BIOLOGY Volume 2 - A Guide to General Biology - 2004

12. MICROBIOLOGY AND BIOTECHNOLOGY

12.9. Laboratory Practical

The purpose of these Practical Sessions is to master some fundamental bacteriological techniques. Milk is recommended as a relatively safe source of Bacteria. Milk serves as an excellent nutrient medium not only for mammals, but also for A wide variety of bacteria.

12.9.1. Bacterial Content of Milk

Even with the strictest adherence to hygiene standards, bacteria inevitably enter milk during milking and subsequent handling. Freshly milked milk is usually chilled immediately to slow down bacterial growth. Raw milk is pasteurized—meaning it is heated to destroy pathogenic bacteria. However, many non-pathogenic bacteria survive this process. The following bacteria are typically present in milk:

1. At 15–30 °C, Streptococcus lactis (a Gram-positive microorganism) predominates, along with various other streptococci (also Gram-positive) and coryneform (club-shaped) bacteria (e.g., Microbacterium, Brevibacterium), which resemble Lactobacillus but feature swollen or clubbed ends.

Streptococcus lactis grows well at 10 °C, but its growth ceases at temperatures above 40 X.

2. At 30–40 °C, various species of Lactobacillus (Gram-positive) and enteric bacteria (Gram-negative), such as E. coli, become dominant.

Streptococcus lactis and Lactobacillus belong to the lactic acid bacteria. Through Fermentation (Anaerobic Respiration), they convert lactose (milk sugar) into lactic acid, which accumulates and causes the milk to sour. Colonies of S. lactis and Lactobacillus are relatively small (maximum colony diameter does not exceed a few millimeters), unpigmented, and chalk-white in appearance. S. lactis forms smooth colonies with entire edges. If finely ground calcium carbonate is added to the nutrient Agar, clear zones form around each colony where the lactic acid dissolves the calcium carbonate. Streptococci are essential for the normal souring of milk. They are spherical in shape and form chains (Fig. 12.12). Lactobacillus Cells are rod-shaped and tend to clump together easily to form long chains (Fig. 12.12). Their colonies have a rough surface and irregular edges.

A number of other bacteria can also be found in milk, including the intestinal rod Alcaligenes (Gram-negative), whose cells may occur singly or in chains. They can be identified on MacConkey agar by the yellowish (alkaline) zone surrounding each colony.

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Fig. 12.12. A. Scanning electron micrograph of Lactobacillus bulgaricus, a rod-shaped Gram-positive bacterium responsible for milk souring. B. Light micrograph of streptococcal chains isolated from milk.