KINETICS OF COLONIZATION OF ADULT QUEENSLAND FRUIT-FLIES (BACTROCERA-TRYONI) BY DINITROGEN-FIXING ALIMENTARY-TRACT BACTERIA

The average total population of bacteria remained constant in the alim entary tracts of adult laboratory-raised Queensland fruit flies (Bactr ocera tryoni) although the insects had ingested large numbers of live bacteria as part of their diet. The mean number of bacteria (about 13 million) present in the gut of the insects from 12 to 55 days after em ergence was not significantly modified when, at 5 days after emergence , the flies were fed antibiotic-resistant bacteria belonging to two sp ecies commonly isolated from the gut of field collected B. tryoni. Fli es were fed one marked dinitrogen-fixing strain each of either Klebsie lla oxytoca or Enterobacter cloacae, and the gastrointestinal tracts o f fed flies were shown to be colonized within 7 days by antibiotic-res istant isolates of K. oxytoca but not E. cloacae. The composition of t he microbial population also appeared to be stable in that the distrib ution and frequency of bacterial taxa among individual flies exhibited similar patterns whether or not the flies had been bacteria fed. Isol ates of either E. cloacae or K. oxytoca, constituting 70% of the total numbers, were usually dominant, with oxidative species including pseu domonads forming the balance of the population. Antibiotic-resistant b acteria could be spread from one cage of flies to the adjacent surface s of a second cage within a few days and had reached a control group s everal meters distant by 3 weeks. Restriction of marked bacteria to th e population of one in five flies sampled from the control group over the next 30 days suggested that the bacterial population in the gut of the insect was susceptible to alteration in the first week after emer gence but that thereafter it entered a steady state and was less likel y to be perturbed by the introduction of newly encountered strains. Al l populations sampled, including controls, included at least one isola te of the dinitrogen-fixing family Enterobacteriaceae; many were disti nct from the marked strains fed to the flies. Nitrogenase activity det ected by the acetylene reduction assay was associated with flies fed d initrogen-fixing bacteria as well as with control groups given either no supplement or free access to a yeast hydrolysate preparation. Nitro gen fixed from the atmosphere may supplement the nutrition of the alim entary tract microbial population of B. tryoni. Transmission electron microscopy showed that the principal site of bacterial colonization in the abdominal alimentary tract was the lumen of the midgut inside the peritrophic membrane. No intracellular symbionts were seen in the gut tissues nor were bacteria found attached to the cuticular folds of th e hindgut. The ultrastructure of the gut resembled that of other fly g enera except that the intercellular spaces between rectal epithelial c ells were more extensive, suggesting a role for unspecialized epitheli um in water and solute uptake in B. tryoni.