Dental double act

Interactions between Actinomyces naeslundii and Streptococcus gordonii in the presence of salivary factors are important for stable dental plaque communities. A study published in FEMS Microbiology Ecology shows that co-aggregation of S. gordonii with A. naeslundii temporarily decreases the intracellular levels of hydrogen peroxide (H₂O₂) in S. gordonii, thereby protecting streptococcal proteins from oxidative damage.

Saliva, from which oral bacteria derive nutrients, contains low concentrations of the amino acid arginine. Recent work has shown that co-aggregation of S. gordonii with A. naeslundii results in the stabilization of S. gordonii arginine biosynthesis genes, enabling the bacteria to grow under conditions of scarce arginine. However, co-aggregation also led to increased activity of the S. gordonii pyruvate oxidase, SpxB, which produces H₂O₂ as a by-product. SpxB activity can produce sufficient H₂O₂ to kill neighbouring bacteria by causing irreversible oxidative damage to amino-acid residues, such as arginine, which leads to a loss of protein function.

Given that streptococci do not produce catalase, which removes H₂O₂, the benefit gained from increased arginine biosynthesis during co-aggregation may be abrogated by the cellular protein damage that is caused by increased H₂O₂ production. To address this issue, Jakubovics and colleagues set out to determine whether co-aggregation with A. naeslundii affected concentrations of H₂O₂ in S. gordonii.

The authors confirmed that co-aggregation with A. naeslundii increased the viability of S. gordonii grown in the absence of arginine. The concentration of H₂O₂ was reduced in co-aggregate cultures compared with S. gordonii monocultures owing to catalase production by A. naeslundii. In co-aggregate cultures, decreased protein oxidation corresponded with the reduction in H₂O₂ concentration, but this protective effect was only temporary. As S. gordonii became dominant in the co-aggregates, the increasing H₂O₂ concentration became toxic to A. naeslundii.

Co-aggregation of S. gordonii with A. naeslundii might have two benefits for the streptococci: first to increase arginine biosynthesis when required and second to reduce H₂O₂ levels. This work reveals the complexity of inter-bacterial interactions in multi-species communities that occur widely in nature.