Staphylococcus aureus


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Staphylococcus aureus


Introduction to bacteria

Bacterial structure and functions

Gram positive bacteria

Staphylococcus epidermidis

Staphylococcus aureus

Staphylococcus saprophyticus

Streptococcus viridans

Streptococcus pneumoniae

Streptococcus pyogenes (Group A Strep)

Streptococcus agalactiae (Group B Strep)


Clostridium perfringens

Clostridium botulinum (Botulism)

Clostridium difficile (Pseudomembranous colitis)

Clostridium tetani (Tetanus)

Bacillus cereus (Food poisoning)

Listeria monocytogenes

Corynebacterium diphtheriae (Diphtheria)

Bacillus anthracis (Anthrax)


Actinomyces israelii

Gram negative bacteria

Escherichia coli

Salmonella (non-typhoidal)

Salmonella typhi (typhoid fever)

Pseudomonas aeruginosa


Klebsiella pneumoniae


Proteus mirabilis

Yersinia enterocolitica

Legionella pneumophila (Legionnaires disease and Pontiac fever)

Serratia marcescens

Bacteroides fragilis

Yersinia pestis (Plague)

Vibrio cholerae (Cholera)

Helicobacter pylori

Campylobacter jejuni

Neisseria meningitidis

Neisseria gonorrhoeae

Moraxella catarrhalis

Francisella tularensis (Tularemia)

Bordetella pertussis (Pertussis/Whooping cough)


Haemophilus influenzae

Haemophilus ducreyi (Chancroid)

Pasteurella multocida


Mycobacterium tuberculosis (Tuberculosis)

Mycobacterium leprae

Mycobacterium avium complex (NORD)

Other bacteria

Mycoplasma pneumoniae

Chlamydia pneumoniae

Chlamydia trachomatis

Borrelia burgdorferi (Lyme disease)

Borrelia species (Relapsing fever)


Treponema pallidum (Syphilis)

Rickettsia rickettsii (Rocky Mountain spotted fever) and other Rickettsia species

Coxiella burnetii (Q fever)

Ehrlichia and Anaplasma

Gardnerella vaginalis (Bacterial vaginosis)


Staphylococcus aureus


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USMLE® Step 1 questions

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High Yield Notes

4 pages


Staphylococcus aureus

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USMLE® Step 1 style questions USMLE

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A 5-day-old female newborn is currently being evaluated in the neonatal intensive care unit because of a rash, fever, and poor feeding. The patient was born full term to a mother taking lamotrigine for seizure prophylaxis. Birth history was otherwise uncomplicated. On day 2 of life, the patient developed macular erythema in the skin folds that progressed to flaccid bullae with thick crusting on days 3 and 4. No mucosal membrane involvement was observed. Vitals are significant for a temperature of 38.3°C (100.9°F). A depiction of the current state of the rash is shown below:

Image reproduced from Wikimedia Commons

Which of the following is the most likely diagnosis?  

External References

First Aid








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



Tanner Marshall, MS

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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