Flashcards

MIBO differential/selective media

Hannah H English 19 cards

-Mannitol Salt Agar (msa) -contains NaCl to select for staphylococci -contains phenol red as pH indicator for fermentation…

Cards in this deck (19)

1—-Mannitol Salt Agar (msa)
-contains NaCl to select for staphylococci
-contains phenol red as pH indicator for fermentation of mannitol
yellow: acidic means it did ferment
red: means it did not ferment
2—-columbia nalidixic acid agar (cna) blood agar
-contains colistin and nalidixic acid to select FOR gram positive
-contains sheep blood to test for hemolysis
-beta: clearing
-alpha:green
-gamma:none
**used to identify staphylococci, streptococci, enterococci
3—Bile Esculin Agar (BEA)
-contains bile salts to select for intestinal organisms
-contains esculin to test for esculinase enzyme--> if positive for enzyme, there will be black brown precipitate surrounding colonies
-often used to identify enterococci and group D streptococci.
reason for brown color: If the organism has the enzyme esculinase, esculin will be digested into esculetin and glucose, the organism will digest the glucose, and the esculetin will react with the ferric citrate in the media to produce a dark color.
4—MacConkey (MAC) Agar
selective: contains crystal violet to select AGAINST G+ and bile salts to select for intestinal organisms
Differential: Contains neutral red as a pH indicator for fermentation of lactose
Neutral to alkaline = opaque growth – lactose negative
Acidic = pink growth – lactose positive
**used to identify E coli and fecal c
5—name: Eosin Methylene Blue (emb) agar
Selective: Contains methylene blue to select AGAINST G+
Differential: Contains methylene blue and eosin Y as pH indicators for fermentation of lactose
Neutral to alkaline = opaque growth – lactose negative
Acidic = purple or black growth – lactose positive
Green metalic sheen- indicates E. coli (and a few other enteric)
** often used to identify e coli and fecal coliforms
6—-Xylose Lysine Desoxycholate (XLD)
Selective: Contains sodium desoxycholate (bile salt) to select for gram neg. enteric
Differential: Contains phenol red as a pH indicator for fermentation of sugars (lactose and sucrose added at high concentration, xylose added at low concentrations) and lysine decarboxylation, and ferric ammonium citrate plus thiosulfate to test for H2S production.
Acidic = yellow colonies, ferments lactose or sucrose (hides lysine decarboxylation) OR xylose fermentation without lysine decarboxylation (rare)
Neutral to alkaline = red colonies,
Does not ferment xylose OR xylose
fermentation plus lysine decarboxylation.
Black precipitate = H2S production
Often used to identify Salmonella species (black with red halo)
7—-Sulfer indole motility (SIM)
Differentiates members of Salmonella and Shigella (Gram negative enteric)
Three Differential Tests:
-Semi-solid medium allows for observation of motility
-Contains Fe as an indicator for H2S production from sulfate reduction or cysteine/methionine fermentation
-Addition of Kovac’s reagent after incubation to detect production of indole from tryptophan:
Amber = negative for indole
Red = positive for indole
8—pH indicator is Methyl Red, red=pH below 4.4
Must add after incubation
Results:
Red = Mixed acid fermentation: large amounts of stable acids produced during fermentation of glucose
Yellow = Less acid production
9—Voges-Proskauer (VP) Test
-Reagents are α-napthol first then KOH
-Results:
Red = Glucose has been fermented to 2,3- butanediol (actually tests for presence of intermediate acetoin)
No color change =
No 2,3-butanediol production
10—-citrate utilization
Media: Simmons Citrate Agar
-Ammonium salt (sole N-source) - Citrate (sole C-source)
-Growth = can use citrate
(Inoculate lightly to avoid false positives)
**Bromothymol blue pH indicator
-Green at pH ~ 7.0
-Blue at pH higher than 7.6
Results:
A. Media stays green = Citrate has not been utilized
B. Media turns blue =Citrate has been utilized, Alkaline by-products
11—-Catalase: Enzyme that converts H2O2 (hydrogen peroxide) to O2
- Present in almost all aerobes and facultative anaerobes.
-Microaerophilic, aerotolerant, and anaerobic microbes typically lack catalase
12—-oxidase test
-Checks for the certain forms of the enzyme cytochrome c oxidase (involved in electron transport chain).
-Reagent (ρ-aminodimethylaniline) turns purple in the presence of oxidase enzyme.
13—-urease test
-Identify bacteria that have urease enzyme
(many uropathogenic organisms).
-Urea + 2 H2O --> CO2 + H2O + 2 NH3
- Includes a pH indicator (phenol red) which turns pink at pH > 8.2 (pH raised due to ammonia release).
14—-Casease Test with Skim Milk Agar
Casein proteins in skim milk are broken down by the enzyme casease to causing a change from cloudy to clear media
Results:
A. Cloudy media: organism does not produce casease
B. Cleared media: organism produces casease
15—-amylase test with starch
-Grow bacteria in starch-rich agar. Add iodine after growth – turns purple in presence of starch
Starch near line streak has been broken down to glucose (and eaten), causing an area that remains clear after addition of iodine.
Results:
A. Solid brown/purple color around and under bacterial growth = no amylase production or starch
hydrolysis
B. Clear zone around and under
bacterial growth = starch hydrolyzed by amylase
16—-DNAse Test Agar
-Tests for organisms having the ability to degrade nucleic acids. Methyl green is indicator, binds DNA to form a green pigment
Dnase activity associated with pathogenicity in Staphylococci
Results:
Green media under and around growth = no DNAses present
Clear zone under and around growth = DNAses present, methyl green depolymerized (colorless)
17—Test for hemolysins (exotoxins that destroy red blood cells and hemoglobin)
Hemolysis – destruction of RBCs and/or hemoglobin
18—Identifies Gram-negative enteric pathogens
Three Differential Tests:
-Fermentation of dextrose(same as glucose), lactose, or sucrose (include pH indicator)
Red = pH > 6.8 (K)
Yellow = acidic, pH < 6.8 (A)
-Production of H2S (via FeSO4 test as in SIM media) Look for formation of a black precipitate
-Gas production: Look for cracks/bubbles in the hard agar
19—Identifies Gram-negative enteric pathogens
Four Differential Tests:
1) Fermentation of glucose (include pH indicator)
Purple = pH > 6.8 (K)
Yellow = acidic, pH < 5.2 (A)
Little glucose is added (like TSI) – fermenters K/A.
2) Production of H2S (via FeSO4 test as in SIM media)
Look for formation of a black precipitate
3)Lysine decarboxylase: following fermentation (requires acid), decarboxylation of lysine raises the pH and turns the butt purple
4) Lysine deaminase: lysine deamination products react with ferric ammonium citrate and turn the slant red
**Note: If organism does not ferment glucose, H2S and lysine decarboxylase are not readable