WorksheetsHEK, MAC, EMB (week 5)
Total questions: 28
Worksheet time: 14mins
Which type of organisms can be differentiated from each other using the HEK media?
Spore-forming gram-positive bacilli
Non-spore-forming gram-negative bacilli
Lactose-fermenting gram-positive cocci
Non-lactose-fermenting gram-positive rods
What was the significant contribution of MacConkey agar?
It inhibited all bacterial growth
It allowed identification of pathogenic enteric bacteria by lack of lactose fermentation
It enhanced the growth of gram-positive bacteria
It was the first media to use antibiotics
Which genera did EMB media allow a visual distinction between?
Bacillus and Clostridium
Escherichia coli and Salmonella
Staphylococcus and Streptococcus
Lactobacillus and Bifidobacterium
What is the primary purpose of Hektoen enteric agar?
To recover gastrointestinal pathogens like Salmonella and Shigella
To enhance the growth of all bacteria
To inhibit the growth of all pathogens
To recover only nonpathogenic bacteria
How does Hektoen enteric agar differentiate between pathogenic and nonpathogenic coliforms?
By color change due to acid production
By size of the colonies
By smell of the colonies
By the speed of growth
What color do colonies appear on Hektoen enteric agar if they are able to ferment lactose, sucrose, or salicin?
Greenish blue
Bright red
Salmon-pink to orange-yellow
Dark purple
What is the role of bile salts in Hektoen enteric agar?
To enhance the growth of all bacteria
To inhibit most gram-positive organisms
To provide nutrients for bacteria
To change the color of the medium
Which organisms are unable to utilize lactose, sucrose, and salicin in Hektoen enteric agar?
Nonpathogenic coliforms
Pathogenic coliforms
Salmonella and Shigella
All gram-negative bacteria
What is the role of thiosulfate and ferric ammonium citrate in HE agar?
To provide nutrients for bacterial growth
To inhibit the growth of all bacteria
To detect Salmonella by producing H₂S gas
To enhance the color of bacterial colonies
What color does a nonpathogenic organism colony appear on HE agar?
Black or with a black center
Orange-yellow, sometimes with a black center
Green with a blue center
Red with a white center
Which organism is expected to show excellent growth and a greenish-blue color on HE agar?
Enterobacter aerogenes
Escherichia coli
Salmonella enterica
Streptococcus faecalis
What does the presence of a black precipitate in the colonies indicate?
Fermentation of lactose
Production of H2S
Presence of bile salts
Absence of pathogens
What does a yellow or orange precipitate around the colonies on HE agar suggest?
The organism is inhibited by bile salts
Bile salts have been precipitated by the organism
The organism is a known pathogen
The organism is nonpathogenic
What does the presence of greenish-blue, light green, or transparent colonies with black centers on HE agar indicate?
Presence of Shigella
Presence of Salmonella
Presence of E. coli
Presence of Lactobacillus
What characteristic appearance do Shigella colonies have on HE agar?
Unchanged agar color (green), with no black spots
Opaque, indicating presence of carbohydrate utilization
Yellow, indicating H₂S production
Red, indicating presence of carbohydrate utilization
What color are the colonies of a gram-negative rod that is capable of fermenting lactose on HE agar?
Yellow orange
Blue green
Red
Purple
Why should the interpretation of carbohydrate utilization be determined within 18 to 24 hours of incubation?
To prevent the exhaustion of carbohydrates by metabolic fermentation
To allow the organism to fully utilize the carbohydrates
To ensure the organism produces more acid
To allow the organism to utilize all proteins
Which organism has a distinctive appearance when grown on EMB agar?
Escherichia coli
Klebsiella pneumoniae
Staphylococcus aureus
Pseudomonas aeruginosa
What dyes does EMB agar use?
Phenol red and nigrosin
Safranin and malachite green
Eosin Y and methylene blue
Methyl red and crystal violet
What purpose do the dyes serve in EMB media?
One prevents the growth of gram-positive bacteria, the other is a pH indicator
One prevents the growth of gram-negative bacteria, the other is a H2S indicator
One is a starch indicator, the other is a pH indicator
One stains acid-fast bacteria, the other stains gram-positive bacteria
What color does eosin y turn in acidic conditions?
From red to yellow
from colorless to pink
from colorless to black
from green to yellow
What kind of results would you expect to see when culturing a gram-positive rod on EMB media?
Black colonies, green media
No growth, no color change in media
Clear colonies, no color change in media
Red colonies, black media
What results would you expect to see from culturing a gram-negative organism that ferments lactose on EMB media?
No bacteria growth, clear media
Weak growth, clear media
Strong yellow colonies, blue media
Strong growth, dark media
Which dyes are used in MacConkey agar?
Iodine and cherry red
Neutral red and crystal violet
Phenol red and nigrosin
Methyl red and malachite green
What purpose do the dyes serve in MacConkey agar?
One inhibits gram-positive bacteria and the other is a pH indicator
One inhibits gram-negative bacteria and the other is a pH indicator
One binds to gram-positive bacteria and the other is a pH indicator
One binds to gram-negative bacteria and the other is a starch indicator
What results would you expect from a gram-negative bacteria that strongly ferments lactose cultured on Mac agar?
Yellow agar, clear colonies
Red/pink agar, red/pink colonies
Yellow agar, red/pink colonies
Clear agar, no colonies
What results would you expect when culturing a gram-negative organism that weakly ferments lactose on Mac agar?
Red/pink colonies, yellow media
Clear colonies, clear media
red/pink colonies with red/pink media
Clear colonies with yellow media
What results would you expect from culturing a gram-positive organism that does not ferment lactose on Mac agar?
Red colonies, red agar
No colonies, unchanged agar
No colonies, pink agar
No colonies, yellow agar
