Lactic acid-forming bacteria

Lactic acid-producing bacteria associated with fermented dairy products have been found to produce antibiotic-like compounds caUed bacteriocins. Concentrations of these natural antibiotics can be added to refrigerated foods in the form of an extract of the fermentation process to help prevent microbial spoilage. Other natural antibiotics are produced by Penicillium wqueforti the mold associated with Roquefort and blue cheese, and by Propionibacterium sp., which produce propionic acid and are associated with Swiss-type cheeses (3). 

Lactose, when fermented by laclic bacteria, is the source of the lactic acid formed in sour milk and whey. Lactose is helpful in establishing a slightly acid reaction in the intestine, which assists in calcium assimilation. 

Homofermentative lactic acid bacteria (such as Lactococcus lactis and Streptococcus lactis) produce (-l-)-L-lactic acid (e.g. in sour cream). Both isomers, (-l-)-L-lactic acid and (-)-D-lactic acid (8-65), are formed during milk fermentation by heterofermen-tative bacteria (lactic acid bacteria are mostly heterofermentative bacteria) and lactic acid thus also occurs as a racemate in sauerkraut, pickled cucumbers, olives and silage. For example, bacteria of the genus Leuconostoc produce d-lactic acid, while bacteria Pediococcus acidilactici and other bacteria produce racemic lactic acid. The content of lactic acid in dairy products is 0.5-1.0%. L-Lactic acid in yoghurt represents about 54% and in sour cream 96% of the total lactic acid content. The total lactic acid content in sauerkraut is 1.5 2.5%, in fermented cucumbers it ranges from 0.5 to 1.5% and fermented green olives contain 0.8 to 1.2% lactic acid. 

It is apparent from this cursory survey that, while lacking the charisma of DNA, iron has played a prominent role in the evolution and development of living forms. One of the few, perhaps the only, species which can challenge the dictum that life and iron are inseparable is the group collectively called the lactic acid bacteria. In 1947 MacLeod, and Snell (103) examined the mineral requirements of representative strains of these organisms. They reported ... 

The method is based on the fact that certain bacteria, fungi, mould or yeast when allowed to grow in a racemic solution, assimilate or consume one of the enantiomers faster than the other. This is why the method is also known as selective assimilation or preferential decomposition. Thus Penicillium glaucum a species of green mould when allowed to grow in ammonium racemate solution consumes the d 0 tartaric acid and leaves the l form, but in a racemic lactic acid it assimilates the l form leaving behind the d form. 

Microorganisms have also been developed to produce alternative products, such as lactic acid [65], propane-1,3-diol [67], 3-hydroxypropionic acid [68], butane-2,3-diol [69] and numerous other intermediates. For instance, bacteria such as the Clostridium acetobutylicum ferment free sugars to C4 oxygenates such as butyric acid or butanol. They form the C4 oxygenates by Aldol condensation of the acetaldehyde intermediates. The Weizmann process exploits this property to ferment starch feedstock anaerobically at 37 °C to produce a mixture of w-butanol, acetone and ethanol in a volume ratio of 70 25 5 [3],... 

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