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.