mazEF is a toxin-antitoxin module located on many bacterial chromosomes, including those of pathogens. Here, we report that Escherichia coli mazEF-mediated cell death is a population phenomenon requiring a quorum-sensing molecule that we call the extracellular death factor (EDF). Structural analysis revealed that EDF is a linear pentapeptide, Asn-Asn-Trp-Asn-Asn. Each of the five amino acids of EDF is important for its activity.

Aphanizomenon ovalisporum (Forti) was identijied and isolated j o m Lake Kinneret upon its first appearance as a dominant bloom in late 1994. This cyanobactm'al species, not previously known to be toxic, was evaluated by a commonly used mouse bioassay and was demonstrated to induce toxic symptoms that were distinguishabb from the typical symptoms of the neurotoxins previously repmted in Aphanizomenon flos-aquae (L.) Ralfs. Mice died 5-24 h after crude extracts were injected intrapm'toneally, and the LDj0 value was estimated as 465 mg dry wt biomass.kg-mouse. A toxicity-guided ji-actionation of the active extract indicated that the potent substance is polar an nature. The structure of the active compound was determined by its mass spectrometry and NMR data. The compound was found to be the sulfate-guanidinium zm'tterion, cylindrosperm@in, previously isolated @om the cyanobacterium Cylindrospermopsis raciborskii (Woloszynska) and recently also reported in Umezakia natans (Watanabe). This is the first time that Aphanizomenon ovalisporum has been reported to contain a toxic compound.Lake Kinneret serves as the main freshwater reservoir in Israel, providing about 30% of the national water requirements, besides being a recreational site and supporting a commercial fishery. The importance of maintaining high water quality led to an intensive and routine monitoring program of the

Microcystins constitute a serious threat to the quality of drinking water worldwide. These protein phosphatase inhibitors are formed by various cyanobacterial species, including Microcystis sp. Microcystins are produced by a complex microcystin synthetase, composed of peptide synthetases and polyketide synthases, encoded by the mcyA-J gene cluster. Recent phylogenetic analysis suggested that the microcystin synthetase predated the metazoan lineage, thus dismissing the possibility that microcystins emerged as a means of defence against grazing, and their original biological role is not clear. We show that lysis of Microcystis cells, either mechanically or because of various stress conditions, induced massive accumulation of McyB and enhanced the production of microcystins in the remaining Microcystis cells. A rise in McyB content was also observed following exposure to microcystin or the protease inhibitors micropeptin and microginin, also produced by Microcystis. The extent of the stimulation by cell extract was strongly affected by the age of the treated Microcystis culture. Older cultures, or those recently diluted from stock cultures, hardly responded to the components in the cell extract. We propose that lysis of a fraction of the Microcystis population is sensed by the rest of the cells because of the release of non-ribosomal peptides. The remaining cells respond by raising their ability to produce microcystins thereby enhancing their fitness in their ecological niche, because of their toxicity.

The EtOAc extract of the whole culture medium of Vibrio parahaemolyticus, which inhabits the toxic mucus of the box fish Ostracion cubicus, afforded a new indole-derived natural product, vibrindole A [1], along with some known cyclic dipeptides and indoles. The structure of 1 was determined by analysis of its physicochemical characteristics.