Apical periodontitis is a chronic inflammatory disorder of periradicular tissues caused by microorganisms surviving in periapical tissues. We isolated Rothia mucilaginosa from a periapical periodontitis lesion where the presence of bacteria was noted in root canals after numerous treatments. Strain DY-18 was randomly selected from this case for further study. Scanning electron micrographs revealed that strain DY-18 possesses unique phenotypic characteristics, exhibiting dense meshwork-like structures on their cells. The results of staining with Calcofluor, a polysaccharide binding dye, suggested that strain DY-18 produces exopolysaccharide (EPS). Our previous studies suggested that EPS is a major component of bacterial biofilms in the oral cavity. High performance liquid chromatography showed that the EPS purified from culture supernatant contained neutral sugars including galactose, mannose, rhamnose and glucose, and amino sugars including glucosamine and galactosamine, but not uronic acid or extracellular DNA. Additionally, there are some genes relating to galactose and mannose metabolism on the genome of strain DY-18. Our results suggest that strain DY-18 has the ability to form a biofilm containing EPS.

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It is well known that biofilm-forming bacteria cause chronic or persistent infections. The attachment property is an important factor in early biofilm development. However, the genes associated with attachment on a variety of abiotic materials remain poorly understood. The aim of this study was to identify gene(s) responsible for attachment to abiotic materials in a clinically isolated Actinomyces oris strain K 20 (strain K 20), which is a biofilm-forming strain that continuously produces mannose-rich exopolysaccharide. We performed a random mutagenesis to strain K 20 by using EZ-Tn 5^<TM> <KAN-2> Tnp Transposome^<TM> (EZ-Tn 5). Screening by adhesion activity on glass or polystyrene surfaces identified one hyper adherent mutant among 904 isolated transposants. The gene disrupted by the transposon insertion was determined by direct genome sequencing with the EZ-Tn 5 transposon-end primers. This mutant, designated as strain M 5-7, had a transposon insertion within a gene encoding a polypeptide with membrane-spanning domains. Hydropathy plot and PSI-BLAST analysis implied that this gene might belong to the transporter family, which includes permease.

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