Enrichment Culture

Enrichment culture is a very valuable technique developed by some giants in microbiology in the early 1900s.  The basic principle involved is that of selection.  Often, it is desired to isolate bacteria that are relatively scarce, or are in fact in very low numbers.  This creates a problem.  Normally, bacteria are isolated from one another using the streak plate technique, in which a sample of bacteria is spread out onto an agar plate in such a way that individual cells are separated from each other.  Each cell multiplies to form a visible colony in which all the cells are the same strain (and essentially clones of each other). Thus you can use a loop to pick some cells from a colony to start a pure culture, a culture in which a single organism (or a few known organisms) is growing.  If the desired organism is a minority in the sample, say 0.1% of the total, one would have to have at least a thousand isolated colonies on a plate to have a chance of seeing just one of the desired bacterium.  This is physically impossible; there is not sufficient room on a Petri dish.

Enrichment culture solves this problem.  The essence of this technique is to provide growth conditions that are very favorable for the organism of interest, and as unfavorable as possible for competing organisms.  For example, if you wish to isolate a bacterium that is a thermophile (prefers to grow at a high temperature such as 55 deg C), incubate the sample at that high temperature.  Organisms that cannot tolerate that temperature will dies or simply fail to grow, while thermophiles will grow and increase in number, over time becoming a large and larger proportion of the total bacterial population in the sample.  This is an example of enrichment by modifying the physical conditions.  
Enrichment can also be carried out by modifying the nutrient content of the culture medium.  Here are two examples.  There are a variety of bacteria present in soil that are capable of converting the otherwise inert form of nitrogen in the atmosphere, N₂, into ammonia.  Such reactions provide the major input of N into the biosphere.  Organisms capable of this transformation of N can be isolated by incubating a soil sample in a culture medium which has all the ingredients necessary for growing except nitrogen. Those bacteria that can "fix" nitrogen and create their own nitrogenous nutrients for growth will have a selective advantage and increase in number compared to those who can't. Nitrogen is likely to present in small amounts as a contaminant in the soil or in the other culture ingredients, so it would be incorrect to say that only nitrogen fixing bacteria will grow.  They will, however, have a great advantage and will eventually outgrow the others given enough time.
Another example of manipulating the culture conditions would be an attempt to isolate bacteria that can break down a xenobiotic (man-made, unnatural compound) such as a pesticide.  Pesticides are organic compounds (most C and H).  Bacteria generally break down organic compounds as a source of building materials and energy.  In a growth medium in which the pesticide in question is the sole carbon source for growth, only those bacteria that can use the compound will grow appreciably, while others will not.  Eventually, the culture will contain a sufficiently high proportion of pesticide-degraders that it would be easy to isolate them using the streak plate technique. 

Microbiologists sometimes argue about what should correctly be called enrichment culture.  A commonly used method in molecular genetics is to select for bacteria which carry a plasmid with an antibiotic resistant gene by growing that bacterium in a medium containing the antibiotic.  This type of selection really isn't what we mean by enrichment culture.

You may have an opportunity to carry out this technique.  The slants of igepon-degrading bacteria that we are studying were isolated in this way.