LAB 21: Selective and Differential Media Quiz

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Questions and Answers

What does the Snyder test specifically measure?

Hemolysis of red blood cells

Acidic fermentation products in the mouth (correct)

Presence of coagulase producing bacteria

Fermentation of mannitol

What is the purpose of using blood agar in culturing bacteria?

To observe hemolysis patterns to differentiate bacteria (correct)

To test for mannitol fermentation

To promote selective growth of only pathogenic bacteria

To identify bacteria based on their coagulase activity

How does coagulase contribute to the pathogenicity of bacteria?

It helps bacteria evade the immune response (correct)

It enhances bacterial growth in blood

It promotes fermentation of mannitol

It increases oxygen availability for metabolism

What is a distinguishing feature of Mannitol Salt Agar (MSA)?

It selects for staphylococci and differentiates mannitol fermenters Signup and view all the answers

Which type of hemolysis indicates complete lysis of red blood cells?

Beta hemolysis Signup and view all the answers

What does SS agar primarily select for?

Salmonella and Shigella species Signup and view all the answers

Which of the following describes the components of blood agar?

Agar, blood, and nutrients necessary for growth Signup and view all the answers

What distinguishes the three categories of hemolysis?

The color change observed in the medium Signup and view all the answers

What is the key differential factor that distinguishes bacteria on SS agar?

By the formation of hydrogen sulfide gas Signup and view all the answers

What indicates a high susceptibility to dental caries in the Snyder test?

A yellow color change within 24 hours Signup and view all the answers

What is the purpose of using low pH in Snyder agar?

To select for acid-producing bacteria Signup and view all the answers

Which component primarily contributes to the differential characteristic of Snyder agar?

Glucose Signup and view all the answers

During the Snyder test, what does the rate of color change signify?

Susceptibility to dental caries Signup and view all the answers

What is the ideal temperature for incubating Snyder agar tubes?

45°C Signup and view all the answers

What happens to the Snyder agar when bacteria ferment glucose?

It turns yellow Signup and view all the answers

What should be done after transferring saliva into the Snyder tube?

Vortex the mixture vigorously Signup and view all the answers

What would result from no color change in Snyder agar after 72 hours?

Low susceptibility to dental caries Signup and view all the answers

Which test is specifically used to differentiate for hemolysis?

Hemolysins test on blood agar Signup and view all the answers

What is the primary component of Snyder agar?

Glucose Signup and view all the answers

What is the primary purpose of MSA (Mannitol Salt Agar)?

To differentiate pathogenic Staphylococcus aureus from non-pathogenic bacteria Signup and view all the answers

What color does MSA agar turn when mannitol fermentation occurs?

Yellow Signup and view all the answers

Which test differentiates Staphylococcus from other Gram-positive bacteria?

Catalase test Signup and view all the answers

What does a positive coagulase test indicate?

The presence of Staphylococcus aureus Signup and view all the answers

What characteristic of MSA agar makes it selective?

It contains salt that inhibits Gram-negative bacteria Signup and view all the answers

Non-mannitol fermenting bacteria on MSA appear how?

Red colonies on red agar Signup and view all the answers

What is the role of thrombin in the coagulase test?

To convert fibrinogen to fibrin and form a clot Signup and view all the answers

Which organism is a typical non-mannitol fermenter found on MSA?

Staphylococcus epidermidis Signup and view all the answers

The acidic products of mannitol fermentation in MSA lead to which change?

Color change in the agar Signup and view all the answers

Which agar is used to differentiate between Salmonella and Shigella?

Salmonella/Shigella agar Signup and view all the answers

What is the primary function of selective media?

To suppress unwanted microbes and encourage desired microbes Signup and view all the answers

What characteristic of differential media allows for the identification of different species?

The ability to change color based on metabolic activity Signup and view all the answers

What does Mannitol Salt Agar (MSA) specifically select for?

Gram-positive bacteria and halophiles Signup and view all the answers

Which statement describes enrichment media?

It enhances the growth of desired bacteria faster than others. Signup and view all the answers

How does Eosin Methylene Blue Agar (EMB) distinguish lactose fermenters?

Through a pH indicator that causes a color change Signup and view all the answers

What is the main function of coagulase in Staphylococcus aureus?

It allows the bacteria to hide from phagocytes. Signup and view all the answers

What role do inhibitors play in selective media?

They suppress the growth of specific undesired bacteria. Signup and view all the answers

Which of the following is a characteristic of MSA (Mannitol Salt Agar)?

