Microbial Growth and Pure Cultures
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Questions and Answers

What is a pure culture?

  • A culture with a variety of microorganisms.
  • A culture without any living organisms.
  • A culture containing only one species or strain. (correct)
  • A culture containing multiple species.

    • Which type of culture media is used to encourage the growth of desired microbes?

  • Complex media
  • Differential media
  • Reducing media
  • Selective media (correct)

    • What is the primary method used for isolating pure cultures?

  • Streak plate method (correct)
  • Spread plate method
  • Dilution method
  • Pour plate method

    • Which of the following describes binary fission?

    <p>Increase in number of cells (A)</p> Signup and view all the answers

    During which phase of growth are cells dividing at their most rapid rate?

    <p>Log phase (C)</p> Signup and view all the answers

    Microbial growth is best defined as an increase in what?

    <p>Number of cells (C)</p> Signup and view all the answers

    Which of the following is NOT a physical requirement for microbial growth?

    <p>Carbon (D)</p> Signup and view all the answers

    What is the optimum growth temperature range for mesophiles?

    <p>25 to 40°C (C)</p> Signup and view all the answers

    Which type of microorganism is most likely to cause food spoilage in a refrigerator?

    <p>Psychrotrophs (A)</p> Signup and view all the answers

    What pH range do most bacteria prefer for optimal growth?

    <p>pH 6.5 to 7.5 (A)</p> Signup and view all the answers

    What is the name given to microbes that grow at very low pH?

    <p>Acidophiles (C)</p> Signup and view all the answers

    Which of the following describes the optimum growth temperature of hyperthermophiles?

    <p>Optimum growth temperature &gt; 80°C (D)</p> Signup and view all the answers

    What happens to a cell in a hypertonic environment?

    <p>The cell membrane shrinks due to water loss. (A)</p> Signup and view all the answers

    What is plasmolysis?

    <p>The shrinking of the cell membrane due to water loss in a hypertonic environment. (C)</p> Signup and view all the answers

    Which of the following is true of halophiles?

    <p>They require moderate to high salt concentrations for growth. (A)</p> Signup and view all the answers

    What is the role of carbon for bacteria?

    <p>Structural backbone of organic molecules. (D)</p> Signup and view all the answers

    What is nitrogen used for in bacteria?

    <p>Component of proteins and DNA. (B)</p> Signup and view all the answers

    Which of the following uses CO2?

    <p>Autotrophs (A)</p> Signup and view all the answers

    Why do obligate aerobes need oxygen?

    <p>To produce energy from nutrients. (A)</p> Signup and view all the answers

    What is a characteristic of a facultative anaerobe?

    <p>They prefer to use oxygen but can grow without it. (D)</p> Signup and view all the answers

    Which of the following is an example of an obligate aerobe?

    <p><em>Pseudomonas</em> (D)</p> Signup and view all the answers

    What conditions do capnophiles require to grow?

    <p>High CO2 (D)</p> Signup and view all the answers

    Which biosafety level requires the most stringent precautions?

    <p>BSL-4 (A)</p> Signup and view all the answers

    What is the purpose of selective media?

    <p>To suppress unwanted microbes (D)</p> Signup and view all the answers

    Saboraud’s Dextrose Agar is used to isolate what type of microbes?

    <p>Fungi (D)</p> Signup and view all the answers

    What is the main function of differential media?

    <p>Distinguish colonies of different microbes (A)</p> Signup and view all the answers

    Which type of culture is primarily used to increase small numbers of a desired organism to detectable levels?

    <p>Enrichment culture (A)</p> Signup and view all the answers

    In a BSL-4 lab, how is exhaust air filtered?

    <p>Twice through HEPA filters (D)</p> Signup and view all the answers

    What basic precaution is required at a minimum in BSL-2 labs?

    <p>Lab coat, gloves, eye protection (B)</p> Signup and view all the answers

    What is a common use for enrichment culture?

