A Novel Exclusion Mechanism for Carbon Resource Partitioning in Aggregatibacter actinomycetemcomitans

Abstract

The bacterium Aggregatibacter actinomycetemcomitans is a common commensal of the human oral cavity and the putative causative agent of the disease localized aggressive periodontitis. A. actinomycetemcomitans is a slow-growing bacterium that possesses limited metabolic machinery for carbon utilization. This likely impacts its ability to colonize the oral cavity, where growth and community composition is mediated by carbon availability. We present evidence that in the presence of the in vivo relevant carbon substrates glucose, fructose, and lactate A. actinomycetemcomitans preferentially metabolizes lactate. This preference for lactate exists despite the fact that A. actinomycetemcomitans grows faster and obtains higher cell yields during growth with carbohydrates. The preference for lactate is mediated by a novel exclusion mechanism in which metabolism of lactate inhibits carbohydrate uptake. Coculture studies reveal that A. actinomycetemcomitans utilizes lactate produced by the oral bacterium Streptococcus gordonii, suggesting the potential for cross-feeding in the oral cavity.Aggregatibacter (Actinobacillus) actinomycetemcomitans is a nonmotile, facultative anaerobic gram-negative bacterium found in the mammalian oral cavity (27). A. actinomycetemcomitans is the putative causative agent of localized aggressive periodontitis, a disease characterized by tissue destruction and tooth loss (38,49). Within the oral cavity, A. actinomycetemcomitans resides in the gingival crevice, the area around the tooth bounded by the tooth surface on one side and the epithelium lining the gingiva on the other. The gingival crevice contains a robust microbial population that is consistently bathed in crevicular fluid, which differentiates it from the tooth surface, which is immersed in saliva. Crevicular fluid is a plasma exudate that passes through the gingiva and flows along the teeth (20) and contains a number of potential carbon sources including sugars, small acids, and amino acids (6,11,20,48). A. actinomycetemcomitans primarily resides in the "moderate" pockets (4 to 6 mm in depth) in the gingival crevice instead of the deeper portions (9). Moderate pockets differ from deeper subgingival pockets by containing oxygen (19), and controlled studies in the laboratory indicate that A. actinomycetemcomitans exhibits enhanced growth in the presence of oxygen (32). Although the importance of A. actinomycetemcomitans as a common commensal and a potential pathogen has been appreciated for some time, virtually nothing is known about the growth and physiology of this bacterium.The mammalian oral cavity is a diverse environment that contains an estimated 500 bacterial species (17). Although the microbiology of the oral cavity has been the subject of study for many years, surprisingly little is known about the basic nutritional preference of oral bacteria. Compared to many common bacteria within the oral cavity, such as the streptococci, A. actinomycetemcomitans grows slowly and possesses the enzymatic capabilities to utilize only a small ...

Polymicrobial interactions stimulate resistance to host innate immunity through metabolite perception

“…A. actinomycetemcomitans strains VT1169 (44), Y4 (45), S. strain Challis DL1.1 (ATCC 49818), E. coli DH5␣, and E. coli SM10 were used in this study. A. actinomycetemcomitans strains were grown in brain heart infusion medium, tryptic soy broth ϩ 0.5% yeast extract (TSBYE), or chemically defined medium (CDM) with 20 mM glucose (15). Culture conditions were at 37°C in a 10% (vol/vol) CO 2 atmosphere with shaking at 165 rpm unless otherwise indicated.…”

“…Indeed, previous studies demonstrated that A. actinomycetemcomitans preferentially utilizes lactic acid over high-energy carbon sources, such as glucose, despite the fact that this bacterium grows significantly more slowly with lactic acid (15). The ability to preferentially use lactic acid not only eliminates caries-causing lactic acid from the oral environment but eliminates the need for A. actinomycetemcomitans to compete with the more numerous and rapidly growing oral streptococci for carbon (10).…”

“…Construction and complementation of the oxyR Ϫ mutant were performed as previously described (15) and are detailed in SI Text. Serum Sensitivity Factor H Binding.…”

Real-time mapping of a hydrogen peroxide concentration profile across a polymicrobial bacterial biofilm using scanning electrochemical microscopy

“…However, recent work has demonstrated that in vitro Sg can grow in coculture with the opportunistic oral pathogen Aggregatibacter actinomycetemcomitans (Aa) (4). In co-culture Aa preferentially utilizes Sg-produced lactic acid (5) and detoxifies Sg-produced hydrogen peroxide using the KatA enzyme (6). Recent work has demonstrated that hydrogen peroxide induces katA expression as well as apiA, which encodes an immunoprotective factor that renders Aa more resistant to killing by host innate immunity (5).…”

“…In co-culture Aa preferentially utilizes Sg-produced lactic acid (5) and detoxifies Sg-produced hydrogen peroxide using the KatA enzyme (6). Recent work has demonstrated that hydrogen peroxide induces katA expression as well as apiA, which encodes an immunoprotective factor that renders Aa more resistant to killing by host innate immunity (5). These studies demonstrated induction of gene expression in mixed species biofilms by Sg-produced hydrogen peroxide.…”