Molecular Characterization of Disease-Associated Streptococci of the Mitis Group That Are Optochin Susceptible

Eight optochin-susceptible (Opt s ) alpha-hemolytic (viridans) streptococcus isolates were characterized at the molecular level. These isolates showed phenotypic characteristics typical of both viridans streptococci and Streptococcus pneumoniae . Comparison of the sequence of housekeeping genes from these isolates with those of S. pneumoniae , Streptococcus mitis , Streptococcus oralis , and Streptococcus pseudopneumoniae suggested that the Opt s isolates corresponded to streptococci of the mitis group. Besides, the Opt s streptococci were negative by a Gen-Probe AccuProbe pneumococcus test and hybridized with specific pneumococcal probes ( lytA and ply ) but also with ant , a gene not present in most S. pneumoniae strains. Moreover, the isolates were insoluble in 1% sodium deoxycholate but completely dissolved in 0.1% deoxycholate. Sequence analysis of the lytA gene revealed that the Opt s streptococci carried lytA alleles characteristic of those present in nonpneumococcal streptococci of the mitis group. The determination of the partial nucleotide sequence embracing the atp operon encoding the F o F 1 H + -ATPase indicated that the optochin susceptibility of the isolates was due to the acquisition of atpC , atpA , and part of atpB from S. pneumoniae by horizontal gene transfer.