Unit 2.7 Homework Reading
Staphylococcus Aureus – Friend or Foe?
Before we start thinking about Staphylococcus Aureus itself, a quick word about commensal
bacteria. Throughout our long history we have coexisted and co-evolved with the microbial
world. During that time our bodies have set aside some spaces as safe to share with microbes.
A healthy human body normally contains thousands of species of microbes - mostly bacteria
and fewer viruses, fungi, and protozoa. They are chiefly located in moist areas of the skin
such as the groin and between the toes, the upper part of the respiratory tract, such as the
nasal cavity, the mouth and large intestine, the lower parts of the urethra and the vagina.
The number of bacteria in these sites varies widely, but the densest possible packing (about a
million million per milliliter) is found in pockets around the teeth and in normal feces, which is
about one-third bacteria by weight. The so-called commensal organisms or normal flora are
extremely complex. For example the intestines of just one person has about 400 distinct species
of bacteria. Most coexist with humans without causing harm. They are there not because of
pathogenicity or virulence but because any microbe will search out the most effective place to
reproduce in. Any newly invading microorganism with plans to colonize the human body must
not only resist host mechanisms that could dislodge or kill them, but also compete with these
normal flora. This is especially true in sites like the intestine, which normally have a wall-to-wall
complement of normal flora. In fact the intestinal flora of any one individual is usually remarkably
constant, and colonization by a new species is infrequent. This suggests that the commensal
bacteria already present withstand challenges from newly ingested bacteria by producing
substances that inhibit the newcomers.
But commensal organisms are not always benign. Under the right circumstances any microbe
that can grow in the body can cause disease and normal flora are no exception. If normal flora
find themselves in unaccustomed sites of the body they may cause opportunistic infections.
Opportunistic infections are very common - physicians see more patients with diseases caused
by the normal flora than from newly acquired microbes. However, not all members of the normal
flora have the same pathogenic potential. For example, if a break in the gut wall releases all the
intestinal bacteria into the peritoneum, the peritonitis is caused by only a few of the bacterial
species present.
Staphylococcus Aureus
The Staphylococcus aureus (S. Aureus) we have been
working with for the past few days is the most common of the
pyogenic, or pus-producing, bacteria and among the most
adaptable of all pathogens. It is one of the hardiest of the
non–spore-forming bacteria and can co-exist with humans in
almost any environment. Staphlyococci can survive on
inanimate objects and surfaces, such as bedding, clothing,
and doorknobs, but their major reservoir is humans. They
grow at high salt and lipid concentrations and are thus well
equipped to colonize the skin. Besides the skin they live in the
nasal cavity, vagina, and in feces. Approximately 30% of
healthy people carry Staphlyococci in their nose and up to
90% will have S. aureus in their nose at some point in their
lives. Some people are carriers for prolonged periods. For
unknown reasons people that work in hospitals are more
prone to be colonized with S. Aureus.
Staph. Aureus, growing on a
nutrient agar plate. Its large
creamy colonies make it easy to
identify.
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Unit 2.7 Homework Reading
Staphylococci cause more frequent and varied types of diseases than any other human
pathogen including abscesses, food poisoning, and toxin-induced diseases such as toxic shock
and scalded skin syndromes. If the bacteria enter the bloodstream, they may cause
osteomyelitis, kidney abscesses, and endocarditis. S. Aureus is difficult to eradicate, and is
responsible for many community- and hospital-associated infections.
S. Aureus only penetrates into deep tissues when the skin or mucous membranes are damaged
by burns, wounds, lacerations, insect bites, surgery, or skin diseases. But if large enough
numbers of S. aureus are present, such as when skin is dirty and moist, they will be able to
penetrate and cause disease even if any abrasions are not visible to the naked eye. S. Aureus
spreads from person to person through direct contact or from aerosols produced by coughing.
Because of this S. Aureus is an important secondary pathogen associated with patients
recovering from influenza and parainfluenza (croup) infections.
S. Aureus causes illness because it produces numerous cell surface and secreted virulence
factors. Viruses can also collaborate with the S. Aureus to increase its pathogenicity. A
bacteriophage is a virus that infects bacteria (we were introduced to bacteriophage in Unit 1).
Bacteriophages often carry a toxin gene able to turn a microbe into a pathogen. S. Aureus can
pick up a toxin from bacteriophage that is called Panton-Valentine leukocidin. This toxin is
found where the bacteria cause particularly severe ‘flesh eating’ (necrotizing) lesions.
Methicillin Resistant S. Aureus (MRSA).
Other plasmids may contain genes that make S. Aureus resistant to specific antibiotics, such as
the methicillin resistance plasmid found in MRSA. It is relatively easy to transfer plasmids
among bacteria via conjugation (see Unit 1) so multi-drug resistant species can also be
produced. It is estimated that half of the MRSA bacteria in the US are also resistant to
erythromycin and tetracycline as well as methicillin and penicillin – three distinct families of
antibiotics. It is now also known that many carriers of infectious agents are themselves
asymptomatic or have subclinical infections. MRSA is a good example – about 30% of all
people carry MRSA without displaying any symptoms.
Interaction between forms of MRSA found in the community with those found in hospitals
because of poor hygiene has produced an unusually virulent hybrid strain. As a result, trivial
scrapes and cuts, if infected, have the potential to progress rapidly to severe sepsis.
Now answer these questions:
1) Would you consider the Staph. Aureus we were culturing over the last few days
to be an infectious agent? Why or Why not?
2) What precautions would be sensible if you had Staph. Aureus as a commensal
bacterium in your nose?
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