Biology Reference
In-Depth Information
cells are extracellular, while others are enclosed within hypertrophied epithelial
cells at the distal end of the caeca.
Among all the gut microbes of insects, only those in termites and cockroaches have been studied
in some detail. Gut bacteria of aphids that were later identiÝed maintain a subtle balance with one
another as well as with mutualistic symbionts inherent in the host.
The gut microbes of termites are housed in an enlarged portion of the hindgut called the paunch.
This region is anaerobic, and all the microbial habitants are obligate or facultative anaerobes. The
paunch of lower termites contains protozoa that are predominantly Þagellate species (Honigberg,
1970). Many of these Þagellates are particular to the hindgut of lower termites. There are also
prokaryotes in the paunch of lower termites that include spirocetes and methanogens (Breznak,
1984). The densities of these microbes are surprisingly high, up to 10
Some
Taphrina
protozoa and 10
to 10
7
9
10
bacteria per milliliter of gut volume. Most of the microbes lie free in the lumen, but some species
adhere to the gut wall. Most remarkably, some spirocetes are associated with the surface of protozoa
on which they confer motility (Smith and Douglas, 1987). Higher termites are distinguished from
lower ones in that their gut microbes are only prokaryotes. It is known that the gut microbes of
termites are discarded at ecdysis of the host insects. However, this is of little consequence to the
individual termite because these insects can easily acquire fresh microbes from the other individuals
in the colony.
Cockroaches contain a complex hindgut microÞora, predominantly of obligate anaerobes, includ-
ing cellulolytic and methanogenic bacteria (Bracke et al., 1979). Whereas most cockroaches are
omnivorous, some species, often called woodroaches, live exclusively on a diet of wood. The hindgut
microbes of woodroaches are very similar to those of termites in that they contain protozoa. Many
lines of evidence suggest that woodroaches and termites share a common ancestor. It is likely that
the association with protozoa had already been established in the ancestor (Smith and Douglas, 1987).
By culturing honeydew, the excreta of aphids, spread over nutritional agar, Harada demonstrated
that pea aphids,
, contained several species of gut microbes (Harada and
Ishikawa, 1993). These were Gram-negative, oxidase-negative, facultative anaerobic, fermentative,
motile, and rod-shaped bacteria with the general characteristics of the family Enterobacteriaceae.
Biochemical tests indicated that these bacteria were related most to
Acyrthosiphon pisum
Erwinia herbicola
and
Pantoea
agglomerans
, which are ectoparasites of many plants (Harada et al., 1997). Keeping aphids under
aseptic conditions for several generations produced aseptic insects that were completely free of the
gut microbes. Such aseptic aphids exhibited somewhat better performance than those with the gut
microbes in terms of both growth rate and fecundity, suggesting that the gut microbes were not
beneÝcial to the host. The result is interesting when taken together with the Ýnding that one of the
gut microbes can be monophyletic with
, a mutualistic, intracellular symbiont of aphids
(see later). It is probable that since aphids have already established a mutualistic association with
Buchnera
Buchnera
, they no longer need its close relatives in the gut. In this context, it is worth noting that
aposymbiotic aphids, which are artiÝcially deprived of
, tend to accumulate many
microbes, including both bacteria and fungi, that otherwise are not detected in the gut (A. Nakabachi,
personal communications). Although it is unknown whether or not some of these microbes substitute
for the mutualistic symbiont, the aposymbiotic aphids, harboring these microbes, live as long as,
or even longer than, symbiotic ones. The tripartite relationship of host insect, mutualistic symbionts,
and gut microbes is a potentially interesting subject of study.
Dynamics among microbes in the gut Þora provide another interesting theme from a micro-
ecological point of view. Each species or strain of microbe from the aphidÔs gut can be cultured
separately and returned to the host by mixing the cells with a synthetic diet that the host ingests.
Interestingly, every species of the gut microbes, when put back into aseptic aphids by itself, tended
to proliferate excessively and eventually kill the hosts (Harada and Ishikawa, 1997). This suggests
that the microbes are potentially harmful to the host and can be compatible with it only when they
form a multispecies community. In other words, it is likely that in the gut Þora microbes keep each
other in check.
Buchnera
