STREPTOCOCCI OCCURRING IN SOUR MILK.

Area of Science:

• Microbiology
• Food Science
• Dairy Microbiology

Background:

• Sour milk contains a distinct group of rod-like and coccoid organisms.
• These organisms are arranged in pairs and chains, suggesting a specific bacterial group.

Purpose of the Study:

• To characterize the morphological and cultural differences of organisms found in sour milk.
• To differentiate between lactic acid streptococci and udder-associated streptococci.
• To understand the competitive dynamics between these bacterial groups in milk.

Main Methods:

• Morphological observation of rod-like and coccoid organisms.
• Cultural characterization, including fermentation of various sugars (dextrose, lactose, maltose, mannitol, salicin, saccharose, raffinose, inulin).
• Growth studies at room temperature and milk coagulation tests.
• Competitive growth experiments in sterile milk, comparing lactic acid and udder streptococci.

Main Results:

• At least three species of streptococci were identified based on fermentation patterns.
• The majority fermented dextrose, lactose, maltose, mannitol, and salicin, but not saccharose, raffinose, or inulin.
• Some strains fermented saccharose, while a few did not ferment mannitol.
• All types grew well at room temperature, coagulated milk, reduced litmus, and produced acid from dextrose.
• Lactic acid streptococci demonstrated superior growth and outcompeted udder streptococci in the milk-souring process.

Conclusions:

• Specific morphological and cultural differences distinguish lactic acid streptococci from udder-associated strains.
• Lactic acid streptococci are the primary drivers of milk souring due to their competitive advantage.
• Udder streptococci are outcompeted and disappear when introduced into milk with lactic acid streptococci.