Novel Mechanism for Conditional Aerobic Growth of the Anaerobic Bacterium Treponema denticola

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Abstract
Treponema denticola , a periodontal pathogen, has recently been shown to exhibit properties of a facultative anaerobic spirochete, in contrast to its previous recognition as an obligate anaerobic bacterium. In this study, the capacity and possible mechanism of T. denticola survival and growth under aerobic conditions were investigated. Factors detrimental to the growth of T. denticola ATCC 33405, such as oxygen concentration and hydrogen sulfide (H 2 S) levels as well as the enzyme activities of gamma-glutamyltransferase, cysteinylglycinase, and cystalysin associated with the cells were monitored. The results demonstrated that T. denticola grew only at deeper levels of broth (≥3 ml in a 10-ml tube), high inoculation ratios (≥20% of culture in medium), and short cultivation times (≤4 days for one passage) and in media containing l -cysteine or glutathione as the substrate for H 2 S production during aerobic growth. The determination of the factors showed that oxygen levels were always lower (0 to 0.6%) with significantly higher concentrations of H 2 S and higher activities of the three enzymes in all cultures grown aerobically. Further data revealed that H 2 S production from the T. denticola enzymes plus their substrates resulted in removal of dissolved O 2 in the growth cultures in a dose-dependent manner. These results demonstrated that T. denticola was able to generate microanaerobic environments in growth media for its survival and growth under aerobic conditions. Furthermore, the organism can be defined as a true obligate anaerobic spirochete. These findings suggest that spirochetes may play a significant role in maintaining the anaerobic environment at diseased sites in periodontitis.