Microbial Growth: Key Concepts and Methods

Microbial Growth

Phases of microbial growth in a closed system, Limited nutrients.

• Lag Phase: This phase has no increase in the number of bacteria.

• Logarithmic Phase: This phase has a rapid increase in the number of bacteria.

• Stationary Phase: This phase has same number of deaths as the same number of alive bacteria.

• Death Phase: This phase has more deaths

Phases of Microbial Growth

Microbial populations in batch culture exhibit characteristic growth phases, each with distinct metabolic activities.

• Lag phase: Adaptation, no increase in cell number.

• Log (exponential) phase: Rapid cell division and population growth.

• Stationary phase: Growth rate slows; cell death equals cell division.

• Death phase: Cell death exceeds new cell formation.

Metabolic activity: Highest during log phase; secondary metabolites often produced in stationary phase.

Classification of Microorganisms by Temperature Preference (Psycho phill Psyched My Temperate Horse)

Microorganisms can be classified into groups based on their preferred temperature ranges for optimal growth. Understanding these groups is essential for culturing microbes and controlling microbial activity in various environments. (

• Psychrophiles: Grow best at low temperatures (0–20°C).

• Mesophiles: Prefer moderate temperatures (20–45°C); most human pathogens are mesophiles.

• Thermophiles: Thrive at high temperatures (45–80°C).

• Hyperthermophiles: Grow optimally above 80°C.

• Psychrotrophs: Can grow at low temperatures but have higher optimal temperatures (15–30°C).

• Optimal temperature: Best temperature for growth.

• Minimum temperature: Lowest temperature for growth.

• Maximum temperature: Highest temperature for growth.

Example: Escherichia coli is a mesophile commonly found in the human gut.

pH and Microbial growth

• Neutrophiles: Grow best at a pH range of 5-8.

• Acidophiles: Grow best at a pH below 5.5.

• Alkaliphiles: Grow best at a pH above 8.5.

Osmotic Pressure and Microbial Growth

Osmotic pressure, influenced by salt and sugar concentrations, affects microbial growth by controlling water movement across cell membranes.

• High osmotic pressure (hypertonic environments) can cause plasmolysis, inhibiting growth.

• Extreme or Obligate Halophiles are adapted to high-salt environments.

• Facultative halophiles: Can tolerate high osmotic pressure.

• Importance: Used in food preservation (e.g., salting, sugaring) to inhibit microbial spoilage.

Chemical requirements for microbial growth.

• Carbon: Carbohydrates

• Nitrogen: Acids/proteins (beef extract)

• Sulfur

• Phosphorus: RNA, DNA, ATP

• Trace Elements

Fastidious (Picky bacteria)

Organic Growth Factors: Organic compounds that an organism is unable to biochemically produce, must be obtained from the environment.

Classification by Oxygen Requirements

Microorganisms are also classified based on their oxygen requirements, which determines their metabolic pathways and environmental niches.

• Obligate aerobes: Require oxygen for growth.

• Obligate anaerobes: Cannot tolerate oxygen.

• Facultative anaerobes: Can grow with or without oxygen but grow better with oxygen.

• Microaerophiles: Require low levels of oxygen.

• Aerotolerant anaerobes: Do not use oxygen but can tolerate its presence.

Key enzymes for oxygen detoxification: Superoxide dismutase (SOD), catalase, peroxidase.

Types of Culture Media

Agar: Complex polysaccharide made from seaweed (algae). Liquefies at 100 degrees Celius, remains liquid at 50 and solidifies at approximately 40 degrees.

Culturing microorganisms requires different types of media, each serving specific purposes in microbiological studies.

• Chemically defined media: Exact chemical composition is known.

• Complex media: Contains extracts (e.g., peptones, yeast extract); composition is not precisely known.

• Selectivity: Selective media favor the growth of specific microbes; differential media distinguish between organisms based on metabolic traits; enrichment media enhance the growth of particular organisms.

Biofilms

• Quorum sensing: The ability of a bacteria colony to sense its size and regulate its activity in response.

• Sewage treatment

• Primary line of defense against pathogens

• Catheters and prostheses

• Dental disease

• Inflammatory bowel disease

• Biosafety: BSL-1 no special precautions, BSL-2 lab coat, gloves, eye protection, BSL-3 biosafety cabinets to prevent airborne transmission, BSL-4 sealed, negative pressure.

Bacterial Growth and Binary Fission

Bacterial growth refers to an increase in the number of cells, primarily through binary fission, a form of asexual reproduction.

• Binary fission: One cell divides into two identical daughter cells.

• Generation time: The time required for a cell to divide and the population to double.

Formula for generation time:

$N = N_0 \times 2^n$

Where $N$ = final cell number, $N_0$ = initial cell number, $n$ = number of generations.

Obtaining pure cultures

• A colony: Is a population of cells arising from a single cell or spore or from a group of attached cells.

• Isolating colonies; streak plate method.

Preserving Bacteria Cultures

• Deep freezing

• Lyophilization (Freeze drying)

Methods of counting Bacteria

• Standard plant count: Reflects the number of viable microbes and assumes that each bacterium grows into a single colon. Uses serial dilutions (30-300 CFUs) *Formula: Original cell density (OCD) = #of colonies/ (dilution factor DF) (amount plated)

• Filtration: Is used to filter out and retain small quantities of bacteria on the surface of a filter. Used in water samples.

• Most Probable Number (MPN) Method: Can be used for microbes that will grow in a liquid medium, it is statistical estimation. Count positive tubes and compare to statistical MPN table.

• Direct Microscopic Count: Microbes in a measured volume of a bacteria suspension are counted with the use of a specially designed slide. Often used for human samples.

• Spectrophotometer: Used to determine turbidity by measuring the amount of light that passes through a suspension of cells. High turbidity- Low absorbance values.

Direct and Indirect Methods for Measuring Microbial Growth

Quantifying microbial populations is essential in microbiology. Methods are classified as direct or indirect.

• Direct methods: Count individual cells (e.g., plate counts, microscopic counts).

• Indirect methods: Estimate cell numbers based on turbidity, metabolic activity, or dry weight.

• Appropriate use: Direct methods are used when accuracy is required; indirect methods are faster for large populations.

Standard Plate Count and Viable Cell Calculation

The standard plate count is a direct method to estimate the number of viable bacteria in a sample.

• Procedure: Serially dilute the sample, plate aliquots, incubate, and count colonies.

• Calculation: Multiply the number of colonies by the dilution factor to determine colony-forming units (CFU) per milliliter.

Formula:

$\text{CFU/mL} = \frac{\text{Number of colonies} \times \text{Dilution factor}}{\text{Volume plated (mL)}}$