Gram-Positive Bugs

Question 1

Gram+, Catalase+, Coagulase+ cocci clusters

Answer 1

Staphylococcus aureus

• protein A binds Fc portion of IgG
• coagulase forms fibrin coat around organisms
• hemolysins and leukocidins destroy RBCs and WBCs

Question 2

Gram+, Catalase+, Coagulase-, Novobiocin sensitive cocci clusters

Answer 2

Staphylococcus epidermidis

• Polysaccharide capsule allows adherence to inserted medical device
• Think S. epidermidis endocardtiis within 60 days of heart valve replacement. After 60 days, think Viridans

“NO SRESs”. Novobiocin - Saprophyticus Resistant, Epidermidis Sensitive

Question 3

Gram+, Catalse+, Coagulase-, Novobiocin resistant cocci clusters

Answer 3

Staphylococcus saprophyticus

• 2nd most common cause of cystitis among sexually active young women (after E. coli); treat with TMP-SMX

“NO SRESs”. Novobiocin - Saprophyticus Resistant, Epidermidis Sensitive

Question 4

Gram+, Catalase-, alpha-hemolytic, Optochin sensitive, Bile soluble (lysed by bile) Lancet-shaped diplococci

Answer 4

Streptococcus pneumoniae

• Capsule impairs phagocytosis (Quellung rxn +)
• IgA proteases
• Causes sepsis in patients with sickle cell or s/p splenectomy
• PPx with Pneumovax capsular polysaccharide vaccine

​”OVRPS”. Optochin - Viridans is Resistant; Pneumoniae is Sensitive

Question 5

Gram+, Catalase-, alpha-hemolytic, Optochin resistant, Bile insoluble (not lysed by bile) cocci chains

Answer 5

Streptocococus mutans/intermedius (Viridans Streptococcus)

• Capsule impaires phagocytosis (Quellung reaction)
• Viridans streptococci (as well as Enterococci) colonize previously-damaged heart valves

“OVRPS”. Optochin - Viridans is Resistant; Pneumoniae is Sensitive

Question 6

Gram+, Catalase-, beta-hemolytic, Bacitracin-sensitive cocci chains

Answer 6

Streptococcus pyogenes (GAS)

• Adheres to pharyngeal epithelium via pili
• Streptokinase - converts plasminogen to plasmin
• M protein - resists phagocytosis
• Hyaluronidase - breaks down connective tissue

“B-BRAS”. Bacitracin - group B strep are Resistant, group A strep are Sensitive

Question 7

Gram+, Catalse-, beta-hemolytic, Bacitracin-resistant cocci chains

Answer 7

Streptococcus agalactiae (GBS)

“B-BRAS”. Bacitracin - group B strep are Resistant, group A strep are Sensitive

Question 8

Gram+, Catalase-, gamma-hemolytic, Growth in bile and 6.5% NaCl, cocci chains

Answer 8

Enterococcus faecalis (Group D streptococci)

• Capsule prevents degregation in bile
• Overgrows locally when normal flora is suppressed by cepahlosporins, leading to UTI and bilitary tract infections
• May colonize previously damaged heart valves s/p Gi/GU procedures
• Vancomycin-resistant enterococci (VRE) are important causes of nosocomial infections

Question 9

Gram+, Catalase-, gamma-hemolytic, Growth in bile but NOT in 6.5% NaCl, cocci chains

Answer 9

Streptococcus bovis (Non-enterococci Group D streptococci)

• Capsule prevents bile salt degredation
• May cause subacute bacterial endocarditis in patients with colon cancer or IBD

Question 10

Gram+, spore-forming, aerobic, motile bacilli

Answer 10

Bacillus cereus

• Heat-stable enterotoxin (cerulide, similar to Staph enterotoxin) causes emesis
• Heat-labile enterotoxin (like E. Coli LT) increases cAMP and causes diarrhea

Question 11

Gram+, spore-forming, aerobic, non-motile bacilli

Answer 11

Bacillus anthracis

• Only bacterium with a polypeptide capsule (contains D-glutamate) instead of polysaccharide capsule
• Protective antigen (PA) binds cell membrane and mediates endocytic entry of EF or LF
• Edema factor (EF) increases cAMP and inhibits PMNs
• Lethal factor (LF) causes cell death

Question 12

Gram+, spore-forming, obligate anaerobe, motile bacilli

Answer 12

• Clostriudium tetani
  
  • Tetanospasmin toxin
  • Tx: antitoxin, DTaP booster, pencillin/metronidazole, diazepam (GABA agonist)
• Clostridium botulinum
  
  • Botulinum toxin (symmetric descending paralysis, floppy baby syndrome after honey ingestion)
  • Tx: antitoxin, respiratory support
• Clostridium difficile
  
  • toxin A (watery diarrhea), toxin B (cytotoxin to epithelial cells, forms pseudomembrane)

Question 13

Gram+, spore-forming, obligate anaerobe, non-motile bacilli

Answer 13

Clostridium perfringens

• Collagenase, hyaluronidase - degredative enzymes that cause slow, painless infection and crepitus
• Alpha toxins (lecithinase) - muscle cell necrosis
• Heat-labile enterotoxin (food-poisoning)

Question 14

Gram+, non-spore-forming, motile bacillus

Answer 14

Listeria monocytogenes

• Heat-resistant organism survives in poorly pasteurized milk, penetrates GI mucosa, and invades phagocytes
• Also may be acquired via transplacental transmission or vaginal transmission during delivery
• “Rocket tails” via actin polymerization allow it to avoid Ab
• Tumbling motility
• Catalase-positive
• Only gram-positive bug that produces LPS

Question 15

Gram+, non-spore-forming, non-motile bacillus

Answer 15

Cornyebacterium diphteriae

• Pseudomembranes in nasopharynx that may block airway
• AB toxin (phage-mediated) ADP ribsoylates EF-2, leading to arrhythmias, mycocarditis, and cranial and peripheral nerve palsy
• Black colonies with Chinese-letter appearance on potassium tellurite culture
• Tx: antitoxin, pencillin/erythromycin, DTaP booster

Question 16

Gram+, non-acid-fast, obligate anaerobe beaded filaments

Answer 16

Actinomyces israelii

• Normal flora of oral cavity, GI tract, female GU tract that can form pus-filled abscesses and draining sinus tracts with disruption of the mucosal border
• Yellow sulfur granules (filamentous bacteria lined by eosinophilic proteinaceous coating) surrounded by PMNs
• Tx: penicillin

Question 17

Gram+, acid-fast, obligate aerobe beaded filaments

Answer 17

Nocardia asteroides, Nocardia brasiliensis

• Frequently found in soil, inhaled, and phagocytosed
• Caseous granulomas form around infectious particles, leading to pneumonia and abscesses in kidney and brain
• Most common in immunocompromised patients
• Tx: TMP-SMX