Oxidoreductases of rumen bacteria : a thesis submitted in partial fulfilment of the requirements for the Degree of Master of Science in the University of Canterbury

Abstract

The presence of fermentable sugars and carbohydrates in the rumen of sheep and cattle favours certain representatives of the ’Lactic acid bacteria’. Under normal rumen conditions the flow of carbon, as a result of micro-biological action, from starch and cellulose results in the volatile fatty acids as final products, together with a number of gases. The principal products found were acetate and carbon dioxide with lesser amounts of propionate, butyrate and methane.

This flow however is the sum of a number of steps in which different groups of bacteria play their role … the cellulolytic and the amylolytic bacteria comprise the initial carbohydrate fermenters.

The fermentation of the simple sugars thought glycolysis is possible the sole energy producing mechanism in some rumen bacteria. Lactate was not found to be an important final end product and investigation thought labelling experiments proved that much was converted into propionate (and acetate) (39).

Nevertheless, pure cultures of some of the numerically important bacteria in the rumen, viz. the gram + ve rods – Lactobacillus spp., and cocci – streptococcus sp., produce large quantities of lactic acid from a glucose energy source. Apart from the relatively small amount of lactate which diffuses thought the rumen epithelium into the host’s blood-stream, most of the lactate is accounted for by lactate fermenting bacteria … Peptostreptococcus elsdenii, Veillonella gazogenes, Propionibacterium spp., and Selenomonas ruminantium var lactilytica.

In the steady state condition, the pool size of lactate is low < 0.12 u mole/ml (39). The end product of one group, the LAB, will be metabolised by another, the lactate fermonters. Gradual increases in the intake of readily fermentable carbohydrate can be accommodated by an increased LAB proportion of the total bacteria, and a consequent increase in lactate fermenters. Sudden or massive increases of starch-like materials in the diet may exceed a critical level. The pH of the rumen contents falls to below 5 (39). S. bovis dies away after a peak in numbers and is replaced by the more acid tolerant Lactobacillus spp., while the lactate fermenters are almost eliminated (39). This is the condition of acidosis or acid indigestion, and may be reversible by withdrawal of the starch-rich diet.

The ability of the LAB to produce, from glucose, lactate in large amounts, suggests an active glycolytic pathway. This investigation aimed at isolating lactate dehydrogenases from lactic acid bacteria and other groups involved in the production or assimilation of lactate and investigating some of their properties. In particular, strains of Streptococcus bovis, Lactobacillus plantarum of the LAB, and Selemomonas lactilytica and Quin’s Oval of the true rumen bacteria were isolated and studied.