Relationship between capsular type, penicillin susceptibility, and virulence of human Streptococcus pneumoniae isolates in mice

We examined the relationship between penicillin susceptibility, peritoneal virulence in Swiss mice, and capsular type in a selection of 122 clinical S treptococcus pneumoniae isolates belonging to 24 serotypes, Regardless of t he serotype, all 32 virulent strains were susceptible to penicillin, and al l 41 strains with diminished susceptibility or resistance to penicillin mer e avirulent, The remaining 49 strains were both susceptible to penicillin a nd avirulent, irrespective of the serotype. On the basis of their capsular type and pathogenic behavior, strains fell into one of four groups. In the group consisting of serotypes 1, 3, and 4 (n = 16), strains were predominan tly virulent (81.3%), and all were penicillin susceptible. In the serotype 6 group (n = 32), the frequency of virulence was significantly lower (34.4 versus 81.3%, P = 0.002), and strains were predominantly penicillin suscept ible (71.9%). In the group composed of serotypes 9, 14, 19, and 23 (n = 50) , all strains were avirulent, and 56% had decreased susceptibility (n = 12) or resistance to (n = 16) penicillin. The fourth group was heterogenous, a s it pooled 24 strains of 15 different serotypes; in this group the frequen cy of virulence was 33.3%, and strains were predominantly penicillin suscep tible (83.3%). These data point to a complex relationship between penicilli n susceptibility and virulence in mice but do not entirely separate these c haracteristics from the role of the capsular type, The possibility that the mechanisms conferring penicillin resistance are related to those leading t o a loss of virulence is supported by these findings.