological and cultural characteristics. There were no statistically significant differences between the growth rates of penicillin susceptible and variants with decreased susceptibility to penicillin, though a delay in multiplication of the laboratory strains during exponential phase of growth was noted. INTERPRETATION & CONCLUSION: Though significant differences in phenotypic characteristics of penicillin susceptible and laboratory variants were noted, the results of this study provides no support to the assumption that variants of BHS with decreased susceptibility to penicillin of BHS were incapable for normal growth. Further studies needs to be done to find out the association between the decreased susceptibility to penicillin in the BHS and decreased growth capability in these bacteria.