The Microbacteria: I. Morphological and Physiological Characteristics.

The genus Microbacterium was proposed by Orla-Jensen in 1919. Four species were described, viz., Microbacterium lacticum, Microbacterium flavum, Microbacterium liquefaciens, and Microbacterium mesentericum. The organisms of this genus presented a physiological gradient with the saccharolytic acid former (M. lacticum) at one extreme and the proteolytic gelatin liquefier (M. liquefaciens) at the other extreme. Later (1921), with reference to the taxonomic position of this genus, Orla-Jensen stated: "Microbacterium is to be understood as merely a provisional collective name for gram-positive rods of size a little smaller than ordinary bacteria. In biological respects some of these rods (Bacillus acidophilus) are closely related to the true lactic acid bacteria, whereas others approach the Tetracoccil or the aerobic bacilli." No further detailed investigations were made on this genus for 14 years. Robertson (1927) encountered M. lacticum in his study of thermoduricorganisms isolated from heated milk. His data agreed well with those given by OrlaJensen (1919) for M. lacticum. An interesting conclusion of Robertson was the statement that M. lacticum and Lactobacillus thermophilus were identical. However, the description of L. thermophilus by Ayers and Johnson (1924) leaves little doubt that these organisms are distinctly different. A valuable contribution to our knowledge of the genus was made by Wittern (1933). Her work confirmed and extended that of Orla-Jensen (1919). A major portion of her investigation dealt with the taxonomic position of M. mesentericum, which she allocated to the genus Mycobacterium. Jensen (1932) made a morphological studv of the genus. His observations led him to the conclusion that M. mesentericum was a proactinomyces and hence should be Proactinomyces mesentericus. Other recommendations by Jensen were that M. lacticum and M. liquefaciens be designated as Corynebacterium lacticum and Corynebacterium liquefaciens. Further studies (1934) led him to the conclusion that M. flavum was actually a Mycobacterium and therefore should be designated as Mycobacterium flavum. Hansen (1938) found that hydrocyanic acid or iodoacetate inhibited the respiration of M. lacticum and indicated that the microbacteria differed from the true lactic acid bacteria not only in catalase content but also in their hemin content. Speck (1943) took issue with the conclusions of Jensen (1932, 1934), stating that to place the Microbacterium in the genera Mycobacterium or Coryne-