The Influence of Growth Rate and Nutrient Limitation on the Microbial Composition and Biochemical Properties of a Mixed Culture of Oral Bacteria Grown in a Chemostat

Publisher Website
Google Scholar
Abstract
A sample of human dental plaque was homogenized in transport fluid and inoculated simultaneously into a glucose-limited and a glucose-excess chemostat maintained at pH 7.0 and a dilution rate (D) of 0.05 h-1. To ensure the establishment of slow-growing bacterial populations, 2 further inoculations of each chemostat with fresh samples of dental plaque took place before a steady-state was attained at this D. D was increased step-wise to D = 0.6 h-1 and then returned directly to D = 0.05 h-1. Contrary to chemostat theory. Microbial communities with a high species diversity were maintained under all of the experimental conditions employed, not all of the bacterial populations present in the inocula established successfully in the chemostat. At each steady-state, the bacteriological composition and biochemical properties (fermentation products, enzyme assays and acid production) of the communities of each chemostat were determined. Higher cell yields and a slightly more diverse community were obtained from the glucose-limited chemostat at all dilution rates. A complex mixture of metabolic end products was obtained from the glucose-limited chemostat, suggesting amino acid catabolism; lactate was the predominant acid of the glucose-excess culture. In washed-cell experiments, communities from the glucose-excess chemostat produced the lower terminal pH values following a pulse of glucose, with the lowest pH values occurring at the higher dilution rates. A film of microorganisms, which accumulated around the neck of the chemostat, was sampled at the end of the experiment. The microbial composition of the films from each chemostat differed marked; both were different from the community of the bulk fluid of the respective chemostat. Spirochetes and a population of yeasts were detected in the films from the glucose-limited and glucose-exces chemostats, respectively. No invertase or glucosyltransferase activity, and little glucoamylase-specific glycogen was detected in the communities from either chemostat; significant endogenous activity, particularly at high dilution rates was obtained with washed-cells from the glucose-excess chemostat. Thus, the chemostat could make a valuable contribution to the study of the ecology of dental plaque.