Fish Microbiology and Quality Assurance

4.6.1. Endosopre formation
Endospore formation is initiated by the vegetative cell under the conditions of nutrient depletion, especially the carbon and nitrogen source. The vegetative cell prepares for sporulation by transforming in to a committed sporulating cell called sporangium. The sporangium actively involves in synthesizing compounds required for spore formation. Most spore formers develop mature spore of complex structure within 6-8 hours. Sporulation usually appears in the late logarthemic phase of growth possibly because of nutrient depletion and accumulation of toxic metabolites.

Structureof endospore
The spore released at the end of sporulation from the mother cell is structurally, biochemically and physiologically different from vegetative cells. The inner core of the spore containing proteins and nucleic acids is surrounded by several layers of varying composition. These include core, spore cortex, spore coat, and outer exosporium.

Exosporium: The outermost spore layer is the exosporium and it varies in size in different species. It is a thin, delicate covering made of protein, polysaccaharide and some lipids.

Spore coat: Following exosporium is the thick and structurally complex spore coat of several layers consisting of proteins with unusual aminoacids. The spore coat protects the inner spore cortex from attack by lytic enzymes, and serves as barrier to oxidizing agents. This layer is not involved in offering resistance to spores from heat or radiation.

Spore cortex: It is made of several layers of loosely cross linked peptodoglycan with calcium and dipicolinic acid.

Core: Core is the central region of the spore and contains DNA, ribosomes, most enzymes, diaminopimelic acid and divalent cations and other macromolecules. Core is also characterized by low water content of 10-30% of vegetative cell which reduces the core cytoplasm to a gel like consistency. The dehydration is responsible for spore dormancy and offering resistance to variety of agents. The core cytoplasm also contains high concentration of small acid-soluble spore proteins (SASP) which bound tightly to DNA and protect it from damaging effect of ultraviolet light, dessication, and dry heat. The SASPs also finction as a source of carbon and energy during germnation of endospore.