The genus Lactobacillus consists of more than eighty species (Figure 2-6). In the last decade, microbial taxonomists have been very busy, proposing and validating new taxa. In one single month (January 2005), seven new species and subspecies were described in the published literature. About all that is common to the species within this genus is that they are all non-sporing rods, but even this description is not wholly satisfactory. There are species that appear rather short (<1.5 ^m), whereas others are more than 5 ^m in length (some are reportedly up to 10 ^m).They may also have a slender, curved, or bent appearance. When viewed microscopically, it is difficult to be sure that a given strain is even a rod. Colony morphology is also variable on agar plates, with some strains producing large round colonies and others producing small or irregular colonies. Not only does this genus contain far more species, with extensive morphological heterogeneity, than any other genera of lactic acid bacteria, but this group is also the most ecologically, physiologically, biochemically, and genetically diverse.

Ecologically, lactobacilli occupy a wide range of habitats. Except for very extreme environments, there are few locales where lactobacilli are not found, and they are often described as being ubiquitous in nature. Some species are normal inhabitants of plant and vegetable material, and they are frequently found in dairy and meat environments, in juice and fermented beverages, and in grains and cereal products. Their presence in the animal and human gastrointestinal tract (as well as in the stomach, mouth and vagina) has led to the suggestion (which has gained substantial scientific support) that these bacteria have broad "probiotic activity," meaning they promote intestinal as well as extra-intestinal health. In foods, they are involved not only in many important fermentations, but are also frequently implicated in spoilage of fermented and non-fermented foods.

The ability of lactobacilli to grow and persist in so many diverse environments and conditions reflects their diverse physiological properties. Although most species are mesophilic, the genus also contains species that are psychrotrophic, thermoduric, or thermophilic. Temperature optima varies widely, from 30°C to 45°C. Some species show high tolerance to salt, osmotic pressure, and low water activity.Acid-tolerance is a common trait of lactobacilli (many strains actually prefer an acidic environment), and some also are ethanol-tolerant or bile-tolerant. Most species are aerotolerant, whereas others require more strict anaerobic conditions.

Like all lactic acid bacteria, lactobacilli are fermentative, but, again, they are more metaboli-cally diverse than other lactics. In fact, one of the major ways in which the genus is subdivided into groups is based on the pathways they use to ferment sugars.As noted previously, lactic acid bacteria, in general, ferment sugars in either homofermentative or heterofermentative fashion. However, some species of Lactobacillus have the genetic and physiological wherewithal to ferment sugars by either pathway. These species, therefore, are referred to as facultative heterofermentative.

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