An concept occurring early in this course that I find students traditionally struggle with is an understanding of microbial nutrition. Nutrition and growth of microorganisms is a critical idea for us to learn and understand, because traditionally if you cannot grow a microorganism in the laboratory, you cannot identify it. There are many microorganism that are considered “unculturable” in the lab; these are organisms that we may be able to observe growing in the field, or perhaps are causing disease in a patient, but when they are brought back into the lab they fail to grow.
So let’s consider the requirements of an actively growing microorganism. As the cell continues to grow, leading into cell division and the formation of two daughter cells, every cell must duplicate all enzymes and structural components in order to carry out normal metabolism, and additionally must duplicate its genome. It has to make an extra copy of every component, and needs the raw materials for all of them.
|DNA or RNA||Genetic info||nucleotides||Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus|
|Proteins||Structure, enzymes||Amino acids||Carbon, Hydrogen, Oxygen, Nitrogen, Sulfur|
|Carbohydrates||Structure, storage||monosaccharides||Carbon, Hydrogen, Oxygen|
|Lipids||Membranes, energy||Glycerol and fatty acids||Carbon, Hydrogen, Oxygen, Phosphorus|
From the building blocks above, larger structures (ribosomes, flagella, cell walls, outer membranes, and plasma membranes) are constructed as the cell grows.
If you examine the column on the right hand side of the table, you might imagine that you could feed an organism those basic elements, and the organism could then use those to create the more complicated compounds found as you move to the left in the table. Certain microorganisms are very good at making all of their cellular components from very simple chemical compounds; we refer to these organisms as non-fastidious microorganisms; they have very few complex nutritional requirements and they can be very easy to grow in the lab. Escherichia coli, a common inhabitant of the mammalian digestive system, is an example of a non-fastidious bacterium. E. coli can be grown in a minimal media containing glucose (a source of carbon,) ammonium chloride (a source of nitrogen,) potassium phosphate (a source of phosphorus,) and a couple of other minerals. The bacterium is able to make all of its cellular components from these simple compounds. Some microorganisms are able to use even simpler sources of carbon and nitrogen, such as carbon dioxide and nitrogen gas taken from the atmosphere. Still, if an organism does not have access to all of the elements on the right, they will be unable to grow, regardless of where they are found.
A fastidious microorganism on the other hand, is unable to make the cellular components necessary for growth from simple chemical compounds, and must have more complex nutritional sources present in order to grow. For example, consider the single amino acid glycine. E. coli is able to make all of the glycine it needs to grow if the organism is fed glucose and ammonium chloride. An organism that is unable to accomplish this must acquire the amino acid glycine from its environment in order to grow, and if we wished to grow it in the laboratory, the growth media would have to be supplemented with glycine to permit growth. An example of a fastidious bacterium is Mycobacterium leprae, which is the causative agent of leprosy. This bacterium is so fastidious that its exact laboratory nutritional requirements are not precisely known, and consequently the organism is most easily cultured in the footpads of the armadillo.