Staphylococcus aureus

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Staphylococcus aureus



Staphylococcus aureus


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USMLE® Step 1 questions

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High Yield Notes

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Staphylococcus aureus

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USMLE® Step 1 style questions USMLE

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One of the major causes of hospital-acquired infections are Staphylococcus aureus species that are resistant to methicillin, oxacillin, and cephalosporins. Which of the following best explains the mechanism by which these bacteria develop antibiotic resistance?  

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Arthritis p. 476

Staphylococcus aureus p. , 133

Bacterial endocarditis p. 320

Staphylococcus aureus p. , 133


Staphylococcus aureus p. , 133

Food poisoning

Staphylococcus aureus p. , 133

Inflammatory diseases

Staphylococcus aureus p. , 133

Osteomyelitis p. 177

Staphylococcus aureus p. , 133

Pneumonia p. 707

Staphylococcus aureus p. , 133

Scalded skin syndrome

Staphylococcus aureus p. , 133

Septic arthritis p. 478

Staphylococcus aureus p. , 133

Staphylococcus aureus p. , 133

bacterial endocarditis p. 321

β -hemolytic nature of p. 133

brain abscesses p. 177

cephalosporins p. 186

cystic fibrosis p. 58, 176

dapsone p. 192

exotoxin production p. 131

food poisoning p. 175

immunocompromised patients p. 176

influenza p. 166

IV drug use p. 177

lung abscess p. 710

nosocomial infection p. 176, 182

osteomyelitis and p. 177

penicillins for p. 185

pigment production p. 127

pneumonia p. NaN

postviral infection p. 176

prophylaxis for p. 195

septic arthritis p. 478

skin infections p. 491

Toxic shock syndrome p. 131

Staphylococcus aureus p. , 133

Virulence factors

Staphylococcus aureus p. , 133


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.


Staphylococcus aureus, also known as "staph," is a gram-positive, beta-hemolytic, catalase, and coagulase-positive bacteria, which is known to cause a wide range of infections in humans. Staph is normally part of the human normal skin and nasal flora, but if it overgrows or if the skin is damaged, then it can cause disease through direct colonization, toxin production, or both.

Staph is known to cause skin infections, organ abscesses, pneumonia, endocarditis, septic arthritis, and osteomyelitis. Staph infections are typically treated with antibiotics, but in some cases, the bacteria may be resistant to the antibiotics that are commonly used. Methicillin-resistant Staphylococcus aureus (MRSA) is a type of staph that is resistant to many antibiotics and is a significant public health concern.


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