Re: How exactly does an endospore become resistant to UV, heat etc.?

It’s interesting how you can study microbiology for years and never think about certain things – I had never even questioned why endospores were so sturdy. The short answer to your question is coat, coat, coat. Unlike the very dynamic plasma membrane/cell wall structure of a vegetative (active) cell, the cell wall of most endospores is much more thick and rugged. The coat is comprised of nearly thirty structural components in B. subtilis, which is a handy model organism for spore-forming rods. Recently (2003) the structure of one of the most important components, CotA, was solved. This protein turns out to be a laccase, which is a copper containing enzyme that can perform redox reactions on a multitude of chemicals. It is also known that CotA is required for UV light resistance, but nobody really knows why this is.

The primary mechanism by which some spores show a heightened resistance to UV radiation appears to be light-absorbing pigments in their coats. This is one of the oldest mechanisms used to protect cells from radiation – a pigment molecule can preferentially absorb certain wavelengths of light. Often (think photosynthesis) the pigment molecule can then release the light as another form of energy, such as excited electrons that the cell can use for energy, but I digress.

Beneath the protein coat, there is an outer membrane that covers a very thick layer of peptidoglycan. Peptidoglycan is normally found in bacterial cell walls, and this modified thick layer seems to play a role in dehydrating the bacterial cell. Dehydration is important to the survival of the spore as it helps protect the cell from thermal damage to its proteins and DNA.

The final trick that endospores employ to enhance their durability is a chemical called dipicolinic acid (DPA). DPA can comprise up to 10% of the dry weight of an endospore. It can offer strong protection to wet heat (think autoclave) and some limited resistance to UV radiation and some chemical assaults such as hydrogen peroxide. Spores that lack DPA expression still maintain a high degree of environmental ruggedness, but somewhat less than those that do express DPA.

As far as germination is concerned, it is important to remember that bacteria most often sporulate in response to environmental hardships. It makes sense, then, to discover that endospores have receptors on their coat to things like protein and sugar that might increase viability of active cells. These molecules are called germinants, because they cause the spore to germinate. In a poorly understood mechanism, the receptors for the germinants somehow cause the cell to release its DPA stores, along with ions such as calcium. When this happens it triggers proteolytic enzymes in the core of the endospore’s wall to degrade the core and release the vegetative cell. Neat, huh?

There are a few links that I thought might be helpful, and I have included them here. I have also included some research articles. Good luck!
Billy.

Endospores and Endospore Staining. http://howie.myweb.uga.edu/structure.html

Bacterial Endospores. http://www.micro.cornell.edu/faculty/Angert/endospores1.html

Enguita FJ, Martins LO, Henriques AO, Carrondo MA. (2003) Crystal Structure of a Bacterial Endospore Coat Component. Journal of Biological Chemistry 278(21). 19416 – 19425.

Moeller R, Horneck G, Facius R, Stackebrandt E. (2005) Role of pigmentation in protecting Bacillus sp. endospores against environmental UV radiation. FEMS Microbiology Ecology. 51(2). 231-236.

Setlow B, Atluri S, Kitchel R, Koziol-Dube K, Setlow P. (2006) Role of dipicolinic acid in resistance and stability of spores of Bacillus subtilis with or without DNA-protective alpha/beta- type small acid-soluble proteins. Journal of Bacteriology. 188(11). 3740 – 3747.

Paidhungat M, Setlow B, Daniels WB, Hoover D, Papafragkou E, Setlow P. (2002) Mechanisms of Induction of Germination of Bacillus subtilis Spores by High Pressure. Applied and Environmental Microbiology. 68(6). 3172 – 3175.