Staphylococcus aureus, sometimes called staph aureus, is coccal, or round-shaped, and grows
in clusters.
In fact, its name, broken down, means “golden cluster of grapes”.
It sorta starts making sense if you look at it under a microscope - it tends to grow in
sticky clusters, and it stains purple when Gram-stained due to its peptidoglycan cell
wall, so it’s Gram positive and it resembles grapes.
As for its “golden” color, when it’s grown on blood agar plates, the colonies have
a distinctive golden-yellow color.
Staphylococcus aureus are Gram positive and facultative anaerobes, meaning that they can
survive in aerobic and anaerobic environments.
They’re non-motile and don’t form spores.
Staphylococci produce an enzyme called catalase which converts hydrogen peroxide to water
and oxygen.
Other common cocci, such as streptococci and enterococci, are catalase negative so they
don’t have this ability and we can use a few drops of hydrogen peroxide to differentiate
them.
Catalase positive bacteria will foam up, while in catalase negative bacteria, nothing happens.
Now, a couple of other staphylococci species, like Staph epidermidis and Staph saprophyticus
are also catalase positive, so to distinguish between them we can look for another enzyme
that’s made by Staph aureus, called coagulase.
Coagulase converts fibrinogen into fibrin.
So let’s say that we stir up some Staph aureus bacteria in a liquid “emulsion”,
and then add a few drops of plasma which contains fibrinogen.
The coagulase positive staph aureus will convert the soluble fibrinogen to sticky fibrin, which
then visibly clumps up, whereas coagulase negative bacteria won’t.
Staph aureus is extremely common and about a quarter of the population is colonized by
it, usually in their nostrils, groin, armpits, and other parts of their skin.
But, most of the time it’s a normal part of our skin flora, and doesn’t cause trouble.
The skin flora is a complex ecosystem of different bacterial species and occasionally, Staph
aureus can begin to dominate that ecosystem.
In individuals that have staph aureus colonization, a number of factors like the pH, humidity,
sweat levels of the skin, as well as presence of other bacteria on our skin, all affect
the amount of staph aureus that’s present.
If more and more Staph aureus is around on the skin, it begins to penetrate through tiny
microfissures in the skin, like you get with eczema, as well as larger breaks in the skin
like you might get after shaving.
In fact, it’s particularly troublesome in terms of causing wound infections where there
is a large break in the skin either from trauma or after a surgery.
So low levels of staph aureus with intact skin leads to colonization, whereas high levels
of Staph aureus with breaks in the skin lead to infections.
When staph aureus invades into the skin it can lead to localized skin infections like
a pimple which can evolve into a furuncle, or a boil.
A bunch of furuncles clustered together make a carbuncle.
There can also be diffuse skin infections, like superficial impetigo which is an infection
of the epidermis, or deeper-reaching cellulitis, which is an infection of the dermis and can
spread over larger surfaces rapidly.
If the infection goes deeper, it can develop into a subcutaneous abscess - a collection
of pus that’s walled off and sometimes develops thin walls within it - called septations.
These abscesses can occur all over the body including in the mouth where they’re called
dental abscesses, and they can develop within various organs like the liver, kidney, spleen,
and brain.
Now if the infection is overlying a muscle, it can spread into the muscle causing a pyomyositis.
If it gets into the bone it can cause osteomyelitis, and if it gets into the joint space it can
cause septic arthritis.
Finally, if Staph aureus gets into the bloodstream, it can cause a septic thrombophlebitis - an
infected blood clot.
In addition, bacteria in the blood is called bacteremia, and it can lead to a number of
serious problems.
There’s typically a widespread immune reaction that causes the blood vessels to expand and
the blood pressure to fall.
The result is hypotension and poor perfusion to various organs - a process called sepsis.
Once it’s in the blood, Staph aureus can also get to various parts of the body.
It can get into the central nervous system - causing bacterial meningitis or an epidural
abscess in the spine.
It can get into the lungs causing a severe pneumonia.
It can start to grow on the heart valves in clumps called vegetations - damaging the valves
- a process called infective endocarditis.
Bits of the vegetations can then chip off and embolize further causing other local infections
around the body.
Now, in addition to invading the body through the skin, Staph aureus can also enter directly
into the bloodstream when a person is getting surgery or having dental work done.
These events occur infrequently, but when they do come up it’s important to take precautions.
For example, individuals at high risk of getting serious disease with Staph aureus - like immunocompromised
individuals or those that are at risk for infective endocarditis - should be given antibiotic
prophylaxis.
Another property of our golden staph is its ability to create biofilm on medical implants
like indwelling intravenous catheters, prosthetic heart valves, and artificial joints.
The biofilm is, essentially, a layer of “slime” within which the Staph aureus live.
It forms when a cluster of Staph aureus adheres to a surface either a natural one like the
surface of a valve or an artificial one like the surface of a catheter.
The bacteria start to produce extracellular matrix made of exopolysaccharides, or EPS,
and over time the cells get completely surrounded by it.
The cells that are surrounded by the gel-like layer of exopolysaccharides, can communicate
with one other through biochemical signals and can even swap genetic information back
and forth - including antibiotic resistance genes.
In addition, Staph aureus thrives but doesn’t divide rapidly within these biofilms, and
it’s relatively hard for antibiotics to penetrate into the biofilms.
Combined that makes it much harder to get rid of these biofilm infections, and often
requires simply removing the surface that they’re growing on, if possible.
If all of this wasn’t enough, S. aureus can also release superantigens or toxins.
In fact, there are five major toxins related to S. aureus - toxic shock syndrome toxin
1, or TSST-1, Panton-Valentine leukocidin toxin, hemolysin, exfoliatin, and enterotoxin…
toxin.
