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Root caries in geriatric patients is a growing problem as more people are maintaining their natural teeth into advanced age. We determined the levels of various bacterial species previously implicated in root caries disease or health using quantitative real-time PCR in a pilot study of 7 patients with 1 to 4 root caries lesions per person. Levels of 12 different species on diseased roots compared to healthy (contralateral control) roots were measured. Four species were found at significantly higher levels on diseased roots (Streptococcus mutans, Veillonella parvula/dispar, Actinomyces naeslundii/viscosus, and Capnocytophaga granulosa) compared across all plaque samples. The level of colonization by these species varied dramatically (up to 1,000-fold) between patients, indicating different patients have different bacteria contributing to root caries disease. Neither of the two species previously reported to correlate with healthy roots (C. granulosa and Delftia acidovorans) showed statistically significant protective roles in our population, although D. acidovorans showed a trend toward higher levels on healthy teeth (P = 0.08). There was a significant positive correlation between higher levels of S. mutans and V. parvula/dispar on the same diseased teeth. In vitro mixed biofilm studies demonstrated that co-culturing S. mutans and V. parvula leads to a 50 to 150% increase in sucrose-dependent biofilm mass compared to S. mutans alone, depending on the growth conditions, while V. parvula alone did not form in vitro biofilms. The presence of V. parvula also decreased the acidification of S. mutans biofilms when grown in artificial saliva and enhanced the health of mixed biofilms.

Many pathogenic microbial ecosystems are polymicrobial, and community function can be shaped by interbacterial interactions. Little is known, however, regarding the genetic determinants required for fitness in heterotypic community environments. In periodontal diseases, Porphyromonas gingivalis is a primary pathogen, but only within polymicrobial communities. Here, we used a transposon sequencing (Tn-Seq) library of P. gingivalis to screen for genes that influence fitness of the organism in a coinfection murine abscess model with the oral partner species Streptococcus gordonii and Fusobacterium nucleatum. Genes impacting fitness with either organism were involved in diverse processes, including metabolism and energy production, along with cell wall and membrane biogenesis. Despite the overall similarity of function, the majority of identified genes were specific to the partner species, indicating that synergistic mechanisms of P. gingivalis vary to a large extent according to community composition. Only two genes were identified as essential for P. gingivalis fitness in abscess development with both S. gordonii and F. nucleatum: ptk1, encoding a tyrosine kinase, and inlJ, encoding an internalin family surface protein. Ptk1, but not InlJ, is required for community development with S. gordonii, and we found that the action of this kinase is similarly required for P. gingivalis to accumulate in a community with F. nucleatum. A limited number of P. gingivalis genes are therefore required for species-independent synergy, and the Ptk1 tyrosine kinase network may integrate and coordinate input from multiple organisms.

Patients with cystic fibrosis (CF) experience lifelong respiratory infections which are a significant cause of morbidity and mortality. These infections are polymicrobial in nature, and the predominant bacterial species undergo a predictable series of changes as patients age. Young patients have populations dominated by opportunists that are typically found within the microbiome of the human nasopharynx, such as nontypeable Haemophilus influenzae (NTHi); these are eventually supplanted and the population within the CF lung is later dominated by pathogens such as Pseudomonas aeruginosa (Pa). In this study, we investigated how initial colonization with NTHi impacts colonization and persistence of Pa in the respiratory tract.Analysis of polymicrobial biofilms in vitro by confocal microscopy revealed that NTHi promoted greater levels of Pa biofilm volume and diffusion. However, sequential respiratory infection of mice with NTHi followed by Pa resulted in significantly lower Pa as compared to infection with Pa alone. Coinfected mice also had reduced airway tissue damage and lower levels of inflammatory cytokines as compared with Pa infected mice. Similar results were observed after instillation of heat-inactivated NTHi bacteria or purified NTHi lipooligosaccharide (LOS) endotoxin prior to Pa introduction. Based on these results, we conclude that NTHi significantly reduces susceptibility to subsequent Pa infection, most likely due to priming of host innate immunity rather than a direct competitive interaction between species. These findings have potential significance with regard to therapeutic management of early life infections in patients with CF.