Abstract
The biofilm mode of growth can lead to diversification of the bacterial population by promoting the emergence of variants. Here we report the identification and characterization of two major subpopulations of morphological variants arising in biofilms of S. aureus. One of these lacked pigmentation (termed white variants; WVs), whilst the other formed colonies on agar that were larger and paler than the parental strain (termed large pale variants; LPVs). WVs were unable to form biofilms, and exhibited increased proteolysis and haemolysis; all phenotypes attributable to loss-of-function mutations identified in the gene encoding the alternative sigma factor, sigB. For LPVs, no differences in biofilm forming capacity or proteolysis were observed compared with the parental strain. Genetic analysis of LPVs revealed that they had undergone mutation in the accessory gene regulator system (agrA), and deficiency in agr was confirmed by demonstrating loss of both colony spreading and haemolytic activity. The observation that S. aureus biofilms elaborate large subpopulations of sigB and agr mutants, both genotypes that have independently been shown to be of importance in staphylococcal disease, has implications for our understanding of staphylococcal infections involving a biofilm component.
Highlights
A growing body of evidence supports the idea that even monospecies biofilms comprise phenotypically and genotypically heterogeneous populations [1,2,3,4]
Two colony morphologies distinct from the parental strain were identified; nonpigmented colonies, which we designated white variants (WVs), and large pale variants (LPVs) (Figure 1)
WVs and LPVs were detected in biofilms of S. aureus SH1000 grown for 96 hrs in a constant flow system, indicating that the appearance of LPVs and WVs is a general feature of staphylococcal biofilm growth, and is not uniquely associated with biofilms forming under static conditions
Summary
A growing body of evidence supports the idea that even monospecies biofilms comprise phenotypically and genotypically heterogeneous populations [1,2,3,4] This heterogeneity suggests that the biofilm mode of growth prompts diversification of the bacterial population by promoting the emergence of variants, and several bacterial species (e.g. Pseudomonas aeruginosa, Streptococcus pneumoniae and Staphylococcus aureus) have been shown to elaborate morphological variants during biofilm growth [1,5,6,7]. It has been demonstrated that the biofilms of some species contain large subpopulations of small colony variants (SCVs) [10,11]; such variants exhibit reduced susceptibility to a variety of antimicrobial agents [6,12], and may play a key role in establishing chronic infection [13]
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