Two bacterial species found in the human mouth can work together to make Aggregatibacter actinomycetemcomitans more infectious, according to findings, published in mBio. The data could help investigators develop strategies to fight bacterial infections that occur in complex communities of bacteria.

Apollo Stacy, a graduate student at the University of Texas in Austin, and colleagues used a laboratory model of 2 oral bacteria, A. actinomycetemcomitans and Streptococcus gordonii (S. gordonii) to assess what effect these species have on each other during infections.

A. actinomycetemcomitans and S. gordonii can be found together in the human mouth, and are also found in abscesses in the lungs and the brain. A. actinomycetemcomitans is a main cause of periodontal disease and can also infect heart valves, while S. gordonii is considered commensal bacteria. Prior studies by the research team have shown that these 2 bacteria do not cause severe disease on their own, but together they are highly infectious.

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The researchers created mutant strains of A. actinomycetemcomitans, and used tansposons to disrupt each gene in its genome. They placed the mutant strains into abscesses on the thighs of mice, either alone or in combination with S. gordonii, and used transposon sequencing to catalogue which genes were required for A. actinomycetemcomitans survival.

The results showed that S. gordonii significantly altered A. actinomycetemcomitans to shift from a primarily fermentative metabolism to a respiratory metabolism that increases growth and persistence. The respiratory metabolism also enhances the fitness of A. actinomycetemcomitans in vivo by increasing the ATP yields with central metabolism and creating a proton motive force.

The investigators noted that commensal bacteria can also provide electron acceptors that promote respiratory and fitness of pathogens in vivo, an interaction which the researchers call “cross-respiration.”

“Now we are starting to understand that a lot of modern infections are caused by a community of organisms,” noted Marvin Whiteley, director of the John Ring LaMontagne Center for Infectious Disease in Austin, in a press release. “These 2 bacteria have evolved to live together, so they give us a way to explore these highly intricate relationships.”


Stacy A, Fleming D, Lamont RJ, et al. A commensal bacterium virulence of an opportunistic pathogen via cross respiration. mBio. 2016. doi: 10.1128/mBio.00782-16.