484. Studies in the bacteriology of milk: IV. The Gram-negative rods of milk

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Abstract
Using non-selective methods of isolation, the species that was detected with greatest frequency in raw milks of varying purity isAlcaligenes viscosus. The majority of the cultures do not produce a ropy condition in milk or other media. In some strains the oxidation of citrate, and hence the production of alkalinity in milk, is slow and erratic, being dependent on the occurrence of a mutation. The only Gram-negative rod detected in laboratory-pasteurized milk wasA. tolerans n.sp., a description of which is given. That species is similar in general character toA. viscosus but differs principally in its greater resistance to heat, its inability to attack fats, and its erratic and usually poor growth on laboratory media. Greatly improved growth on nutrient agar is obtained by the addition of lactate and relatively high concentrations of ferrous or ferric salts.