Description of optochin-resistant Streptococcus pneumoniae due to an uncommon mutation in the atpA gene and comparison with previously identified atpC mutants from Brazil

Optochin susceptibility testing is a major assay used for presumptive identification of Streptococcus pneumoniae . Still, atypical optochin-resistant (Opt r ) pneumococci have been reported and this phenotype has been attributed to nucleotide substitutions in the genes coding for the F 0 F 1 ATPase. While substitutions in the atp C gene (c-subunit of ATPase) are more common and better characterized, data on mutations in the atp A (a-subunit) are still limited. We have characterized five Opt r isolates presenting alterations in the atp A (Trp 206 Cys in four isolates and Trp 206 Ser in one isolate), constituting the first report of such mutations in Brazil. Most of the Opt r isolates consisted of heterogeneous populations. Except for Opt MICs and the nucleotide changes in the atp A gene, Opt r and Opt s subpopulations originating from the same culture had identical characteristics. In addition, we compared phenotypic and genetic characteristics of these atp A mutants with those of atp C mutants previously identified in Brazil. No structural alterations were detected among predicted proteins, regardless of mutations in the coding gene, suggesting that, despite the occurrence of mutations, protein structures tend to be highly conserved, ensuring their functionalities. Phylogenetic analysis revealed that atypical Opt r strains are true pneumococci and Opt resistance does not represent any apparent selective advantage for clinical isolates.