|MadSci Network: Microbiology|
An excellent question which illustrates the fact that even closely related bacteria have different properties. The genus Streptococcus consists of Gram-positive spherical bacteria that grow in chains and ferment sugars to lactic acid. It is this last property, converting sugars into lactic acid during growth, which is useful when streptococci are used to convert milk into curds for cheesemaking or into yogurt. The species Streptococcus lactis is the most common bacterium used for this purpose.
Other species of the genus Streptococcus, however, are responsible for a variety of diseases in humans. These range from bacterial strept throat to the massively destructive disease called necrotizing fasciitis in which the layer of tissue between your skin and muscle is digested and your skin sloughs off (this is the infamous "flesh eating" bacterial infection). The most important streptococcal species that is responsible for disease in humans is Streptococcus pyogenes.
Although S. lactis and S. pyogenes are closely related species, they have vastly different properties when it comes to causing disease. Why? It turns out that S. pyogenes has a special set of genes which encode "virulence factors". Virulence factors are special gene products that allow a bacterium to successfully infect and cause disease in a human host. For example, certain virulence factors give the bacterium the ability to stick to cells in the body, allow the bacterium to secrete toxins that damage cells and make you feel sick, and confer properties that help the bacteria survive the immune response that the human body mounts against invading organisms. Since S. lactis lacks these virulence factors you can eat billions of them in a spoonful of yogurt without any ill effects, while the few thousand S. pyogenes on the surface of a splinter could conceivably rapidly multipy and result in a "flesh eating" streptococcal infection (remember massive flesh eating-type of infections are extremely rare and most S. pyogenes infections can be dealt with by using good wound care and antibiotics).
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