An ffh mutant of Streptococcus mutans is viable and able to physiologically adapt to low pH in continuous culture.

Previously, we described in Streptococcus mutans strain NG8 a 5-gene operon (sat) that includes ffh, the bacterial homologue of the eukaryotic signal recognition particle (SRP) protein, SR54. A mutation in ffh resulted in acid sensitivity but not loss of viability. In the present study, chemostat-grown cells of the ffh mutant were shown to possess only 26% and 39% of the parental membrane F-ATPase activity and 55% and 75% of parental glucose-phosphotransferase (PTS) activity when pH-7 and pH-5-grown cells, respectively, were assayed. Two-dimensional-gel electrophoretic analyses revealed significant differences in protein profiles between parent and ffh-mutant strains at both pH 5 and pH 7. It appears that the loss of active SRP (Ffh) function, while not lethal, results in substantial alterations in cellular physiology that includes acid tolerance. Copyright 2004 Federation of European Microbiological Societies