It differentiates based on mannitol fermentation and salt tolerance. Signup and view all the answers

Which bacteria does Mannitol Salt Agar differentiate based on mannitol fermentation?

Gram-positive mannitol fermenters Signup and view all the answers

What type of colonies would you expect to see on SS agar when testing for Salmonella typhimurium?

Black colonies due to hydrogen sulfide production. Signup and view all the answers

Why might one use selective and differential media together?

To isolate specific types of bacteria while also differentiating them based on metabolic activity Signup and view all the answers

Which bacteria cannot metabolize either lactose or cysteine?

Shigella Signup and view all the answers

Which media type would best encourage the growth of halophiles?

Mannitol Salt Agar Signup and view all the answers

Selective media can utilize which of the following components to suppress unwanted bacterial growth?

High salt concentrations Signup and view all the answers

In the context of Snyder test, what does acidic fermentation indicate?

Possible dental caries due to acid-producing bacteria. Signup and view all the answers

What is the result of a positive catalase test?

It means the organism can break down hydrogen peroxide. Signup and view all the answers

Study Notes

Selective and Differential Agar

Selective media encourages the growth of certain bacteria types while inhibiting the growth of others.
Differential media differentiates between bacterial types growing on the same agar.
Examples of enrichment media include antibiotics, nutrients, salt, pH, and oxygen.
Selective media uses inhibitors that suppress the growth of certain bacteria types.
Differential media usually has a pH indicator that changes color.

Snyder Test

The Snyder test measures susceptibility to dental caries using a selective and differential agar.
The medium selects for acidiphiles by inhibiting mesophile growth at low pH.
The medium differentiates the amount of acidic producing bacteria by the rate of color change in the agar.
The Snyder agar contains a high percentage of glucose and a low percentage of agar.
Uninoculated Snyder agar is initially green in color.
Bacteria ferment glucose, producing acid that lowers the pH, turning the agar yellow.
The rate of color change indicates the susceptibility to dental caries: no color change indicates no dental caries, 24 hours indicates high susceptibility, 48 hours indicates moderate susceptibility, and 72 hours indicates low susceptibility.

MSA (Mannitol Salt Agar)

MSA is a selective and differential medium that differentiates pathogenic Staph.aureus from non-pathogenic resident skin bacteria.
Staph.aureus is a mannitol fermenter that produces acid during fermentation, turning the agar yellow.
MSA selects for Gram-positive halophiles, as the salt inhibits Gram-negative bacteria.
MSA agar contains mannitol and peptides.
The pH indicator in the agar turns yellow due to acidic mannitol fermentation.
Colonies with red agar indicate no mannitol fermentation, suggesting bacteria like S.epidermidis or M.luteus.
Colonies with yellow agar possibly contain S.aureus.

Coagulase Test

The coagulase test helps identify pathogenic species of Staphylococcus.
Coagulase is an enzyme produced by certain bacteria that activates prothrombin to thrombin.
Thrombin then activates fibrinogen to fibrin, forming a clot.
Staph.aureus produces coagulase, allowing it to hide within blood clots from phagocytes.
A positive coagulase test results in solidification when inoculated serum.
A positive catalase test differentiates staphylococcus from other Gram-positive bacteria.
Staph.aureus is catalase positive, producing oxygen bubbles when hydrogen peroxide is dripped on it.

SS (Salmonella/Shigella) Agar

SS agar is a selective and differential agar used to differentiate pathogenic Shigella, Salmonella typhimiurium, and E.coli.
Salmonella can metabolize the amino acid cysteine.
E.coli can metabolize lactose.
Shigella cannot metabolize either cysteine or lactose.
SS agar selects for enteric bacteria, as bile salts inhibit other bacteria.
SS agar contains lactose and cysteine peptides.
Acidic lactose fermentation causes the pH indicator in the agar to precipitate, producing purple colonies, indicating the presence of E.coli.
Cysteine fermentation produces hydrogen sulfide, which reacts with iron in the media, forming iron sulfide and black colonies, indicating the presence of Salmonella typhimiurium.
White colonies on SS agar indicate the presence of Shigella, as they are negative for both lactose and cysteine metabolism.

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Description

Test your knowledge on selective and differential agar, including the principles behind enrichment media and their applications like the Snyder test. Understand how different media types promote or inhibit bacterial growth and how they can indicate susceptibility to dental caries.