    <p>Fecal and soil samples (B)</p> Signup and view all the answers

    What is the pH of Saboraud’s Dextrose Agar?

    <p>5.6 (C)</p> Signup and view all the answers

    What is a chemically defined media?

    <p>Media where the exact chemical composition is known. (A)</p> Signup and view all the answers

    What is a characteristic of fastidious organisms?

    <p>They require many growth factors. (C)</p> Signup and view all the answers

    What is a complex media?

    <p>Media made from extracts and digests of yeasts, meat, or plants. (C)</p> Signup and view all the answers

    Which of the following is an example of a complex medium?

    <p>Nutrient agar (A)</p> Signup and view all the answers

    What is the purpose of using reducing media?

    <p>To cultivate anaerobic bacteria. (C)</p> Signup and view all the answers

    What chemical is used to reduce oxygen in anaerobic growth media?

    <p>Sodium thioglycolate (B)</p> Signup and view all the answers

    What is done to reducing media to remove oxygen?

    <p>Heated (D)</p> Signup and view all the answers

    What type of organism is Escherichia coli?

    <p>Chemoheterotroph (B)</p> Signup and view all the answers

    Which of the following is a component of nutrient agar?

    <p>Beef extract (B)</p> Signup and view all the answers

    In what form is the agar added to nutrient agar?

    <p>Solid (C)</p> Signup and view all the answers

    Flashcards

    Microbial Growth

    Increase in the number of cells, not cell size.

    Physical Requirements for Growth

    Factors that affect microbial growth include temperature, pH, and osmotic pressure.

    Temperature Ranges

    Microbes thrive in specific temperature ranges: minimum, optimum, and maximum.

    Psychrophiles

    Microorganisms that grow best at cold temperatures, typically (-15 to 20°C).

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    Acidophiles

    Microbes that grow in very acidic environments, pH 0.1 to 5.4.

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    Neutrophiles

    Bacteria that grow best at neutral pH between 5.4 to 8.5; includes many human pathogens.

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    Alkaliphiles

    Microorganisms that favor alkaline environments, often with pH above 7.

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    Osmotic Pressure

    Pressure exerted by solutes in a solution affecting water movement.

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    Hypertonic Environment

    A solution with higher solute concentration than the cell, causing plasmolysis.

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    Plasmolysis

    The process where cells shrink due to water loss in a hypertonic solution.

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    Hypotonic Environments

    Solutions with lower solute concentration, causing water to enter the cell.

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    Halophiles

    Organisms that thrive in high salt concentrations.

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    Obligate Halophiles

    Bacteria that require very high salt concentrations (20-30%).

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    Facultative Halophiles

    Bacteria that can tolerate but do not require high salt concentrations for growth.

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    Nitrogen Fixation

    The process some bacteria use to convert nitrogen gas (N2) into usable forms.

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    Obligate Aerobes

    Organisms that require oxygen for survival and energy production.

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    Chemically Defined Media

    Growth media with known chemical composition for specific organisms.

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    Selective Media

    Growth media that suppresses unwanted microbes while encouraging desired ones.

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    Streak Plate Method

    Technique to isolate pure cultures from a mixed sample.

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    Generation Time

    The time it takes for a bacterial cell to divide, ranging from 20 minutes to 24 hours.

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    Phases of Bacterial Growth

    Includes lag, log, stationary, and death phases in bacterial growth cycle.

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    Fastidious organisms

    Organisms that need many growth factors to grow.

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    Complex media

    Media composed of extracts with variable composition.

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    Nutrient broth

    A liquid complex medium for culturing bacteria.

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    Nutrient agar

    A solid complex medium for growing bacteria.

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    Reducing media

    Media that depletes oxygen for anaerobic bacteria growth.

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    Sodium thioglycolate

    A chemical in reducing media that combines with oxygen.

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    Anaerobic growth methods

    Techniques used to grow bacteria that do not require oxygen.

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    Anaerobic jar

    A device used to cultivate anaerobic bacteria in petri dishes.