TSST-1 is produced at the site of infection, and can enter the bloodstream.
TSST-1 binds to major histocompatibility complex type II or MHC II, which is a receptor found
on immune cells called antigen presenting cells.
When TSST-1 binds to the MHC II receptors on these cells, it really stimulates them,
making them release loads of pro-inflammatory chemicals called cytokines, creating a cytokine
storm.
The cytokine storm results in a number of physiologic changes like fevers, a sunburn
like rash, low blood pressure, and poor end-organ perfusion that can result in death - and together
this is called toxic shock syndrome.
Panton-Valentine leukocidin toxin, or PVL, punches holes in leukocytes - our immune cells,
killing them, and giving the toxin its name.
Leukocytes with holes in their cell membrane, die through necrosis and that triggers inflammation.
When that happens throughout the tissue, like in a necrotizing pneumonia, large chunks of
the tissue begin to die off and the organ cannot do its job properly leading to organ
failure.
Hemolysin is another pore-forming toxin that destroys erythrocytes or red blood cells,
releasing their hemoglobin, which contain iron, into the blood.
Staph aureus uses iron in its own metabolism, so this is one way it gets access to that
coveted heavy metal.
Exfoliatin toxin causes staphylococcal scalded skin syndrome, SSSS, or Ritter’s disease.
It creates painful patches of red skin, with fluid filled blisters, but it often resolves
within a few weeks.
Finally, there’s enterotoxin.
Staph aureus might land on food and start to generate enterotoxin.
The enterotoxin is quite stable in the environment, and can remain active even after the bacteria
are killed off by cooking.
The toxin can even withstand boiling at 100 degrees Celsius for a few minutes!
If the toxin is eaten, it can cause food poisoning - with symptoms like vomiting and diarrhea
a few hours after ingestion.
And, in rare situations, if the enterotoxin somehow gets into the bloodstream, it can
cause toxic shock syndrome, much like TSST-1.
The main treatment for infections is antibiotics, but Staph aureus adapts quickly and has developed
resistance to a number of antibiotics.
Penicillins, which are beta lactam antibiotics, were tried early on.
They work by disrupting disabling the enzyme DD-transpeptidase, which is a penicillin binding
protein, or PBP.
PBP is in charge of crosslinking the long strands of amino polysaccharides that make
the peptidoglycan cell wall, running in parallel.
These are made of segments of N-acetylglucosamine, or NAG, and N-acetylmuramic acid, or NAM,
in an alternating pattern - so, NAG, NAM, NAG, NAM, and so on, like a pearl necklace.
Protruding from the tips of the NAM subunits are tetrapeptide and pentapeptide chains.
These peptide chains can link to other peptide chains from the neighboring strands through
a process known as transpeptidation.
If PBPs aren’t allowed to work, no peptidoglycan is made, and the cell wall becomes leaky.
So when the bacteria try to divide and make more cell wall - they simply can’t and they
die.
Nowadays, practically all strains of Staph aureus make beta lactamases which allow them
to simply disable beta lactam antibiotics by breaking their beta lactam ring.
Next, antibiotics called beta lactamase inhibitors, like clavulanic acid, were used to bind and
disable the beta lactamases.
In addition, new types of beta lactam antibiotics, like methicillin, were identified that couldn’t
be easily destroyed by the beta lactamases.
Unfortunately, some S. aureus strains found a way around this as well by evolving and
expressing a gene called mecA, which encodes special PBPs - that are unaffected by beta
lactam antibiotics.
Methicillin and older penicillins can’t physically fit into them, rendering them ineffective.
These methicillin resistant staph aureus strains are called MRSA for short - and have become
more common around the world.
There’s two major categories of MRSA - the health-care associated MRSA, or HA-MRSA, and
community associated MRSA, or CA-MRSA.
HA-MRSA is found in places like hospitals and nursing homes, where there are lots of
chronically ill patients and a high usage of antibiotics.
CA-MRSA is found in the community, and is thought to be due to rampant antibiotic use
on factory farms, and overprescription of antibiotics.
Both HA-MRSA and CA-MRSA pose a major problem because infections with those strains cannot
be treated with beta lactam antibiotics.
As a result, another class of antibiotics - glycopeptide antibiotics, like vancomycin,
are often used instead.
Unfortunately, it’s not as effective and comes with problematic side effects.
To make matters worse, some Staph aureus strains have developed intermediate resistance to
vancomycin, and they’re called “vancomycin intermediate S. aureus”, or VISA for short.
Not the type of VISA anyone wants to get.
And strains with complete resistance are called “vancomycin resistant S. aureus”, or VRSA.
The search is on for new antibiotics and vaccines to prevent staph aureus infections, but the
overuse of antibiotics makes rapid resistance an ongoing problem.
Ultimately, treatment of Staph aureus infections involves testing the bacteria against a handful
of antibiotics and then choosing the most appropriate antibiotic from them.
Sometimes, hospitals put together an antibiogram which shows how frequently bacteria are resistant
to a particular antibiotic.
Typically when treating MRSA, antibiotics like clindamycin or vancomycin are chosen,
but alternatives include tetracyclines, trimethoprim/sulfamethoxazole, linezolid, tigecycline, daptomycin, and quinupristin-dalfopristin.
All right, quick recap!
S. aureus is a gram positive coccus that grows in clusters.
It’s a part of the normal skin and nasal flora in about a quarter of the population,
but if it overgrows or if the skin is damaged, then it can cause disease through direct colonization,
toxin production, or both.
Staph aureus is very adaptive and it has become resistant to various antibiotics.
Methicillin resistant strains are often classified as HA-MRSA or CA-MRSA, and vancomycin intermediate
and vancomycin resistant strains are called VISA and VRSA.