Micro - Clinical Bacteriology (Gram + Cocci)

Question 1

When you see gram-positive cocci in clusters, what should you think?

Answer 1

Staphylococcus (and be particularly alert to possibility of S. aureus, which is the most important pathogen)

Question 2

What is the main virulence factor of S. aureus, and how does it function?

Answer 2

PROTEIN A - binds Fc-IgG, inhibiting complement fixation and phagocytosis

Question 3

What categories of disease does S. aureus cause? Where applicable, specify exotoxins involved.

Answer 3

(1) Inflammatory disease - skin infections, organ abscesses, pneumonia (2) Toxin-mediated disease - toxic shock syndrome (TSST-1), scalded skin syndrome (exfoliative toxin), & rapid-onset food poisoning (enterotoxins) (3) MRSA (methicillin-resistant S. aureus) = important cause of serious nosocomial and community-acquired infections

Question 4

What is MRSA? What does it cause? What makes it so uniquely important?

Answer 4

Methicillin-resistant S. aureus; Important cause of serious nosocomial and community-acquired infections; Resistant to methicillin and nafcillin because of altered penicillin-binding protein

Question 5

To what bacteria does TSST belong? What is TSST? How does it function?

Answer 5

S. aureus; Toxic shock syndrome toxin = superantigen; Binds to MHC II and T-cell receptor, resulting in polyclonal T-cell activation

Question 6

What causes toxic shock syndrome? How does it present? What predisposes to toxic shock syndrome?

Answer 6

S. aureus’ TSST; Presents as fever, vomiting, rash, desquamation, shock, end-organ failure; Use of vaginal or nasal tampons predisposes to toxic shock syndrome

Question 7

What causes S. aureus food poisoning? What is the incubation period? What relevance does cooking have in this disease, if any?

Answer 7

Ingestion of preformed toxin (enterotoxin) = short incubation period (2-6 hr). Enterotoxin is heat stable => not destroyed by cooking

Question 8

What bacteria causes inflammatory disease (skin infections, organ abscesses, & pneumonia) and toxin-mediated disease (toxic shock syndrome, scalded skin syndrome, & rapid-onset food poisoning)? What other prominent diseases does it cause?

Answer 8

S. aureus; Acute bacterial endocarditis, Osteomyelitis

Question 9

What does S. aureus make that distinguishes it from other Staphylococcus? What is the result of this production?

Answer 9

Bad staph (aureus) make coagulase and toxins. Coagulase = forms fibrin clot around self, which can lead to abscess (Note: It also forms toxins, but the coagulase is what is used in labs to identify S. aureus)

Question 10

What is Staphylococcus epidermidis known to infect, and how?

Answer 10

Prosthetic devices and intravenous catheters, by producing adherent biofilms

Question 11

What kind of contamination does S. epidermidis frequently cause, and why does this make sense?

Answer 11

Contaminates blood culture; S. epidermidis = component of normal skin flora

Question 12

Of what diseases is Streptococcus pneumoniae that most common cause?

Answer 12

(1) Meningitis (2) Ottis Media (3) Pneumonia (4) Sinusitis; For, S. pneumoniae Think: “MOPS are Most OPtochin Sensitive”

Question 13

What are the defining characteristics that identify S. pneumoniae on gram stain?

Answer 13

Lancet-shaped, gram-positive diplococci

Question 14

What is S. pneumoniae’s main virulence factor?

Answer 14

IgA Protease

Question 15

Does S. pneumoniae have a capsule? Why is this important?

Answer 15

Yes, it is encapsulated; No virulence without capsule

Question 16

With what characteristic clinical presentation is pneumococcus associated? With what condition, and in what particular patient population is it commonly associated?

Answer 16

“Rusty” sputum, sepsis in sickle cell anemia &/or splenectomized patients (due to it being encapsulated)

Question 17

Compare and contrast Viridans group streptococci with S. pneumoniae.

Answer 17

Both are alpha-hemolytic BUT S. pneumoniae is optochin sensitive and Viridans is optochin resistant

Question 18

Where are Viridans group streptococci normally found?

Answer 18

Normal flora of oropharynx; Think: Viridans group strep live in the mouth because they are not afraid of-the-chin (op-to-chin resistant)

Question 19

With what conditions/diseases are Viridans group streptococci associated? What are specific examples of bacteria in Viridans group streptococci that commonly cause each of this?

Answer 19

(1) Dental caries - Streptococcus mutans (2) Subacute bacterial endocarditis - S. sanguis

Question 20

What kind of bacteria is S. sanguis? What disease does it cause, and how?

