Clinical Bacteriology: Gram Positive Bacteria - Staphylococci and Streptococci Flashcards Preview

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Flashcards in Clinical Bacteriology: Gram Positive Bacteria - Staphylococci and Streptococci Deck (28):

What color are gram positive bacteria on the gram stain?

Gram negative?

Gram positive bacteria are purple/blue.

Gram negative bacteria will be pink.


What differentiates staphylococcus from streptococcus by initial testing?

Staphylococcus is catalase + (in clusters)

Streptococcus is catalase - (in chains)


(Catalase catalyzes the reaction 2H2O2 -> 2H20 + O2)


What is the major differentiation between staph aureus and the other staphylococci?

S. Aureus is coagulase positive.

The other staphylocicci (S. epidermidis and S. saprophyticus) are coagulase negative.


How can you differentiate staph epidermidis from staph saprophyticus?

NOvobiocin - Saprophyticus is Resistant

Epidermidis is Sensitive


"On the office's "staph" retreat, there was NO StRESs"


How are the major divisions of streptococci formed by lab testing?

What does each category mean?

Their behavior by hemolysis.

Partial hemolysis AKA α-hemolysis: (green ring)

Complete hemolysis AKA β-hemolysis (clear)

No hemolysis AKA γ-hemolysis (no change in surrounding agar)


What are the α-hemolytic streptococci?

How are they differentiated?

Streptococcus pneumoniae: Capsule. Optochin sensitive, bile soluble (lysed by bile)

Viridans streptococci (e.g., S. mutans): No capsule, optochin resistant, bile insoluble (not lysed by bile)


Optochin - Viridans is Resistant; Pneumoniae is Sensitive: OVRPS (overpass)


What are the β-hemolytic streptococci?

How are they differentiated?

Group A: Streptococcus Pyogenes, bacitracin sensitive

Group B: Streptococcus Agalactiae, bacitracin resistant


Bacitracin - group B strep are Resistant; group A strep are sensitive: B-BRAS


What are all the β-hemolytic bacteria?

How to differentiate?

Staphylococcus Aureus: Catalase and coagulase positive

Streptococcus pyogenes - Group A Strep (catalase negative, bacitracin sensitive)

Streptococcus agalactiae - Group B strep (catalase negative, bacitracin resistant)

Listeria monocytogenes: Tumbling motility, meningitis in newborns, unpasteurized milk


An unknown organism is gram stained. It is coagulase positive. What organism is this?

What would you predict about catalase activity?

What is its most common virulence factor?

What does it commonly colonize?

Q image thumb

Staphylococcus aureus - gram-positive (purple) cocci in clusters.

Catalase (staph) and coagulase (S. aureus) positive.

Protein A exotoxin: Binds Fc-IgG, inhibiting complement activation and phagocytosis.

Colonizes the nose.


What are some inflammatory diseases caused by Staph aureus?

Skin infections, organ abscesses, pneumonia (often after influenza virus infection), endocarditis, and osteomyelitis.


What are some toxin-mediated diseases caused by Staph Aureus?

What is the toxin that causes each?

Toxic Shock Syndrome: Toxic Shock Syndrome Toxin (TSST-1)

Scalded skin syndrome: Exfoliative toxin

Rapid-onset food poisoning: Enterotoxins


What predisposes to toxic shock syndrome, caused by S. Aureus?

What is its major exotoxin, and how does it work?

Toxic shock syndrome predisposed by vaginal or nasal tampons

Toxic shock syndrome toxin (TSST-1): Superantigen that binds MHC II and T-cell receptor, resulting in polyclonal T-cell activation.


Presents with fever, vomiting, rash, desquamation, shock, end-organ failure.


What causes S. aureus food poisoning?

What is special about the toxin?

Ingestion of preformed enterotoxins.

Short incubation period (2-6 hours)

Enterotoxin is heat stable -> not destroyed by cooking. Unique among exotoxins for heat stability.


What types of infections are characteristic of Staphylococcus epidermidis?

What antibiotic is it sensitive/resistant to?

Staphylococcus epidermidis: Infects prosthetic devices and intravenous catheters by producing adherent biofilms. Component of normal skin flora.


S. epidermidis is Novobiocin sensitive.


What types of infections are characteristic of Staphylococcus saprophyticus?

What antibiotic is it resistant/sensitive to?

