Re: Is it true that spores are protected by the hardest organic substance know?

hi Trevor,

Thanks for your very interesting question. To start with - no, spores are not composed of the "hardest" known organic substance. In fact several components make up a spore, but we will get to that in a moment. I was unable to find a reliable reference as to what is the hardest known organic substance. Some sources claim that it is tooth enamel, others claim that it is carnauba, a substance derived from a Brazilian plant and used in automotive wax. Some sites will tell you that it is diamond, but I'm sure you know that organic chemistry is the study of carbon compounds and diamond is not an organic compound - it's a pure form of carbon which is an element.

So onto spores. The word spore describes a number of different biological structures. It can mean a minute grain from a non-flowering plant, similar in function to a seed. It can also describe a mode of reproduction found in Protozoa and some bacteria, in which cells fragment into spores. In diatoms (unicellular algae with a silicaceous coat), we find auxospores which grow in length, acquire their coat and become new diatom cells. You might want to try Google and look for macrospore, microspore, oospore, sphaerospore, swarmspore tetraspore, zoospore and zygospore for some other definitions of spores!

However when most people refer to spores they mean endospores - a structure in which a cell has lost most of its water, is in a quiescent state and is surrounded by a hard protective coat. Sporulation is a fascinating biological process, most studied in species of Bacillus and Clostridium, both of which include pathogenic species. When these cells are depleted of nutrients, they undergo a series of morphological and biochemical changes to form a spore. The protective coat around a spore is composed of several organic compounds - predominantly peptidoglycans (molecules of sugars and peptides), calcium dipicolinic acid and a protein similar to keratin (the protein in skin, hair and nails). These molecules interact to form a tightly-packed, cross-linked lattice structure and so spores are extremely resistant to the types of treatment that we would normally use to kill cells - heat, UV or ionising radiation and biocidal compounds. Some more information about spores is at this web page. So it is not one organic compound that makes the spore coat tough, but the interactions between several types of compound.

Could bacterial spores survive interplanetary space travel? There is currently a lot of speculation about this topic. Perhaps the most cited story is that of the lunar spaceprobe Surveyor 3. The camera from this unmanned probe was retrieved by the crew of Apollo 12 when they landed on the moon and when returned to earth, there were reports that scientists had revived cells of the bacterium Streptococcus mitis from the camera, after 31 months in space. However, there is some uncertainty as to this result because of the small numbers of cells and the chance of contamination by handling. In addition, this species does not form spores and could only have survived by a freeze-drying type process. Similarly, there are reports of bacterial spores on earth being revived after 25 million or even 250 million years old. Again, these reports are very contentious due to the chance of contamination.

Microbial space travel is a key part of the Panspermia hypothesis and NASA certainly take it seriously - their unmanned probes are sterilised before launch and the Galileo probe was steered into Jupiter at the end of its life to prevent possible microbial contamination of the Jovian moon Europa. So spores are certainly tough - but the jury is still out on just how long they can survive in space.