During Na + stress both parent and mutant cells contained higher levels of intracellular methionine sulfoxide, attributable to the treatment rather than a mutation. Methionine is modified to methionine sulfoxide by reactive oxygen species and to repair oxidative damage the sulfoxide is converted back to methionine by methionine sulfoxide reductases. Interestingly, in the response of B. fragilis to air exposure methionine sulphoxide reductase was induced more than ten-fold [33], however, there were no significant changes in the expression of this gene in P. gingivalis strains under Na + stress (data not shown). As an asaccharolytic organism, P. gingivalis preferentially uses dipeptides as carbon, nitrogen and energy sources. The metabolite **data** indicated that in addition to the preferred aspartate and glutamate containing peptides, Na + stressed cells showed an increased ability to use diverse peptides, notably alanyl peptides with leucine and glutamate. Whether this expansion of growth substrates is an adaptation that offsets growth limiting conditions is as yet unknown.