Quiz: Bacterial Culture and Media — 17 questions

1. What is the primary purpose of bacterial culture in a clinical laboratory?

A. To measure bacterial growth requirements before selecting a culture medium

B. To determine whether a specimen contains antibiotic-resistant genes

C. To observe bacterial cells without separating them from the original specimen

D. To obtain a pure isolate for examination and species or strain identification

Answer: (D)

Explanation

Bacterial culture is performed to obtain a pure bacterial isolate that can be examined and identified at the species or strain level. A mixed culture contains multiple bacterial species, so it cannot provide the same degree of identification certainty.

2. Which finding represents a microscopic examination of a bacterial culture?

A. The protein spectrum generated by MALDI-TOF mass spectrometry

B. The size, colour, and shape of colonies on an agar plate

C. The arrangement and staining reaction of cells after Gram staining

D. The resistance pattern produced by an antibiotic susceptibility test

Answer: (C)

Explanation

Gram staining reveals microscopic characteristics of bacterial cells, including their staining reaction and appearance. Colony morphology is instead a cultural observation made by examining visible growth on a medium.

3. A bacterium is placed in a medium with adequate nutrients and water but a pH of 5.0. Which growth requirement is most clearly unsuitable?

A. The temperature must be between 30°C and 40°C for every bacterial species

B. The salt concentration is below the usual optimal level of 0.5%

C. The atmosphere contains too much carbon dioxide for bacterial multiplication

D. The pH is outside the usual optimal range of 7.2–7.4

Answer: (D)

Explanation

Most bacterial growth described here is optimized at a pH of 7.2–7.4, so pH 5.0 is outside that range. Temperature and atmospheric requirements also matter, but the stated problem directly concerns the unsuitable pH.

4. A bacterial species grows both in the presence and absence of oxygen. How should its atmospheric requirement be classified?

A. Microaerophilic

B. Capnophilic under increased carbon dioxide

C. Obligately anaerobic

D. Facultatively anaerobic

Answer: (D)

Explanation

Facultatively anaerobic bacteria can grow with or without oxygen. Obligately anaerobic bacteria require anaerobic conditions, while microaerophilic and capnophilic bacteria have more specific atmospheric preferences.

5. Which temperature profile best describes a psychrotrophic bacterium?

A. It has a mesophilic optimum but can grow at temperatures near 0°C

B. It has an optimum below 0°C and cannot grow at mesophilic temperatures

C. It requires temperatures above 50°C while tolerating brief exposure to refrigeration

D. It grows optimally between 30°C and 40°C but is unable to grow at lower temperatures

Answer: (A)

Explanation

Psychrotrophic bacteria are mesophilic bacteria that retain the ability to grow at lower temperatures, sometimes close to 0°C. Their classification does not mean that their optimum temperature is necessarily below freezing.

6. What is the role of the indicator in an anaerobic culture system?

A. It changes colour to verify that anaerobic conditions have been reached

B. It produces hydrogen and carbon dioxide to remove oxygen from the system

C. It catalyzes the chemical reaction that establishes the anaerobic atmosphere

D. It supplies nutrients that support multiplication of obligately anaerobic bacteria

Answer: (A)

Explanation

The indicator changes colour to show whether the system has achieved anaerobic conditions. The envelope generates hydrogen and carbon dioxide, while the palladium catalyst participates in establishing the anaerobic atmosphere.

7. Which sequence correctly describes how an anaerobic culture envelope establishes and verifies suitable conditions?

A. Palladium generates nutrients, the envelope measures oxygen, and the indicator supports bacterial multiplication

B. The indicator generates hydrogen and carbon dioxide, the envelope changes colour, and palladium identifies the organism

C. The envelope removes bacteria, palladium performs Gram staining, and the indicator detects antibiotic resistance

D. The envelope generates hydrogen and carbon dioxide, palladium acts as a catalyst, and an indicator verifies the result

Answer: (D)

Explanation

An anaerobic system uses an envelope that generates hydrogen and carbon dioxide, a palladium catalyst that participates in establishing the atmosphere, and an indicator that verifies the condition. The indicator is a verification device rather than the component that generates the gases.

8. Which culture medium is best suited for isolating individual bacterial colonies and examining their colony morphology?

A. Semisolid medium containing 0.1–0.5% agar

B. Liquid medium used for biochemical testing

C. Solid medium containing 1.5–2% agar

D. Liquid medium used for enrichment

Answer: (C)

Explanation

Solid media with 1.5–2% agar allow cells to remain spatially separated, producing individual colonies that can be examined. Liquid media are more appropriate for enrichment or biochemical testing, while semisolid media are used for motility and transport.

