Microbes As Direct Food Sources

It is extremely difficult to characterize the degree to which microbes are used as food. Ingested microbes may be digested, take up temporary residence, or pass through; many live as commensals and symbionts. Studies of cockroaches as disease vectors indicate that some bacteria fed to cockroaches are passed with feces, while others could not be recovered even if billions were repeatedly ingested (Roth and Willis, 1957). A mushroom certainly qualifies as food, but so does any microbe that dies within the digestive system, releasing its nutrients to be assimilated by the cockroach host, other microbes resident in the gut, or a coprophage feeding on a subsequent fecal pellet. We do not know the degree to which cockroaches feeding on dead plant material handle the substrate/microbe package in bulk (the gourmand strategy) versus pick through the detrital community, ingesting only the relatively rich microbial biomass (the gourmet strategy). If the latter, they are not detritivores, because they feed primarily on living matter and on material of high food value (Plante et al., 1990). The gourmet strategy may be common among the youngest cockroach nymphs in tropical rainforests. Many of them never leave the leaf litter (WJB, pers. obs.), and small browsers can be highly selective

(Sibley, 1981). Even if a cockroach is a gourmand, however, it may only digest and assimilate the microbial biomass, and pass the substrate in feces relatively unchanged, "like feeding on peanut butter spread on an indigestible biscuit" (Cummins, 1974).

Regardless of the strategy, it is generally agreed that for most detritivores microorganisms are the major, if not sole source of proteinaceous food, and are assimilated with high efficiency, 90% or more in the case of bacteria (White, 1985,1993; Bignell, 1989; Plante et al., 1990). On a dry weight basis, fungi are 2-8% nitrogen, yeasts are 7.5-8.5%, and bacteria are 11.5-12.5% (Table 5.1). These levels are comparable to arthropod tissue and may exceed cockroach tissue (about 9.5% in C. punctulatus adults) (Nalepa and Mullins, 1992). In addition to being rich sources of nitrogen, microbes contain high levels of macronutrients such as lipids and carbohydrates, and critical micronutrients, such as unsaturated fatty acids, sterols, and vitamins (Martin and Kukor, 1984). Even if the ingested biomass is small, the nutrient value may be highly significant (Seastedt, 1984; Ullrich et al., 1992). Irmler and Furch (1979), for example, pointed out that a litter-feeding cockroach in Amazonia would need to consume impossible amounts (30-40 times its energy requirement) of litter to satisfy its phosphorus requirement; it is known, however, that microbial tissue is a rich source of this element (Swift et al., 1979).

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