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    Anaerobic chamber

    A specialized workspace for working with anaerobic conditions.

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    Capnophiles

    Microbes that require high CO2 conditions for growth.

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    Biosafety Level 1 (BSL-1)

    Basic teaching labs with no special precautions needed.

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    Biosafety Level 2 (BSL-2)

    Labs requiring lab coat, gloves, and eye protection.

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    Biosafety Level 3 (BSL-3)

    Labs with biosafety cabinets to prevent airborne transmission.

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    Biosafety Level 4 (BSL-4)

    Highly secure labs with sealed negative pressure, a 'hot zone'.

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    Differential Media

    Media that allows distinguishing colonies of different microbes on the same plate.

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    Enrichment Culture

    A method that encourages the growth of a desired microbe to detectable levels.

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    Saboraud’s Dextrose Agar

    A selective medium with pH of 5.6 to prevent bacterial growth, used for fungi isolation.

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    Selective and Differential Characteristics

    Some media have features that both suppress unwanted microbes and distinguish different microbes.

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    Study Notes

    Microbial Growth

    • Microbial growth is an increase in the number of cells, not cell size.

    Requirements for Growth

    • Physical Requirements:

      • Temperature:
        • Minimum growth temperature
        • Optimum growth temperature
        • Maximum growth temperature
      • pH: Most bacteria grow between pH 6.5 and 7.5; molds and yeasts grow between pH 5 and 6; acidophiles grow in acidic environments; acidity inhibits most microbial growth; alkalinity inhibits microbial growth but is not commonly used for food preservation.
      • Osmotic pressure: Hypertonic environments (higher solute concentration in the surrounding environment than inside a cell) cause plasmolysis. Halophiles require moderate to large salt concentrations, obligate halophiles need high salt concentrations, and facultative halophiles tolerate salt.

    • Chemical Requirements:

      • Carbon: Structural backbone of organic molecules; chemoheterotrophs use organic molecules; autotrophs use CO2
      • Nitrogen: Component of proteins, DNA, and ATP; most bacteria decompose protein materials for nitrogen; some use NH4+ or NO3–; others, use molecular nitrogen
      • Sulfur: Used in amino acids, thiamine, and biotin; most bacteria decompose protein for the sulfur source; some use SO4 2- or H2S.
      • Phosphorus: Used in DNA, RNA, and ATP; found in membranes; PO43– is a source of phosphorus.
      • Oxygen:
        • Obligate aerobes require oxygen to survive. A disadvantage is that oxygen isn't found in all environments and dissolves poorly in water
        • Facultative anaerobes prefer oxygen but can survive without it
        • Obligate anaerobes cannot use oxygen and are harmed by its presence
        • Aerotolerant anaerobes cannot use oxygen but tolerate its presence
        • Microaerophiles require oxygen, but at low concentrations, are sensitive to oxygen.

    • Trace Elements: Inorganic elements needed in small amounts, usually as enzyme cofactors, including iron, copper, molybdenum, and zinc.

    • Organic Growth Factors: Vitamins, amino acids, purines, and pyrimidines obtained from the environment.

    Biofilms

    • Microbial communities that grow in slime or hydrogel layers adhering to surfaces.
    • Bacteria communicate via quorum sensing and share nutrients.
    • Shelters bacteria from harmful environmental factors.
    • Found in digestive systems, sewage treatment systems, and can clog pipes.
    • Resistant to microbial control methods and are involved in 70% of infections.

