Classical Propionic Acid Bacteria

Cell anatomy of the classical propionic acid bacteria is typical for Grampositive cells (Figs. 1.9, 1.10). Thickness of the cell wall varies a little with the age of culture, being in the range of 20.0-30.0 nm. In cells treated with a lead fixative two layers of the cell wall can be observed. Cell division begins with a centripetal formation of a division septum so that the new cell wall appears before the cell division is complete. 

Cell extracts of P. shermanii VKM-103 (wild type), P. shermanii KM-82 mutant (superproducer of vitamin B12), P. acnes CCM 322 and propionic add cocci P. coccoides) were tested as possible sources of antimutagens. The first two strains represented classical propionic acid bacteria, and P. acnes represented cutaneous propionibacteria. In the assay for antimutagenic activity S. typhimuhum TA 1535 his mutant) was used as the test organism (base pair substitution mutations tester). 

To summarize, corrinoids in propionic acid bacteria are involved not onh in fermentation, but also in such important anabolic processes as protein and DNA synthesis and DNA methylation. In this respect, corrinoids differ from other related tetrapyrrole compounds by their polyfunctionalit. The involvement of corrinoid-dependent enzymes in different metabolic processes in propionibacteria explains the propensity of anaerobic strains of the classical propionic acid bacteria to synthesize large amounts of corrinoids under suitable conditions. 

Differentiation between the species is shown above, in Table 1.3. Electrophoretic separation of cellular proteins for the identification and differentiation of propionic acid bacteria was suggested by Baer (1987). Every strain was characterized by a specific spectrum of proteins that was compared with the spectrum of standard strains . This method is more rapid and accurate than the classical microbiological analysis of propionibacteria, although it could not differentiate between the strains of P. freudenreichii subsp. freudenreichii and P. freudenreichii subsp. shermanii. Modem methods of molecular biology are being used at present for the identification of propionibacteria (see below). 

The classical manufacturing process of the Swiss-type cheese did not rely on special introduction of propionibacteria (as the starter), but sufficient numbers of propionic acid bacteria were present naturally in raw milk and rennet extract. At present, pasteurized milk is used in cheese making, and during heating at 71 C for 15 s most of the propionibacteria are killed (Alekseeva et al, 1983). A standard requirement for the content of propionibacteria is 2T0 -4T0 per g Soviet cheese, that is why it is necessary to add propionic acid bacteria with high acid-, gas- and lipolytic... 

Propionic acid bacteria represent a distinct and ancient evolutionary branch ascending from an actinomycetous ancestor. It is a bridge that links the past and present. Three subgroups of propionic acid bacteria classical, cutaneous, and P. propionicum comprise the genus Propionibacterium, The classical propionibacteria inhabit mainly milk and cheese (hence their second name—dairy propionic acid bacteria), the cutaneous species inhabit human skin and the rumen of ruminants, and P. propionicum dwells in the soil. 

The most important property of the genus Propionibacterium is the production of propionic acid as a result of the propionic acid fermentation dependent on coenzyme B12. If the dependence on coenzyme Bn is disregarded, some clostridial strains that do not form spores may be erroneously attributed as propionibacteria. For example, Cl botulinum, Cl propionicum and some other species can produce propionic acid, but propionibacteria have the GC-type DNA (65-67 mol% G+C in classical and 53-62 mol% in cutaneous bacteria), while clostridial DNA is of the AT-type (25-30 mol% G+C). 

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