Lactate dehydrogenase-deficient mutants of serotype g Streptococcus mutans

Effect of nicotine on cariogenic virulence of Streptococcus mutans

Nicotine has well-documented effects on the growth and colonization of Streptococcus mutans. This study attempts to investigate the effects of nicotine on pathogenic factors of S. mutans, such as the effect on biofilm formation and viability, expression of pathogenic genes, and metabolites of S. mutans. The results demonstrated that addition of nicotine did not significantly influence the viability of S. mutans cells. The biofilms became increasingly compact as the concentrations of nicotine increased. The expression of virulence genes, such as ldh and phosphotransferase system (PTS)-associated genes, was upregulated, and nlmC was upregulated significantly, while ftf was downregulated. The lactate concentration of S. mutans grown in 1 mg/mL of nicotine was increased up to twofold over either biofilm or planktonic cells grown without nicotine. Changes in the metabolites involved in central carbon metabolism from sucrose indicated that most selected metabolites were detectable and influenced by increased concentrations of nicotine. This study demonstrated that nicotine can influence the pathogenicity of S. mutans and may lead to increased dental caries through the production of more lactate and the upregulation of virulence genes.

Cariogenicity of a lactate dehydrogenase-deficient mutant of Streptococcus mutans serotype c in gnotobiotic rats

A lactate dehydrogenase-deficient (Ldh-) mutant of a human isolate of Streptococcus mutans serotype c was tested in a gnotobiotic rat caries model. Compared with the wild-type Ldh-positive (Ldh+) strains, it was significantly (alpha less than or equal to 0.005) less cariogenic in experiments with two different sublines of Sprague-Dawley rats. The Ldh- mutant strain 044 colonized the oral cavity of the test animals to the same extent as its parent strain 041, although its initial implantation was slightly but not significantly (P greater than or equal to 0.2) less. Multiple oral or fecal samples plated on 2,3,5-triphenyltetrazolium indicator medium revealed no evidence of back mutation from Ldh- to Ldh+ in vivo. Both Ldh+ strain 041 and Ldh- strain 044 demonstrated bacteriocinlike activity in vitro against a number of human strains of mutans streptococci representing serotype a (S. cricetus) and serotypes c and e (S. mutans). Serotypes b (S. rattus) and f (S. mutans) and strains of S. mitior, S. sanguis, and S. salivarius were not inhibited. Thus, Ldh mutant strain 044 possesses a number of desirable traits that suggest it should be investigated further as a possible effector strain for replacement therapy of dental caries. These traits include its stability and low cariogenicity in the sensitive gnotobiotic rat caries model, its bacteriocinlike activity against certain other cariogenic S. mutans (but not against more inocuous indigenous oral streptococci), and the fact that it is a member of the most prevalent human serotype of cariogenic streptococci.

Serotype c Streptococcus mutans mutatable to lactate dehydrogenase deficiency

Three lactate-dehydrogenase-deficient mutants of serotype c S. mutans were made by using, as parents, two serotype c strains that produced unusually large amounts of ethanol, acetic acid, and acetoin, and very little lactic acid, when grown in broth containing a limiting amount of glucose. The mutants, obtained with N-methyl-N'-nitro-N-nitrosoguanidine, were stable during 12 weeks of daily subculture in broth. Crude cell-free extracts of the mutants had less than 1% of the LDH-specific activity of their parent strains. The serotype c mutants resembled serotype g mutants in having molar growth yields at least as high as those of their parents. However, in contrast to the g mutants, the c mutants produced cell crops (cell mass per ml medium) that were as high as those of their parent strains.