Complex relationship between acquisition of beta-lactam resistance and loss of virulence in Streptococcus pneumoniae

In a previous study of a murine peritonitis model, no Streptococcus pneumon iae strains were found that were both clinically penicillin resistant and v irulent. This study assessed the relationship between acquired resistance a nd virulence in single- and double-isogenic penicillin-resistant (Peni-R) m utants obtained by transformation of a virulent penicillin-susceptible reci pient strain with pbp2b and pbpX polymerase chain reaction fragments from a Peni-R donor strain. Sequence analysis results of the pbp2b and pbpX allel es from these strains were in keeping with acquired penicillin resistance. The virulence of these strains was significantly reduced, which shows a rel ationship between beta -lactam resistance and loss of virulence. The phenot ype of the 23.2x mutant remained stable after in vivo passage, which sugges ts that the pbpX gene is involved in growth, whereas virulent revertants of the 23.2b and 23.2b.2x mutants had no change in MIC. Compensatory mutation s are implicated in the revival of virulence.