ABSTRACT The optochin susceptibility test is a key method for differentiating Streptococcus pneumoniae from other α-hemolytic streptococci; however, optochin-resistant (Optr) S. pneumoniae have been reported in the last two decades. In this study, we investigated the isolation frequency of Optr S. pneumoniae in the North Kyushu area of Japan, and biochemically and genetically characterized Optr S. pneumoniae clinical isolates. Seven (0.68%) out of 1032 S. pneumoniae isolates collected by the North Fukuoka Infectious Diseases Working Group were found to be Optr S. pneumoniae. Resistant strains had MICs of optochin 2- to 64- fold higher than susceptible strains, possessed different antimicrobial resistance profiles, and belonged to different serotypes. All the seven Optr isolates had mutations in the nucleotide sequence code for subunit c of F0F1 ATPase. Three isolates had mutations in codon 48 (deduced amino acid substitution of valine with phenylalanine) and two isolates had mutations in codon 49 (substitution of alanine with threonine or serine). Of the remaining two isolates, one had mutation in codon 50 (substitution of phenylalanine with leucine) and the other had mutation in codon 44 (substitution of methionine with isoleucine, which was a novel mutation in this position). From these results, we identified the mutation in the H+-ATPase subunit c gene (atpC) of S. pneumoniae, which was not recognized earlier, and determined that Opt resistance among Japanese pneumococcal isolates is not related to a specific pneumococcal serotype or antimicrobial resistance profile. Furthermore, the results indicate that when α-hemolytic streptococci resistant to optochin are isolated from patients with invasive infectious diseases, such as meningitis and pneumonia, we should perform additional examinations such as bile solubility tests or PCR assays before confirming isolates as viridans streptococci. This is the first report of the characterization of Optr S. pneumoniae in Japan.
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