Bacterial Growth and Culturing

Chapter 6: Bacterial Growth, Nutrition, and Differentiation

• Nutrition and environmental factors impact microbial growth and differentiation.
• Microbial classifications are based on nutritional needs and environmental limits.
• Understanding microbial growth helps identify disease-causing pathogens.
• Biofilms' significance in infectious diseases is important.

Culturing and Counting Bacteria

• Pure cultures are essential in medicine.
• Synthetic, complex, selective, and differential media types are used in clinical microbiology.
• Bacterial growth is measured in various ways; each method has its advantages and disadvantages.

Growing Bacteria in Culture

• Culture involves growing bacteria, considering nutrition, growth temperature, pH, pressure, osmotic balance, and oxygen levels.
• Solid media facilitates colony formation by bacterial cells with agar added to create a firm surface.
• The streak plate method aids in separating colonies to obtain pure cultures.
• Selective media is used.
• Differential media is used.
• MacConkey Agar contains selective agents like bile salts and crystal violet, and lactose as a differential agent.

Determining the Number of Cells (Dead or Alive)

• To determine the number of cells, one can preform direct or indirect cell counts.
• Viable cell counts can be done by serial dilution and spread plate method.
• In a case history, urine samples from healthy patients usually have fewer than 10,000 bacteria per milliliter.

The Growth Cycle

• The growth rate measures the number of cells in a population over time.
• Binary fission occurs.
• There is symmetrical and asymmetrical division
• The bacterial growth curve shows changes in growth rate over time.
• During the lag phase, bacteria prepare their cell machinery for growth.
• The log phase exhibits exponential growth.
• In the stationary phase, cells cease growth and activate stress responses to maintain viability.
• The death phase is when cells begin to die at an exponential rate.
• Population size doubles at a fixed rate during exponential growth, resulting in an exponential curve when plotted.
• Generation or doubling time varies with organism type, medium, and environmental factors.

Environmental Limits on Microbial Growth

• Environmental changes can alter the shape of a growth curve.
• Microbes are classified by environmental niches (pH, temperature, and salt).
• Extremophiles are growth considerations to environmental limits.
• Growth considerations: temperature, pressure, osmotic balance, pH level, nutrients.
• Psychrophiles grow in the cold.
• Mesophiles grow in midrange temperatures.
• Thermophiles grow in the heat.
• Hyperthermophiles grow in extremely hot temperatures.
• Barophiles can survive at high barometric pressure.
• Halophiles thrive in high osmotic pressure.
• All known halophiles are Archaea.
• Neutralophiles are most human pathogens.
• Acidophiles grow in low pH.
• Alkaliphiles grow in high pH.

Living with Oxygen

• Anaerobes and aerobes are differentiated, and how they are cultivated.
• Aerobes and anaerobes can cause disease.
• The basic differences between respiration and fermentation affect where an organism grows.
• Strict aerobes require oxygen.
• Strict Anaerobes cannot survive with oxygen.
• Microaerophiles need small amounts of oxygen.
• Aerotolerant anaerobes do not use oxygen but can survive.
• Facultative anaerobes can survive with or without oxygen.

Microbial Communities and Cell Differentiation

• Biofilms develop and the role of quorum sensing in the process.
• Biofilms are important to infection.
• Sporulation occurs and spores impact certain infections.
• Biofilms are surface-attached communities of single or multiple species, such as dental plaques, Pseudomonas aeruginosa and Staphylococci.
• Cells communicate and coordinate actions through quorum sensing.
• Endospores are seen in some Gram-positive bacteria.
• Endospores are dormant and can last for decades or centuries.
• Bacillus, such as B. anthracis and Clostridium, such as C. botulinum and C. tetani form endospores.