Poured agar plates are stored upside down in the refrigerator mainly to:

Stop condensed water dripping onto the agar

Allow carbon dioxide to collect

Prevent light from destroying the dyes

Chocolate agar grows Haemophilus influenzae better than blood agar because heating the blood:

Releases X and V factors from the red cells

Adds extra glucose to the medium

Kills bacteria present in the blood

Bacteriology: morphology, growth and culture media

32 Kerala PSC Lab Technician Gr II practice questions on Bacteriology: morphology, growth and culture media, part of Microbiology. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Medium
Poured agar plates are stored upside down in the refrigerator mainly to:
1. AKeep the agar from sliding out
2. BAllow carbon dioxide to collect
3. CPrevent light from destroying the dyes
4. DStop condensed water dripping onto the agar
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Answer: D · Stop condensed water dripping onto the agar
As plates cool, water condenses on the lid. With the lid underneath, the water collects there instead of falling on the agar, where it would make colonies run together and spread contaminants. Plates are kept at 2-8 °C in sealed bags to slow drying, and each batch is labelled with its preparation date. Before use, plates are brought to room temperature and their surface dried.
- A: Agar sticks to the dish and does not slide out.
- B: Storing plates has nothing to do with collecting carbon dioxide.
- C: Light-sensitive media are protected by dark storage, not by inverting.
Question 2 · Easy
To what final pH are most routine bacteriological media adjusted?
1. A5.0-5.6
2. B6.0-6.4
3. C8.4-8.8
4. D7.2-7.6
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Answer: D · 7.2-7.6
Most pathogenic bacteria grow best near the pH of human tissues, so routine media such as nutrient agar and blood agar are set at about pH 7.2-7.6. The pH is adjusted before sterilization and the final pH is checked at room temperature with a pH meter or comparator. Some media are deliberately different: Sabouraud agar (about pH 5.6) favours fungi, and alkaline peptone water and TCBS agar (about pH 8.6) favour vibrios.
- A: pH 5.0-5.6 favours fungi, as in Sabouraud agar.
- B: pH 6.0-6.4 is too acidic for most pathogenic bacteria.
- C: pH 8.4-8.8 is used for vibrios, not for routine media.
Question 3 · Easy
Robertson's cooked meat medium is used mainly for which organisms?
1. AAnaerobic bacteria
2. BVibrio cholerae
3. CMycobacteria
4. DDermatophyte fungi
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Answer: A · Anaerobic bacteria
Robertson's cooked meat broth contains pieces of heart muscle under nutrient broth. The meat supplies reducing substances such as glutathione and unsaturated fatty acids that take up oxygen, so anaerobes such as clostridia can grow. Saccharolytic clostridia turn the meat pink, while proteolytic species blacken and digest it. The broth is also used to keep stock cultures of anaerobes.
- B: V. cholerae is grown on alkaline media such as TCBS agar.
- C: Mycobacteria are grown on egg-based media such as Lowenstein-Jensen.
- D: Dermatophytes are grown on Sabouraud dextrose agar.
Question 4 · Medium
Chocolate agar grows Haemophilus influenzae better than blood agar because heating the blood:
1. AReleases X and V factors from the red cells
2. BAdds extra glucose to the medium
3. CMakes the medium more alkaline
4. DKills bacteria present in the blood
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Answer: A · Releases X and V factors from the red cells
H. influenzae needs both X factor (haemin) and V factor (NAD). Heating the blood to about 75-80 °C lyses the red cells and frees both factors into the agar, and it also inactivates enzymes in blood that break down NAD. On ordinary blood agar the organism grows poorly except as satellite colonies around Staphylococcus aureus, which releases V factor. Chocolate agar is therefore a routine medium for CSF and respiratory specimens.
- B: Heating the blood adds no glucose.
- C: The pH is not the reason; the released growth factors are.
- D: Blood for media is collected aseptically; heating is not meant to sterilize it.
Question 5 · Medium
Which culture method is used to estimate the number of viable bacteria in a sample?
1. ALawn culture
2. BPour plate culture
3. CStreak plate culture
4. DStab culture
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Answer: B · Pour plate culture
In the pour plate method, measured volumes of serial dilutions are mixed with melted agar cooled to about 45-50 °C and poured into plates. Each viable bacterium grows into a separate colony, so the colonies are counted and multiplied by the dilution factor. The method is used for counts in water, milk and other samples. An ordinary streak plate separates colonies but does not count them.
- A: Lawn culture spreads growth evenly for disc susceptibility tests.
- C: A streak plate separates colonies for isolation, not for counting.
- D: Stab culture is used to show motility or keep stock cultures.
Question 6 · Hard
A new batch of blood agar comes out chocolate-brown instead of bright red, and hemolysis cannot be read. The most likely error is that:
1. AThe plates were stored in the refrigerator
2. BThe agar base was autoclaved at 121 °C
3. CThe blood was added to agar that was too hot
4. DDefibrinated sheep blood was used
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Question 7 · Easy
Lipopolysaccharide (endotoxin) is a component of the:
1. ACapsule of pneumococci
2. BSpore coat of clostridia
3. COuter membrane of gram-negative bacteria
4. DCell wall of gram-positive bacteria
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Question 8 · Hard
Nutrient agar made with the correct 1.5% agar is autoclaved at 121 °C for 15 minutes and poured at 50 °C, but it does not set. A pH check of the leftover medium reads 4.5 (should be 7.4). The most likely cause is:
1. AHolding time too short to dissolve agar
2. BAgar hydrolysed by heating at acid pH
3. CMedium poured at too high a temperature
4. DAgar concentration too low to gel
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Question 9 · Easy
Bacteria with a tuft of flagella at one end are described as:
1. ALophotrichous
2. BPeritrichous
3. CAmphitrichous
4. DMonotrichous
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Question 10 · Easy
During which phase of the bacterial growth curve do the cells divide at a constant, maximum rate?
1. ADecline phase
2. BLag phase
3. CStationary phase
4. DLog phase
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Question 11 · Hard
A broth culture in log phase contains 1,000 bacteria/mL. Three hours later it contains 64,000 bacteria/mL. What is the generation time?
1. A3 minutes
2. B20 minutes
3. C30 minutes
4. D60 minutes
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Question 12 · Easy
Selenite F broth, used for stool samples, is an example of:
1. AAn indicator medium
2. BAn enriched medium
3. CAn enrichment medium
4. DA transport medium
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Question 13 · Medium
In MacConkey agar, which ingredient inhibits most gram-positive bacteria?
1. ABile salts
2. BLactose
3. CPeptone
4. DNeutral red
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Question 14 · Medium
Why is sodium thioglycollate added to fluid thioglycollate medium?
1. ATo raise the pH of the medium
2. BTo inhibit gram-negative bacteria
3. CTo supply X factor for growth
4. DTo lower the redox potential
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Question 15 · Easy
Blood agar is an example of which type of medium?
1. ATransport medium
2. BSelective medium
3. CEnriched medium
4. DEnrichment medium
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Question 16 · Hard
A water sample is diluted to 10^-6 and 0.1 mL of this dilution is spread on nutrient agar. After incubation, 156 colonies grow. What is the viable count of the sample?
1. A1.56 x 10^8 CFU/mL
2. B1.56 x 10^10 CFU/mL
3. C1.56 x 10^7 CFU/mL
4. D1.56 x 10^9 CFU/mL
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Question 17 · Easy
Most bacteria that cause disease in humans grow best at 37 °C and are therefore called:
1. AHalophiles
2. BMesophiles
3. CThermophiles
4. DPsychrophiles
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Question 18 · Medium
In a McIntosh and Fildes anaerobic jar, the reduced methylene blue indicator shows that anaerobic conditions have been reached when it:
1. ATurns pink
2. BTurns yellow
3. CBecomes colourless
4. DTurns deep blue
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Question 19 · Easy
TCBS agar is a selective medium for the isolation of:
1. AVibrio cholerae
2. BCampylobacter jejuni
3. CSalmonella Typhi
4. DShigella dysenteriae
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Question 20 · Medium
Which medium prevents the swarming of Proteus in urine cultures?
1. ACLED agar
2. BBlood agar
3. CNutrient agar
4. DChocolate agar
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Question 21 · Medium
Loeffler's serum slope is preferred for a throat swab in suspected diphtheria because C. diphtheriae on it:
1. AIs the only organism that can grow
2. BGrows in 6-8 hours with well-formed granules
3. CShows a wide zone of beta hemolysis
4. DForms black colonies with a brown halo
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Question 22 · Easy
Lowenstein-Jensen medium, which contains egg, is sterilised by:
1. AAutoclaving
2. BFiltration
3. CInspissation
4. DHot air oven
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Question 23 · Medium
Transferable drug resistance passes between gram-negative bacteria by conjugation mainly through:
1. AVolutin granules
2. BMesosomes
3. CR plasmids
4. DRibosomes
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Question 24 · Easy
Which bacteria have no cell wall and so are not affected by penicillin?
1. AChlamydia
2. BMycoplasma
3. CRickettsia
4. DTreponema
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Question 25 · Easy
Lactose-fermenting colonies on MacConkey agar turn pink because of the pH indicator:
1. APhenol red
2. BBromothymol blue
3. CMethyl red
4. DNeutral red
Answer and explanation in LearnFree account

