IMIS

Bacilli are soil bacteria that exist in their habitat either as vegetative cells or, more abundantly, as spores (23, 30). The spore is a dormant state that confers in vitro resistance to a broad range of insults that are bactericidal for the vegetative form of bacteria, including extreme temperatures, mechanical stresses, or exposure to chemicals (5). The ability of spores to resist these treatments is at least partially attributed to the spore coat, a proteinaceous layer composed of at least 70 protein species in Bacillus subtilis (11). The coat is organized in two distinct layers, an outer coat and an inner coat. The proper assembly of the spore coat requires the expression of at least five morphogenetic proteins (11), including CotE that controls the recruitment and the binding of a large subset of proteins to the coat (36). A ΔcotE mutation results in spores that lack the outer coat and frequently display an aberrant inner coat that appears to be disconnected from the outer surface of the forespore (6, 36). Further, the precise timing of CotE expression is critical for the proper recruitment of proteins to the coat during assembly, and delaying CotE expression during sporulation results in spores lacking the outer spore structures (3).

The modifications in the coat structure observed in the ΔcotE mutant spores are assumed to be responsible for their sensitivity to peptidoglycan-degrading enzymes, such as hen egg white lysozyme (36). Lysozymes are widely distributed among organisms and are considered effectors of innate immunity in metazoa or as digestive enzymes in unicellular and multicellular eukaryotes (27). In soil, Bacillus species encounter different types of predator organisms, including protozoa and bacteriophagous nematodes. Recently, the relevance of in vivo studies in understanding the role of the spore coat structures (15) was demonstrated using the protozoan Tetrahymena thermophila. The bacteriovorous nematode Caenorhabditis elegans appears to interact with soil bacteria like the endospore-forming Bacillus cereus in the wild (9). C. elegans expresses and presumably secretes into its intestinal lumen bacteriolytic proteins, including lysozyme-like proteins (19, 20) encoded by numerous genes (>10) in its genome that show homology to either c-type, i-type, or Ch-type lysozymes (M.-H. Laaberki and J. Dworkin, unpublished data). In the present study, we investigated the fate of Bacillus subtilis cells during C. elegans predation. We tested the ability of C. elegans to ingest and digest vegetative Bacillus subtilis cells or spores. In addition, we characterized the role of coat structures in the resistance to C. elegans ingestion.