Experimental aspiration pneumonia induced by human supragingival dental plaque in mice

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Human dental plaque consists of a wide variety of microorganisms, some of which are believed to cause systemic infections, including aspiration pneumonia. To confirm this hypothesis experimentally, we examined the pneumonia-inducing ability of dental plaque in mice, the microbial features of the infectious locus produced by the plaque. Aliquots of a suspension of supragingival dental plaque containing 1 x 10^7 CFU of bacteria were delivered into the lower part of the trachea of mice. All mice exhibited signs of illness immediately after inoculation such as ruffled fur, labored respiration, and appeared waste. In total, 12 out of 18 mice (66.7%) died within 3 days after inoculation with dental plaque suspension. All the 3 human dental plaque samples tested induced pneumonia in mice. Bacterial cells were recovered from the lungs of all the mice that died, and the number was 4.44 ± 0.77 log_<10>CFU/mg of lung. From 5 out of 6 survived mice, bacterial cells were recovered from the lungs and the number was 0.38±0.38 log_<10>CFU/mg of lung. Various microbial species were recovered from infected lungs examined, and no characteristic bacterial distribution was observed both in the survived mice and the mice that died within 3 days after plaque inoculation. Representative isolates of Streptococcus oralis, Streptococcus intermedius and Gemella morbillorum recovered from lungs showed ability to induce pneumonia, respectively. These results suggest that human supragingival dental plaque is a source of the infectious pathogens that cause pneumonia.