there was a signi¢cant di¡erence between the foregut
and the hindgut, though the number of culturable
microorganisms in the latter part of the digestive
tract only reached 2U103 cells per digestive tract
(Table 1). Colonies recovered from the media for
total aerobic, total anaerobic, glucose-fermenting or
succinate-degrading bacteria appeared to be highly
homogeneous. In addition, the numerations on these
various media gave similar results (between 95 and
250 cfu per foregut and between 1250 and 2400 cfu
per hindgut). Twelve isolates were randomly chosen
for further characterization. All tested isolates were
found to belong to the Enterobacteriaceae (short
rods, Gram-negative, catalase-positive, oxidase-negative)
and were identi¢ed as Serratia sp., apparently
belonging to the S. proteomaculans group. The isolates
could grow either aerobically or anaerobically
on a wide range of carbon sources, including N-acetylglucosamine.
In addition, all isolates grew, though
slowly, on chitin as sole carbon source. The number
of bacteria increased in the direction from the foregut
to the hindgut, strongly suggesting that these
microorganisms do not serve as a feeding source
for B. spinosa. Indeed, such a situation would imply
a large number of cells in the foregut (as is the case
for Branchinecta gigas [13]) and the presence of cell
debris in the hindgut. On the other hand, the observed
chitinolytic activity of the Serratia isolates
from this study suggests three di¡erent possible functions
for these microorganisms: (1) an active role in
the renewing of the peritrophic matrix: in the sugar
beet root maggot Tetanops myopaeformis, Serratia
liquefaciens and S. marcescens were found to be associated
with all development stages [14], and the