Staphylococcus aureus

Staphylococcus aureus, species of bacteria in family Staphylococcaceae that is considered to be one of the most important pathogens occurring in humans and the most dangerous member of the genus Staphylococcus in terms of its ability to cause disease. Staphylococcus aureus occurs naturally in the environment and in the nose and other mucous membranes in about 30 percent of humans. However, it also is an opportunistic pathogen and can cause a variety of infections, ranging from mild to life-threatening. Among the most worrisome strains of S. aureus are those that have become resistant to antibiotics, such as methicillin-resistant S. aureus (MRSA), which can cause severe infections leading to sepsis and death.

Characteristics

S. aureus, similar to other members of its genus, tends to aggregate into grapelike clusters. The name staphylococcus, in fact, is derived from the Greek words staphulē, meaning “bunch of grapes,” and kokkos, meaning “berry.” The word aureus is Latin and means “golden,” which refers to the bacteria’s pigmentation. S. aureus is characterized as gram-positive, non-spore-forming, nonmotile, and facultative anaerobic (not requiring oxygen). It ranges in size from 0.5 to 1.5 μm in diameter, and it has high variability in growth parameters—different strains can grow, for example, at temperatures ranging from 4 to 48 °C (roughly 39 to 118 °F) and at pH ranging from 4.2 to 9.3.

S. aureus is unique among staphylococcus bacteria, owing to its ability to produce an enzyme known as coagulase; when secreted by infectious S. aureus, coagulase causes blood to clot, thereby enabling the bacteria to persist in tissues and cause the development of abscesses. The coagulase test is used to determine whether an infection is caused by S. aureus.

S. aureus is further distinguished by its high propensity to develop antibiotic resistance. Resistance can develop through any of various mechanisms, depending on the type of antibiotic to which the bacteria is exposed. Such mechanisms include the use of efflux pumps, which actively remove antibiotic from the bacterial cell; alteration of the bacterial protein that is targeted by the antibiotic, which results in reduced affinity for the antibiotic; and enzymatic inactivation of the antibiotic, which essentially renders it ineffective. Antibiotic resistance may also be acquired through spontaneous genetic mutations and positive selection of resistant bacterial cells and through horizontal gene transfer, in which a bacterial cell acquires genes that confer resistance from the genomes of other entities, such as bacteriophages.

Infections and transmission

On healthy skin, S. aureus is generally harmless. However, under certain circumstances, it can cause infections, such as when the bacteria come into contact with an open wound or sore. In such instances, S. aureus infection can give rise to skin conditions such as carbuncles, cellulitis, or impetigo. In rare cases, typically those involving newborns or immunocompromised adults, staphylococcal scalded skin syndrome (SSSS; or Ritter disease) develops, causing the skin to turn red and blister, giving it a burned or scalded appearance. S. aureus skin infections are often associated with the transmission of bacteria from person to person. This occurs primarily through direct contact, such as a health care worker with contaminated skin caring for a patient with an open wound. Various means of touch can result in transmission, particularly contact with contaminated objects, such as door handles and handrails.

Infection of the bloodstream by S. aureus can lead to serious, potentially life-threatening conditions, including endocarditis (infection of the heart valves), meningitis (inflammation of the meninges in the brain), osteomyelitis (bone infection), pneumonia, and sepsis. In health care settings, S. aureus can be introduced into the bloodstream via contaminated medical equipment, such as intravenous fluid catheters or surgical instruments.

Toxic shock syndrome (TSS), a type of inflammatory disease caused by a toxin released from S. aureus (and S. pyogenes), is associated with the use of certain highly absorbent materials, such as those found in products used during menstruation in women; however, children and men are also susceptible to TSS, which can be acquired through open wounds, a nosebleed, surgery, or other instances in which packed gauze or other absorbent materials are applied to control bleeding. S. aureus also causes urinary tract infections, food poisoning, and mastitis, which occurs during breastfeeding, when bacteria from the nursing baby’s mouth enters the mother’s bloodstream through a crack in the nipple.

Treatment and prevention

Infections involving S. aureus are treated with antibiotics. The specific antibiotics used are determined by whether resistant strains are present in the infection. Drug-resistant strains require cautious treatment approaches, which in many instances involves the antibiotic vancomycin, to which most antibiotic-resistant strains remain susceptible. For non-resistant strains of S. aureus, penicillin is generally used.

Management of S. aureus infections in health care settings is focused on decontamination and efforts to halt transmission. The spread of S. aureus can be reduced by handwashing and sanitization of hands and surfaces with alcohol-based solutions and by reducing the risk that items such as towels, razors, and toothbrushes are inadvertently shared.