Catalast-Positive GPCs (chapter 14) Flashcards
Catalase pos GPCs (4 genera)
Staphylococci
Micrococci
Rothia (formerly Stomatococcus)
Planococcus
Catalase neg GPCs (6 genera)
Streptococci Aerococci Enterococci Leuconostoc Pediococcus Gemella
catalase reaction
3% H2O2 –> O2 + H2O
Other catalase pos organisms
Liseteria
Diphtheroids
(blood causes false pos)
Coagulase positive gpc
Staph aureus
coagulase neg GPC
Coag-neg staph (S. lugdunensis, S. intermedius, S. hyicus, S. schleiferi)
Micrococcus
Rothia
Planococcus
tests for bound coagulase (clotting factor)
slide method
tests for free coagulase
tube method
What organism causes a positive reaction in both positive and negative coagulase wells?
Staph saprophyticus
What substrate is used in coagulase test?
Rabbit serum
How does coagulase test work?
Protein A reacts with antibody-coated latex beads and causes clumping
Positive reaction for:
Tube coagulase =
Slide coagulase =
tube: Clot in tube in less than 4 hours (may look neg after 4 hours due to fibrinolysin)
slide: clumps within 10 seconds
Staph Aureus virulence factors (9)
coagulase Alpha toxin enterotoxins capsular antigen penicillinase (beta-lactamase) TSST 1 hyaluronidase Exfoliatin protein A
Coagulase (SA virulence factor)
enhances evasion & survival, coats PMNs with fibrin, protects bac from phagocytosis. Bound and free forms.
alpha toxin (SA virulence factor)
disrupts smooth muscle in blood vessels.
Toxic to RBCs, WBCs, plts & hepatocytes.
Demonecrotic action- breaks down cell membrane
Thanks to alpha toxin, SA infections are often ____ and ____
inflammatory, necrotic
Enterotoxins (SA virulence factor)
resistant to hydrolysis by gastric and intestinal enzymes. Often found in milk products. Associated with psuedomembranous colitis & TSS. Gastroenteritis, food poisoning.
capsular antigen (SA virulence factor)
inhibits phagocytosis, creates biofilm.
Extreme cause of virulence. Allows bacteria to cling to inorganic surfaces & avoid effects of Abx.
Penicillinase (SA virulence factor)
(beta-lactamase) Destroys beta-lactams by hydrolysis before drug can bind to PBPs in cell membrane
TSST 1 (SA virulence factor)
(pyrogenic exotoxin C)
superantigen- makes t cells produce cytokines that cause major systemic events.
most common TSST
Enterotoxin F; causes fever, low BP, loss of skin
in TSS & SSS, infection _______, but toxins ________
localized, throughout body
Hyaluronidase (SA virulence factor)
enhances evasion & survival in tissues. Aids in spread of infection
Exfoliatin (SA virulence factor)
Exotoxin, causes SSS. Serine protease.
Caused extensive sloughing of skin, usually in infants. Burn-like effects with large, watery blisters.
Protein A (SA virulence factor)
anti-phagocytic surface protein, bound to cytoplasmic membrane.
High affinity for IgG & complement. Provides a mechanism for bac to bind to immune molecules, decreasing clearance from infection site.
Staph epi and saprophyticus are differentiated by:
Novobiocin
staph epi is Novobiocin _____
sensitive
Staph saprophyticus is novobiocin _______
reisistant
3 things that help differentiate Micrococcus from staphylococcus
Microdase
Lysostaphin
Bacitracin
Micrococcus is lysostaphin _______
resistant
Micrococcus is bacitracin _______
sensitive
staph is bacitracin _________
reisistant
virulence factors of Staph epi
biofilm production (exopolysaccharide) mecA gene/Abx resistance
Micrococcus environmental requirement
strict aerobe
Modified oxidase test
Use on bright yellow colonies (micrococcus suspected)
Colonies should be 18 to 24 hours old
Tests for cytochrome C production, blue color reaction within 2 minutes.
Micrococcus treatment
no guidelines exist, usually susceptible to beta-lactams
Bacitracin test
0.5 McFarland suspension plated to BAP, topped with 0.04-U Bacitracin disk. Incubate for 24 hours.
Zone of 10 or more = susceptible, think micrococcus.
Staph typically resistant- no zone of inhibition
D test
0.5 MF suspension, Mueller Hinton plate
Used to see if clinda can be reported as susceptible (negative D test)
Erythromycin & Clinda disks placed 15 mm apart (12 mm for strep) and incubated overnight.
positive test = clinda resistant
MRSA MIC results
Methicillin >8
Oxacillin >4
(resistance to oxacillin indicates resistance to methicillin, all beta-lactams, clindamycins, gentamycin, cephalosporin)
Mannitol Salt Agar
Selective for some Staph
7.5% Nacl
Mannitol & phenol red (pH indicator)
*Fermentation of mannitol turns agar yellow
MRSA chromagar
Positive = denim blue color change after 24 hour incubation
PBP2a test
Rapid immunochromatographic qualitative assay for detection of PBP2a
Used on direct SA isolates to detect MRSA (if pt not previously identified as having MRSA)
Limitation/consideration of PBP2a test
Only test on SA isolates, other staph species could be positive
HA-MRSA often:
multi-drug resistant, including oxacillin
CA-MRSA infections
Skin infections, boils
CA-MRSA sensitive to
non-penicillins (previously)
SXT, clindamycin, ciprofloxacin, tetracycline, vanco, linezolid or daptomycin
**more sensitive to non-betalactams than HA strains
Daptomycin
IV only, not for respiratory infections- inhibited by lung surfactancts
Vanco susceptibilities on vitek
susceptible = 2 or less intermediate = 4-8 resistant = 16 or higher
VRSA mechanism
plasmid-mediated transfer of vanA gene confers vanco resistance
VISA mechanism
unusually thickened cell wall, increased drug bound on wall from dipeptides, drug doesn’t affect cell as much.
VISA/VRSA treatment
Daptomycin or Linezolid
Linezolid
IV or oral Abx, can use for respiratory infections
