Growth, development, and gene expression in a persistent Streptococcus gordonii biofilm

Abstract

A model for the protracted (30-day) colonization of smooth surfaces by Streptococcus gordonii that incorporates the nutrient flux that occurs in the oral cavity was developed. This model was used to characterize the biphasic expansion of the adherent bacterial population, which corresponded with the emergence of higher-order architectures characteristic of biofilms. Biofilm formation by S. gordonii was observed to be influenced by the presence of simple sugars including sucrose, glucose, and fructose. Real-time PCR was used to quantify changes in expression of S. gordonii genes known or thought to be involved in biofilm formation. Morphological changes were accompanied by a significant shift in gene expression patterns. The majority of S. gordonii genes examined were observed to be downregulated in the biofilm phase. Genes found to be upregulated in the biofilm state were observed to encode products related to environmental sensing and signaling.

FIG. 1.

Confocal scanning laser micrographs of the development of S. gordonii Challis DL1 biofilms over 26 days. The biofilms were generated on glass coverslips in 0.5× TY medium supplemented with 10 mM sucrose and were stained immediately prior to microscopy with acridine orange. Numbers indicate the age of the biofilm in days. Magnification, ×63.

FIG. 2.

Change in thickness of biofilms with time, as measured by confocal microscopy. The depth of the biofilm represents the mean of five randomly chosen sites within each biofilm.

FIG. 3.

Change in CFU within S. gordonii Challis DL1 biofilms cultured in 0.5× TY medium supplemented with sucrose, which was replaced at 24-h intervals. Cells were enumerated over 30 days of cultivation. Numbers of CFU for biofilms harvested at each time point are expressed as means of triplicate determinations. Error bars represent the standard errors of the means and are not detectable where the error is smaller than the symbol.

FIG. 4.

Effects of medium composition on partitioning of S. gordonii DL1 into biofilm and planktonic phases after 4 days of cultivation. Reduction in the yeast extract and tryptone contents of the medium resulted in increased numbers of CFU occurring in the biofilm phase in all cases where the medium was supplemented with 10 mmol of a simple sugar (sucrose, glucose, or fructose). Lack of carbohydrate supplementation of either TY or 0.5× TY resulted in comparable numbers in the biofilm and planktonic phases, and supplementation of TY with sucrose resulted in an unexpected increase in the number of planktonic cells, with no net change in numbers in the biofilm phase.