General Microbiology - Schlegel, H. 1987

Prokaryotes: A Systematic Survey
Non-spore-forming Gram-positive rods (Group 16)

Gram-positive, non-spore-forming Bacteria capable of growing in the presence of oxygen and fermenting CARBOHYDRATES (such as glucose and lactose) primarily into lactic acid are collectively known as lactic acid bacteria. This group includes the genera Lactobacillus, Leuconostoc, Streptococcus, Pediococcus, and Bifidobacterium. However, in Bergey's Manual of Systematic Bacteriology, Group 16 lists only the rod-shaped lactic acid bacteria of the genus Lactobacillus; the coccoid genera are assigned to Group 14, while Bifidobacterium is classified among the actinomycetes (Group 17). All of these bacteria will be discussed in the context of Lactic acid Fermentation (Section 8.2).

Based on Cell Morphology and other physiological characteristics, Gram-positive bacteria can be arranged in a continuous series, with rod-shaped lactic acid bacteria positioned at the very beginning. The majority of lactic acid bacteria are regular rods. A tendency toward club-shaped and faintly branched Cells is merely incipient here, whereas in coryneform bacteria—which include the genera Corynebacterium and Arthrobacter—such morphological Variability has already become the standard. The capacity for branching is even more pronounced in mycobacteria. This links them to proactinomycetes, which transiently form a mycelium, while actinomycetes occupy the end of this series. Many representatives of these groups are readily identified by their cellular and colonial morphology as well as physiological traits, whereas others can only be identified through various biochemical markers.

As an additional note, mention should be made of the bacterium Caryophanon latum, which possesses flagella and forms trichomes; it is typically isolated from cow manure. These are Gram-positive, septate rods (Fig. 3.1) of unusually large dimensions (3×15 $\mu m$).

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Fig. 3.1. Caryophanon latum, a peritrichously flagellated bacterium forming filaments. (Photograph by M.P. Peshkov.)