9. Which pairing correctly matches a culture-medium example with its physical form?

A. Brain-heart infusion—solid medium

B. Slant agar—liquid medium

C. Meat-peptone broth—solid medium

D. Peptone water—liquid medium

Answer: (D)

Explanation

Peptone water is an example of a liquid culture medium. Slant agar is solid medium held in a tube, and the listed broths are liquid rather than solid.

10. A pathogen is present in very low numbers and must be multiplied before it is placed on agar; which medium is most appropriate first?

A. A diagnostic indicator medium

B. A differential agar

C. A selective agar

D. An enrichment broth

Answer: (D)

Explanation

Enrichment broth increases the number of a target pathogen before agar inoculation. Selective agar instead suppresses competing organisms during growth on a solid surface.

11. What feature enables a differential medium to distinguish colonies of different organisms growing in the same culture?

A. A broth formulation that multiplies scarce pathogens

B. A high concentration of agar that prevents spreading

C. A diagnostic compound or coloured indicator

D. A selective agent that suppresses every competing species

Answer: (C)

Explanation

Differential media contain diagnostic compounds or coloured indicators that produce observable differences among growing organisms. A selective agent has a different function: it suppresses unwanted growth rather than distinguishing colonies that grow.

12. Why does the last segment of a streak plate commonly provide the best material for obtaining a pure bacterial culture?

A. Cells have been progressively diluted until isolated colonies form

B. Cells multiply more rapidly in the final segment because agar is softer

C. The first segment contains the highest concentration of selective agents

D. The last segment receives mixed cells that remain together during growth

Answer: (A)

Explanation

Streaking progressively dilutes cells across the agar surface, so the later segments can contain isolated colonies suitable for subculture. The first segment generally has denser mixed growth rather than the best-separated colonies.

13. Which inoculation method is primarily used to quantify bacteria as colony-forming units?

A. Stab inoculation

B. Broth enrichment inoculation

C. Streak inoculation

D. Pour or spread plating

Answer: (D)

Explanation

Pour plates and spread plates are used to count bacteria by measuring colony-forming units. Streak inoculation is designed to isolate single colonies, while stab inoculation is used with liquid or semisolid media.

14. Which combination correctly describes the examination of colony morphology and the medium required to assess haemolysis?

A. Colony age, inoculum density, incubation atmosphere, and pH tolerance; haemolysis is assessed on bile-esculin agar

B. Size, shape, margin, elevation, texture, colour, smell, consistence, and haemolysis; haemolysis is assessed on blood agar

C. Growth rate, oxygen requirement, motility, staining reaction, and enzyme activity; haemolysis is assessed on MacConkey agar

D. Cell size, spore position, capsule thickness, and metabolic pathway; haemolysis is assessed on XLD agar

Answer: (B)

Explanation

Colony morphology includes features such as size, shape, margin, elevation, consistence, pigmentation, smell, texture, and haemolysis, with haemolysis examined on blood agar. MacConkey and XLD agars serve selective or differential purposes rather than being the standard medium for assessing haemolysis.

15. A colony completely clears the red cells surrounding it on blood agar; which haemolysis pattern does this indicate?

A. Beta haemolysis

B. Alpha haemolysis

C. Partial haemolysis

D. Gamma haemolysis

Answer: (A)

Explanation

Beta haemolysis is complete destruction of red cells, producing clear zones around colonies on blood agar. Alpha haemolysis is incomplete and usually produces a greenish change, whereas gamma haemolysis indicates no haemolysis.

16. A freshly isolated, highly virulent culture produces moist, sticky colonies, while an older laboratory culture produces dry, granular colonies; which interpretation best fits these appearances?

A. The moist colonies suggest capsule or slime production, whereas the dry colonies are rough forms

B. The moist colonies represent rough mutants, whereas the dry colonies represent freshly isolated wild strains

C. The moist colonies indicate absent lipopolysaccharide, whereas the dry colonies indicate strong virulence

D. The moist colonies indicate pigment loss, whereas the dry colonies indicate active capsule production

Answer: (A)

Explanation

Mucous colonies are associated with capsule or slime production and commonly occur in highly virulent fresh cultures, while rough colonies are dry and granulated and may occur in old cultures or laboratory mutants. Rough colonies therefore do not indicate active capsule production.

17. A red colony with a black centre grows on XLD agar; which organism and metabolic feature are most consistent with this result?

A. Enterococcus degrading esculin to black esculetin

B. Salmonella producing hydrogen sulfide

C. A Gram-positive organism growing despite bile salts

D. A lactose fermenter producing abundant acid

Answer: (B)

Explanation

XLD agar is selective-differential for Salmonella, whose colonies can appear red with black centres because of hydrogen sulfide production. Yellow to orange colonies on this medium are associated with lactose fermenters, not the characteristic Salmonella reaction.