Unmodified and recombinant strains of <i>Lactobacillus plantarum</i> are rapidly lost from the rumen by protozoal predation

<jats:p> A genetically‐manipulated strain of <jats:italic>Lactobacillus plantarum</jats:italic> and the unmodified parent strain were introduced into the rumen of sheep at an initial inoculum level of 1 times 10 <jats:sup>7</jats:sup> cfu ml <jats:sup>‐1</jats:sup> 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 <jats:sup>‐1</jats:sup> (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 <jats:italic>vitro</jats:italic> revealed that the inoculants persisted in sterile rumen fluid with a loss rate of 0.044 and 0.057 h <jats:sup>‐1</jats:sup> 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. </jats:p>