Role of a nosX homolog in Streptococcus gordonii in aerobic growth and biofilm formation.

Oral streptococci such as Streptococcus gordonii are facultative anaerobes that initiate biofilm formation on tooth surfaces. An isolated S. gordonii::Tn917-lac biofilm-defective mutant contained a transposon insertion in an open reading frame (ORF) encoding a homolog of NosX of Ralstonia eutropha, a putative maturation factor of nitrous oxide reductase. Located downstream are two genes, qor1 and qor2, predicted to encode two putative NADPH quinone oxidoreductases. These three genes are cotranscribed, forming a putative oxidative stress response (osr) operon in S. gordonii. Inactivation of nosX, qor1, or qor2 resulted in biofilm-defective phenotypes. Expression of nosX, measured by the beta-galactosidase activity of the nosX::Tn917-lac mutant, was growth-phase dependent and enhanced when grown under aerobic conditions or in the presence of paraquat. Real-time reverse transcription-PCR revealed that nosX-specific mRNA levels were increased approximately 8.4 and 3.5 fold in biofilm-derived cells grown on plastic and glass, respectively, when compared to planktonic cells. Expression of nosX increased 19.9 fold in cells grown under aerated aerobic conditions and 4.7 fold in cells grown under static aerobic conditions. Two ORFs immediately adjacent to the osr operon encode a putative NADH oxidase (Nox) and a putative thiol-specific antioxidant enzyme (AhpC, for alkyl hydroperoxide peroxidase C). Expression of nox and ahpC was also significantly increased in cells grown under aerated and static aerobic conditions when compared to anaerobic conditions. In addition, nox expression was increased in biofilm cells compared to planktonic cells. These genes may be part of an island that deals with oxidoreductive response, some of which may be important in S. gordonii biofilm formation.