Online Textbook Bacteriology is continuously updated and includes information on Staphylococcus, MRSA, Streptococcus, E. coli, anthrax, cholera, tuberculosis, Lyme disease and other bacterial diseases of humans.
Kenneth Todar is the author of the Online Textbook of Bacteriology and an emeritus lecturer at the University of encourages people to wear a FDA approved face mask during the Covid-19 pandemic.
The Online Textbook of Bacteriology is a general and medical microbiology text and includes discussion of staph, MRSA, strep, Anthrax, E. coli, cholera, tuberculosis, Lyme Disease and other bacterial pathogens.
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Tag words: Staphylococcus aureus, Staphylococcus, staph, staphylococcal, S. aureus, MRSA, CA-MRSA, superbug, staph infection, wound infection, food poisoning, toxic shock syndrome, antibiotic resistance, Staph epidermidis, normal flora, skin bacteria, bacteriology, microbiology

Staphylococcus aureus

Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Staphylococcaceae
Genus: Staphylococcus
Species: S. aureus

Common References: Staphylococcus, Staph, MRSA, Superbug

Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interest include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Staphylococcus (page 3)

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Adherence to Host Cell Proteins

S. aureus cells express surface proteins that promote attachment to host proteins such as laminin and fibronectin that form the extracellular matrix of epithelial and endothelial surfaces. In addition, most strains express a fibrin/fibrinogen binding protein (clumping factor) which promotes attachment to blood clots and traumatized tissue. Most strains of S. aureus express both fibronectin and fibrinogen-binding proteins. In addition, an adhesin that promotes attachment to collagen has been found in strains that cause osteomyelitis and septic arthritis. Interaction with collagen may also be important in promoting bacterial attachment to damaged tissue where the underlying layers have been exposed.

Evidence that staphylococcal matrix-binding proteins are virulence factors has come from studying defective mutants in adherence assays. Mutants defective in binding to fibronectin and to fibrinogen have reduced virulence in a rat model for endocarditis, and mutants lacking the collagen-binding protein have reduced virulence in a mouse model for septic arthritis, suggesting that bacterial colonization is ineffective. Furthermore, the isolated ligand-binding domain of the fibrinogen, fibronectin and collagen receptors strongly blocks attachment of bacterial cells to the corresponding host proteins.


The invasion of host tissues by staphylococci apparently involves the production of a huge array of extracellular proteins, some of which may occur also as cell-associated proteins. These proteins are described below with some possible explanations for their role in invasive process.

Membrane-damaging toxins

alpha toxin (alpha-hemolysin) The best characterized and most potent membrane-damaging toxin of S. aureus is alpha toxin. It is expressed as a monomer that binds to the membrane of susceptible cells. Subunits then oligomerize to form heptameric rings with a central pore through which cellular contents leak.

In humans, platelets and monocytes are particularly sensitive to alpha toxin. Susceptible cells have a specific receptor for alpha toxin which allows the toxin to bind causing small pores through which monovalent cations can pass. The mode of action of alpha hemolysin is likely by osmotic lysis.

ß-toxin is a sphingomyelinase which damages membranes rich in this lipid. The classical test for ß-toxin is lysis of sheep erythrocytes. The majority of human isolates of S. aureus do not express ß-toxin. A lysogenic bacteriophage is known to encode the toxin.

delta toxin is a very small peptide toxin produced by most strains of S. aureus. It is also produced by S. epidermidis. The role of delta toxin in disease is unknown.

Leukocidin is a multicomponent protein toxin produced as separate components which act together to damage membranes. Leukocidin forms a hetero-oligomeric transmembrane pore composed of four LukF and four LukS subunits, thereby forming an octameric pore in the affected membrane. Leukocidin is hemolytic, but less so than alpha hemolysin.

Only 2% of all of S. aureus isolates express leukocidin, but nearly 90% of the strains isolated from severe dermonecrotic lesions express this toxin, which suggests that it is an important factor in necrotizing skin infections.

Coagulase and clumping factor

Coagulase is an extracellular protein which binds to prothrombin in the host to form a complex called staphylothrombin. The protease activity characteristic of thrombin is activated in the complex, resulting in the conversion of fibrinogen to fibrin. Coagulase is a traditional marker for identifying S aureus in the clinical microbiology laboratory. However, there is no overwhelming evidence that it is a virulence factor, although it is reasonable to speculate that the bacteria could protect themselves from phagocytic and immune defenses by causing localized clotting.

There is some confusion in the literature concerning coagulase and clumping factor, the fibrinogen-binding determinant on the S. aureus cell surface. Partly the confusion results from the fact that a small amount of coagulase is tightly bound on the bacterial cell surface where it can react with prothrombin leading to fibrin clotting. However, genetic studies have shown unequivocally that coagulase and clumping factor are distinct entities. Specific mutants lacking coagulase retain clumping factor activity, while clumping factor mutants express coagulase normally.


Many strains of S aureus express a plasminogen activator called staphylokinase. This factor lyses fibrin. The genetic determinant is associated with lysogenic bacteriophages. A complex formed between staphylokinase and plasminogen activates plasmin-like proteolytic activity which causes dissolution of fibrin clots. The mechanism is identical to streptokinase, which is used in medicine to treat patients suffering from coronary thrombosis. As with coagulase, there is no strong evidence that staphylokinase is a virulence factor, although it seems reasonable to imagine that localized fibrinolysis might aid in bacterial spreading.

Other extracellular enzymes

S. aureus can express proteases, a lipase, a deoxyribonuclease (DNase) and a fatty acid modifying enzyme (FAME). The first three probably provide nutrients for the bacteria, and it is unlikely that they have anything but a minor role in pathogenesis. However, the FAME enzyme may be important in abscesses, where it could modify anti-bacterial lipids and prolong bacterial survival.
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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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