What is Staphylococcus aureus?
Staphylococcus aureus is a bacterial species that belongs to the genus Staphylococcus. The Greek word “staphyle” refers to a bunch of grapes, whereas the Latin word “coccus” refers to spherical bacteria. Therefore, the name Staphylococcus describes the peculiar grape bunch-like colonies that the round bacteria in this genus create. The Latin word “aureus” means golden. Therefore, Staphylococcus aureus is also sometimes called the “golden staph“.
Staphylococcus aureus (commonly abbreviated as S. aureus) is the most common cause of infections caused by the Staphylococcus genus in humans. However, the mere presence of S. aureus on the surface of the body does not cause infection. About 20% to 30% of healthy individuals have S. aureus on their skin and nasal mucosa as part of the normal flora. These individuals are considered to be Staphylococcus carriers.
Even a minor breach in the skin or mucosal barrier is sufficient to cause Staphylococcus aureus infections such as pimples, styes, folliculitis, boils, furuncles, swimmer’s ear, sinusitis, whitlow, epiglottitis, cellulitis, impetigo, breast infection, scalded skin syndrome and genital infections.
Read more on staph skin infections.
Other internal infections caused by Staphylococcus include arthritis, urinary tract infections, lymphangitis, lymphadenitis, thrombophlebitis, osteomyelitis, encodarditis, meningitis, toxic shock syndrome, necrotizing fasciitis and sepsis.
Along with Staphylococcus epidermidis, Staphylococcus aureus is the most common cause of hospital-acquired infections. Such infections can be acquired during surgery from catheters, artificial heart valves and prosthetic joints. Bloodstream infections and pneumonia can also be caused by these bacteria. Food poisoning can also happen due to enterotoxins from S. aureus.
Physical Characteristics of Staphylococcus aureus
Staphylococcus aureus is a Gram-positive bacteria with a thin capsule and a thick cell wall. It is a spherical bacteria with a diameter of about 1 micron. Staphylococcus aureus is a facultative anaerobe that can survive in both aerobic and anaerobic conditions. This bacteria is immobile and grows in pairs, chains or clusters. The colonies of S. aureus are usually 6 to 8 millimeters in diameter, have a smooth and rounded shape, and display golden, pale yellow or orange color.
Even though S. aureus does not form spores, it can survive on non-living materials (such as computer keyboards and beddings) for many weeks. It is noteworthy that Staphylococcus aureus can live in the body without causing any disease. However, it is an opportunistic pathogen that causes infection and disease when there is an injury or the immunity is lowered.
Staphylococcus aureus is resistant to conditions such as high salt concentration (<10%), high temperatures (50 °C or 122 °F), and drying. The cell wall of this bacteria has three layers: an inner cytoplasmic membrane, a middle murein or peptidoglycan layer, and an outer capsule made up of polysaccharides. A “fuzzy coat” made up of embedded teichoic acid and proteins is seen on the outer side of the cell wall.
Virulence Factors of Staphylococcus aureus
The ability of a microbe to cause disease by invading the tissues of a host is technically referred to virulence. Individual S. aureus bacteria may differ considerably in the degree of virulence factors. However, as a species Staphylococcus aureus possesses the following virulence factors that facilitates the ability for it to cause disease in humans:
- The proteins present on the surface of the S. aureus bacteria help it to attach to the damaged tissues of the host.
- A variety of factors help the S. aureus bacteria in evading the phagocytic defense processes of the host. These include the bacterial capsule, the peculiar binding mode of Protein A to IgG, formation of biofilm or slime layer by polysaccharides secreted by the bacteria, presence of catalase enzyme to break down peroxide, and presence of yellow carotenoid pigment to protect against free radicals.
- S. aureus also has coagulase that causes coagulation of plasma and clumping of bacteria. The coagulase exists in both bound and free forms. A variety of invasins (hyaluronidases, kinases, and fatty acid modifying enzymes) help S. aureus to invade and spread through the host tissues.
- Toxins released by S. aureus cause lysis of host cell membranes. Erythrocyte and leukocyte lysis can occur due to these cytotoxins. Toxins from Staphylococcus may also cause scalded skin syndrome (due to exfoliatin toxin), massive non-specific T-cell response (due to superantigen toxins), toxic shock syndrome (due to TSST-1)and food poisoning (due to enterotoxins).
Mode of Staphylococcus aureus infection
Staphylococcus aureus infection happens when a person comes in contact with a substance that is contaminated with the bacteria. Normally the skin is an effective barrier against bacteria provided that the skin is healthy. However, when there is a break in the skin or mucosal barrier, it then allows the bacteria to penetrate inside the body and lead to an infection.
Examples of modes of Staphylococcus aureus infection include skin contact, and sharing of towels and other personal clothing items. S.aureus infection does not spread through the air. Also, immunodeficiency enhances the chances of lethal S. aureus infection (sepsis). In healthy individuals, S. aureus infection is usually localized, and heals on its own within time. However, if the immune system is weak, these infections can become deadly.
Treatment of Staphylococcus aureus infections
Antibacterial ointments can be used to treat localized skin infections. If abscesses are present at the sites of infection, then they need to be drained. Surgery may be required to drain deep abscesses. If the infection is widespread or systemic, oral or parenteral antibiotic treatment can be used.
However, the sensitivity of S. aureus to various antibiotics needs to be tested before prescribing appropriate antibiotic therapy. This is because these bacteria can be resistant to certain widely used antibiotics. Hospitalization is required in case of systemic infections that require intravenous administration of antibiotics.
Some of the antibiotics used for the treatment of S. aureus infections include penicillin, amoxicillin, oxacillin, cephalosporins, clindamycin, erythromycin, mupirocin, and ciprofloxacin. Some of the antibiotics may not be suitable for treating S. aureus infection in pregnant women. If S. aureus infections occur in vein catheters and artificial heart valves, then these may need to be removed and replaced.
Staphylococcus strains that are resistant to some commonly used antibiotics present challenges in the treatment of Staphylococcus infections. Penicillin resistance was seen in Staphylococcus within two years of introducing penicillin in the market. It is estimated that more than 90% of the Staphylococcus bacteria today have penicillin resistance (due to the presence of penicillase enzyme that breaks down penicillin). These resistant bacteria need to be identified and treated with penicillase-resistant penicillin, methicillin, cephalosporins, and other antibiotics.
Methicillin-resistant Staphylococcus aureus (abbreviated as MRSA) has now emerged worldwide (especially in hospital settings). Some rare vancomycin-resistant strains have also been reported to occur. These “superbugs” do not cause any unique signs and symptoms, and are, therefore, indistinguishable from non-resistant S. aureus. These cases pose a challenge in prescribing the right antibiotic to treat S. aureus infections.
Prevention of Staphylococcus aureus infections
The following are some of the preventive measures against S. aureus infections:
- Regular bathing and hand washing can help. However, antibacterial soaps are not essential. Regular soaps are effective.
- Any wounds on the skin should be cleaned and covered to prevent S. aureus infections.
- Preparation of food should not be done by people who are carriers of S. aureus. Food workers should be regularly tested for the presence of S. aureus, and treated.
- Disinfection of surfaces and instruments in hospitals must be done with alcohol, iodine, or quaternary ammonium compounds.
- Cow milk should be tested for the presence of S. aureus. Milk should be pasteurized before consumption.
Images sourced from atlasdermatologico.com.br.