Soil Microbiology


This webpage was created by Justin Wilson

(1) Overview Of Enrichment Cultures

Most environments contain very diverse and numerous population of microorganisms. To effectively study the role an organism plays in its environment you need to understand its metabolic and genetic characteristics.  The best way to do this is to study them as a pure culture in a laboratory.  Often the organism you want to study is found in small numbers in its environment and cannot be isolated by using general all - purpose media.  The problem is they are usually outgrown by more numerous species, and obtaining a pure culture becomes nearly impossible.

Isolating organisms in the laboratory requires a knowledge of their nutritional and environmental conditions which favor their growth, these include specific energy sources and whether they are aerobic or anaerobic .  Enrichment - culture technique makes use of this knowledge to design enrichment cultures so that a particular type of microorganism will grow faster then all other in a given sample.  For example, if you wanted to isolate a sulfate reducing microorganism from a soil solution you would use a liquid media with sulfate as the only energy source and you would incubate under anaerobic conditions.

Similar to enrichment cultures are "Selective Media."  These media are designed to inhibit the growth of undesired organism so that only the desired type can grow.  Enrichment media, on the other hand, are designed to favor the growth of the desired organism without deliberately repressing other forms.  Sometimes the effect are the same, other organisms might not grow because of the specific makeup of the enrichment medium.

There are three general strategies in favoring the growth of a particular microorganism in an enrichment culture, these are chemical, physical, and biological strategies.

1. Chemical Strategies, include using a specific energy source such as sulfate that   enriches for organisms with specific abilities to utilize it as an energy source. Controlling pH would be another method, by keeping pH between 4.0 and 5.4 you could select for acidophelic organisms.

2. Physical Strategies, include incubating at high or low temperature which would select for Thermophilic (heat loving) or Psychrophilic (cold loving) organisms. Incubating without oxygen would be another strategy this would select for anaerobic organisms.

3. Biological Strategies, include using a live host to enrich for a particular type of bacterial virus.

(2) Overview Of Enrichment Culture Procedures

Enrichment cultures can be used to enumerate microbeal populations in a sample.  Using an Dilution Endpoint series (see illustration below) and a spectometer, Most Probable Numbers can be calculated.

In a dilution endpoint series the inoculum is subjected to serial dilution in a sterile medium, and a large number of tubes of medium are inoculated with aliquots of each successive dilution.  The goal is to inoculate a series of tubes with a microbial suspension so dilute that the probability of introducing even one individual into a given tube is very small. The endpoint is the last dilution which will show growth of the organism of interest.

Once the inoculated; enrichment medium has been incubated for an adequate period of time, growth of the microorganism will show up as turbidity which is a clouding of the media due to an increase in both mass and cell number of the organism.

This turbidity can be quantified using a spectrophotometer which is an instrument that measures the amount of light transmitted directly through a sample. Measurements are usually begun by inserting into the instrument a cuvette (a special test tube) that contains only sterile medium and adusting the galvanometer to read 100 percent transmittance.  Then a second cuvette filled with a growing culture in the same medium is placed in the light path. The turbid culture scatters more light and produces a lower transmittance reading on the galvanometer. There is an inverse relationship between cell numbers and percent transmittance.

Enumeration of the microorganism can be estimated by correlating turbidity with changes in cell numbers. A standard curve can be constructed which within the concentration range may be used to estimate the microbial population from observed turbidity values.

            Source: Fisher Scientific

(3) Pros And Cons

The major pro with enrichment cultures is that it provides a relative easy way in which to isolate pure cultures of  scarce microbial types from mixed culture environments.  Enrichment cultures can also be used in the search for new microbeal types by selecting for organisms with specific capabilities.

The two biggest cons with using enrichment cultures are that they become very easily contanamated, and the munbers of microorganisms isolated never represent the true numbers found in their environment.

(4) Additional Sources Of Information

Lovelock, D. W., and Davies, R. 1978. Techniques for the Study of Mixed Populations. Academic Press, N. Y.

Jennings, D., and Isaac, S. 1995. Microbial Cultures.  Bios Scientific Publishers Ltd, Oxford, UK.

Claus, G. W. 1995. Understanding Microbes A Laboratory Textbook for Microbiology. W.H. Freeman and Company, N.Y. pp.203 - 206

O' Leary, W. 1989. Practical Handbook of Microbiology. CRC Press, Boston. 337-348.

(5) Links To Other Sites On Enrichment Cultures

None available on the web at this time.