Controlling Microbial Growth

Introduction to Microbial Growth Control

  • Microbial growth is essential to understanding microbiology, particularly focusing on methods to control growth in vitro.

Factors That Affect Microbial Growth

1. Availability of Nutrients

  • Required for sustaining life; nutrients serve as energy sources through chemical bond breakdown.

2. Moisture

  • Water is vital for metabolic processes.

  • Some microorganisms like bacterial endospores survive desiccation.

3. Temperature

  • Each microorganism has an optimum temperature for growth, primarily influenced by enzymatic activity.

  • Types of microorganisms based on temperature preferences:

    • Thermophiles: Thrive at high temperatures.

    • Mesophiles: Optimal growth around 37 °C.

    • Psychrophiles: Prefer cold environments (e.g., deep ocean).

    • Psychrotrophs: Prefer refrigerator temperatures (4 °C).

    • Psychroduric: Prefer warmth but can survive cold conditions.

4. pH

  • pH indicates solution acidity/alkalinity.

  • Most prefer pH 7.0 to 7.4.

  • Acidophiles thrive at pH 2-5, while alkaliphiles prefer pH > 8.5.

5. Osmotic Pressure and Salinity

  • Osmotic pressure: Pressure exerted on cell membranes by external and internal solutions.

  • Plasmolysis is a condition in which the cell membrane

    and cytoplasm of a cell shrink away from the cell

    wall; occurs when bacteria with rigid cell walls are

    placed into a hypertonic solution.

  • if it does burst, the cytoplasm escapes – this process is

    known as plasmoptysis

  • Barometric Pressure

    – Microbes that can survive in high atmospheric pressure (> 14.7 psi) are know as piezo philes

  • Solutions:

    • Hypertonic: Higher solute concentration outside cell; causes plasmolysis.

    • Hypotonic: Lower solute concentration outside cell; could cause plasmoptysis.

    • Isotonic: Equal concentrations outside and inside.

  • Halophilic: Prefer salty environments; haloduric can survive in them but do not prefer.

6. Gaseous Atmosphere

  • Oxygen requirements vary:

    • Obligate aerobes: Need oxygen (20-21%).

    • Microaerophiles: Require lower oxygen levels (~5%).

    • Obligate anaerobes: Killed by oxygen.

    • Capnophiles: Prefer higher CO2 levels (5-10%).

Encouraging Microbial Growth in Vitro

Culturing in Laboratory:

  • Growth measurements reflect an increase in the number of organisms, not size.

  • Binary fission: Dividing of cells under optimal conditions; E. coli takes about 20 minutes.

    • Culture Media: Used in microbiology; can be: solid or liquid

      • Chemically defined: All ingredients known.

      • Enriched: Contains rich nutrients for specific organisms (e.g., chocolate agar).

      • Selective: Contains inhibitors to restrict certain organisms (e.g., PEA agar).

      • Differential: Differentiates organisms based on appearances (e.g., MacConkey agar).

      • Thioglycollate broth (THIO) is a popular liquid medium in bacteriology labs; it supports the growth of all categories of bacteria from obligate aerobes to obligate anaerobes.

      • Inoculation involves adding a portion of a specimen to the

        medium. After media are inoculated, they must be placed into an incubator which will maintain the appropriate

        atmosphere, temperature, and moisture level; the

        process is known as incubation

        • 3 types of incubators are used in clinical microbiology laboratories:

          – A CO2 incubator (contains 5-10% CO 2)

          – A non-CO2 incubator (contains room air)

          – An anaerobic incubator (the atmosphere is devoid of oxygen

        • A spectrophotometer can be used to determine growth by measuring the turbidity of the medium.

        • A viable plate count is used to determine the number of viable bacteria in a liquid sample by making serial dilutions of the liquid and inoculating onto nutrient agar; after overnight incubation, the number of colonies is counted

        • A population growth curve for any particular species of bacterium may be determined by growing a pure culture of the organism in a liquid medium at a constant temperature.

        • Obligate intracellular pathogens are microbes that can only survive and multiply within living cells (called host cells)

        • Fungi (including yeasts, moulds and dimorphic fungi) grow on and in a variety of solid and liquid culture media.

        • Most microbiology laboratories do not culture protozoa; some research and reference labs do,

Inhibiting Microbial Growth in Vitro

  1. Definitions:

  • Sterilization: Complete destruction of all forms of life (cells, spores, viruses).

  • Disinfection: Removal of pathogens from non-living objects.

    • Disinfectants are chemical substances that eliminate

      pathogens on inanimate objects.

    • Antiseptics are solutions used to disinfect skin and

      other living tissues.

  1. Definition of Terms, cont.

    • The suffix –cide or –cidal refers to “killing.”

    • Germicidal agents, biocidal agents, and microbicidal

    agents are chemicals that kill microbes.

    • Bactericidal agents are chemicals that specifically kill

    bacteria, but not necessarily bacterial endospores.

    • Sporicidal agents kill bacterial endospores.

    • Fungicidal agents kill fungi, including fungal spores.

    • Algicidal agents kill algae.

    • Viricidal agents destroy viruses.

    • A microbistatic agent is a drug or chemical that inhibits growth and reproduction of microbes.

    • A bacteriostatic agent is one that specifically inhibits the metabolism and reproduction of bacteria.

    • Lyophilization is a process that combines dehydration (drying) and freezing. This process is widely used in industry to preserve foods, antibiotics, microorganisms, and other biologic materials.

    • Sepsis refers to the presence of pathogens in blood or tissues, whereas asepsis means the absence of pathogens.

    • Antisepsis is the prevention of infection

  2. Physical Methods:

  • Heat: Effective sterilization

    • Moist heat (autoclaving) destroys all microbes at higher temperatures.

    • Dry heat: Relies on high temperatures over extended times.

  • The autoclave: A large metal pressure cooker that uses steam under pressure to completely destroy all microbial life

  • Cold; most microorganisms are not killed, but their metabolic activities are slowed.

  • Desiccation; many dried microorganisms remain viable, but they cannot reproduce.

  • Radiation; an ultra-violet (UV) lamp is useful for reducing the number of microbes in the air.

  • Ultrasonic waves; used in hospitals and medical and dental clinics to clean equipment.

  • Filters; used to separate cells/microbes from liquids or

    gases.

  • Gaseous atmosphere; can be altered to inhibit growth.

3. Chemical Agents:

  • Chemical disinfection refers to the use of chemical agents

    to inhibit the growth of pathogens, either temporarily or

    permanently.

    Disinfectants are affected by:

    – Prior cleaning of the object or surface

    – The organic load (e.g., feces, blood, pus)

    – The bioburden; types and numbers of microbes

    – Concentration of the disinfectant

    – Contact time

    – Physical nature of the object being disinfected

    – Temperature and pH

  • Broad-spectrum, fast-acting, stable, and effective against microorganisms.

  • Antiseptics: Safe for use on skin; help reduce organism numbers but do not penetrate deep.

Maintaining Safety and Control

  • Aseptic Techniques: Strict practices to prevent contamination during culturing.

  • Kitchen Safety: Awareness of foodborne pathogens like E. coli and Salmonella; maintain hygiene by washing hands and sanitizing surfaces.

  • Controversies: Resistance due to widespread antibiotic use in livestock and household products; childhood exposure to microbes debated for immune development.