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  Section: Genetics » Sexuality and Recombination in Bacteria and Viruses
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Culture media and mutant strains

Sexuality and Recombination in Bacteria and Viruses
Three Methods for Transfer of Genetic Material 
Sexual conjugation in bacteria 
Culture media and mutant strains
Discovery of gene transfer
Discovery of linkage in bacteria
Donor and recipient strains
- Mechanism of chromosome transfer
- Physical structures involved in chromosome transfer
Linkage maps in bacteria
Conjugation mapping through interrupted mapping
Circular linkage map
Linkage information from transformation
Recombination after gene transfer
High resolution mapping
Linear order of genes
Replication and recombination in viruses 
Replication of bacteriophages
Lysogenic bacteria
Recombination in viruses
Circular genetic maps in viruses

Sexual Conjugation in Bacteria
Culture media and mutant strains
Most of the important work on genetics of bacteria was atleast initially done on colon bacillus (Escherichia coli = E. coli). These bacteria can be grown in petri-dishes or in culture tubes on a simple culture medium consisting of glucose, ammonium salts and a few additional substances needed in traces. From these simple substances, other complicated substances like vitamins and amino acids can be manufactured by the bacteria themselves. The strains of bacteria which can grow on this simplest medium, are called prototrophs. However, certain mutant strain may be incapable of manufacturing a particular substance, so that this specific substance will have to be added to the simplest (minimal) medium for the growth of the mutant strain. Such a mutant strain is called an auxotroph. While a prototroph can grow on the minimal medium, the auxotrophs will always grow on supplemented media.

There may be available mutants of other kinds as well. For instance a strain may be a mutant in regard to its capacity to resist infection by a virus or it may be resistant to an antibiotic like streptomycin. These resistant strains are though mutants, can still grow on minimal medium and will therefore be called prototrophs and not auxotrophs. Auxotrophs are only those mutants which are deficient only in their nutritional requirements.

Bacteria can be grown either in liquid medium or on the surface of medium which is made solid by means of agar (a gelatin like substance). In liquid medium, cells will always remain scattered, but on a solid medium, the products of cell division will remain together to form a colony, so that ultimately each colony will represent descendants of a single cell (Fig. 12.2). Availability of these culture techniques enable detection of a few prototrophs among millions of cells. If millions of auxotrophic cells containing a few prototrophs are placed on an agar plate with minimal medium, only the prototrophs will grow, so that if the number of cells plated is known, the relative proportion of prototrophs can be easily worked out.
Technique of growing single cell bacterial colonies in the laboratory (spreader is used to spread a drop of suspension culture, on agar plate, giving single cells by repeated spreading).
Fig. 12.2. Technique of growing single cell bacterial colonies in the laboratory (spreader is used to spread a drop of suspension culture, on agar plate, giving single cells by repeated spreading).

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