chapter 4 microbiology

Chapter 4: Microbial Growth

Exodus Reference

• Exodus 10:5: Cultural impact of swarms covering the land. Emphasizes the influence and dominance of microorganisms.

A Glimpse of History

• Robert Koch (1843–1910):

  • German physician known for studying disease-causing bacteria.

  • Awarded Nobel Prize in 1905.

  • Developed methods to cultivate bacteria on solid media.

  • Early Experiments:

    • Utilized potatoes but faced nutrient limitations.

    • Introduced gelatin to solidify media, faced melting temperature issues.

  • Agar (suggested by Fannie Hess in 1882):

    • Became the standard for solidifying jelly due to its superior properties.

Introduction to Microbial Growth

• Prokaryotic Diversity:

  • Thrive in extreme environments (ocean depths, volcanic vents, polar regions).

  • Study of these microbes offers insights into potential extraterrestrial life.

  • Growth requires specific nutrients and conditions for culture, highlighting both medical and industrial significance.

Principles of Bacterial Growth

• Binary Fission:

  • Process by which prokaryotic cells divide.

  • Exhibits exponential growth; population doubles each division.

• Generation Time:

  • Varies by species and environmental conditions.

  • Example: Foodborne pathogens can quickly multiply, from 10 cells to 40,960 in 4 hours under optimal conditions.

Growth Calculation

• Formula: Nt = N0 x 2^n

  • Nt = final number of cells

  • N0 = initial number of cells

  • n = number of divisions

• Example in potato salad shows emphasis on pathogen growth dynamics under optimal conditions.

The Power of Exponential Growth

• Rapid Increase:

  • Optimal conditions can lead to enormous populations in short times.

  • Generation time is crucial in understanding microbial population dynamics.

Prokaryotic Growth in Nature

• Adaptability:

  • Microbes in dynamic environments adjust via sensing changes.

  • They can live individually or within aggregates called biofilms.

Biofilm Formation

• Process:

  • Planktonic (free-floating) bacteria adhere to surfaces.

  • Produce extracellular polymeric substances (EPS).

  • Channels form for nutrient and waste exchange.

  • Detachment and formation of new biofilms can occur.

Characteristics of Biofilms

• Architecture:

  • Channels facilitate nutrient flow and waste disposal.

• Significance:

  • Dental plaque and infections can stem from biofilms.

  • Industrial challenges with pipe clogging.

  • Protective structures make microbes more resistant to treatment.

Interactions in Mixed Communities

• Cooperative Interactions:

  • Some species support each other’s growth; oxygen consumption by one helps anaerobes grow.

• Competitive Interactions:

  • Species may produce toxins to inhibit others.

Obtaining a Pure Culture

• Definition: A population derived from a single cell, essential for studying a specific microbial species.

• Techniques:

  • Aseptic techniques to avoid contamination.

  • Use of culture media (broth or solid) that contains necessary nutrients.

Growing Microorganisms on Solid Medium

• Essential to have the right conditions for individual cells to create visible colonies (~1 million cells).

• Agar Properties:

  • Not destroyed by high temperatures, solidifies below 45°C.

  • Effective for isolating pure cultures.

Streak-Plate Method

• Most common isolation technique.

• Involves spreading cells to achieve separation and individual colonies.

Maintaining Stock Cultures

• Streak-plate method allows pure cultures to be stored.

• Can be kept in a refrigerator or frozen at -70°C for long-term study.

Prokaryotic Growth Conditions

• Closed Systems:

  • Batch cultures with no nutrient renewal or waste removal.

• Open Systems:

  • Continuous cultures that allow consistent nutrient addition and waste removal.

The Growth Curve

• Stages:

  • Lag phase: no growth, metabolic preparation.

  • Log/exponential phase: rapid cell division.

  • Stationary phase: equilibrium between growth and death.

  • Death phase: viable cells decrease.

  • Prolonged decline: potential survival of some cells.

Colony Characteristics

• Differences exist between colonies based on environmental conditions.

• Outer edges have more nutrients and oxygen compared to the center.

Continuous Culture Systems

• Chemostat: Device that maintains continuous growth through regulated nutrient input and waste removal.

Environmental Factors Influencing Growth

• Extremophiles: Microbes thriving in harsh conditions.

• Key Conditions: Temperature, atmosphere, pH, water availability.

Temperature Requirements

• Each species has an optimal temperature range for growth:

  • Psychrophiles, psychrotrophs, mesophiles, thermophiles, hyperthermophiles.

Protein Thermostability

• Thermophilic proteins resist denaturing, crucial for survival in high temperatures.

Oxygen Requirements

• Reactive Oxygen Species (ROS):

  • Byproducts of aerobic respiration potentially harmful to cells.

  • Organisms have adapted mechanisms such as superoxide dismutase and catalase to mitigate damage.

pH Influence on Microbial Growth

• Most microbes prefer neutral pH (around 7).

• Microbial Adaptations: Neutrophiles, acidophiles, and alkaliphiles exist to thrive in varying pH levels.

Water Availability

• Water is essential for microbial growth; solute concentrations can impact cell health.

• Certain microbes thrive in high salt concentrations (halotolerant and halophiles), demonstrating varied survival strategies.