Genes in Viruses and Bacteria

Part 2

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Counting Viruses

Viruses can only be cultured (grown for study) in the presence of living cells. To grow bacteriophages you must first start with a healthy layer or "lawn" of susceptible bacteria.

Only then can the virus be introduced the bacteria covered agar (usually in a petri dish). Each active virus infects just one bacterium, but within a few hours hundreds of bacteria will be infected and dying. The death and destruction of the bacteria leave a small but visible "hole" in the bacterial lawn.

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Classification of Viruses

Microbiologists use a wide variety of characterists to identify and classify viruses including:

• Shape - examples: polyhedral, spherical, rodshaped, polyhedral + tail
• Nucleic acid content: DNA or RNA
• Presence or absence of an envelop
• type of host affected
• method of transmission (vectors)

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