MCB 2024 C Microbiology Lab Practical Midterm Prof. Curtis
(Valencia)
1. Phenylethyl Alcohol Agar: Selective Agent: Phenylethyl Alcohol Favors the
growth of Gram (+) by inhibiting the growth of gram Organisms:
Staphylococcus aureus - gram (+)Enterococcus faecalis - gram
(+)Escherichia coli - gram (-)
2. Columbia CNA with 5% Sheep Blood Agar: Selective Agents: Colistin
(antibiotic) and Nalidixic (antibiotic) Acid (CNA) Favors growth of Gram (+) -
inhibiting the growth of gram (-).Useful in identifying the pathogenic:
-Staphylococcus aureus - beta hemotolic hemolysis pattern - Complete degradation
-Streptococcus mitis - alpha hemolysis pattern - incomplete degradation
-Enterococcus faecalia - gamma hemolysis pattern - no degradation,
3. What is the differential agent and indicator in
Columbia CNA with 5% sheep blood agar?:
Differential Agent: 5% Blood (gives cells a nice
red, opaque color. Purpose is to identify cells
capable of breaking down red blood cells.
Indicator: hemolytic activity
4. Hemolytic Activity: ² (beta) hemolysis:
complete degradation ± alpha) hemolysis:
incomplete degradation( ³ (gamma) hemolysis:
no degradation
5. Mannitol Salt Agar: Selective Agent: 7.5% NaCl
Favors the growth of halo-tolerant Staphylococcus
A: Halo-tolerant bacteria
, MCB 2024 C Microbiology Lab Practical Midterm Prof. Curtis
(Valencia)
6. What is the differential agent and indicator in mannitol salt agar?:
Differential Agent: Mannitol
Indicator: Phenol Red
7. How does manitol salt agar differentiate between microbes?: Identifies
microbes that can ferment mannitol and produce acid.
Acid production will cause a yellow color change as the pH of the media drops below
6.8
Indicative of Staphylococcus aureus
8. MacConkey Agar (MAC): Selective Agents: Bile Salts and Crystal Violet
Favors growth of Gram (-)
9. What is the differential agent and indicator in MacConkey Agar?: Differential
Agent: Lactose
Indicator: Neutral Red (acid causes a shift from colorless to red as the pH drops
below 6.8) (as seen in 1 & 2)
10. How does MacConkey Agar differentiate microbes?: Identifies microbes
that can ferment lactose and produce acid as a byproduct
Acid causes a shift from colorless to red as the pH drops below 6.8
Indicates a possible coliform (causes UTIs if it gets outside the intestinal tract. E.
coli can cause UTIs
11. MacConkey Agar (MAC) - (favors gram (-)): 1) Enterobacter
aerogenes/pink/gram negative/fermentation
2) Escherichia coli / pink / gram negative /fermentation
3) Enterococcus faecalis / No growth / gram positive
4) Salmonella typhimurium - growth, no color change / gram negative / no
fermentation
Fermentation will produce acid. When they ferment sugar they will make acid as a
byproduct.
(Valencia)
1. Phenylethyl Alcohol Agar: Selective Agent: Phenylethyl Alcohol Favors the
growth of Gram (+) by inhibiting the growth of gram Organisms:
Staphylococcus aureus - gram (+)Enterococcus faecalis - gram
(+)Escherichia coli - gram (-)
2. Columbia CNA with 5% Sheep Blood Agar: Selective Agents: Colistin
(antibiotic) and Nalidixic (antibiotic) Acid (CNA) Favors growth of Gram (+) -
inhibiting the growth of gram (-).Useful in identifying the pathogenic:
-Staphylococcus aureus - beta hemotolic hemolysis pattern - Complete degradation
-Streptococcus mitis - alpha hemolysis pattern - incomplete degradation
-Enterococcus faecalia - gamma hemolysis pattern - no degradation,
3. What is the differential agent and indicator in
Columbia CNA with 5% sheep blood agar?:
Differential Agent: 5% Blood (gives cells a nice
red, opaque color. Purpose is to identify cells
capable of breaking down red blood cells.
Indicator: hemolytic activity
4. Hemolytic Activity: ² (beta) hemolysis:
complete degradation ± alpha) hemolysis:
incomplete degradation( ³ (gamma) hemolysis:
no degradation
5. Mannitol Salt Agar: Selective Agent: 7.5% NaCl
Favors the growth of halo-tolerant Staphylococcus
A: Halo-tolerant bacteria
, MCB 2024 C Microbiology Lab Practical Midterm Prof. Curtis
(Valencia)
6. What is the differential agent and indicator in mannitol salt agar?:
Differential Agent: Mannitol
Indicator: Phenol Red
7. How does manitol salt agar differentiate between microbes?: Identifies
microbes that can ferment mannitol and produce acid.
Acid production will cause a yellow color change as the pH of the media drops below
6.8
Indicative of Staphylococcus aureus
8. MacConkey Agar (MAC): Selective Agents: Bile Salts and Crystal Violet
Favors growth of Gram (-)
9. What is the differential agent and indicator in MacConkey Agar?: Differential
Agent: Lactose
Indicator: Neutral Red (acid causes a shift from colorless to red as the pH drops
below 6.8) (as seen in 1 & 2)
10. How does MacConkey Agar differentiate microbes?: Identifies microbes
that can ferment lactose and produce acid as a byproduct
Acid causes a shift from colorless to red as the pH drops below 6.8
Indicates a possible coliform (causes UTIs if it gets outside the intestinal tract. E.
coli can cause UTIs
11. MacConkey Agar (MAC) - (favors gram (-)): 1) Enterobacter
aerogenes/pink/gram negative/fermentation
2) Escherichia coli / pink / gram negative /fermentation
3) Enterococcus faecalis / No growth / gram positive
4) Salmonella typhimurium - growth, no color change / gram negative / no
fermentation
Fermentation will produce acid. When they ferment sugar they will make acid as a
byproduct.
