Explanation:

Idli and dosa:
Idli and dosa are popular traditional South Indian dishes made from fermented rice and lentil batter. Fermentation is an essential step in the preparation of idli and dosa as it contributes to the unique texture, taste, and nutritional value of these dishes.

Bacteria used for fermentation:
The fermentation of idli and dosa batter is primarily carried out by lactic acid bacteria (LAB) which convert the carbohydrates present in the batter into lactic acid. This fermentation process imparts the characteristic sour taste and fluffiness to the final product.

Leuconostoc mesenteroides:
Leuconostoc mesenteroides is a type of LAB that is commonly used for fermenting idli and dosa batter. It is a mesophilic bacterium, which means it grows well at moderate temperatures (20-30°C).

Role of Leuconostoc mesenteroides:
1. Acid production: Leuconostoc mesenteroides produces lactic acid by fermenting the carbohydrates in the batter. The lactic acid lowers the pH of the batter, creating an acidic environment that inhibits the growth of harmful bacteria.
2. Gas production: During fermentation, Leuconostoc mesenteroides also produces carbon dioxide gas, which gets trapped in the batter and contributes to the fluffy texture of idli and dosa.

Other bacteria:
While Leuconostoc mesenteroides is the primary bacterium used for fermenting idli and dosa batter, other LAB species may also be present. These include:
- Lactobacillus bulgaricus: This LAB species is commonly used for fermenting yogurt and other dairy products. It produces lactic acid and contributes to the tart flavor of fermented foods.
- Lactobacillus acidophilus: Another LAB species commonly found in fermented dairy products. It also produces lactic acid and contributes to the fermentation process.

Streptococcus thermophiles:
Streptococcus thermophiles is not typically used for fermenting idli and dosa batter. It is commonly used for fermenting dairy products like cheese and yogurt at higher temperatures (thermophilic).

Conclusion:
In conclusion, Leuconostoc mesenteroides is the primary bacterium used for fermenting idli and dosa batter. Its ability to produce lactic acid and carbon dioxide gas contributes to the unique texture and taste of these South Indian dishes. While other LAB species like Lactobacillus bulgaricus and Lactobacillus acidophilus may also be present, they play a secondary role in the fermentation process.