Staphylococcus

TRANSMISSION ROUTE: Direct contact

Definition

Staphylococcus is a genus of gram-positive cocci, or bacteria, with a thick peptidoglycan layer in their cell walls (gram-positive) that appear under the microscope as clusters of spherical cells (cocci). The staphylococci can grow in the presence or absence of oxygen and are distinguished from bacteria in the genus Streptococcus in the laboratory by the production of the enzyme catalase by staph species.

Natural Habitat and Features

Staphylococci are present among the normal microbiota of human skin and the mucous membranes of the respiratory and gastrointestinal tracts. Approximately 90 percent of infants become carriers of staphylococci within ten days of birth. For some staphylococcal species, the carrier rate is higher among medical personnel than among the general population.

Pathogenicity and Clinical Significance

While S. epidermidis is a universal symbiotant of the human skin, it can cause opportunistic infections. Other staph species are responsible for sinusitis, various infections of the skin, food poisoning, infections of the blood and bone, and toxic shock syndrome.

S. aureus is responsible for most staph infections in humans. Most people carry it in their nose and spread it to the surface of the skin or clothing. S. aureus invades the epidermis at a site of injury, at a gland opening, or at a hair follicle. White blood cells and plasma enter the area by passing through dilated capillaries, producing the swelling and redness of boils, furuncles, and infected wounds. Pus formation results when white blood cells and dead bacteria accumulate at the site.

Usually, a fibrin clot forms around the infection site and prevents the spread of bacteria into the blood. A pus-filled blemish near the surface of the skin is called a boil. When the infection penetrates into deeper layers of the skin, it is referred to as a furuncle. Boils and furuncles are treated by drainage through a surgical incision in an effort to avoid applying pressure and pushing bacteria into uninfected tissue or the blood.

Staphylococcal infection of newborns is a serious concern. S. aureus is readily transmitted between carriers to newborns by direct contact. The sites infected most frequently are infants’ eyes and umbilical cords. The increased prevalence of antibiotic-resistant strains has led to widespread use of commercial disinfectants in hospital settings.

Severe staphylococcal infections may occur as complications of trauma that breaks the skin, such as trauma from surgery, accident, or burn. Illnesses that leave the immune system weakened, such as cancer, acquired immunodeficiency syndrome, or cirrhosis of the liver leave people susceptible to pneumonia, bone infections (osteomyelitis), deep tissue abscesses, and infections of the surface of the heart (endocarditis) and the spinal cord and brain (meningitis).

Staphylococcal food poisoning occurs when food is left unrefrigerated. The origin of the bacteria is usually a cook or other food preparer who unknowingly carries S. aureus and contaminates food during its preparation. The bacteria grow in the food and produce a toxin. Dairy items such as creamed food products and custard, and bread stuffing, support the growth of S. aureus cells and their production of the toxin. Once the toxin contaminates food, rewarming does not make the food safe. While the bacterial cells may be killed, the toxin remains. Staphylococcal food intoxication occurs a few hours after eating contaminated food. Sudden nausea, vomiting, and diarrhea that lasts twenty-four to forty-eight hours characterize this common cause of food poisoning.

S. aureus and tampons are linked to the occurrence of toxic shock syndrome (TSS) in menstruating girls and women. In 1980, the public took note when there were 299 TSS cases and 25 deaths in the United States. Nearly all the women became ill during their menstrual period while using super absorbent tampons. The Centers for Disease Control and Prevention recommended that women avoid using tampons continuously during their menstrual cycle. Presumably, the super absorbent tampons injured the vaginal mucous membrane and facilitated the growth of S. aureus, and the uptake of bacterial toxin into the bloodstream. The symptoms of TSS include the sudden onset of high fever, nausea, vomiting, diarrhea, and muscle cramps (myalgia). A sunburn-like rash develops eight to ten days after the symptoms first appear. In severe cases, a person can go into shock.

Methicillin-resistant S. aureus (MRSA) and oxacillin-resistant S. aureus (ORSA) infections are caused by strains of bacteria resistant to several of the antibiotics commonly used to treat ordinary staph infections. These infections occur most often in people who have been in hospitals, nursing homes, and dialysis centers. When an MRSA occurs in these settings, it is known as health-care-acquired MRSA (HA-MRSA).

Since 2000, another type of MRSA infection has become more prevalent in otherwise healthy people. This form, community acquired MRSA (CA-MRSA), often begins as a skin lesion. It is spread by skin-to-skin contact. People at risk for CA-MRSA include high school wrestlers, child-care staff, and those who live in crowded conditions.

Drug Susceptibility

Antibiotics are not effective against staphylococcal infections of the skin because of the fibrin clot that walls off the infection from the normal circulation. An exception is staphylococcal impetigo, which can be treated with topical antibiotics.

After the introduction of penicillin, the strains of S. aureus circulating in the United States were rapidly replaced with antibiotic-resistant ones. Staphylococcal pneumonia, osteomyelitis, deep tissue abscesses, endocarditis, and meningitis are treated with prolonged and intensive antibiotic therapy. Nafcillin is administered for methicillin-sensitive strains, and intravenous vancomycin is used for MRSA strains. Trimethoprim-sulfamethoxazole or rifampin is used with vancomycin in severe cases and in CA-MRSA infections. Linezolid is frequently used for treating staph meningitis.

Bibliography

Brachman, Philip S., and Elias Abrutyn, eds. Bacterial Infections of Humans: Epidemiology and Control. 4th ed. Springer, 2009.

Crossley, Kent B., Kimberly K. Jefferson, and Gordon L. Archer, eds. Staphylococci in Human Disease. John Wiley & Sons, 2009.

Jorgensen, James H., et al. Manual of Clinical Microbiology. 11th ed. ASM, 2015.

"Staphylococcus Aureus Basics." Centers for Disease Control and Prevention, 15 Apr. 2024, www.cdc.gov/staphylococcus-aureus/about/index.html. Accessed 24 Jul. 2024.

"Staphylococcus Aureus Infection." National Library of Medicine, 17 Jul. 2023, www.ncbi.nlm.nih.gov/books/NBK441868/. Accessed 24 Jul. 2024.