    Culture Media

    • Culture media: Nutrients prepared for microbial growth.
    • Sterile: No living microbes
    • Inoculum: Introduction of microbes into a medium.
    • Culture: Microbes growing in or on a culture medium.
    • Agar: A solidifying agent in Petri plates, slants, and deeps that is often generally not metabolized by microbes. It liquefies at 100°C and solidifies around 40°C
    • Chemically defined media: Exact chemical composition is known ; fastidious organisms require many growth factors
    • Complex media: Extracts and digests of yeasts, meat, or plants; chemical composition varies.
    • Nutrient broth: Liquid medium.
    • Nutrient agar: Solid medium.
    • Reducing media: Used for cultivating anaerobic bacteria; contains chemicals like sodium thioglycolate that deplete oxygen; heated to remove oxygen
    • Anaerobic chamber: Used for cultivating anaerobic bacteria. This environment is fully enclosed and has conditions that eliminate oxygen (e.g. CO2).

    Special Culture Techniques

    • Capnophiles: Microbes requiring high CO2 conditions (CO2 packet, candle jar)
    • Biosafety levels (BSL): Standards for lab safety in handling microbes (BSL-1 to BSL-4). BSL-4 labs are fully enclosed, and air is filtered prior to exhaust, are highly dangerous for the organisms housed and personnel.

    Selective and Differential Media

    • Selective media: Suppress unwanted microbes and encourage a specific microbe (e.g., Sabouraud's Dextrose Agar, pH of 5.6 prevents bacterial growth and used to isolate fungi).
    • Differential media: Allow distinguishing colonies of different microbes on the same plate.
    • Some media are both selective and differentiated.

    Enrichment Culture

    • Encourages the growth of a desired microbe, even in low numbers, to detectable levels.
    • Unlike selective media, does not suppress the growth of other microbes.
    • Often used for fecal and soil samples.

    Obtaining Pure Cultures

    • A pure culture has only one species or strain.
    • A colony is a population of cells arising from a single cell (or spore) or a group of attached cells . A colony is often called a colony-forming unit (CFU).
    • The streak plate method isolates pure cultures.

    Preserving Bacterial Cultures

    • Deep-freezing: -50° to -95°C
    • Lyophilization (freeze-drying): Frozen (-54° to -72°C) and dehydrated in a vacuum.

    Bacterial Division

    • Increase in the number of cells, not cell size.
    • Binary fission is the most common method.
    • Budding, conidiospores (actinomycetes).
    • Fragmentation of filaments.

    Generation Time

    • Time needed for a cell to divide.
    • 20 minutes to 24 hours
    • Binary fission doubles the number of cells each generation.
    • Growth curves are represented logarithmically.

    Phases of Growth

    • Lag phase: Adjustment to new conditions; little or no cell division; intense metabolic activity.
    • Log (or exponential) phase: Rapid growth—constant minimum division time. The number of cells produced is greater than the number dying.
    • Stationary phase: Population stabilizes; number produced equals number dying. Factors that decrease growth include accumulation of toxic waste, acidic pH, limited nutrients, and limited oxygen.
    • Death phase: Population decreases; number of cells dying is greater than the number of cells produced.Cells lose their ability to divide; although, some cells may remain alive for long periods

    Direct Measurement of Microbial Growth

    • Plate counts: Counting colonies (CFUs) on plates with 30-300 colonies. Serial dilutions are used to ensure the right number of colonies are counted. Pour plate method and spread plate method are used.
    • Filtration: Solution passed through a filter, filter collected bacteria and transferred to the plate.
    • Most probable number (MPN) method: Statistical estimation of bacteria in a sample.
    • Direct microscopic count: Volume placed; average bacteria per viewing field; Petroff-Hausser cell counter.

    Estimating Bacterial Numbers by Indirect Methods

    • Turbidity: Measuring cloudiness of culture with a spectrophotometer
    • Metabolic activity: Amount of metabolic products (e.g., glucose uptake) is proportional to the number of bacteria.
    • Weight (dry weight): Bacteria filtered, dried, and weighed (used for filamentous organisms).

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    Microbiology An Introduction, Chapter 6, Microbial Growth PDF

    Description

    Test your knowledge of microbial growth requirements, pure culture techniques, and the phases of bacterial growth. This quiz covers physical and chemical factors influencing microbial propagation, methods for isolating pure cultures, and adaptations to extreme environments.

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