PATHOGENIC POTENTIAL OF LACTOBACILLI

Lactobacilli are often considered to be commensal or beneficial partic ipants in human microbial ecology and considerable research is being c arried out into the effects of the use of lactobacilli as additives in both human and animal diets. However, lactobacilli also cause some hu man diseases (e.g. dental caries, rheumatic vascular disease, septicae mia and infective endocarditis (IE)), and have recently been identifie d as potential emerging pathogens in elderly and immunocompromised pat ients, particularly those receiving broad spectrum antibiotic therapy. The identification of potential pathogenic traits amongst lactobacill i will therefore facilitate the use of the organisms for probiotic pur poses. The ability to aggregate human platelets is considered to be a possible pathogenic trait in the progression of IE. A comparison of ba cterial cell surface properties amongst L. rhamnosus strains showed th at platelets were aggregated by 5/5 IE strains and 8/16 laboratory str ains. For the L. paracasei subsp. paracasei strains the respective num bers were 2/5 and 2/9. However two strains, morphological mutants of a non-aggregating strain, which had been re-isolated after passaging th rough rats were found to aggregate platelets. No loss of aggregating f unction occurred on extensive subculturing of IE strains. Aggregation also occurred with 11/14 strains for five other species, namely, Lacto bacillus acidophilus, Lactobacillus fermentum, Lactobacillus oris, Lac tobacillus plantarum and Lactobacillus salivarius, with each species b eing represented indicating that the property is not uncommon in the g enus. A comparison of IE and oral isolates of L. rhamnosus and L. para casei subsp. paracasei and seven other Lactobacillus species, has show n that the binding of both fibronectin and fibrinogen by lactobacilli is greatly increased, up to 50 fold, when the pH is reduced from 7.0 t o 5.0. Re-exposing the lactobacilli to a neutral pH environment releas es most of the bound proteins, but the amount still remaining bound to the cell is several times more than is bound at neutral pH. Lactobaci lli will also bind to the proteins that make up the extracellular matr ix of endothelial cells. Lactobacilli bound significantly better to co llagen types I and V than to types III and IV (p < 0.01). Further, str ains isolated from IE cases, particularly L. rhamnosus strains, bound significantly better to types I and V than did 'normal' strains (p < 0 .02). Type V collagen has been demonstrated at the sites of endothelia l damage. Thus the binding of lactobacilli, particularly L. rhamnosus to these collagen types may be of importance in the early stages of co lonization of the damaged heart valve. The research therefore suggests that lactobacilli being considered for probiotic purposes should be s creened for these potentially pathogenic traits.