Staphylococcus aureus


Life History and Characteristics

Staphylococcus aureus is a gram positive bacterium that is usually found in the nasal passages and on the skin of 15 to 40% of healthy humans, but can also survive in a wide variety of locations in the body. This bacterium is spread from person to person or to fomite by direct contact. Colonies of S. aureus appear in pairs, chains, or clusters. S. aureus is not an organism that is contained to one region of the world and is a universal health concern, specifically in the food handling industries.


The most common health concern associated with S. aureus is food poisoning caused by the release of enterotoxins, even in small doses, into food. Release of less than 1 microgram of toxin is sufficient to contaminate food enough to illicit symptoms of food poisoning. The infective dose of toxin is generally present when food is contaminated with an excess of 100,000 bacteria per gram of food. The intensity and variety of symptoms resulting from S. aureus food poisoning differ from individual to individual, but some of the most common symptoms are nausea, vomiting, abdominal cramping, and prostration (complete physical or mental exhaustion). It usually takes 2 or 3 days to recover from S. aureus food poisoning, but in some instances individuals will require more time to fully recover.

Even though S. aureus is mainly associated with food poisoning, the bacterium can penetrate the skin or other mucous membranes to invade a range of tissues which will cause a variety of infections. Superficial infection of the skin can cause boils, impetigo, styes (infection of the glands or hair follicles of the eyelids), folliculitis, and furnacles. All of these infections are characterized by varying degrees of inflammation of the skin cells, or associated structures such as hair follicles. The release of two exotoxins from certain strains of S. aureus can lead to Staphylococcal scaled skin syndrome (SSSS), which is characterized by blistering skin. Invasion into the body can lead to more serious health problems including pneumonia (a frequent complication of influenza), mastitis, phlebitis (inflammation of the veins), meningitis, and urinary tract infections. If the bacterium is allowed to colonize even deeper tissues more serious conditions such as osteomyelitis and endocarditis may result. The most serious consequences of these deeper tissue infections occur when the bacterium invades the bloodstream leading to septic shock and possibly death.

Virulence Factors 

The most important virulence factor of S. aureus is the specific surface proteins that allow the organism to attach to host proteins. The surface proteins of this bacterium allow it to attach to host proteins such as laminin and fibronectin, which form the extracellular matrix of epithelial and endothelial cells. S. aureusalso produces a number of membrane damaging toxins that allow the organism to further invade and harm the host, of which the alpha- toxin is the most well studied and is the protein responsible for septic shock. The alpha- toxin is a protein that binds to a specific receptor in platelets and monocytes in humans, forming pores that eventually destroy the cell.

A second toxin that is significant in human infection is the gamma- toxin (also called leukotoxin) which works in combination with leukocidin. These two proteins work together, but are expressed at different loci in the genome. Only 2% of all of S. aureus isolates express leukocidin, but nearly 90% of the strains isolated from severe skin lesions express this toxin. This bacterium also possesses a surface protein (Protein A) which binds to IgG in the incorrect orientation, disrupting the body’s normal phagocytic activity. S. aureus also secretes six antigenic types of enterotoxins that cause diarrhea and vomiting when ingested.

Epidemiology: The incidence of food poisoning due to the presence of S. aureus in food is unknown due to poor reporting by those infected. Many who come down with food poisoning due to S. aureus never report it to a health care provider. However, since this organism is part of the normal flora of many individuals, S. aureus infection is one of the most common nosocomial infections due to surgical wounds or indwelling medical devices. The most effective way to control food poisoning caused by S. aureusis to employ proper sanitation and food preparation. Heating food to above 60 degrees C and storing food below 7.2 degees C are two effective ways of controling S. aureusgrowth in food. The most important recent epidemiological information concerning this organism involves the increasing resistance to antibiotics. Methicillin-resistant S. aureus (MRSA) is the most common of these antibiotic resistant organisms. The effects of MSRA are the same as any other S. aureus infection, however, MSRA infections are a difficult to treat because there are few effective antibiotics available. MSRA infections are generally not life threatening, however in some extreme cases death can occur.