Characterization of fecal lactic acid bacteria isolated from healthy Ethiopian infants : bacteriocin production and antibiotic suscepebility

In this thesis lactic acid bacteria (LAB) were isolated from fecal samples of healthy Ethiopian infants, identified to species level, screened for bacteriocin production and tested for antibiotic susceptibility. Enterococci were also screened for the production of cytolysin and gelatinase. Among 150 LAB isolates, 81 lactobacilli, 54 enterococci and 15 streptococci were identified by 16S rRNA gene sequence analysis. Most of the lactobacilli were obtained from breast-fed infants, while most of the enterococci and the streptococci were from mixed-fed infants. Lactobacillus fermentum, Enterococcus avium and Enterococcus faecalis were the most frequently isolated species. About 10% of the LAB produced bacteriocins. The majority of the producers were enterococci, indicating a higher prevalence of bacteriocin production in enterococci compared to lactobacilli. For some of the bacteriocins, the producers were found to occur in high numbers in fecal samples, suggesting that bacteriocin renders the producers the capacity to outcompete other bacteria. Six new bacteriocins were detected, two of which showed strong activity against pathogenic bacteria and thus were purified and characterized at biochemical and genetic levels. These are a 4288.2 Da pediocin-like bacteriocin (avicin A) and a 3466.55 Da trypsin-resistant, nisin-like lantibiotic bacteriocin (salivaricin D). Avicin A, produced by E. avium strains, is active against the food-borne pathogen Listeria monocytogenes. Sequence analysis showed that avicin A gene was found on a 7 Kb locus that contains genes encoding bacteriocin synthesis, export, immunity and regulation. Avicin A is similar to mundticin KS and enterocin CRL 35, but its locus resembles that of sakacin X. We showed that the production of avicin A is inducible and regulated by a quorum sensing regulatory system. Salivaricin D is produced by S. salivarius isolates which appeared to account for over 60% of the fecal LAB flora of an infant. It is active against the important pathogens S. pyogenes and S. pneumoniae, which cause a wide variety of diseases. Sequence analysis showed that salivaricin D is located on a 16.5 kb locus that consists of genes responsible for its synthesis, modification, export, processing, immunity and regulation. The primary structure and locus of salivaricin D is similar to that of nisin Q. Avicin A and salivaricin D may be potential chemotherapeutic agents that might control infections due to the respective pathogens they inhi