Economic demands for milk and meat products force farmers to increase the carbohydrate content of grains fed to animals. One of the consequences of this intervention is the lactic acidosis condition in animals fed the high-concentrate diets, which is the accumulation of lactic acid in the rumen. Symptoms of the condition include lameness, bloatedness, epistaxis and dilated pupils. Methods such as ionophore antibiotics, gradual transition from high to low-concentrate diets and yeast cultures have in the past been used to avert this condition but all had their disadvantages. Microorganisms may develop resistance to the ionophore antibiotics; gradual transition time may be too long for production and yeast cultures have been reported to be ineffective when used alone. Megasphaera elsdenii, a major lactate utiliser of the rumen of animals, has been used as a direct-fed microbial in the management of lactic acid levels. Studies on M. elsdenii NCIMB 41125 have shown that the bacterium is an effective lactic acid utiliser. Megasphaera elsdenii NCIMB 41125 was cultured in a Biostat Braun B fermenter where growth yields were attempted to be optimised by using a pulse-and-shift method. A semi-defined lactate (SDL) and corn steep liquor (CSL) media, which contained reducing agents, to ensure anaerobiosis, were used in the optimisation and shelf-life studies. Culture stability studies were performed on samples from a fermenter, and subsequently in stainless-steel kegs. Samples for analysis were then taken from the kegs. Preservation of M. elsdenii NCIMB 41125 and prevention of cell settlement methods were also evaluated using a combination of sodium lactate / glycerol and pure xanthan gum / gelatin, respectively. The cultures were harvested using either continuous or fed-batch fermentations. Shelf-life was better for cultures grown on SDL medium with a lower concentration of lactic acid, a finding which related to the substrate affinity of M. elsdenii NCIMB 41125. Higher growth yields were obtained from secondary cultures which had been continuously harvested into stainless-steel kegs. Shelf-life results obtained from the use of corn steep liquor (CSL) medium were almost similar to those obtained when SDL medium was used, however, the problem with CSL data was the variability between batches. None of the preservation or prevention of cell settlement methods resulted in positive responses, although pure xanthan gum preserved cultures for the six days evaluated. In order to avert a sudden reduction of viable cells when high concentrations of lactic acid are used, it could be necessary to harvest cells during the secondary growth phase. / Dissertation (MSc)--University of Pretoria, 2010. / Microbiology and Plant Pathology / unrestricted