Culture

One aspect of the forensic examination of samples or of a crime or accident scene can involve determining whether or not a particular microorganism is present. Disease-causing (pathogenic) micro-organisms including bacteria and viruses are capable of causing illness and death, or may have contaminated a food or water source.

Modern techniques exist that rely on the detection of the genetic material of the microorganism and do not require the growth of the organism. Indeed, the organism can be dead and still remain detectable. However, the more traditional growth-dependent identification techniques are reliable, inexpensive, and are still widely used.

Bacteria require a food source to grow. Depending on the type of bacteria, the liquid or solid food source (growth medium) can be very general or highly specific, requiring the presence of certain types of amino acids, carbon sources, and other compounds. As well, some bacteria require the presence of oxygen (aerobic bacteria), while others require the complete absence of oxygen (anaerobic bacteria).

When the bacteria-containing sample is added to the medium in the step called inoculation, living bacteria will begin to assimilate the nutrients and use them to repair damaged components and construct new components. As a result, the bacteria will begin to grow and divide to produce two progeny bacteria.

Over the course of hours, the cycle of growth and division is repeated thousands of times. With each round of division, cell numbers double (i.e., growth is exponential). This rate of growth quickly leads to huge numbers of bacteria in the liquid medium or on the solid medium. This causes the liquid to become cloudy. On the surface, the countless growth and division cycles lead to the formation of a visible mound of bacteria that is known as a colony.

Bacteria can be cultured in different types of media and the various resulting biochemical reactions can be used to identify the type of organism that is present. Differing appearance of the colonies on the solid medium or the production of various compounds in the presence of specific nutrients can all be clues to the identity of the microoganism. Depending on the type of bacterium, culture-based identification can take from several days to weeks.

Viruses can also be cultured and identified (typically by their shape). However, since viruses cannot grow independently, they require the presence of a host cell. For example, poliovirus is cultured using cells found in eggs. Some viruses known as bacteriophages require a bacterial host.