Competition between Streptococcus mutans and Lactobacillus casei in mixed continuous culture

Area of Science:

• Oral microbiology
• Dental caries research
• Bacterial community dynamics

Background:

• Oral ecosystems feature bacterial succession, notably Streptococcus mutans and Lactobacillus species.
• These successional patterns are linked to the development of dental caries.
• Understanding inter-species interactions under varying conditions is crucial for caries prevention.

Purpose of the Study:

• To investigate the interaction between Streptococcus mutans (S. mutans) and Lactobacillus casei (L. casei).
• To explore how environmental pH and fluoride influence these interactions in continuous culture.
• To determine the impact of pH fluctuations on the dominance of these key oral bacteria.

Main Methods:

• Continuous culture of S. mutans and L. casei strains (SF2452 and RB1014) isolated from carious lesions.
• Manipulation of environmental pH (from 7.0 down to 4.8) and observation of bacterial population dynamics.
• Assessment of bacterial succession and dominance under glucose-limited conditions.

Main Results:

• Initially, S. mutans dominated at pH 7.0.
• A decrease in pH to 5.1 led to L. casei predominance.
• Rapid pH reduction to 4.8 significantly reduced S. mutans, allowing L. casei to take over; pH restoration reversed this trend.

Conclusions:

• Environmental pH is a critical factor regulating the competitive balance between S. mutans and L. casei.
• L. casei demonstrates a competitive advantage over S. mutans under acidic conditions.
• The findings provide insights into bacterial succession in the oral cavity and its relation to caries development.