Evidence for Growth of Strains of the Plant Epiphytic BacteriumErwinia herbicolaand Transconjugation among the Bacterial Strains in Guts of the SilkwormBombyx mori

Abstract

Growth of plant epiphytic bacteriaErwinia herbicolaandPseudomonas syringaein guts of the silkworm,Bombyx mori,was studied. Fifth instar silkworm larvae were fed artificial diets supplemented with these bacteria for 6 to 12 h followed by uncontaminated diets. At 1, 3, and 6 days after feeding, bacteria were isolated from insect guts and feces. A much larger population ofE. herbicolawas detected in the samples collected 3 and 6 days after the inoculation than in samples collected after 1 day, indicating that these bacteria grew in the insect gut, whileP. syringaewas unable to survive. Transconjugation betweenE. herbicolastrains in the insect gut was also examined. First, either a donor or a recipient strain was fed to the insects in artificial diets containing the bacteria during 12 h, and then pairing strains were fed during 12 h after starvation for 12 h. The conjugative plasmid pBPW1::Tn7was transferred into recipient cells at very high frequencies (10−1/recipient after 3 days and 10−3after 6 days) in insect guts. Indigenous plasmids ofE. herbicolamobilized RSF1010 plasmid into recipient cells at frequencies of 10−4in insect guts. These transconjugants were detected in the feces of the insects. Thus, plasmid-mediated gene transfer among the epiphytic bacteria in insect guts was demonstrated. The results obtained suggest thatin insectagene transfer may play an important role in the evolution of plant epiphytic bacteria.