Pediococcus

The pediococci are similar, in many respects, to other coccoid-shaped, obligate homofermenta-tive lactic acid bacteria, with one main excep-tion.When these bacteria divide, they do so in two "planes" (and in right angles).Thus, tetrads are formed, which can be observed visually. Cells may appear as pairs (and always spherical in shape), but chains are not formed, as they are for lactococci, streptococci, and leuconos-tocs. Pediococci, like other lactic acid bacteria, are facultative anaerobes, with complex nutritional requirements. They have optimum growth temperatures ranging from 25 °C to 40°C, but some species can grow at temperatures as high as 50°C. Several of the pediococci are also distinguished from other lactic acid bacteria by their ability to tolerate high acid (growth at pH 4.2) and high salt (growth at 6.5% NaCl) environments (Table 2-4).

The pediococci can be found in diverse habitats, including plant material, milk, brines, ani mal urine, and beer. There are six recognized species of Pediococcus (Figure 2-5); several are important in food fermentations. Two species, Pediococcus acidilactici and Pediococcus pen-tosaceus, are naturally present in raw vegetables, where, under suitable conditions, they play a key role in the manufacture of sauerkraut and other fermented vegetables.These same species may also be added to meat to produce fermented sausages. Despite their inability to ferment lactose, P. acidilactici and P. pentosaceus are frequently found in cheese, where they may participate in the ripening process. Pediococci are also important as spoilage organisms in fermented foods, in particular, beer, wine, and cider. One species, Pediococcus damnosus, is especially a problem in beer, where it produces diacetyl, which in beer is a serious defect.

Plasmids are frequently present in pedio-cocci.The genes located on these plasmids may encode for functions involved in sugar metabolism (e.g., raffinose and sucrose) and production of bacteriocins. The latter (described in Chapter 6) are defined as anti-microbial proteins that inhibit closely related bacteria, including the meat-associated pathogens Listeria monocytogenes, Staphylococcus aureus, and Clostridium botulinum. Bacteriocin

32 Microbiology and Technology of Fermented Foods Table 2.4. Characteristics of Pediococcus1

Property

Pediococcus acidilactici

Pediococcus pentosaceus

Pediococcus damnosus

% G+C

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