Selective and Differential Media
Results seen EMB plate reasonings: Large amounts of acid Small amounts of acid No fermentation
green metallic sheen pink colorless
Describe a catalase positive test
hydrogen peroxide will bubble and can convert hydrogen peroxide to water and oxygen if positive
Describe results of TSI slant
if organisms produce an acid by product the color changes
Describe Chocolate Agar
isolates and cultivates fastidious bacteria like Neisseria and Haemophilus bacteria growth factors and rbc's encourage growth of these species
Organisms unable to ferment on the HE the three sugars look like what?
make blue to green colors and no color change in the medium
Visual Mac Conkey Agar
pink: positive and ferments lactose
Results of VP test
red color= positive
Positive vs. negative indole
red= positive
What encourages the growth of staph in msa
salt
What does HE agar contain
sodium thiosulfate and ferric iron
Eosin and Methylene Blue
these are dyes which INHIBIT the growth of most Gram positive bacteria. They also react with any acidic products resulted from lactose fermentation to color the colonies.
What do coagulase positive tests do? ex
they clump; staph aureus
Methyl red results
yellow to red color change
Fermentation of mannitol produces what color and give an example of what bacteria does this
yellow, staph aureus
How does the HE agar look if the organism ferments ?
yellow/orange colonies
How is the Hektoen Enteric Agar both selective and differential
D: distinguishes between the ability of species to ferment the sugars lactose, sucrose and salicin, and the ability of species to reduce sulfur organisms that can do all three produce and acid by product S: contains dyes and bile salts that inhibit the growth of g+ bacteria
What does Mac Conkey agar distinguish among
Enterobacteriales
Describe EMB Agar plate
Eosin methylene blue agar (EMB) is both a selective and differential medium used for the detection and isolation of Gram-negative intestinal pathogens.
Examples of bacteria that ferment lactose
Escherichia coli : produce small, dark colonies with a green metallic sheen
gram postive
In MacConkey agar the bile salts and crystal red inhibit what
lactose fermenters
In MacConkey agar..it differentiates for?
hemolysins
In the Blood agar test...you are looking to see if your bacteria produces what?
gram positive
In the EMB test, eosin Y and methylene blue inhibit what?
red
In the MR test, what color is a positive result?
staphylococcus (spp.)
In the MSA test, it contains 7.5% NaCl which selects for what type of bacteria?
differential
In the MSA test, it contains Mannitol which differentiates between S. aureus and S. Epi, making it what type of media?
S. aureus that ferments mannitol
In the MSA test.....the bright yellow growth on this plate is a positive result for what?
pH indicator
In the MSA test...it contains phenol red which acts as a ____________ and turns yellow at a low pH
staphylococcus other than s. aureus
In the MSA test...the pink growth is positive for what?
beta hemolysis streptococcus pyogenes and staph aureus
In the blood agar test, complete destruction of RBCs and hb is what result? give ex
gamma hemolysis
In the blood agar test, no hemolysis is what result?
Alpha hemolysis strep pneumoniae
In the blood agar test...a green sheen is considered what? give ex
IMViC staands for what
Indole, Methyl-Red, Voges-Proskauer, Citrate
What reagent used in indole test?
Kovac's reagent
Why is mannitol salt agar considered a selective as well as a differential medium?
MSA is selective because it contains 7.5% salt-a high salt concentration that promotes the growth of some organisms while discouraging the growth of others. MSA is a differential medium because it contains the sugar mannitol and the pH indicator phenol red.
Examples of differential media types
Mac Conkey (MCK) agar, Eosin Methylene Blue (EMB) agar and MSA
Gram negative
MacConkey agar is selective for
Describe Mac Conkey agar
MacConkey's Agar is both a selective and differential media; it is selective for Gram negative bacteria and can differentiate those bacteria that have the ability to ferment lactose.
isolate
Phenylethel alcohol agar is used to _________ staph and strep
difference between selective and differential media
Selective media allows for the growth of specific organisms, while differential media is used to distinguish one organism from another.
MSA distinguishes between what genera of bacteria
Staphylococci
gram positive
The Hekoten Enteric Agar bile salts inhibit what?
enterics(Salmonella and Shigella)
The Hektoen enteric agar differentiates for?
How does the process of the TSI work
- medium is prepared as a slant with a deep bottom(butt) which is inoculated by stabbing (anaerobic environment) and then the slant itself(aerobic)
How does the TSI detect gas production and what does a black precipitate mean
- some anaerobic organisms may life the but of the slant or crack the agar surface - a black precipitate forms when H2S combines with ferric iron when organisms cannot use the sulfur as a source of an electron receptor
gram negative
What does the EMB test select for?
mixed acid fermenation
What does the MR test for?
neutral fermenation products
What does the VP test or
Gram positive
What is the Phenylethyl alcohol agar test selective for?
EMB test
What test is being performed here?
MSA (Mannitol Salt Agar)
What test is being performed here?
enteric
What type of bacteria does the EMB test differentiate for?
selective, differential
What type of media (2 types) is the Eosin Methylene Blue agar?
What organisms are catalase positive
aerobes and facultative anaerobes do this due to the ETC
What reagents are in the V-P test
alpha naphthol and KOH
What causes the appearance of red or pink colonies in Mac Conkey agar
bacteria such as Escherichia coli, Enterobacter and Klebsiella will produce acid in the medium, which lowers the pH of the agar below 6.8 and results in the appearance of red or pink colonies
Results of citrate test
blue media
How is gas production seen in durham tubes?
bubbles are formed due to fermentation
Organisms that ferment that carb. in the durham tubes does what?
changes from pink to yellow
Citrate test
differential based on ability of an organism to use citrate as its sole carbon source
Methyl-Red test
differential based on glucose fermentation by means of mixed-acid fermentation (glucose and buffer)
Describe the Oxidase test. what is it used to detect
differential between microbes that can and cannot produce the enzyme oxidase; aerobes and facultative anaerobes make Add tetramethyl-p purple : oxidase positive
What type of test is the catalase test
differential between organisms that can and cannot produce the enzyme catalase
Carbohydrate broths with Durham Tubes are differential or selective
differential for the ability to ferment a variety of carbohydrates (lactose, sucrose, or glucose) and a pH indicator
How do you interpret TSI Slants
differential test used to distinguish among enterobacteriales order of bacteria based on ability to ferment glucose, lactose, or sucrose
Describe the coagulase test
differential test used to distinguish between organisms that can and cannot make the enzyme coagulase
Indole test
differential used to distinguish between microbes that can and cannot digest the amino acid tryptophan using tryptophanase
Voges-Proskauer
distinguishes between organisms that can and cannot ferment glucose to acetone
What does the coagulase test detect
distinguishes between staph aureus and other staph species
How does MSA differentiate
distinguishes between the ability to ferment mannitol
What does IMViC series distinguish among
enterobacteriales
Bile Esculin Agar distinguishes between what species
enterococci has bile that inhibits the growth of g+ species (Selective) distinguishes between the ability of an organism to hydrolyze esculin(differential) -----> makes brown color if it hydrolyzes esculin