ABSTRACT
In bacteria, exotoxins are proteins that are secreted during the exponential phase of growth or are released when the bacterium undergoes lysis or during the transition from a vegetative organism to a spore or the reverse. Exotoxins usually are encoded by extra-chromosomal genetic elements such as plasmids, prophages, and pathogenicity islands. Exotoxins are cytotoxins when they kill the cells to which they bind and leukotoxins when they kill leukocytes. Exotoxins not shown to be essential for virulence by gene inactivation, but which contribute to pathogenesis, are more appropriately termed determinants of pathogenesis. Some of these exotoxins may prove to contribute significantly to virulence once the effects of gene inactivation are explored. Exotoxins that damage cell membranes do so by forming pores through the cell membrane. Repeats in toxin exotoxins are produced by many genera of gram-negative bacteria. Toxins produced by filamentous fungi growing in food are termed mycotoxins. Unlike bacterial exotoxins, mycotoxins are non-protein secondary metabolites.
