Staphylococcus aureus is a
Gram-positive, coccus-shaped facultative
anaerobe and a member of the Staphylococcaceae
family and Bacillales order (Figure 1). S.
aureus is frequently part of the skin flora
found in the nose and on skin. It can, however,
cause a wide range of illnesses from minor skin
infections, such as pimples,
scalded skin syndrome, to life-threatening
diseases such as pneumonia,
syndrome. Infections of the blood can carry the
bacteria to various deep tissues, including
bones, joints, organs, and the respiratory
system. Furthermore, S. aureus is
catalase-positive and mannitol-positive. A
coagulase test determines pathogenic from
non-pathogenic staphylocci, in which S.
aureus would test positive.
This scanning electron micrograph shows
Staphylococcus aureus bacteria.
Colonial morphology of S. aureus
in culture is typically raised, growing smooth
medium to large colonies that are slightly
translucent with creamy-yellow pigmentation.
Most strains are beta-hemolytic, suggesting that
they are able to lyse red blood cells
and grow on blood agar medium.
Pathogenicity: Polymers of glycerol or
ribitol phosphate (teichoic acids) are attached
to peptidoglycan molecules on the cell wall of
S. aureus, making it antigenic. Recall
that teichoic acid can be used by bacteria to
attach to mucosal membranes, and its release
during bacterial cell lysis into the bloodstream
may cause fever, decrease in blood pressure due
to blood vessel dilation, and possibly toxic
shock (Figure 2). Furthermore, some strains of
S. aureus vary from others in
morphology by the presence of a capsule composed
of protein A (SpA), which can impede host
polymononuclear phagocytosis by binding to
immunoglobulin G (IgG) Fc (constant region). SpA
is also a potent activator of tumor necrosis
factor α (TNF-α) receptor 1 (TNFR1) signaling,
inducing both chemokine expression and
TNF-converting enzyme-dependent soluble TNFR1
shedding, which has anti-inflammatory
consequences, particularly in the lung.
Moreover, aggregation of S. aureus
bacteria is attributed to the presence of
coagulase and fibrogenin on their cell wall
surface, which bind together to produce the
clusters observed microscopically (Figure 1).
This patient presented with a 'strawberry
tongue', and was diagnosed with Toxic Shock
Syndrome caused by Staphylococcus aureus.
In fact, 'Menstrual' Toxic Shock Syndrome (TSS)
emerged as a public health threat to women of
reproductive age in 1979 to 1980.
Depending on the particular
strain, there are several kinds of toxins
attributed to S. aureus virulence.
Exotoxins can include toxic shock syndrome
toxin-1 (TSST-1), exfoliatins, and enterotoxins.
Others may include alpha-toxin, beta-toxin,
delta-toxin, and bicomponent toxins such as
Panton-Valentine leukocidin. Factors including
protein A, Staphyloxanthin pigment, clumping
factor, coagulase, hyaluronidase, leukocidin,
and biofilm production can also affect the
virulence (Forbes et al., 2007).
Exotoxin TSST-1 causes toxic
shock syndrome by stimulating the release of
large amounts of interleukin-1 (IL-1) by human
monocytes, interleukin-2 (IL-2), and tumour
necrosis factor. Similarly, it induces the
expression of IL-2 receptors and the
proliferation of human T lymphocytes. It does
this by binding to MHC class II molecules and
the exotonin is produced by most strains of
S. aureus (Scholl et al., 1989).
In general, the toxin is not produced by
bacteria growing in the blood; rather, it is
produced at the local site of an infection, and
then enters the bloodstream.
Exfoliative toxins are the
sole agents responsible for staphylococcal
scalded skin syndrome (SSSS), a disease
predominantly affecting infants and
characterized by the loss of superficial skin
layers, dehydration, and secondary infections.
Exfoliatins are serine proteases with high
substrate specificity, which selectively
recognize and cleave (hydrolyse) desmosomal
cadherins only in the superficial layers of the
skin, which is directly responsible for the
clinical manifestation of SSSS (Bukowski et
Alpha-toxin secreted by this
pathogen binds to cell surface receptors and
form the heptameric pores. This pore allows the
exchange of monovalent ions, resulting in DNA
fragmentation and eventually apoptosis. Higher
concentrations result in the toxin absorbing
nonspecifically to the lipid bilayer and forming
large, Ca2+ permissive pores. This,
in turn, results in massive necrosis and other
secondary cellular reactions triggered by the
uncontrolled Ca2+ influx. Similarly,
the cytotoxin Panton-Valentine leukocidin forms
β-pores in target cells, causing necrotic
lesions involving the skin or mucosa, including
necrotic hemorrhagic pneumonia. More
specifically, this cytokin assembles in the
membrane of host defense cells, particularly
white blood cells, monocytes and macrophages.
The two subunits fit together and form a ring
with a central pore through which cell contents
leak and which acts as a superantigen.
As mentioned earluer, S. aureus
can be found in the normal flora of humans.
Anterior nares, the nasopharynx, the perineal
area, and the skin commonly house small colonies
of S. aureus (Forbes et al.,
2007). This organism can be carried by a
symptomless host and transmitted via direct
contact with a carrier or indirectly by touching
a contaminated item. Lipoteichoic adherence
complexes are used to hold to and colonize
within the housing tissues. In addition, spread
of the bacteria from its normal site of
habitation to an abnormal site can result from a
tear in the tissue from a wound or surgery. This
species is often associated with
immunocompromised patients, since many outbreaks
occur in healthcare facilities due to contact
between patients and asymptomatic healthcare
Bukowski, M., Wladyka, B., &
Dubin, G. (2010). Exfoliative Toxins of
Staphylococcus aureus. Toxins, 2(5):
Forbes, B.A., Sham, D.F. &
Weissfeld, A. S. (2007). Bailey & Scott's
diagnostic microbiology (12th ed.).
St. Louis: Mosby.
Scholl, P., Diez, A. et
al. (1989). Toxic shock syndrome toxin 1
binds to major histocompatibility complex class
II molecules. Proceedings of the National
Academy of Sciences USA, 86: 4210-4214.