Lab Exam 2: Ex. 4-1; 4-2

Question 1

What is the most common means of isolating an organism from a mixed culture?

Answer 1

streak for isolation

Question 2

selective media

Answer 2

media designed to enhance the isolation procedure by inhibiting growth of some organisms while encouraging the growth of others.

Question 3

differential media

Answer 3

selective media that contains indicators to expose differences between organisms

Question 4

Give an example of a situation in which selective media would be used.

Answer 4

Clinical microbiologists, who are familiar with human pathogens and the types of infections they cause choose selective media that will screen out normal flora that also are likely to be in a sample.

Question 5

defined

Answer 5

“chemically defined” - each of its ingredients is known and in exactly what amounts.

Question 6

undefined

Answer 6

“complex” - it contains one or more ingredient(s) of unknown composition and/or amount such as yeast extract, beef extract, digest of gelatin, etc.

Question 7

What is the purpose of nutritional components?

Answer 7

Nutritional components are selected to obtain the optimum growth of the organisms being tested for or suspected of being in the sample.

Question 8

What do inhibitors do?

Answer 8

make the medium selective - they are designed to exploit weaknesses in specific groups of organisms and thus prevent or inhibit their growth, while allowing other organisms to grow.

Question 9

In what ways do inhibitors function?

Answer 9

1. interrupting DNA synthesis or expression of a gene.
2. at the enzymatic level, interfering with a critical reaction.
3. interfere with membrane permeability, thus upsetting homeostasis and starting a cascade of catastrophic changes inside the cell.

Question 10

What can be added to inhibit organisms that can’t tolerate high osmotic pressures?

Answer 10

sodium chloride

Question 11

What component almost always makes the medium differential?

Answer 11

substrate

Question 12

What does differentiation and identification of organisms frequently rely on?

Answer 12

their differing abilities to perform a specific chemical reaction or set of reactions and to do them in a way that can be observed.

Question 13

indicators

Answer 13

make a desired or expected reaction visible

Question 14

What is Phenylethyl Alcohol Agar?

Answer 14

an undefined, selective medium that inhibits the growth of many Gram-negative and some Gram-positive organisms (primarily selects for Gram-positive microbes). It is not a differential medium because no distinction is made between organisms that successfully grow on it.

Question 15

What does PEA stand for?

Answer 15

Phenylethyl Alchohol Agar

Question 16

What is Phenylethyl Alcohol Agar (PEA) used for?

Answer 16

it is used to isolate staphylococci and streptococci (including enterococci and lactococci) from specimens containing mixtures of bacterial flora.

Question 17

What contaminants does PEA typically screen out?

Answer 17

Escherichia coli and Proteus species

Question 18

What microbes did we use in our PEA lab and what was the result?

Answer 18

Escherichia coli and Staphlyococcus epidermidis
- There was growth of both microbes but considerably more growth of the Se which is Gram-positive on the PEA plate (all growth on the nutrient agar plate)

Question 19

What is Columbia CNA with 5% sheeps blood agar?

Answer 19

an undefined, differential, and selective medium that allows growth of Gram-positive organisms (especially staphylococci, streptococci, and enterococci) and stops or inhibits growth of most Gram-negative organisms.

Question 20

What is the selective agent of PEA?

Answer 20

Phenylethyl alcohol - the PEA itself makes it selective

Question 21

What does Columbia CNA select for?

Answer 21

Gram-positive microbes

Question 22

What does PEA Select for?

Answer 22

Gram-positive microbes

Question 23

What are the selective agents for Columbia CNA?

Answer 23

colistin and nalidixic acid

Question 24

What are the differential agents for Columbia CNA?

Answer 24

sheeps blood

Question 25

What is Columbia CNA differentiating?

Answer 25

whether or not a microbe is hemolytic or not

Question 26

What microbes did we use in our CNA lab and what was the results?

Answer 26

Enterococcus faecalis - medium growth on CNA plate, Escherichia coli - no growth on CNA plate, Staphylococcus epidermidis - heavy growth on CNA plate
(all growth on regular nutrient agar plate)

Question 27

On the CNA plate how can you distinguish a hemolytic microbe from a non-hemolytic microbe?

Answer 27

The non hemolytic microbe will grow normal; the hemolytic microbe will have space around it in the red agar.