Staphylococcus aureus

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.