Lists
Lists
Anyogu, Amarachukwu (2011) Some aspects of survival of Salmonella during lactic fermentation of cassava. Doctoral thesis, London Metropolitan University.
Survival of Salmonella enterica organisms; S. Typhimurium, S. Enteritidis and S. Stanley during controlled lactic cassava fermentations has been investigated. To develop laboratory model fermentations, lactic acid bacteria (LAB) were isolated from unprocessed and fermented African cassava products. LAB (15) isolates were identified and characterized by phenotyping and genotyping and were found to include species of Lactobacillus, Leuconostoc, Enterococcus, Pediococcus and Weissella. LAB isolates (8) were screened for antimicrobial properties with particular reference to inhibitory activity against three Salmonella serovars. Antimicrobial activity was observed to be due to rapid acid production and not to antimicrobial peptides such as bacteriocins. During controlled fermentations, all LAB strains showed ability to bring about desired changes in cassava with respect to reduction in pH and increase in titratable acidity. Two strains of L. plantarum and one strain of P. acidilactici were selected as starter cultures as they were rapid acid producers and showed best antimicrobial activities against Salmonella. Growth and survival characteristics of S. Typhimurium, S. Enteritidis and S. Stanley were determined during both cassava fermentation; as influenced by temperature (25°C, 30°C and 37°C) and in the final fermented product during storage (4°C and 25°C). Irrespective of fermentation temperature, similar growth patterns were noted for all three S. enterica serovars, however there was extended survival at 25°C. Salmonella enterica inoculated post-fermentation survived during 4°C storage, but not 25°C storage. Survival characteristics of acid adapted and unadapted Salmonella enterica serovars during cassava fermentation and in acidified broths was studied. Salmonellae pre-adapted to low pH showed a tendency for better survival in pre-fermented cassava and acidified broths. In contrast, acid adaptation did not influence survival when inoculated prior to fermentation. In all 'survival' experiments, S. Stanley was noted to exhibit an acid sensitive phenotype compared to the other two serovars. Genetic differences between test strains were investigated with regard to the role of rpoS. While S. Stanley exhibited an rpoS mutant phenotype, sequencing of the entire gene did not reveal significant mutations. Growth and survival characteristics of wild type S. Enteritidis was compared with S. Enteritidis rpoS mutant (SErpoS) during cassava fermentation and in fermented cassava stored at 4°C and 25°C. The ability of S. enterica serovars to survive in fermented cassava was dependent on the activity of the stationary phase sigma factor, RpoS. Salmonella Enteritidis possessing wild type RpoS were more resistant to acidic conditions in fermented cassava during storage at both refrigeration and ambient temperatures than mutant strain suggesting an RpoS dependent mechanism role in survival. These studies have increased the knowledge of survival of Salmonella in traditional African acid fermented foods which is of relevance to public health.
 |
View Item |