Km. Murphy et al., KINETICS OF COLONIZATION OF ADULT QUEENSLAND FRUIT-FLIES (BACTROCERA-TRYONI) BY DINITROGEN-FIXING ALIMENTARY-TRACT BACTERIA, Applied and environmental microbiology, 60(7), 1994, pp. 2508-2517
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.