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Ex. 14: Skin Cultures and Importance of Selective and Differential Media for Isolating Gram-Positive Cocci. Objectives??. MSA: Selective and Differential Medium for Gram + Cocci. - PowerPoint PPT PresentationTRANSCRIPT
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Ex. 14: Skin Cultures and Importance of Selective and
Differential Media for Isolating Gram-Positive Cocci
Objectives??
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Table 14-2: Bacteria Commonly Found on Human Skin
Bacterium Frequency of presence
Propionibacterium spp. ++Staphylococcus epidermidis ++Staphylococcus aureus +Micrococcus spp. +Corynebacterium spp. ++Streptococcus pyogenes +/++ = nearly always present; + = commonly present; +/ = rare
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MSA: Selective and Differential Medium for Gram + Cocci
Composition: meat extract, casein, peptones; 7.5% NaCl, mannitol as only fermentable carbohydrate, phenol red indicator.
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Table 14-1: Mannitol Salt Agar (MSA) Results and InterpretationResult Interpretation Presumptive IDPoor growth or no growth (P)
Organism is inhibited by NaCl Not Staphylococcus
Good growth (G) Organism is not inhibited by NaCl
Possible Staphylococcus
Medium turns yellow (Y)
Organism produces acid (A) from mannitol fermentation
Possible pathogenic Staphylococcus aureus
Medium remains red (R)
Organism does not ferment mannitol. No reaction (NR)
Nonpathogenic Staphylococcus
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Staphylococcus saprophyticus on MSA plate
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Materials needed per team of two students: Three MSA plates Two sterile cotton swab Two tubes with sterile saline
Materials needed per table: Slant cultures of the following bacterial species: 1.Micrococcus luteus2.Staphylococcus aureus3.Staphylococcus
epidermidisWork as a team of two for the control organisms, then perform the skin swab
individually!
Day 1
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Control Organisms Skin Inoculum
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Materials needed per table: Gram staining reagents Hydrogen peroxide (3%) , glass slides Rabbit plasma vials for coagulase test
Day 2
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Catalase Test
H2O2 is by-product of aerobic respiration. Lethal to the cells.
Most aerobic organisms produce catalase for protection
Catalase test differentiates among morphologically similar Gram+ cocci.
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Table 14-2: Catalase Test Results and InterpretationResult Interpretation SymbolBubbles Catalase is present +No bubbles Catalase is absent
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Coagulase TestCoagulase activates fibrin and leads to clot formation Protective barrier around bacterial cells Virulence factor
Test determines presence of coagulase enzymeUsed for differentiating S. aureus from other Staphylococci.
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Tube Coagulase Test
check no later than 24 hours after inoculation!
Tests for bound of free coagulase
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Slide Coagulase TestTests for bound coagulasePositive if agglutination within 1 to 2 minutes
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Table 14-5: Results of MSA Plates
Colony Colony description Pigment Mannitol
fermentation Gram stain Catalase reaction
S. aureus
S. epidermidis
M. luteus
Skin isolate #1
Skin isolate #2
Skin isolate #3