Taxonomy and important features of probiotic microorganisms in food and nutrition

Lactic acid bacteria are among the most important probiotic microorganisms typically associated with the human gastrointestinal tract. Traditionally, lactic acid bacteria have been classified on the basis of phenotypic proper ties, eg, morphology, mode of glucose fermentation, growth at different tem peratures, lactic acid configuration, and fermentation of various carbohydr ates. Studies based on comparative 16S ribosomal RNA sequencing analysis, h owever, showed that some taxa generated on the basis of phenotypic features do not correspond with the suggested phylogenetic relations. Thus, some sp ecies are not readily distinguishable by phenotypic characteristics. This i s especially true for the so-called Lactobacillus acidophilus group, the La ctobacillus casei and Lactobacillus paracasei group, and some bifidobacteri a, strains of which have been introduced in many probiotic foods, eg, the n ovel yogurt-like commodities. Consequently, modem molecular techniques, inc luding polymerase chain reaction-based and other genotyping methods, have b ecome increasingly important for species identification or for the differen tiation of probiotic strains. Probiotic strains are selected for potential application on the basis of particular physiologic and functional propertie s, some of which may be determined in vitro. The classification and identif ication of a probiotic strain may give a strong indication of its typical h abitat and origin. The species, or even genus name, may also indicate the s train's safety and technical applicability for use in probiotic products. M olecular typing methods such as pulsed-field gel electrophoresis, repetitiv e polymerase chain reaction, and restriction fragment length polymorphism a re extremely valuable for specific characterization and detection of such s trains selected for application as probiotics.