Competition between Fluoride-Resistant and Fluoride-Sensitive Streptococcus mutans in Rat Dental Plaque

Abstract

The experiments were designed to study the effects of fluoride on the competition between a fluoride-sensitive strain of Streptococcus mutans (C180–2) and a fluoride-resistant mutant (S. mutans C180-2FR) in rat dental plaque. Competition was induced either by superinfecting rats or by co-caging rats infected with either strain with non-infected rats. Sodium fluoride was applied in diet and drinking water (60 ppm F), as daily brushings with a 1% F solution, or as a combination of both administrations. The fluoride applications favoured S. mutans C180-2FR, but not enough to outcompete the parent strain successfully under all conditions. Fluoride protected rat dental plaque harbouring S. mutans C180-2FR against colonization by the parent strain. In rats not previously infected with S. mutans, the fluoride in diet and drinking water in combination with the fluoride brushings favoured S. mutans C180-2FR in colonizing rat dental plaque. However, the fluoride applications did not benefit S. mutans C180-2FR in colonizing plaque in which the parent strain was already present. Human dental plaque may be colonized by fluoride-resistant S. mutans strains after spontaneous mutation in response to a fluoride pressure. Our data suggest that this pressure may also provide an ecological advantage to this mutant to colonize the dental plaque.

Fluoride resistance inStreptococcus mutans: a mini review

“…Van Loveren (1989) reported less severe dentinal lesions in rats superinfected with a fluoride-resistant strain than in rats superinfected with the wild-type strain [50]. This result was in line with the results of the competition test, in which the same strains were examined [55,58]. However, when the number of all visible lesions, including enamel and dentine lesions, were taken into account, the two strains showed similar cariogenic potentials, irrespective of whether fluoride was included in the diet [50].…”

“…Controversial results have also been reported for the competition test, which examined the competition between fluoride-resistant and fluoride-sensitive strains to bind to enamel. A fluoride-resistant strain was unable to bind to teeth when its wild-type strain was present [55,58]. This result was challenged by Hoelscher, who found another fluoride-resistant strain to be capable of binding to the same extent as the wild-type strain when they had been mixed in equal proportions [47].…”

“…Likely, the growth rate of each strain determines the outcome of the competition. In the former study [55,58], the growth rate of the fluoride-resistant strain was reported to be lower than its wild-type strain, while the strains in the latter study exhibited similar generation time [47]. …”

Identification and Functional Analysis of Genome Mutations in a Fluoride-Resistant Streptococcus mutans Strain

“…mutans C180-2 [ 22 ] and the derived fluoride-resistant strain C180-2FR [ 7 ]. These two strains were chosen because the process of obtaining a C180-2 fluoride-resistant strain in the laboratory is well documented [ 23 ]. The phenotypic differences between the two strains have been described previously [ 6 , 19 ].…”

The Fitness Cost of Fluoride Resistance for Different Streptococcus mutans Strains in Biofilms

“…These fitness costs made antimicrobial-resistant microbes less competitive than susceptible strains when the selective pressure from antimicrobials was removed, which might eventually result in the loss of the antimicrobial resistance ( Andersson and Hughes, 2010 ). In order to understand the effect of fluoride resistance on bacterial fitness, several laboratory-derived fluoride-resistant S. mutans strains were compared to their isogenic wild-type strains on growth, acidogenicity, and surface adherence in vitro or in animal models ( Rosen et al, 1978 ; Lau and Kral, 1987 ; Van Loveren et al, 1991a ; Liao et al, 2015 ). However, the results from these studies were inconsistent.…”