Gene transfer between bacteria within digestive vacuoles of protozoa

The occurrence of horizontal gene transfer between bacteria within digestive vacuoles and faecal pellets of the protozoan Tetrahymena pyriformis was investigated. More than 90% of the egested faecal pellets of T. pyriformis, added as predator to a suspension of Escherichia coli, contained viable bacteria. In a mixed population, containing donor (plasmid RP4) and recipient E. coli cells, the presence of T. pyriformis increased conjugational gene transfer by three orders of magnitude. Since the protozoa formed an average of 12–13 digestive vacuoles per cell, each protozoan had statistically egested one or more transconjugants. Thus, we show for the first time that digestive vacuoles of free-living protozoa appear to be an important ecological micro-niche, where gene transfer by conjugation (or retromobilisation) will be favoured. So far, digestive vacuoles have been ignored in genetic and ecological studies. This micro-biotope provides a selective pressure which might enhance the acquisition of virulence genes in cases of mutual interactions between genetically modified micro-organisms and wild-type pathogens. This finding is important for biosafety considerations.