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Bacteriology: morphology, growth and culture media

Poured agar plates are stored upside down in the refrigerator mainly to:

To what final pH are most routine bacteriological media adjusted?

Robertson's cooked meat medium is used mainly for which organisms?

Chocolate agar grows Haemophilus influenzae better than blood agar because heating the blood:

Which culture method is used to estimate the number of viable bacteria in a sample?

A new batch of blood agar comes out chocolate-brown instead of bright red, and hemolysis cannot be read. The most likely error is that:

Lipopolysaccharide (endotoxin) is a component of the:

Nutrient agar made with the correct 1.5% agar is autoclaved at 121 °C for 15 minutes and poured at 50 °C, but it does not set. A pH check of the leftover medium reads 4.5 (should be 7.4). The most likely cause is:

Bacteria with a tuft of flagella at one end are described as:

During which phase of the bacterial growth curve do the cells divide at a constant, maximum rate?

A broth culture in log phase contains 1,000 bacteria/mL. Three hours later it contains 64,000 bacteria/mL. What is the generation time?

Selenite F broth, used for stool samples, is an example of:

In MacConkey agar, which ingredient inhibits most gram-positive bacteria?

Why is sodium thioglycollate added to fluid thioglycollate medium?

Blood agar is an example of which type of medium?

A water sample is diluted to 10^-6 and 0.1 mL of this dilution is spread on nutrient agar. After incubation, 156 colonies grow. What is the viable count of the sample?

Most bacteria that cause disease in humans grow best at 37 °C and are therefore called:

In a McIntosh and Fildes anaerobic jar, the reduced methylene blue indicator shows that anaerobic conditions have been reached when it:

TCBS agar is a selective medium for the isolation of:

Which medium prevents the swarming of Proteus in urine cultures?

Loeffler's serum slope is preferred for a throat swab in suspected diphtheria because C. diphtheriae on it:

Lowenstein-Jensen medium, which contains egg, is sterilised by:

Transferable drug resistance passes between gram-negative bacteria by conjugation mainly through:

Which bacteria have no cell wall and so are not affected by penicillin?

Lactose-fermenting colonies on MacConkey agar turn pink because of the pH indicator: