Do Primocolonizing Bacteria Enable Bacteroides thetaiotaomicron Intestinal Colonization Independently of the Capacity To Consume Oxygen?

Aerobic bacteria are frequent primocolonizers of the human naive intestine. Their generally accepted role is to eliminate oxygen, which would allow colonization by anaerobes that subsequently dominate bacterial gut populations. In this hypothesis-based study, we revisited this dogma experimentally in a germfree mouse model as a mimic of the germfree newborn. We varied conditions leading to the establishment of the dominant intestinal anaerobe Bacteroides thetaiotaomicron. Two variables were introduced: Bacteroides inoculum size and preestablishment by bacteria capable or not of consuming oxygen. High Bacteroides inoculum size enabled its primocolonization. At low inocula, we show that bacterial preestablishment was decisive for subsequent Bacteroides colonization. However, even non-oxygen-respiring bacteria, a hemA Escherichia coli mutant and the intestinal obligate anaerobe Clostridium scindens, facilitated Bacteroides establishment. These findings, which are supported by recent reports, revise the long-held assumption that oxygen scavenging is the main role for aerobic primocolonizing bacteria. Instead, we suggest that better survival of aerobic bacteria ex vivo during vectorization between hosts could be a reason for their frequent primocolonization.