The tentative recognition of psychrotrophic Gram-negative bacteria in 48 h by their surface growth at 10°C

Abstract

Employing 153 species and strains of Gram-negative bacteria representing 20 genera, 79 produced visible growth on brain heart infusion agar (BHIA) at 7°C in 10 days and thus conformed to the definition of psychrotrophs. All but 5 of the 79 (93.7%) grew on BHIA plates at 10°C in 2 days. At 12°C, 13 psychrotrophs did not grow in 24 h while in 2 days 28 nonpsychrotrophs grew. Surface plating for colony enumeration generally required 3 days, and pour plating 4 days for countable colonies. It is suggested that those bacteria that grow rapidly at 7°C but not at 40°C be designated stenopsychrotrophs while those that can grow at 40°C or above and generally more slowly at 7°C be designated eurypsychrotrophs. The use of 10°C for 2 days may be used to make a rapid and tentative designation of Gram-negative psychrotrophs when turbid broth cultures are streaked on BHIA plates.