SELECTION CRITERIA OF LACTOBACILLI FOR PROBIOTIC USE

The present knowledge of research makes it possible to use a whole ran ge of stabilized micro-organisms and their metabolites for the product ion of preparations known as probiotics. The mechanism of the effect o f probiotics, although it is not fully elucidated, is closely connecte d with the properties of the production strains. When selecting them, it is necessary to respect the origin of the strain used, its ability to adhere to the epithelial cells of the gut and to produce the inhibi tory substances, the ability to survive and grow in the respective eco logical units. The strain should be genetically stable, it should have good growth properties in vitro and in vivo, to maintain its high via bility at processing, lyophilization and storage. Lactobacilli belong to the micro-organisms most frequently used to prepare the probiotics. The statement that lactobacilli inhibit the colonization of pathogeni c bacteria upon the intestinal epithelium has been confirmed by many e xperiments on experimental germ-free and/or gnotobiotic animals. This inhibitory process known as ''competitive exclusion'' can be explained by the competition for the adherence sites on the intestinal mucosa b etween pathogens and lactobacilli and by the production of inhibitory substances. The host specificity as well as different degree of the ex pression of adherent phenotype, which is conditioned, in addition to t he effect of the external environment also by the presence of plasmids of adherence, present the important property of the adherence of lact obacilli. The host specificity of the lactobacilli is closely connecte d with the presence of the specific molecules of receptors on the host cells which can be distinguished by means of specific molecules of th e bacterial cells. The competition for the nutrients to be found on th e intestinal epithelium, which present the growth substrates for both the probiotic strains and the pathogens, may present an important fact or influencing the colonization of lactobacilli. The inhibitory compon ents of the lactobacilli comprise the production of bacteriocins, toxi c metabolites of the oxygen and organic acids. The inhibitory spectrum of the bacteriocins is different; the overwhelming majority of them i nhibits Gf bacteria within the genus or taxonomically close to the gen us of Lactobacillus, others such as bulgaricin, acidolin, laktocidin, acidophilin and non-protein substance reuterin are active against a wi de spectrum of G(+), G(-) bacteria, yeasts and fungi. Of toxic oxygen metabolites, the production of hydrogen peroxide is of importance. The hydrogen peroxide along with lactoperoxidase-thiocyanate milk system exerts bacteriocid effects on most pathogens. The production of organi c acids (lactic, acetic) by lactobacilli is considered to be the prima ry regulator of the gut microbial activity in animals. The lactic acid as a product of the intestinal fermentation is further metabolized by lactate utilizing bacteria to VFA, thus the environment of the digest ive tract is acidified and the acidosensitive pathogens cannot exert t heir effect. When pH of the environment falls, the concentration of mo lecular (non-dissociated) form of weak organic acids (acetic acid, VFA ) elevates. The above acids diffuse through the cell membrane into a c ell, tihereby exhibiting higher inhibitory capacity. In case the mecha nism of the effect of probiotics is fully elucidated, the criteria of the selection could be more precisely defined and thereby also the eff iciency of probiotics would be improved.