Light micrograph (phase contrast) of the bacteria Lactobacillus bulgaricus, taken from live yoghurt. These rod-shaped, gram-positive bacteria appear as the thin, black lines in the picture. The bacteria grow in acid media & are capable of producing lactic acid from the fermentation of carbo- hydrates. Lactic acid produced by the fermentation of milk is responsible for the preservation & flavour of cheese, yoghurt & other fermented milk products. The bacteria are found in fermenting animal & plant products & in the alimentary tract & vagina of humans & other animals, where they serve a protective role against more dangerous bacteria. Magnification: x400 at 35mm size.