Bacteria and a protist used.

A development of grazing resistance in Escherichia coli K-12 was examined in the presence of a bacterivorous protist, Spumella sp. TGKK2. Two transformants were generated from E. coli K12 for grazing experiments. One was E. coli K-12-TGF, which possesses tetracycline resistance and green fluorescence. The other was E. coli K-12-KRF with kanamycin resistance and red fluorescence. These strains can be selectively colonized on antibiotic-containing agar media and further confirmed by their fluorescent colors. First, we added protist-untouched E. coli K-12-KRF to protist-touched residual E. coli K-12-TGF that had been attacked by Spumella sp. TGKK2 in a batch test. Then the survivability of the respective strains was investigated. Consequently, E. coli K-12-KRF was predated preferentially. On the other hand, E. coli K-12-TGF in the same tube was less predated, indicating some grazing resistance. Similar phenomena were observed when the conditions of these two strains of bacteria were reversed. Also, a continuous culture device supplied with a glucose-containing medium as a substrate was operated. The device connected two complete mixed reactors in series. E. coli K-12-TGF was cultivated in the first reactor, and then grown E. coli K-12-TGF was predated by Spumella sp. TGKK2 in the second reactor. The effluent in the second reactor containing residual E. coli K-12-TGF and Spumella sp. TGKK2 was supplemented with batch-cultured E. coli K-12-KRF. Consequently, it was confirmed that bach-cultured E. coli K-12-KRF never exposed to protist was predated preferentially. These findings reveal that E. coli K12 acquires some predation resistance through coexistence with the bacterivorous protist.