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Chemical Changes in Bacteria Heated in Milk as Related to Loss of Stainability

Field Crops and Animal Products Branch, Plant Industry Station, USDA, Beltsville, Maryland

ABSTRACT

The effect of heating bacteria in milk on direct microscopic clump counts was determined. The periodic acid-bisulfite-toluidine blue (PST) and the aniline oil-methylene blue (AOMB) stains were compared. The PST stain gave quantitative recovery in all cases, whereas the AOMB procedure did not stain gram-negative rods that had been heated. Gram-negative rods were poorly stained by the PST method, but could be counted. The proteose-peptone fraction of milk was primarily responsible for the loss of stainability. Mineral salts were less effective. Loss of stainability was associated with extraction from the bacterial cells of nitrogen and phosphorus-containing materials, identified as nucleic acids. The superior results obtained with the PST stain are attributed to the fact that polysaccharides as well as nucleic acids are stained. Strains of bacteria which contain little polysaccharide stain poorly by this procedure after nucleic acids are removed.