Strict anaerobes cannot grow in the presence of greater than 5 μM dissolved oxygen. Despite this growth inhibition, many strict anaerobes of the Bacteroides class of eubacteria can survive in oxygenated environments until the partial pressure of O2 (PO2) is sufficiently reduced. For example, the periodontal pathogens Porphyromonas gingivalis and Tannerella forsythensis colonize subgingival plaques of mammals, whereas several other Bacteroides species colonize the gastrointestinal tract of animals. It has been suggested that pre-colonization of these sites by facultative anaerobes is essential for reduction of the PO2 and subsequent colonization by strict anaerobes. However, this model is inconsistent with the observation that Bacteroides fragilis can colonize the colon in the absence of facultative anaerobes. Thus, this strict anaerobe may have a role in reduction of the environmental PO2. Although some strictly anaerobic bacteria can consume oxygen through an integral membrane electron transport system, the physiological role of this system has not been established in these organisms. Here we demonstrate that B. fragilis encodes a cytochrome bd oxidase that is essential for O2 consumption and is required, under some conditions, for the stimulation of growth in the presence of nanomolar concentrations of O2. Furthermore, our data suggest that this property is conserved in many other organisms that have been described as strict anaerobes.
Bibliographical noteFunding Information:
Acknowledgements We thank L. de Bonis and D. Pilbeam for comments, help, discussions and for providing documents and comparative materials; P.-O. Antoine for the identification of large mammals; C. Vozenin-Serra for the identification of fossil wood; and M. Ponce de Léon and C. Zollikofer for the CT-Scan sections. This work is supported by the Fyssen and Leakey foundations, the Department of Mineral Resources (Bangkok), the Thai-French TRF-CNRS Biodiversity Project (PICS Thaïlande) and the C.N.R.S. ‘Eclipse’ Program.
Acknowledgements We thank A. L. Sonenshein for critical review of the manuscript, and D. W. Lazinski and A. Camilli for comments on the manuscript. This study was supported by a US Public Health Service Grant.
Copyright 2008 Elsevier B.V., All rights reserved.