Microbio Lab 9

Question 1

Describe the selective and differential properties of mannitol salt agar (MSA) for the isolation and identification of staphylococci.

Answer 1

As a selective medium, MSA has a high concentration of salt (7.5% NaCl), which is inhibitory to most bacteria other than the staphylococci. As a differential medium, MSA contains the sugar mannitol as a substrate for fermentation and phenol red as a pH indicator to detect the production of acid. S. aureus ferments mannitol and changes the color of the medium from red to yellow. Most other staphylococci do not ferment mannitol.

Question 2

Describe the differential property of blood agar for the isolation and identification of staphylococci.

Answer 2

Staphylococcus aureus produces a toxin that lyses red blood cells whereas Staphylococcus epidermidis and S. saprophyticus do not produce the toxin. Colonies of S. aureus will produce β-hemolysis on blood agar.

Question 3

Why is the coagulase test considered to be the definitive test for S. aureus?

Answer 3

Virtually all strains of S. aureus are coagulase positive whereas all other species are negative.

Question 4

What is the role of coagulase in the pathogenesis of S.aureus?

Answer 4

S. aureus produces coagulase during infection to surround itself in a coat of clotted blood protein, which protects it against host defenses as the bacterium multiplies in the host.

Question 5

What is the role of alpha toxin in the pathogenesis of S. aureus?

Answer 5

Alpha toxin is an exotoxin that destroys red blood cells to release nutrients and growth factors that increase S. aureus multiplication in the host bloodstream

Question 6

What are health care acquired infections?

Answer 6

Nosocomial infections are infections acquired in a hospital or
healthcare
setting.

Question 7

Why are the staphylococci among the leading causes of nosocomial infections?

Answer 7

S. aureus is an opportunistic pathogen that causes infections of compromised hosts (e.g. surgical patients, burn and trauma patients) in the hospital. Thirty percent of the normal population carries S. aureus so endogenous infections in patients and transmission from healthcare worker to patient are common.

Question 8

Why are staphylococcal infections becoming increasingly difficult to treat?

Answer 8

Numerous strains of S. aureus have developed multiple resistances to common antibiotics including penicillins and aminoglycosides and the resulting infections have become difficult to treat effectively.

Question 9

Why might hospital patients be tested for nasal carriage of S. aureus?

Answer 9

At-risk patients are tested for S. aureus nasal carriage because of the risk for endogenous infection.

Question 10

Describe results from a coagulase, DNase, and novobiocin test that would suggest a mixed culture was used for the tests, as opposed to a pure culture.

Answer 10

Mixed cultures of S. aureus and S. saprophyticus would produce positive results for coagulase and DNase and be resistant to novobiocin. A pure cultures of S. aureus would be positive for coagulase and DNase but sensitive to novobiocin. A pure culture of S. saprophyticus would be negative for coagulase and DNase but resistant to novobiocin.

Question 11

Why is MRSA not confined to transmission only in hospitals?

Answer 11

…

Question 12

When bacteria from a throat swab are streaked on blood agar, why is the agar stabbed several times with the loop?

Answer 12

Stabbing of the blood agar creates an anaerobic environment for the streptococci and improves the development of hemolysis.

Question 13

Differentiate between alpha and beta hemolysis

Answer 13

Alpha hemolysis is partial lysis of red blood cells and will create a halo of greenish discoloration around colonies on the blood agar. Beta hemolysis is complete lysis of red blood cells and will create a clear halo around colonies on the blood agar.

Question 14

What was Rebecca Lancefield’s contribution to the study of streptococci?

Answer 14

Lancefield classified streptococci into several major groups (e.g. group A, B, and C) based upon surface carbohydrate antigens (Lancefield antigens).

Question 15

In the CAMP reaction, which organism produces the CAMP factor? What substance does the CAMP factor react with to cause enhanced breakdown of RBCs?

Answer 15

Streptococcus agalactiae, the CAMP factor acts synergistically with staphylococcal hemolysins to cause an enhanced breakdown of blood cells

Question 16

Humans may carry both staphylococci and streptococci as normal microbiota. How might you differentiate between the two genera?

Answer 16

a. Staphylococci occur in clusters and are catalase positive.
b. Streptococci occur in chains and are catalase negative.

Question 17

Two tests that are useful in differentiating S. pyrogens and S. agalactiae

Answer 17

Lancefield-antigen testing (S. pyogenes is group A, S. agalactiae is group B), bacitracin susceptibility (S. pyogenes is susceptible, S. agalactiae is resistant), and CAMP reaction (S. pyogenes is negative, S. agalactiae is positive)

Question 18

Name two tests that are useful in differentiating of pneumococci and oral viridian’s streptococci

Answer 18

Optochin susceptibility (S. pneumoniae is susceptible, oral viridans streptococci are resistant) and hemolysis (oral viridians streptococci are weakly alpha hemolytic, while S. pneumonia cultures exhibit strong alpha hemolysis.

Question 19

What test can be performed to differentiate the enterococci from other group D streptococci?

Answer 19

Enterococci are tolerant to 6.5% NaCl whereas other group D streptococci are not.

Question 20

What test can be performed to differentiate between group A and group C streptococci?

Answer 20

SXT sensitivity (group A strep are resistant, group C strep are sensitive)

Question 21

Describe the appearance of an S. agalactiae colony grown on blood agar. Describe how that colony would differ in appearance from a colony of S. pyrogenes

Answer 21

S. agalactiae colonies on blood agar are large with a narrow zone of β- hemolysis
S. pyogenes colonies are small with a large zone of β-hemolysis