Answer 20

Viridans group streptococci (Gram + chains, alpha-hemolytic, optochin resistant); Subacute bacterial endocarditis - S. sanguis sticks to damaged valves by glycocalyx; Think: Sanguis = blood, There is lots of blood in the heart (endocarditis)

Question 21

What is another name for group A Streptococci?

Answer 21

Streptococcus pyogenes

Question 22

What categories of disease do S. pyogenes cause? What are examples of specific diseases within each category?

Answer 22

(1) PYOGENIC - pharyngitis, cellulitis, impetigo (2) TOXIGENIC - scarlet fever, toxic shock-like syndrome, necrotizing fasciitis (3) IMMUNOLOGIC - rheumatic fever, acute glomerulonephritis

Question 23

Is S. pyogenes bacitracin resistant or sensitive? How could this be used to differentiate S. pyogenes from another kind of Strep?

Answer 23

Sensitive; Both Group A and Group B Strep are Beta-hemolytic, but Group A Strep is bacitracin sensitive and Group B Strep is bacitracin resistant

Question 24

What bacteria causes rheumatic fever? What is the mechanism of disease?

Answer 24

S. pyogenes; Antibodies M protein enhance host defenses against S. pyogenes but can give rise to rheumatic fever

Question 25

What detects recent S. pyogenes infection?

Answer 25

ASO (i.e., Antistreptolysin O) titer

Question 26

What are the criteria for diagnosing rheumatic fever collectively called? What are the specific criteria?

Answer 26

JONES criteria; (1) Joints - polyarthritis (2) O (in shape of heart) - carditis (3) Nodules (subcutaneous) (4) Erythema marginatum (5) Sydenham’s chorea

Question 27

What diseases can follow S. pyogenes pharyngitis?

Answer 27

(1) Rheumatic fever (2) Glomerulonephritis; Think: “PHaryngitis –> rheumatic PHever & glomerulonePHritis”

Question 28

What is important to remember about impetigo versus pharyngitis in terms of subsequent diseases?

Answer 28

Impetigo more commonly precedes glomerulonephritis than pharyngitis

Question 29

What signs/symptoms are seen in scarlet fever?

Answer 29

Scarlet rash with sandpaper-like texture, strawberry tongue, circumoral pallor

Question 30

What is another name for group B streptococci?

Answer 30

Streptococcus agalactiae

Question 31

Are S. agalactiae bacitracin sensitive or resistant?

Answer 31

Bacitracin resistant

Question 32

What kind of hemolysis does S. agalactiae display on blood agar? Where in the human body does it colonize?

Answer 32

Beta-hemolytic; Vagina

Question 33

What diseases does S. agalactiae cause? In what population does it mainly cause them?

Answer 33

Pneumonia, meningitis, & sepsis, mainly in babies; Think: “group B for Babies”

Question 34

What is the role of CAMP factor as it relates to idenftification of S. agalactiae? What other lab test is positive for S. agalactiae?

Answer 34

S. agalactiae produces CAMP factor, which enlarges the area of hemolysis formed by S. aureus (Note: CAMP stands for the authors of the test, not cyclic AMP); Hippurate test positive

Question 35

Who should be screened for S. agalactiae, and when? What should be done in cases of positive culture result?

Answer 35

Screen pregnant women at 35-37 weeks; Patients with positive culture receive intrapartum penicillin prophylaxis

Question 36

What groups are included in Lancefield group D streptococci? What is the basis of Lancefield typing?

Answer 36

(1) Enterococci (2) Nonenterococci; Differences in the C carbohydrate on the bacterial cell wall

Question 37

What are names of species of enterococci?

Answer 37

(1) E. faecalis (2) E. faecium

Question 38

Where are enterococci normally found? What conditions/diseases do they cause?

Answer 38

Normal colonic flora; Think: “Entero = intestine, faecalis = feces”; (1) UTI (2) Biliary tract infections (3) Subacute endocarditis

Question 39

To what antibiotic are enterococci classically resistant?

Answer 39

Penicillin G

Question 40

What kind of hemolysis do enterococci display?

Answer 40

Variable (alpha or gamma hemolysis)

Question 41

What kind of enterococci are an important cause of nosocomial infection?

Answer 41

Vancomycin-resistant enterococci

Question 42

On what lab tested environments can enterococci grow?

Answer 42

(1) 6.5% NaCl (2) Bile

Question 43

What kind of bacteria is Streptococcus bovis?

Answer 43

Group D streptococci (nonenterococcal)

Question 44

Where is S. bovis normally found? What diseases can it cause, and in what patient population?

Answer 44

Colonizes the gut; Bacteremia & subacute endocarditis in colon cancer patients; Think: “Bovis in the Blood = Cancer in the Colon”

Question 45

What is the clinical relevance of Staphylococcus saprophyticus? How is it distinguished from S. epidermidis?

Answer 45

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