Microbiology (2002), 148, 467–472 Printed in Great Britain Fusobacterium nucleatum supports the growth of Porphyromonas gingivalis in oxygenated and carbon-dioxide-depleted environments P. I. Diaz, P. S. Zilm and A. H. Rogers Author for correspondence : A. H. Rogers. Tel : 61 8 8303 5104. Fax: 61 8 8303 3444. e-mail : tony.rogersadelaide.edu.au Microbiology Laboratory, Dental School, Adelaide University, North Terrace, Adelaide, South Australia 5005, Australia The authors compared the differences in tolerance to oxygen of the anaerobic periodontopathic bacteria Fusobacterium nucleatum and Porphyromonas gingivalis, and explored the possibility that F. nucleatum might be able to support the growth of P. gingivalis in aerated and CO 2 -depleted environments. Both micro-organisms were grown as monocultures and in co-culture in the presence and absence of CO 2 and under different aerated conditions using a continuous culture system. At steady state, viable counts were performed and the activities of the enzymes superoxide dismutase and NADH oxidase/peroxidase were assayed in P. gingivalis. In co-culture, F. nucleatum was able to support the growth of P. gingivalis in aerated and CO 2 -depleted environments in which P. gingivalis, as a monoculture, was not able to survive. F. nucleatum not only appeared to have a much higher tolerance to oxygen than P. gingivalis, but a significant increase in its numbers occurred under moderately oxygenated conditions. F. nucleatum might have an additional indirect role in dental plaque maturation, contributing to the reducing conditions necessary for the survival of P. gingivalis and possibly other anaerobes less tolerant to oxygen. Additionally, F. nucleatum is able to generate a capnophilic environment essential for the growth of P. gingivalis. Keywords : oxidative stress, oral bacteria, anaerobes, interactions, continuous culture INTRODUCTION It is well known that elevated oxygen tensions within bacterial cells increase the enzymic and non-enzymic reduction of molecular oxygen to superoxide anions (O - ), which can form, by dismutation, H O and O . H O , in turn, reacts with O - to form OH in the presence of iron complexes (Rosen & Klebanoff, 1979). These oxygen species are highly reactive and can cleave nucleic acids and oxidize essential proteins and lipids (Brawn & Fridovich, 1981 ; Harley et al., 1981). Strictly anaerobic micro-organisms do not possess the anti- oxidant systems needed to detoxify such reactive oxygen species. However, the susceptibility of anaerobes to oxygen varies even among closely related micro-or- ganisms, and it has been suggested that it correlates with the levels of anti-oxidant enzymes present, superoxide dismutase (SOD) in particular (Park et al., 1992). ................................................................................................................................................. Abbreviations : SEM, scanning electron microscopy ; SOD, superoxide dismutase. As the oral cavity is an overtly aerobic environment, it is therefore likely that oral anaerobes encounter residual amounts of oxygen both in the early stages of dental plaque development and in established periodontal pockets (Marquis, 1995). Indeed, periodontal pockets have been reported to possess residual oxygen at one-tenth the level in air-saturated water (which is 0021 μmol ml -) (Mettraux et al., 1984) and the average E h (redox potential) in subgingival plaque appears to be only somewhat negative at about 50 mV (Kenny & Ash, 1969). Moreover, there is evidence in dental plaque of open channels that could deliver oxygen deep into the biofilm (Massol-Deya et al., 1994). Therefore, the survival of anaerobic bacteria in the mouth might be dependent on the specific tolerance to oxygen of each species and on microbial interactions within the com- munity. Porphyromonas gingivalis and Fusobacterium nuclea- tum belong to the group of strictly anaerobic bacteria associated with periodontal diseases (Ximenez-Fyvie et al., 2000). Due to its numerous putative virulence 0002-5170 2002 SGM 467