Some studies on microbacteria from Iowa dairy products.

Microbacteria have attracted attention because they are among the most heat-resistant, non-sporeforming types of bacteria found in dairy products. As they grow poorly, if at all, at 37 C, they usually were overlooked when the standard plate count procedure called for incubation of the plates for 48 hr at that temperature. However, they form small colonies when incubation temperatures of 32 to 35 C are employed. Because of their thermoduric nature, microbacteria have assumed increasing importance as contributors to high postpasteurization counts on milk products, as the use of low incubation temperatures has become more general. Also, their common association with poorly cleaned equipment has been responsible for increased interest. The significance of microbacteria in milk was first brought to light by Robertson (1927) in his researches on the thermophilic and thermoduric flora of pasteurized milk. Thomas et at. (1950) have reviewed the literature appearing up to 1950 on the incidence and thermoduric character of these organisms as they appear in dairy products and also have reviewed much of the literature in classification. The taxonomic position of this group of bacteria has been the subject of much controversy since OrlaJensen (1919) gave them the generic name Microbacterium. Other workers who were interested primarily in the taxonomic aspects of microbacteria included Jensen (1932, 1934), Wittern (1933), Speck (1943), Doetsch and Pelczar (1948) and Abd-El-Malek and Gibson (1952). Speck (1943) concluded that M. lacticum and M. flavum constitute legitimate species of the genus Microbacterium, which should be classified close to the genera Propionibacterium and LactobaciUus. Later, Doetsch and Pelczar (1948) confirmed and extended the studies of Speck. They also proposed another species which was designated Microbacterium sp., being less heat-resistant than M. lacticum but more fermentative. Abd-El-Malek and Gibson (1952) found that what they termed the saprophytic heat-resistant corynebacteria of milk were indistinguishable from M. lacticum. They favored discarding the genus Microbacterium as had been proposed by Jensen (1932, 1934). They grouped the organisms they encountered under Corynebacterium lacticum.