The purpose of practical 3 is to isolate a bacteriophage capable of infecting a specific host bacterium. The enrichment of environmental sample i.e. the sewage sample, for the presence of bacteriophages infecting a specific host can be carried out by inoculating environmental samples with the host organism and waiting for lysis to occur. This experiment allows the determination of bacteriophage particles capable of initiating productive infections of their host bacterium in the original sample. Should we plate the sample directly or begin with an enrichment?
A selective enrichment used in this experiment increases the probability that colonies of the desired organism will be isolated upon subsequent streak-plating and not crowded out by others. Selective enrichment media are useful in recovering organisms which are in very low numbers, and they are often formulated like their corresponding isolation media. Selective enrichments can also be used for certain organisms that are not necessarily in low numbers; the "most probable number" method of quantitation involves their use.
Water samples are passed through a filter which is then placed on the appropriate selective plating medium. This method is useful in the direct plating of samples containing low numbers of the desired organism.
Recognition of the presence of bacteriophages depends on the cytopathic effects of the virus on its host organism. For bacteriophages, the most easily identifiable symptom is of infection is by lysis of the host cell, which is identified most easily by the formation of a plaque on a lawn of bacteria growing on solid medium. Each plaque arises from a single viral particle in the original sample. By counting the number of plaques produced, the number of infective virus particles in the original sample can be estimated. Since the particle concentration may be great, practical4 will require the...