Staphylococcus (often simply called “staph”) infections are usually the result of two species – S. aureus and S. epidermidis. The latter was originally called S. albus to differentiate its white colonies from the golden orange colonies of S. aureus. The bacterium was not studied until the late 19th century when Sir Alexander Ogston of the Royal Aberdeen Infirmary named the organisms after the Greek words for grape (staphyle) and berry (kokkos) because they grow like bunches of grapes. About half of all adults carry the organism in their nose as part of their normal flora, and about 20% have them on their skin.
Of the two species, S. aureus is the more virulent. Although S. aureus produces a number of toxins, the most important one is Staphylococcal enterotoxin B, or SEB responsible for a noticeable eruption of food poisoning within thirty minutes of ingestion. Other diseases caused by S. aureus include the following: pimples and impetigo, boils and carbuncles, wound infections and abscesses (whether from surgical/accidental/or burns), septicemia, osteomyelitis, endocarditis, meningitis, and respiratory infections (pharyngitis, laryngitis, bronchitis, and pneumonia). It is particularly prevalent in hospitals and care facilities, where infection rates rival those of E. coli outbreaks. Most people fight off the infection, but it is particularly lethal for the weak – infants, the elderly, the malnourished, obese, and the already ill. It is particularly worrisome to burn victims, as well as those with foreign objects in their bodies such as sutures, tampons, artificial valves or joints, IVs, etc.
Staphylococcus aureus is a sperical-shaped, anaerobic, nonspore-forming, Gram-positive bacterium. Only some strains have capsules, but all are highly contagious. Genetic traits are readily transferred between strains of S. aureus by plasmids and phages. This has led to the various strains becoming resistant to common antibiotics. In addition, the presence of protein A on virulent staph may help protect them from antibodies. What turns this normally harmless bacterium into a pathogen appears to be associated with the production of various enzymes and toxins that include such substances as the following: hemolysins, coagulase, leukocidin, hyaluronidase, and a fibrinolysin. Some strains of S. aureus secrete a toxin called exfoliatin that causes the skin to peel in layers. About half of the strains can secrete a number of enterotoxins that cause food poisoning.
S. aureus can survive for months in dried pus or other body fluids and is very resistant to common disinfectants. In open wounds, staph infection can give off a musty or mousy odor. If colonies of S. aureus penetrate into the skin via a hair follicle, sweat gland, or an abrasion, it can produce a furuncle (boil), stye, carbuncle, or abscess depending on the site.
Squeezing a boil or a pus-filled pimple can force the Staph organisms into the bloodstream where it can cause more serious infections. It can settle in the lungs, causing pneumonia, or in the pelvis of the kidneys, causing pyelonephritis. In young children, bone infections are common, resulting in osteomyelitis, which often becomes a chronic condition. If it lodges in bone joints, the disease is septic arthritis; in the heart chambers, it is called endocarditis; in the meningeal tissues that live in the Central Nervous System (CNS), it becomes meningitis.
Another common childhood complaint is impetigo, which is caused by either group A streptococcus or Staphylococcal aureus or both. Bullous impetigo is caused only by S. aueus, which gives off a toxin that attacks the skin surface, causing blisters and crusting. Bullous impetigo starts as a tiny pus-filled blister on noninjured skin surfaces. This will break in a day or two, leaving a thin crust that can be mistaken for an allergic reaction. It is very contagious, and is transmitted by direct contact. Nonbullous impetigo starts as a red pimple on an area of skin that has been scratched or bitten. The pimple becomes thick and creamy, and crusts over. If the crust is removed, there will be a red base that may bleed and drain a reddish fluid. These lesions can appear anywhere on the body, and are usually ignored until other complications develop. Both are very contagious.
S. aureus differs from other Staphylococcus organisms in that it has the ability to clot plasma, by secreting an enzyme called coagulase, which activates the clotting mechanism of normal plasma. Staph that do not secrete coagulase are classified into about twenty-six species, with the most common being S. epidermidis and S. saprophyticus. Both are becoming more common in human infection, especially in relation to implants (heart valves, joints, IV catheters, feeding lines, etc.) S. saprophyticus looks very much like S. epidermidis, except it is resistant to the antibiotic novobiocin. This organism is found more frequently in sexually active young women. Treatment of these infections not only include chemotherapy, but may include surgery to remove an offending device, surgery on an area of skin or bone, or it may involve the lancing of an abscessed lesion.
Bacillus cereus was once indistinguishable from S. aureus. It is a spore-forming aerobic Gram-positive rod often contaminating rice. The spores survive the boiling process and germinate overnight when refrigerated rice is saved to be used the next day as refried rice. B. cereus is very heat stable, withstanding temperatures of 126C (258.8°F) for more than an hour and a half. However, some strains produce an enterotoxin that temperatures of 56C (132.8°F) can quickly destroy. The short-duration illness produced is often referred to as a “Chinese restaurant syndrome,” and is often confused with a Staph food poisoning.