Genetic Characterization of Optochin-Susceptible Viridans Group Streptococci

Abstract

ABSTRACT Two clinical isolates of viridans group streptococci (VS) with different degrees of susceptibility to optochin (OPT), i.e., fully OPT-susceptible (Opt s ) VS strain 1162/99 (for which the MIC was equal to that for Streptococcus pneumoniae , 0.75 μg/ml) and intermediate Opt s VS strain 1174/97 (MIC, 6 μg/ml) were studied. Besides being OPT susceptible, they showed characteristics typical of VS, such as bile insolubility; lack of reaction with pneumococcal capsular antibodies; and lack of hybridization with rRNA (AccuProbe)-, lytA -, and pnl -specific pneumococcal probes. However, these VS Opt s strains and VS type strains hybridized with ant , a gene not present in S. pneumoniae . A detailed characterization of the genes encoding the 16S rRNA and SodA classified isolates 1162/99 and 1174/97 as Streptococcus mitis . Analysis of the atpCAB region, which encodes the c , a , and b subunits of the F 0 F 1 H + -ATPase, the target of optochin, revealed high degrees of similarity between S. mitis 1162/99 and S. pneumoniae in atpC , atpA , and the N terminus of atpB . Moreover, amino acid identity between S. mitis 1174/97 and S. pneumoniae was found in α helix 5 of the a subunit. The organization of the chromosomal region containing the atp operon of the two Opt s VS and VS type strains was spr1284 - atpC , with spr1284 being located 296 to 556 bp from atpC , whereas in S. pneumoniae this distance was longer than 68 kb. In addition, the gene order in S. pneumoniae was IS 1239 -74 bp -atpC . The results suggest that the full OPT susceptibility of S. mitis 1162/99 is due to the acquisition of atpC , atpA , and part of atpB from S. pneumoniae and that the intermediate OPT susceptibility of S. mitis 1174/97 correlates with the amino acid composition of its a subunit.