EFFECT OF MUCIN ON EXPONENTIAL GROWTH, STATIONARY PHASE SURVIVAL AND BIOFILM FORMATION IN STREPTOCOCCUS MUTANS

Abstract

Streptococcus mutans is a member of the dental plaque and is the principal causative agent of dental caries. It can metabolize a wide array of sugars which results in the production of acid that causes demineralization of the tooth surface. S. mutans can also persist for extended periods of starvation, which may occur in different niches within the oral cavity. Previous studies have shown that mucin prolonged the survival of S. mutans in batch cultures and biofilms. Our laboratory has shown that the pyruvate dehydrogenase (pdh) operon is upregulated in stationary phase and is important for prolonged survival during nutrient starvation in chemically defined medium (CDM). In this study, I found that mucin enabled S. mutans to grow in a minimal chemically defined medium (MCDM) containing glucose and lacking all amino acids. A pdh mutant was found to be impaired in survival in CDM or MCDM supplemented with mucin. Additionally, I have shown that a galactose utilization mutant was impaired in survival in CDM with mucin and no exogenous glucose; galactose is the most abundant utilizable sugar residue in mucin. Mucin, supplemented in minimal medium with sucrose, also enabled S. mutans to form biofilms. Survival of the biofilms was pdh independent. Although biofilm formation was not seen in the absence of sucrose, cells did adhere to the glass coverslip; their survival was found to be pdh dependent. Using a previously constructed reporter strain, pdh expression was observed in a majority of cells in this condition. The expression of the pdh operon was also monitored in batch cultures. The starting inoculum size affected the percentage of fluorescent cells and the outcome of survival in media (CDM or MCDM) supplemented with mucin and no sugar. Greater than 50% of the stationary phase population expressed pdh in CDM with galactose. Less than 1% expressed pdh in CDM with glucose. However, galactose was unable to prolong survival of S. mutans in batch cultures in contrast to the effect of mucin. These results show that mucin may have a metabolic role in promoting long term survival of S. mutans. However it is possible that different components of mucin can be utilized in different conditions.