Some microorganisms have the enzymes and biochemical pathways needed to synthesize all cell components using minerals and sources of energy, carbon, nitrogen, phosphorus, and sulfur.
Other microorganisms lack one or more of the enzymes needed to manufacture indispensable constituents. Therefore they must obtain these constituents or their precursors from the environment.
Organic compounds that are essential cell components or precursors of such components but cannot be synthesized by the organism are called growth factors.
There are three major classes of growth factors:
- amino acids.
- purines and pyrimidines.
Amino acids are needed for protein synthesis; purines and pyrimidines for nucleic acid synthesis. Vitamins are small organic molecules that usually make up all or part of enzyme cofactors and are needed in only very small amounts to sustain growth.
The functions of selected vitamins, and examples of microorganisms requiring them, are given in table.
Some microorganisms require many vitamins; for example, Enterococcus faecalis needs eight different vitamins for growth. Other growth factors are also seen; heme (from hemoglobin or cytochromes) is required by Haemophilus influenzae, and some mycoplasmas need cholesterol.
Enzymes Understanding the growth factor requirements of microbes has important practical applications. Both microbes with known, specific requirements and those that produce large quantities of a substance (e.g., vitamins) are useful. Microbes with a specific growth factor requirement can be used in bioassays for the factor they need.
A typical assay is a growth-response assay, which allows the amount of growth factor in a solution to be determined. These assays are based on the observation that the amount of growth in a culture is related to the amount of growth factor present.
Ideally, the amount of growth is directly proportional to the amount of growth factor; if the growth factor concentration doubles the amount of microbial growth doubles.
For example, species from the bacterial genera Lactobacillus and Streptococcus can be used in microbiological assays of most vitamins and amino acids.
The appropriate bacterium is grown in a series of culture vessels, each containing medium with an excess amount of all required components except the growth factor to be assayed. A different amount of growth factor is added to each vessel.
The standard curve is prepared by plotting the growth factor quantity or concentration against the total extent of bacterial growth.
The quantity of the growth factor in a test sample is determined by comparing the extent of growth caused by the unknown sample with that resulting from the standards. Microbiological assays are specific, sensitive, and simple.
They still are used in the assay of substances like vitamin B12 and biotin, despite advances in chemical assay techniques.
On the other hand, those microorganisms able to synthesize large quantities of vitamins can be used to manufacture these compounds for human use. Several water-soluble and fat-soluble vitamins are produced partly or completely using industrial fermentations.
Good examples of such vitamins and the microorganisms that synthesize them are riboflavin (Clostridium, Candida, Ashbya, Eremothecium),
coenzyme A (Brevibacterium),
vitamin B12 (Streptomyces, Propionibacterium, Pseudomonas),
vitamin C (Gluconobacter, Erwinia, Corynebacterium),
Beta- carotene (Dunaliella),
vitamin D (Saccharomyces).
Current research focuses on improving yields and finding microorganisms that can produce large quantities of other vitamins.