Whole genome sequencing of penicillin-resistant Streptococcus pneumoniae reveals mutations in penicillin-binding proteins and in a putative iron permease

Background

Penicillin resistance in Streptococcus pneumoniae is mediated by a mosaic of genes encoding altered penicillin-binding proteins (PBPs). Nonetheless, S. pneumoniae has also developed non-PBP mechanisms implicated in penicillin resistance. In this study, whole genome sequencing of resistant organisms was used to discover mutations implicated in resistance to penicillin.

Results

We sequenced two S. pneumoniae isolates selected for resistance to penicillin in vitro. The analysis of the genome assemblies revealed that six genes were mutated in both mutants. These included three pbp genes, and three non-pbp genes, including a putative iron permease, spr1178. The nonsense mutation in spr1178 always occurred in the first step of the selection process. Although the mutants had increased resistance to penicillin, the introduction of altered versions of PBPs into a penicillin-susceptible strain by sequential transformation led to strains with a minimal increase in resistance, thus implicating other genes in resistance. The introduction by transformation of the non-PBP recurrent mutations did not increase penicillin resistance, but the introduction of the nonsense mutation in the putative iron permease spr1178 led to a reduced accumulation of reactive oxygen species following exposure to penicillin and to other bactericidal antibiotics as well.

Conclusions

This study indicates that the selection of resistance to penicillin in S. pneumoniae involves the acquisition of mutations conferring tolerance to the antibiotic-induced accumulation of oxidants, which translates into an increased survival that putatively enables the selection of major resistance determinants such as mutations in PBPs.