Cases of emesis associated with bacterial contamination of an infant breakfast cereal product.

A commercial product for infants containing cereal mixed with dried infant formula was diagnosed as producing rapid projectile vomiting in two infants. Analysis of multiple samples of the cereal product revealed significant contamination with two spore-forming species, Bacillus subtilis and a strain of Bacillus cereus. The latter is the most likely cause of the emetic food poisoning, but we were unable to detect B. cereus emetic toxin. This raises the possibility of the cause being either a new cereulide-type toxin, or the bacterial load, in which case the presence of B. subtilis could have been a contributing factor.

The PDZ domain of the SpoIVB transmembrane signaling protein enables cis-trans interactions involving multiple partners leading to the activation of the pro-sigmaK processing complex in Bacillus subtilis.

In sporulating cells of Bacillus subtilis, the serine peptidase SpoIVB is the essential component of a transmembrane signaling cascade between the two intracellular compartments (forespore and mother cell) that leads to activation of the sigmaK transcription factor in the mother cell chamber. This regulatory process, referred to as the sigmaK checkpoint, is essential for ensuring proper development of the spore and introduces an appropriate level of fidelity to the developmental process. This work unravels the signaling process and establishes how SpoIVB interacts with other protein partners in the sigmaK checkpoint. SpoIVB is synthesized as a zymogen that is autoproteolytically activated and carries a PDZ domain that is responsible for at least three distinct binding reactions, a phenomenon not previously demonstrated for an individual PDZ domain. First, binding to the SpoIVB NH2 terminus to maintain the protein in its zymogen form. Second, following secretion across a spore membrane, binding in trans to the COOH terminus of another SpoIVB molecule. Binding in trans facilitates the first cleavage event of SpoIVB near the NH2 terminus releasing it from the inner forespore membrane. We show that at least two further cis cleavage events occur at specific sites near the NH2 terminus after which the PDZ domain targets SpoIVB to the pro-sigmaK processing complex in the outer forespore membrane. Specifically, SpoIVB binds to the COOH terminus of BofA. In turn, this allows SpoIVB to cleave the COOH terminus of SpoIVFA an event pivotal to activating the SpoIVFB zinc metalloprotease by disruption of the heteroligomeric pro-sigmaK complex.

SpoIVB-mediated cleavage of SpoIVFA could provide the intercellular signal to activate processing of Pro-sigmaK in Bacillus subtilis.

SpoIVB is the critical determinant for intercompartmental signalling of pro-sigmaK processing during sporulation in Bacillus subtilis. We show here that the SpoIVB serine peptidase can cleave the SpoIVFA protein, which is one component of the pro-sigmaK processing complex. SpoIVFA has been shown elsewhere (Rudner, D.Z., and Losick, R., 2002, Genes Dev 16: 1007-1018) to tether BofA and SpoIVFB in a membrane-embedded heteroligomeric complex in which BofA directly inhibits the activity of SpoIVFB. Cleavage of SpoIVFA would provide the necessary signal to dissolve this complex and release BofA-mediated inhibition on the zinc metalloprotease, SpoIVFB, that is responsible for cleaving pro-sigmaK to its mature form. We also show that the SpoIVB PDZ domain is required for self-recognition and trans cleavage of SpoIVB and is probably also used to target an internal motif within the C-terminal region of SpoIVFA exposed in the space between the inner and outer forespore membranes. This work reveals the mechanism of intercompartmental signalling and provides a unified model as to how sigmaK-directed gene expression in the mother cell is co-ordinated with events in the forespore chamber.