Mechanism of resistance to fluoroquinolone in oral streptococci.

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Mechanisms of resistance to fluoroquinolones in oral streptococci such as Streptococcus sanguis and Streptococcus anginosus were studied. S. sanguis QS-951 and S. anginosus QS-701, susceptible to ofloxacin, were serially transferred on agar, with each sample containing gradually increased concentrations of ofloxacin or DU-6859a. Although the resistances of these strains to fluoroquinolones were markedly induced with both drugs, MIC values of DU-6859a-induced resistant strains of S. sanguis QS-951 and S. anginosus QS-701 were 4-8 fold higher than those of ofloxacin-induced strains. The gyrA was analyzed to elucidate the mechanisms of resistance to fluoroquinolones in these strains. The mutation of serine at codon 83 to phenylalanine or tyrosine was found in each of the resistant strains and occurred by inductions with either ofloxacin or DU-6859a, and the additional mutation of glutamic acid at codon 87 to lysine was found in high-level resistant strains by induction with DU-6859a. Marked differences in types of gyrA mutation and in levels of resistance were found in the in vitro resistant strains of oral streptococci, when different kinds of fluoroquinolones were used as the inducers.
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