Media

Question 1

In what way is MacConkey’s Agar selective?

Answer 1

MacConkey’s Agar inhibits the growth of Gram +ve bacteria.

Question 2

MacConkey’s Agar is selective.

These two chemical agents contribute to its selectivity.

Answer 2

crystal violet and bile salts

inhibit the growth of Gram +ve bacteria.

Question 3

In what way is MacConkey’s Agar differential.

Answer 3

MacConkey’s Agar contains lactose and a ph color change indicator. Bacteria able to ferment lactose create an acidic byproduct resulting in a local ph change, and change in color of the colony under growth. Lactose fermenters appear pink on Mac’.

Question 4

CNA agar is selective for

Answer 4

Gram +ve bacteria

Question 5

CNA agar is differential for what?

Answer 5

Hemolysis (Beta, Alpha, Gamma)

Question 6

What mechanisms drive the selectivity of CNA agar?

Answer 6

colistin and nalidixic acid

disrupt the cell membrane of gram-negative organisms.

nalidixic acid

blocks DNA replication in susceptible gram-negative bacteria.

Question 7

Is blood agar selective or differential?

Answer 7

Blood agar is differential for hemolysis.

Question 8

PEA agar contains

Answer 8

phenylethyl alcohol (agar)

Question 9

TMP / SMX contains

Answer 9

TriMethoPrim SulfaMethoXazole

Question 10

Todd-Hewitt broth contains antimicrobials.

What are those antimicrobials.

Answer 10

gentamicin

nalidixic acid

Question 11

Todd-Hewitt broths contain antimicrobials in order to suppress what?

Answer 11

Normal vaginal microbiota, while allowing the growth GBS (group B Streptococci).

Antimicrobials gentamicin & nalidixic acid are specific for the suppression of normal vaginal microbiota, while allowing the growth GBS (group B Streptococci)

Question 12

Is bile esculin media selective or differential?

Answer 12

Bile esculin in BOTH selective and differential.

Question 13

To what end is bile esculin agar (BEA) employed?

Answer 13

isolate and identify Enterococci and group D Streptococci (based on their ability to hydrolyze esculin)

Question 14

Esculin hydrolyzes to what?

Answer 14

glucose and esculeTIN Esculetin then reacts with an iron salt (ferric citrate) in the medium to form a phenolic iron complex which produces a dark brown or black color. Esculin + Acid → β-D-glucose + Esculetin Esculetin + Fe3+ → Dark Brown Color

Question 15

A positive bile esculin test reveals what color? Why?

Answer 15

Positive test is dark brown or black. Esculetin reacts with iron salt (Ferric Citrate) creating the dark color.

Question 16

Todd Hewitt Broth primarily is used for the growth of what?

Answer 16

beta-hemolytic streptococci Streptococcus pyogenes Streptococcus pneumoniae Streptococcus agalactiae

Question 17

The ability to hydrolyze hippurate is an alternative test for the presumptive identification of _________ .

Answer 17

S. agalactiae

Question 18

Streptococci other than S. agalactiae may also be hippurate hydrolysis positive, particularly ___________ .

Answer 18

viridans group streptococci.

Question 19

All of the viridans group streptococci are leucine aminopeptidase positive and __________ .

Answer 19

pyrrolidonyl arylamidase negative.

Question 20

A positive bile esculin test is generally indicative of what?

Answer 20

Species belonging to the S. bovis group or enterococci

Question 21

This enzyme, associated with Gram + bacteria, neutralizes the bactericidal effects of hydrogen peroxide.

Answer 21

catalase

Question 22

Catalase is a common enzyme which catalyzes the decomposition of _____________ to ________ and _________.

Answer 22

hydrogen peroxide

to

water and oxygen gas

2H₂O₂ → 2H₂O + O₂

Question 23

The enzyme catalase mediates the breakdown of hydrogen peroxide into what?

Answer 23

oxygen and water

Question 24

The morphologically similar Enterococcus or Streptococcus and Staphylococcus can be differentiated using what test?

Answer 24

catalase test

Enterococcus or Streptococcus (catalase -ve)

Staphylococcus (catalase +ve)

Question 25

Lactose fermenting strains grow as what color on MacConkey Agar?

Answer 25

red or pink

Lactose non-fermenting strains, such as Shigella and Salmonella are colourless and transparent

Question 26

Urease (as a virulence factor) acts to break down what?

Answer 26

Urease dissociates

Urea into carbon dioxide and amonia

Urea ⇒ Carbon dioxide and amonia

CH₄N₂0 ⇒ CO₂ + NH₃