Structure of Endospores

Structure of Endospores

	Structure of Endospores - The endospore, consists of a protoplast or core surrounded by a spore integument or envelope. The protoplast contains the DNA of one chromosome and components of the protein synthesis machinery like ribosomes, tRNAs, accessory factors and enzymes. No mRNA has, however, been detected. There is virtual absence of amino acids and their biosynthetic enzymes. The amino acids are supplied during germination by hydrolysis of a low molecular weight storage protein. From the inner to the outer side, the spore envelope consists of the inner membrane, the cortex, the outer membrane and the spore coat. In some species, an additional covering called the exosporium is present.
The inner membrane (germ cell membrane) is a unit membrane bounding the spore cytoplasm and nuclear material. The cortex is a multilaminated structure which develops between the inner and the outer membranes. It consists of many layers of peptidoglycan. The outer layer of peptidoglycan is loosely cross linked and is hydrolyzed during germination. It is probably laid down by the outer membrane. The inner layer of peptidoglycan, which comprises 20% of the total peptidoglycan in Bacillus subtilis, is tightly cross-linked, and is retained during germination. This stable peptidoglycan is laid down by the inner membrane.
The loose cortical peptidoglycan contains three repeating subunits, an alanine subunit, a tetrapeptide subunit and a muramic lactam subunit, in the proportion of about 55%, 15% and 30%, respectively (i) To the alanine subunit is attached an L-alanyl residue. (ii) The tetrapeptide subunit bears the sequence L-ala-D -glu- meso DAP-- D-ala. (iii) The muramic lac tam subunit does not have any attached amino acids. The cortex also contains large amounts of dipicolinic acid (2, 6pyridine carboxylic acid) and calcium ions, probably in the form of calcium dipicolinate.
The outer membrane is the membrane of the cell enclosing the endospore which has grown around the spore cell during endospore formation. The spore coat is an electron-dense layer formed outside the outer membrane. It is made up of keratin-like protein which constitutes as much as 80% of the total spore protein. The spore coat proteins are rich in cysteine and hydrophobic amino acids. They are unaffected by chemical treatments which solubi1ize most proteins and are responsible for the resistance of spores to chemicals. Exosporium: In the B.cereus group a thin exosporium is formed around the spore coat. This is a polysaccharide protein membrane (with small amounts of lipids) which loosely covers the spores.

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