Unmodified and recombinant strains of Lactobacillus plantarum are rapidly lost from the rumen by protozoal predation.

A genetically-manipulated strain of Lactobacillus plantarum and the unmodified parent strain were introduced into the rumen of sheep at an initial inoculum level of 1 x 10(7) cfu ml-1 of rumen fluid. There were no significant differences between the viable counts of the two inoculants throughout a 24 h sampling period. The rates of loss were 0.36 and 0.29 h-1 (proportion of colony-forming units lost, measured over the first 2 h) for the parent strain and recombinant strain respectively, and within 24 h of inoculation neither of the strains were detectable in rumen fluid. Further experiments in vitro revealed that the inoculants persisted in sterile rumen fluid with a loss rate of 0.044 and 0.057 h-1 for the parent strain and the recombinant strain respectively. Incubations with rumen fluid alone, protozoa-free rumen fluid and protozoa-enriched rumen fluid revealed that protozoal predation was the most significant factor in the loss of the introduced population. The loss rates from protozoa-free rumen fluid were not significantly different (P < 0.05) from those observed in sterile rumen fluid.