Microbial Culture and Media Quiz

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Questions and Answers

What is the purpose of selective media in microbiology?

To enhance the growth of all bacteria present

To recover and isolate specific bacteria from a mixed sample (correct)

To allow the identification of all organisms in a sample

To suppress the growth of specific bacteria while promoting others

Which of the following media is specifically selective for the isolation of mycobacteria?

Mannitol salt agar

Blood agar

CHROMagar

Lowestein-Jensen medium (correct)

What is the role of malachite green in Lowestein-Jensen medium?

To promote the growth of mycobacteria

To enhance lactose fermentation

To suppress gram-positive bacteria (correct)

To inhibit the growth of mycobacteria

What characteristic is often used to differentiate bacteria in differential media?

The coloration or pH change in the medium Signup and view all the answers

What is the minimal inhibitory concentration (MIC) in antibiotic susceptibility testing?

The lowest concentration of an antibiotic that prevents visible bacterial growth Signup and view all the answers

What is the primary purpose of microbial culture in the laboratory?

To multiply microbial organisms under controlled conditions. Signup and view all the answers

Which type of culture media is specifically designed for the cultivation of fastidious bacteria?

Complex culture media Signup and view all the answers

What characteristic is unique to agar as a solidifying agent in culture media?

It has solidifying properties from red-purple marine algae. Signup and view all the answers

Which category of culture media is classified based on its ability to support the growth of certain bacteria while inhibiting others?

Selective media Signup and view all the answers

What type of culture media contains only known and well-defined chemical constituents?

Synthetic culture media Signup and view all the answers

What information can be obtained from in vitro cultures of pathogenic bacteria?

Morphology of the colonies and antimicrobial susceptibility. Signup and view all the answers

What distinguishes complex culture media from synthetic culture media?

Complex media contain unknown amounts of nutrients. Signup and view all the answers

Which culture media type is described as supporting the growth of microorganisms for further study?

Artificial growth media Signup and view all the answers

Which characteristic of agar makes it suitable for use in microbiological applications?

Physiologically stable upon usage and storage Signup and view all the answers

At what temperature does agar liquefy?

100°C Signup and view all the answers

What is the primary purpose of a chemostat in microbiological research?

To maintain the Log growth phase of a bacterial population Signup and view all the answers

What does the zone of inhibition indicate in a disk diffusion test?

The efficacy of the antimicrobial agent used Signup and view all the answers

Which agar is recommended for antibiotic susceptibility testing?

Mueller-Hinton agar Signup and view all the answers

What process is catalyzed by hemolysins observed in blood agar?

Lysis of erythrocytes Signup and view all the answers

In a disk diffusion test, how much antimicrobial sample is typically loaded onto each disc?

10 µL Signup and view all the answers

What is a primary function of enriched nonselective media like blood agar?

To test bacteria’s ability to lyse red blood cells Signup and view all the answers

Study Notes

Bacterial Cultures

Microbial culture is a method to grow microorganisms (bacteria and fungi) in a lab.
In vitro cultures of bacteria allow for:
- Isolation and cultivation of pathogenic bacteria
- Morphology analysis of colonies and phenotypic characteristics for identification
- Antibacterial susceptibility testing for choosing suitable antimicrobial agents
Microbial culture identification is crucial to determine the causative agent of a disease.

Culture Media

Culture media are nutrient media, artificial growth mediums for microbial propagation.
Types of culture media are categorized by consistency (solid, semi-solid, liquid/broth).
Categorized by nutritional components:
- Complex media (undefined): contain unknown nutrients, used for fastidious bacteria (requiring extra nutrients like blood, serum, yeast or meat extract).
- Synthetic media (chemically defined): contain known and well-defined chemical constituents.
Categorized by function/property:
- Selective media: promotes growth of certain bacteria while inhibiting others.
- Differential media: allows differentiation of bacteria based on their metabolic activities (e.g. lactose fermentation).
- Indicator media: show the presence of a specific substance in the medium.
- Enriched media: support the growth of specific species that need extra nutrients.
- Composite media: combination of properties.
- Storage media: maintain viability of cultures.
- Transport media: for safely carrying specimens for analysis.

Liquid Culture Media (Nutrient Broth)

Nutrient Broth: liquid medium for nonfastidious microorganisms.
Can be supplemented with blood, serum, sugars, etc. for special purposes.
Typical formula (g/L): Beef Extract (1.0), Peptone (5.0), Yeast Extract (2.0), Sodium Chloride (5.0).
Final pH 6.8 ± 0.2 at 25°C.
Principle: components provide essential nutrients (amino acids, nitrogen, carbon, vitamins, minerals) and maintain osmotic balance for growth.

Solid Culture Media (Agar)

Agar: unique solidifying agent from red-purple marine algae.
Properties:
- Stable during sterilization
- Physiologically stable upon usage and storage
- Nontoxic to bacterial growth.
- Liquifies at 100°C, solidifies at 50°C
Crucial for growing bacteria in agar plates for specific tests and observation.

Solid Medium for Bacteria (Nutrient Agar)

Nutrient Agar ISO 6579: a non-selective medium for cultivating various microorganisms.
Typical formula (g/L): Meat Extract (3.0), Peptone (5.0), Agar (15.0).
Final pH 7.0 ± 0.2 at 25°C.
Principle: Components supply essential nutrients (amino acids, nitrogen, carbon, minerals, vitamins) for microbial growth. Agar is the solidifying agent.

Microbial Growth Curve

Stages: Lag phase, Log phase (exponential growth), Stationary phase, and Decline phase.

Continuous Culture Device (Chemostat)

Chemostat: maintains a constant Log phase of bacterial growth.
Flow control of fresh medium and outflow of spent medium.

Bacterial Growth vs Oxygen Concentration

Obligate aerobes: require oxygen
Obligate anaerobes: cannot tolerate oxygen
Facultative anaerobes: can grow with or without oxygen
Aerotolerant anaerobes: tolerate oxygen but do not use it for growth.

Bacterial Growth in Solid Media

Observed as colony formation.
Isolated colonies grow from single bacterial cells.
Colony characteristics (e.g., shape, elevation, margin) aid in bacterial identification.

Bacterial Colony Morphology

Categorized by form, elevation, and margin shape

Enriched Nonselective Media

Blood agar: contains 5% sheep/horse blood to test hemolysis.
Chocolate agar: modified blood agar via heating blood to a chocolate-brown colour.
Mueller-Hinton agar: recommended for antibiotic susceptibility testing.

Disk Diffusion Test (Kirby-Bauer Test)

Antimicrobial susceptibility test performed on Mueller-Hinton agar.
Antibiotic discs are placed on the agar plate containing the microorganisms.
Inhibition zones are measured to determine the effectiveness of different antibiotics.

Antibiotic Susceptibility Testing in Liquid Medium (MIC)

Serial 2-fold dilutions of antibiotics in Mueller-Hinton broth.
Inoculate with a known amount of microorganisms.
Determine the lowest antibiotic concentration that inhibits bacterial growth (MIC).

Selective Media and Differential Media

Selective media: specifically designed to recover particular bacteria from a mixture.
Examples: Mannitol salt agar, Lowenstein-Jensen agar, CHROMagar.
Selective media can also be differential to visually identify and differentiate.

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Description

Test your knowledge on microbial culture techniques and the various types of culture media used in laboratories. Learn about the importance of isolating pathogenic bacteria and the characteristics of different culture media. Enhance your understanding of microbial propagation methods and their applications in microbiology.