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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)




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USMLE® Step 1 questions

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High Yield Notes

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USMLE® Step 1 style questions USMLE

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A 68-year-old man presents to the emergency department with a several week history of malaise, subjective fevers, and fatigue. He has a history of renal cell carcinoma diagnosed 5 months ago. The patient reports he had a recent stent placed in the left ureter approximately two months ago due to worsening metastatic disease burden. The hospital course was complicated by damage to the ureter, which was repaired surgically. Temperature is 37.7 °C (100.6 °F) , pulse is 102/min, respirations are 22/min and blood pressure is 130/62 mmHg. On physical examination the patient is noted to have conjunctival pallor. A new holosystolic murmur is appreciated at the cardiac apex. Which of the following is the most common organism implicated in this patient’s disease process?  

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Biliary tract infections

Enterococci p. 135


enterococci p. 135

Enterococci p. 135

penicillins for p. 185

vancomycin p. 187

vancomycin-resistant (VRE) p. 135

Enterococcus spp.

Gram-positive algorithm p. 132

UTIs p. 179

Nosocomial infections p. 182, 280

enterococci p. 135

Subacute endocarditis

enterococci p. 135

Urinary tract infections (UTIs) p. 179, 625

enterococci as cause p. 135

VRE (vancomycin-resistant enterococci)

enterococci p. 135

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Content Reviewers

Viviana Popa, MD


Alexandru Duhaniuc, MD

Evan Debevec-McKenney

Tanner Marshall, MS

With Enterococcus, entero- refers to the intestines, while -coccus means round shape.

So Enterococcus is a genus of round bacteria that commonly colonizes the gut of humans and animals.

Enterococcus is also called Group D streptococcus in Lancefield classification developed by an American microbiologist Rebecca Lancefield.

There are two species that can cause infections in humans and these are Enterococcus faecalis, amounting for the majority of infections, and Enterococcus faecium, which causes disease more rarely.

Now, looking at an individual bacterium, Enterococcus has a thick peptidoglycan cell wall, which takes in purple dye when Gram stained - so this is a gram-positive bacteria.

When there’s more of them, Enterococci grow in short chains, usually in pairs.

They’re non-spore forming, facultative anaerobes, meaning that they can survive in both aerobic and anaerobic environments and catalase negative, which means they don’t produce an enzyme called catalase.

Enterococci also can tolerate extreme environmental conditions including high sodium chloride concentrations, high pH and even high temperatures.

They can survive at 60 degrees Celsius for up to 30 minutes!

Ok, now, enterococcus is pyrrolidonyl arylamidase positive, because it makes an enzyme called L-pyrrolidonyl arylamidase.

To test for this, a small sample is taken from a suspected bacterial colony, and then inoculated to a disk pad that’s embedded with pyrrolidonyl beta naphthylamide - another joy of a word.

With Enterococcus, pyrrolidonyl arylamidase hydrolyzes pyrrolidonyl beta-naphthylamide to produce beta-naphthylamide.

Try saying that 3 times fast! Finally, another reagent called N-methylamino-cinnamaldehyde is added to the disk, and it reacts with beta-naphthylamide, resulting in a bright red color that confirms Enterococcus is pyrrolidonyl arylamidase positive.

Now, most commonly, Enterococci are gamma hemolytic which means that when cultivated on blood agar they don’t induce hemolysis, so the agar under and around the colony remains unchanged.

But sometimes, they can induce alpha hemolysis, also called partial hemolysis, which means that the agar under the colony turns dark and greenish.


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