Insight into a unique carbon resource partitioning mechanism in Aggregatibacter actinomycetemcomitans

Aggregatibacter actinomycetemcomitans is a Gram negative bacterium found exclusively in the mammalian oral cavity where it resides in the gingival crevice, the space between the tooth and gum tissue. Though it has historically been considered a common commensal organism, it is now appreciated that A. actinomycetemcomitans is an opportunistic pathogen associated with the diseases periodontitis and endocarditis. To cause infection, A. actinomycetemcomitans must interact and compete with neighboring bacteria for space and nutrients, though little is known about the physiology it employs within the gingival crevice. Using A. actinomycetemcomitans grown in a chemically defined medium containing carbon sources found in vivo, I use transcriptome analyses and growth studies to show that A. actinomycetemcomitans preferentially utilizes lactate over the phosphotransferase system (PTS) sugars glucose and fructose. Additionally, the presence of lactate or pyruvate inhibits the transport and metabolism of these sugars in a post-transcriptionally controlled process I have termed PTS substrate exclusion. Since lactate is an energetically inferior carbon source, PTS substrate exclusion appears to be a carbon resource partitioning mechanism that allows A. actinomycetemcomitans to avoid competition for energetically favorable sugars with other species, and I propose a model to describe this phenomenon. To begin to understand the mechanism of PTS substrate exclusion, I examine the first step of the proposed model by purifying and characterizing the L-lactate dehydrogenase (LctD) from A. actinomycetemcomitans. I demonstrate that, unlike other studied lactate dehydrogenases, the LctD from A. actinomycetemcomitans does not exhibit feedback inhibition in the presence of physiologically relevant concentrations of pyruvate, which supports my hypothesis that elevated intracellular pyruvate levels inhibit the PTS. The results of my studies provide insight into a new regulatory mechanism governing carbon utilization in this bacterium.