Microbial Growth

7.1 Basics

Microbial growth refers to the increase in the number of cells, not the size of individual cells. Understanding the basics of microbial growth is essential for studying how microorganisms multiply and form populations in various environments.

• Microbial growth: Increase in the number of cells (not cell size).

• Lab vs. Nature:

  • Lab: Pure cultures (single species grown in isolation).

  • Nature: Mixed communities, often forming biofilms (complex, surface-attached microbial communities).

• Biofilms: Sticky layers where microbes live together (e.g., catheters, heart valves).

• Generation time: Time for one cell division.

  • Example: Escherichia coli ≈ 20 minutes.

  • Mycobacterium tuberculosis ≈ 15–20 hours.

• Growth curve (batch culture):

  1. Lag phase: Adjustment, no division.

  2. Log (exponential) phase: Rapid cell division.

  3. Stationary phase: Nutrient depletion, growth = death.

  4. Death phase: Cells die exponentially.

Growth Requirements

7.2 Growth Requirements

Microbial growth depends on several environmental and nutritional factors. Understanding these requirements helps in culturing and controlling microbes.

• Temperature ranges:

  • Psychrophiles: Cold-loving (–20 to 10°C).

  • Psychrotrophs: Fridge temps (0–30°C), cause food spoilage.

  • Mesophiles: 10–50°C (most pathogens, human body).

  • Thermophiles: 40–75°C.

  • Extreme thermophiles: 65–120°C.

• pH:

  • Acidophiles: pH < 5.

  • Neutrophiles: pH 5–8 (most pathogens).

  • Alkaliphiles: pH 9–11.

• Salt:

  • Halophiles: Love salt (e.g., Dead Sea).

  • Facultative halophiles: Tolerate salt (e.g., Staphylococcus aureus).

• Oxygen requirements:

  • Obligate aerobes: Need O2.

  • Facultative anaerobes: Use O2 if present, but can grow without it.

  • Obligate anaerobes: Killed by O2.

  • Microaerophiles: Need small O2 amounts.

  • Aerotolerant anaerobes: Don’t use O2, but tolerate it.

• Nutritional types:

  • Autotrophs: Make their own carbon (fix CO2).

  • Heterotrophs: Need organic carbon (food).

  • Growth factors: Must be imported if not made (e.g., vitamins).

• Energy sources:

  • Phototrophs: Light.

  • Chemotrophs: Chemicals.

Growing & Counting Microbes

7.3 Growing & Counting Microbes

Microbiologists use various media and methods to culture and quantify microbes. The choice of media and counting technique depends on the organism and the purpose of the study.

• Media types:

  • Liquid (broth), solid (agar), semisolid (motility tests).

  • Complex media: Not chemically defined (beef extract, etc.).

  • Defined media: Known chemical composition.

  • Selective media: Only some microbes grow.

  • Differential media: Show differences between microbes.

  • Streak plate: Isolate colonies.

• Counting methods:

  • Direct: Microscopy, flow cytometry, plate counts.

  • Indirect: Turbidity (cloudiness), metabolic activity, dry weight.

Microbial Control

7.4 Microbial Control

Controlling microbial growth is crucial in healthcare, food safety, and laboratory settings. Methods include physical and chemical approaches to reduce or eliminate microbes.

• Decontamination: Reduce microbes.

• Sterilization: Kill all microbes, including endospores.

• Disinfection: Remove most microbes from surfaces.

• Antiseptic: Reduce microbes on living tissue.

• Physical control:

  • Temperature:

    • Refrigeration/freezing = slows growth.

    • Heat kills microbes:

      • Pasteurization: Reduces pathogens in food.

      • Boiling: Kills most microbes (not endospores).

      • Autoclave: Steam + pressure = sterilize (kills endospores).

      • Dry heat: Ovens, incineration.

  • Radiation:

    • Ionizing (X-rays, gamma) = sterilize.

    • Non-ionizing (UV) = damages DNA.

  • Filtration: Removes microbes from air/liquids (HEPA, LifeStraw).

• Chemical control:

  • Low-level: Detergents/soaps.

  • Intermediate: Alcohols, phenolics.

  • High-level: Aldehydes, peroxides, chlorine.

• Microbial resistance:

  • Mycobacteria: Hard to kill (waxy coat).

  • Endospores: Very resistant; autoclaving/sporicides needed.

  • Viruses:

    • Enveloped: Easy to kill (detergents, heat).

    • Naked: Harder (need chlorine).

  • Protozoa: Some resistant, need filtration/UV.

  • Prions: Very resistant, need extra harsh sterilization.

Table: Microbial Temperature Classifications

Additional info:

• Biofilms are important in medical settings because they can protect microbes from antibiotics and immune responses.

• Autoclaving is the gold standard for sterilization in laboratories and hospitals.

• Prions are infectious proteins and are the most difficult to destroy; standard sterilization is often ineffective.