Abstract: We inventoried the bacteria community and the effects of diet on bacterial composition in larval midguts of two strains (the monophagous C108 and the polyphagous SCN2) of the silkworm, Bombyx mori L., an economically important insect, using 16S PCR-DGGE and clone libraries. DNA sequence analysis indicated that the PCR-amplified 16S rDNA genes represented 41 phylotypes of bacteria. Most of them belonged to the Proteobacteria and Lactobacillales, whereas less dominant taxa including members of the Deinococcus- Thermus, Bacillales, Clostridiales and Actinobacteria were also found in the midgut bacterial community. Phylotypes of Enterococcus were the most dominant group in both strains tested, followed by Thermus. Bacterial composition or population structure of Enterococcus varied between the two strains and in the same strain fed with different diets. Especially, when strain SCN2 was fed with mulberry-free artificial diet, a midgut phylotype belonging to Staphylococcus appeared with a fairly high proportion. DGGE profiles demonstrated that there were differences in the bacterial pattern between young and old larvae. This might be related with the differences of physiology between the two developmental stages. The results suggested that the appearance of special bacteria might associate with the special phagy of the silkworm, and unfit diet might destroy the balance of midgut microbiota to block normal growth of the silkworm.

Key words: Bombyx mori, monophagous, polyphagous, midgut bacteria, DGGE, 16S rDNA liBraries