Evidence that Accumulation of Mutants in a Biofilm Reflects Natural Selection Rather than Stress-Induced Adaptive Mutation

Abstract

The accumulation of mutant genotypes within a biofilm evokes the controversy over whether the biofilm environment induces adaptive mutation or whether the accumulation can be explained by natural selection. A comparison of the virulence of two strains of the dental pathogen Streptococcus mutans showed that rats infected with one of the strains accumulated a high proportion (average, 22%) of organisms that had undergone a deletion between two contiguous and highly homologous genes. To determine if the accumulation of deletion mutants was due to selection or to an increased mutation rate, accumulations of deletion mutants within in vitro planktonic and biofilm cultures and within rats inoculated with various proportions of deletion organisms were quantified. We report here that natural selection was the primary force behind the accumulation of the deletion mutants.

Competence-Dependent Endogenous DNA Rearrangement and Uptake of Extracellular DNA Give a Natural Variant of Streptococcus mutans without Biofilm Formation

“…S. mutans is organized on biofilms, where the cells are closely packed together and multiple species of microorganisms coexist. A previous study indicates that the S. mutans WIG Ϫ variant has a stronger presence within biofilm cells than among planktonic cells (3). Our data are in agreement with this (Fig.…”

“…Previous studies have shown that recombination of gtfB and gtfC causes the formation of the WIG Ϫ strain and is RecA dependent (3). Previous studies with Streptococcus pneumoniae found the that recA gene was induced at competence (23,24), and competence-specific induction of recA was also demonstrated in Bacillus subtilis (22).…”

Antimicrobial effects of the combination of chlorhexidine and xylitol

“…23 Furthermore, S. mutans is able to demineralise hydroxyapatite, thus demonstrating its relevance in the development of caries. 1,8,24 This key role played by S. mutans in the formation of cariogenic biofilm is a hypothesis already accepted in dentistry, since it is the best explanation for caries aetiology. 25 Therefore, reducing the levels of this micro-organism in the oral cavity seems to be crucial for controlling caries in general.…”

“…It induces mineral loss due to its strong adhesion to the tooth surface and to the acid production resulting from fermentable carbohydrates, which keeps local pH low. 1 The biofilm produced by this pathogen accounts for S. mutans adhesion, and is therefore considered to be cariogenic as well. 2 Chlorhexidine (CHX) 3,4 and xylitol (XYL) 5 have been used as strategies to prevent and reduce carious lesions.…”

Bacterial SOS response: a food safety perspective

“…Adaptive mutagenesis or natural selection of resistant cells may occur in bacterial biofilms and some evidence exists that these biofilms are responsible for the persistence of adapted strains in the industry or in human infections [47,48]. While the underlying mechanism of adaptation is not known exactly, biofilms are a source of natural diversity within bacterial populations [49].…”