“Staph” is an abbreviation for the bacteria Staphylococcus. There are various species, but the main one which causes disease is Staph aureus (aka “coagulase-positive”). So whenever we say “Staph,” in medicine, that’s what we’re referring to. It’s very common, and many of us harbor it on our skin or in our noses, where it lives harmlessly. It only causes illness (infection) if it gets under the skin, and inside us. (see also our topic about types of Germs).
Most skin infections are caused by Staph. These include abscesses, boils, impetigo (yellow crusts around the nose), cellulitis (a red, hot, painful patch), and wound infections. Staph can also cause bone and joint infections, and an uncommon but severe type of pneumonia. Some strains of Staph produce toxins, which can cause food poisoning, or septic shock if it gets into the bloodstream (toxic shock is a form of this).
One strain of Staph is called “MRSA” (pronounced “MER-sa”), which stands for “Methicillin-resistant Staph aureus.” When penicillin was invented in the mid-1940’s, it killed all Staph, but the bacteria quickly acquired resistance. So they invented new antibiotics to fight the germ, the first being methicillin. Methicillin no longer exists, although related drugs do, and they worked fine until around 2005, when the MRSA strain emerged & spread. There’s nothing special about MRSA, it’s the same as any other Staph, except that we need to use other antibiotics to treat it.
The main oral antibiotics that cure Staph (which today is usually a MRSA strain) include trimethoprim-sulfamethoxazole (Septra®, Bactrim®, etc.), doxycycline (a form of tetracycline), and usually clindamycin. Others like cephalexin (Keflex®) or amoxacillin + clavulanate (Augmentin®) can cure non-MRSA strains of Staph, but not MRSA. Penicillin and amoxicillin almost never work anymore.
Other strains of Staphylococcus bacteria, like “coagulase-negative” Staph, Staphylococcus epidermidis, and others are usually just contaminants from the skin and not the cause of disease. When we see them in a culture, we ignore them (unless it’s a blood culture for heart infection or sepsis, & we find the bacteria in several specimens).