Microbial Diversity in the Human Mouth: Dominant Genera and their Interactions.

Investigations seeking definition of the microbial diversity present in the human oral cavity have been underway for many years. These investigations seek to define the normal microflora of the oral cavity, as well as the stability of the microflora. Predominantly, these kinds of studies have focused on the differences in microbial carriage between healthy and diseased oral cavities and have identified several species which may be causative and promote oral diseases such as periodontitis and halitosis. Less is known about how co-habitants of a healthy oral cavity interact and how those interactions are mediated and controlled. The genes that may be involved in microbial interactions and their regulation and roles within response pathways in oral microbes is an interesting area of research to pursue, as it could give valuable insight into the changing genetic and potentially physiological state of oral microbes.

An evaluation of oral microbial diversity performed during this study by Restriction Fragment Length Polymorphism (RFLP) and Fatty Acid Methyl Ester (FAME) profiling analyses carried out on whole mouth rinse samples identified many bacterial genera from six phyla. These genera included Streptococcus, Gemella, Veillonella, Eubacterium, Porphyromonas, Neisseria, Rothia and Prevotella. The three prevalent genera, as defined by the number of times observed from mouth rinse samples, were Streptococcus, Neisseria and Veillonella.

Solid-phase co-culture experiments using isolates of the three prevalent genera revealed a strong interaction between Streptococcus and Neisseria, with Streptococcus exerting a negative effect on Neisserial growth. Liquid phase co-culture experiments between these two genera confirmed initial observations and it was hypothesised that the interaction was mediated by toxic hydrogen peroxide produced by Streptococcus. Further experiments showed that Neisseria were protected from Streptococcal killing upon addition of exogenous catalase, confirming that the interaction was mediated by hydrogen peroxide.

The roles of the key genes of the oxidative stress response of Neisseria were investigated by assessing the interactions between Streptococcus pneumoniae and several mutant Neisseria meningitidis, defective in one or more genes of the oxidate stress response. These experiments resulted in observations differing to those previously published, particularly in relation to the prx gene (encoding the peroxidase peroxiredoxin), which lead to the proposal of multiple pathways of stress response existing in N. meningitidis. It was observed that N. meningitidis survival was increased in the absence of the bcp gene (encoding the peroxidase bacterioferritin co-migratory protein) and it was hypothesised that this was due to an up-regulation of other peroxidases in this mutant. Real-Time PCR (RT-PCR) experiments showed increased expression of prx in a bcp-deficient N. meningitidis mutant in the presence of hydrogen peroxide, but this up-regulation was only marginal in the un-stressed mutant strain.

Furthermore, it was shown that in the presence of high-levels of hydrogen peroxide (1 mM), N. meningitidis and the various mutant strains examined, behaved as previously observed. This gives rise to the notion that the hydrogen peroxide levels produced in co-culture with S. pneumoniae are not high enough to induce the typical oxidative stress response in N. meningitidis, indicating that another pathway(s) must exist in order to deal with low / intermediate-level hydrogen peroxide / organic peroxides experienced during co-colonisation of the oral cavity.