Pathogenic potential of lactobacilli

Abstract

Lactobacilli are often considered to be commensal or beneficial participants in human microbial ecology and considerable research is being carried out into the effects of the use of lactobacilli as additives in both human and animal diets. However, lactobacilli also cause some human diseases (e.g. dental caries, rheumatic vascular disease, septicaemia and infective endocarditis (IE)), and have recently been identified as potential emerging pathogens in elderly and immunocompromised patients, particularly those receiving broad spectrum antibiotic therapy. The identification of potential pathogenic traits amongst lactobacilli will therefore facilitate the use of the organisms for probiotic purposes. The ability to aggregate human platelets is considered to be a possible pathogenic trait in the progression of IE. A comparison of bacterial cell surface properties amongst L. rhamnosus strains showed that platelets were aggregated by 5 5 IE strains and 8 16 laboratory strains. For the L. paracasei subsp. paracasei strains the respective numbers were 2 5 and 2 9 . However two strains, morphological mutants of a non-aggregating strain, which had been re-isolated after passaging through rats were found to aggregate platelets. No loss of aggregating function occured on extensive subculturing of IE strains. Aggregation also occurred with 11 14 strains for five other species, namely, Lactobacillus acidophilus, Lactobacillus fermentum, Lactobacillus oris, Lactobacillus plantarum and Lactobacillus salivarius, with each species being represented indicating that the property is not uncommon in the genus. A comparison of IE and oral isolates of L. rhamnosus and L. paracasei subsp. paracasei and seven other Lactobacillus species, has shown 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 to 5.0. Re-exposing the lactobacilli to a neutral pH environment releases most of the bound proteins, but the amount still remaining bound to the cell is several times more than is bound at neutral pH. Lactobacilli will also bind to the proteins that make up the extracellular matrix of endothelial cells. Lactobacilli bound significantly better to collagen types I and V than to types III and IV ( p<0.01). Further, strains 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 endothelial damage. Thus the binding of lactobacilli, particularly L. rhamnosus to these collagen types may be of importance in the early stages of colonization of the damaged heart valve. The research therefore suggests that lactobacilli being considered for probiotic purposes should be screened for these potentially pathogenic traits.