Second most common cause of uncomplicated UTI in young women (first is E. coli)

S. saprophyticus is Novobiocin resistant.


A patient's sputum smear is described as "rusty" and shows lancet-shaped gram positive diplococci.

What description is this characteristic for?

Q image thumb

Streptococcus pneumoniae.


What diseases does Streptococcus pneumoniae cause?

What antibitoic is it resistant/sensitive to?

Most common cause of:


Otisis media (in children)




"S. pneumoniae MOPS are Most Optochin Sensitive".


What are the virulence factors for Streptococcus pneumoniae?

What conditions can be complicated by infection with S. pneumoniae?

IgA protease (cleaves IgA, allows respiratory infection)

Capsule (no virulence without capsule)


Associated with sepsis in sickle cell anemia and splenectomy. (Encapsulated organism, should give vaccine).


An unknown organism is a gram positive cocci that is catalase negative.

It is shown to be alpha-hemolytic and resistant to optochin.

What is this organism? What diseases does it cause?

Viridans streptococci.

Normal flora of the oropharynx - cause dental caries (S. mutans)

Also cause subacute bacterial endocarditis at damaged valvues (S. sanguinis. Sanguis = blood, lots of blood in the heart (endocarditis). Makes dextrans, which bind to fibrin-platelet aggregates on damaged heart valves.


Viridans group strep live in the mouth because they are not afraid of-the-chin (op-to-chin resistant).


Gram positive cocci, catalase negative, beta hemolytic, bacitracin sensitive.

Name that organism.

What diseases does it cause?

Streptococcus pyogenes (group A strep)


Pyrogenic: pharyngitis, cellulitis, impetigo

Toxigenic: Scarlet fever, toxic shock-like syndrome, necrotizing fasciitis

Immunologic - Rheumatic fever, acute glomerulonephritis.


Patient presents with a widespread erythematous rash with sandpaper-like texture, strawberry tongue, and circumoral pallor.

What is this a classic description of? Describe the etiology.

Scarlet Fever

Caused by erythrogenic toxin from Streptococcus pyogenes (Group A strep)


What is the believed etiology of rheumatic fever?

How would you diagnose rheumatic fever?

History of pharyngitis with Streptococcus pyogenes. Antibodies to M protein enhance host defences against S. pyogenes but may give rise to rheumatic fever. 

ASO titer (anti-streptolysin O) detects recent S. pyogenes infection.


Jones criteria:

Joints - polyarthritis

O - carditis

Nodules (subcutaneous)

Erythema marginatum

Sydenham chorea


What commonly precedes S. pyogenes-induced glomerulonephritis?

Impetigo more commonly precedes glomerulonephritis than pharyngitis.


Pharyngitis can result in rheumatic "phever" and glomerulonephritis.


Gram-positive cocci, catalase negative, β-hemolytic, bacitracin resistant.

What is that organism?

Where is it found and what diseases does it cause?

Streptococcus agalactiae (Group B strep).

Colonizes the vagina; causes pneumonia, meningitis, and sepsis mainly in babies.


"Group B for Babies!"


What are some other lab tests that are positive in Group B strep (streptococcus agalactiae).

Produces CAMP factor that enhances area of hemolysis produced by S. aureus. (NOT cyclic AMP, the name is from the authors)

Positive Hippurate test. (Can hydrolyze Hippurate)


When should pregnant women be screened for Streptococcus agalactiae (Group B strep)?

What should you do if the culture is positive?

Test at 35-37 weeks.

Give intrapartum penicillin prophylaxis.


A gram-positive cocci that is catalase negative is found to be able to grow in bile and 6.5% NaCl

What is that bug?

What diseases does it cause?

Enterococcus (Group D strep). Include Enterococcus fecalis and E. faecium).

Normal colonic flora that are penicillin G resistant and cause UTI, biliary tract infections, and subacute endocarditis (following GI/GU procedures).


VRE (vancomycin resistant enterococci) are important cause of nosocomial infection.


Entero = intestine

Faecalis = feces

Strepto = twisted (chains)

Coccus = berry


A gram-positive cocci that is catalase negative is found to be able to grow in bile but not in 6.5% NaCl.

Where is it found, and what diseases does it cause?

Nonenterococcus, AKA Streptococcus Bovis (Group D strep)

Colonizes the gut. Can cause bacteremia and subacute endocarditis in COLON CANCER patients.


"Bovis in the blood = cancer in the colon".