Monday, 12 September 2016 11:28

Scraping away at the complex microbial communities that cause periodontal disease

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Published in mBiosphere

Though both gingivitis and periodontitis are diseases of the gums, the related ailments are not simply different severities of the same disease, finds a new study published in Applied and Environmental Microbiology. Researchers confirmed this by investigating the bacterial composition of the supragingival plaque through high- throughput sequencing.

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First author Liam Shaw and senior author Nigel Klein looked at the supragingival plaque (plaque on the tooth surface above the gum line) in a low-income community that suffers poor oral hygiene and a variety of gum diseases. Poor oral hygiene is tied to a number of systemic health conditions, including heart disease, so diagnosing and treating this relatively superficial infection can have major health benefits.

Gingivitis, inflammation of the gums surrounding the teeth, is always a feature of periodontitis, which occurs when the gums pull away from the teeth and open up pockets in which bacteria can grow. In periodontitis, inflamed gums are accompanied by loose teeth and pain while chewing, which will eventually culminate in tooth loss if left untreated. “But diagnosing periodontitis visually is impossible and it doesn’t usually give any symptoms until it has developed so far that teeth become mobile, which is very late for any treatment,” said study coauthor Ulla Harjunmaa.

Further, gingivitis always precedes periodontitis – but it doesn’t always progress to this more serious disease. This further complicates diagnoses. The scientific team wanted to investigate whether the bacterial communities associated with supragingival plaque could be associated with healthy gums, gingivitis, or periodontitis.

To test this idea, the researchers tested plaque samples from a cohort of over 900 Malawian women with various stages of oral health. The bacterial composition of their plaque was determined using 16S rRNA gene sequencing. Sequencing from severe gingivitis and periodontitis showed a significant shift in bacterial populations from the plaque of healthy gums.

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Disease type and severity correlated with these population shifts. The most severe gingivitis was associated with the presence of Peptostreptococcus stomatis, an anaerobic Gram-negative bacterium that often lives as part of the microbiota without causing disease (see Gram-stained image, left). Over 75% of plaque samples in moderate and severe gingivitis showed increases on P. stomatis populations.

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Co-occurrence of periodontal-associated bacteria with periodontitis symptoms

Periodontitis was associated with increases in Lautropia mirabilis, Rothia aeria, Streptococcus pyogenes, Streptococcus mutans, and seven members of Actinomyces. Sequencing identified red complex member Treponema denticola, but not Porphyromonas gingivalis, which the researchers speculate may be due to primer mismatch. Treponema, Prevotella, and Selemonas genera were uniquely seen in periodontitis patients and not patients with gingivitis or healthy gums. The researchers could therefore use the association of these periodontitis-associated bacteria to identify gingivitis patients likely to harbor this more difficult to diagnose disease (see right).

The red complex, a microbial group whose members are associated with severe periodontal disease, was defined in a study of subgingival plaque (plaque below the gumline). These important studies looked at the site of periodontitis, which is when periodontal pockets of bacterial growth appear below the gumline. However, no study has looked at bacterial composition in such a large patient cohort. Additionally, differentiating bacterial communities of the more accessible supragingival plaque could allow dentists to assess gum disease progression in both a more cost-effience way for low-resource environments and an easier, less painful way for patients.

The discovery of unique bacterial community compositions in gingivitis versus periodontitis ‘confirm that periodontitis cannot be considered simply an advanced stange of gingivitis, even when only considering supragingival plaque,” said Shaw. “Where diseases can have multiple severities and clinical features that are also found in non-disease (like bleeding gums in periodontitis), this kind of modeling will be increasing important in microbiome research.”

Photo credits: Plaque schematic, Peptostreptococcus Gram Stain, Figure from AppEnvMicro report

Julie Wolf

ASM Communications Social Media Specialist Julie Wolf spent her research career focused on medical mycology and infectious disease. Broadly interested in microbiology and scientific communication, she has taught at Long Island University and the community biolab Genspace and has written for the Scientista Foundation and Scholastic’s Science World magazine. Follow her on Twitter for more ASM and Microbiology highlights at @JulieMarieWolf.