Microbial growth is influenced by various requirements which are broadly categorized into:
Physical Requirements
Temperature
Minimum growth temperature
Optimum growth temperature
Maximum growth temperature
pH
Osmotic Pressure
Chemical Requirements
Carbon
Nitrogen, Sulfur, and Phosphorus
Trace Elements
Oxygen
Organic Growth Factors
Organisms exhibit distinct temperature preferences:
Psychrophiles: Cold-loving; can grow below 20°C; found in polar regions and oceans.
Psychrotrophs: Grow at 0°C to 30°C; significant in food spoilage.
Mesophiles: Prefer temperatures between 20°C and 45°C; includes pathogens and normal microbiota.
Thermophiles: Optimal growth at 45°C to 80°C; found in hot springs and composts.
Hyperthermophiles: Optimal growth above 80°C; thrive in extreme heat environments.
Bacterial growth optimal between pH 6.5 and 7.5.
Molds and yeasts prefer lower pH (5-6).
Certain preservation methods exploit acid production by bacteria (e.g., in sauerkraut and pickles).
Acidophiles thrive in acidic environments.
Hypertonic environments lead to plasmolysis (cell shrinkage).
Extreme halophiles require high salt concentrations (up to 30% NaCl).
Facultative halophiles tolerate salt concentrations (2-10% NaCl).
Fundamental as a backbone for organic molecules.
Chemoheterotrophs: use organic molecules as carbon and energy sources.
Autotrophs: utilize carbon dioxide as their carbon source.
Essential for proteins and nucleic acids (DNA, RNA).
Many bacteria decompose proteins to obtain nitrogen.
Some utilize nitrogen from organic compounds, and a few can perform nitrogen fixation.
Required for producing amino acids, thiamine, and biotin.
Acquired through decomposing proteins or directly from organic sources.
Key element in DNA, RNA, ATP, and membranes (phospholipids).
Inorganic elements needed in small amounts; serve as enzyme cofactors.
Include iron, copper, molybdenum, and zinc.
Types of Bacteria based on oxygen requirement:
Obligate aerobes: Require oxygen.
Facultative anaerobes: Grow with or without oxygen, utilizing respiration or fermentation.
Anaerobes: Cannot use oxygen; often harmed by it.
Aerotolerant anaerobes: Tolerate oxygen but do not use it.
Microaerophiles: Require lower oxygen levels than atmospheric concentration.
Organic compounds necessary for growth, including vitamins, amino acids, purines, and pyrimidines.
Communities of microbes that form slime or hydrogels on surfaces.
Utilize quorum sensing for communication and nutrient sharing.
Provide protection against environmental factors and are involved in approximately 70% of infections (e.g., medical devices, dental issues).
Culture Medium: Nutrients prepared for microbial growth.
Sterile Media: Free from living microbes.
Inoculum: Introduction of microbes to the medium.
Agar: A solidifying agent derived from algae; used in Petri plates.
Chemically Defined Media: Exact chemical composition is known.
Complex Media: Composition varies; contains extracts from yeast or plants.
Selective Media: Encouage growth of certain microbes, suppressing others.
Differential Media: Differentiate between microbial colonies based on metabolic activity.
Enrichment Culture: Enhances growth of specific organisms from low initial concentrations.
A pure culture originates from a single species or strain, often isolated using the streak plate method.
Growth represented by an increase in the number of cells (not size).
Binary Fission: Most common method of bacterial division.
Budding: Used by some species, primarily non-pathogenic.
Bacterial growth is observed in phases:
Lag Phase: Initial adjustment; no increase in numbers but high metabolic activity.
Log Phase: Rapid population increase; constant generation time.
Stationary Phase: Growth slows; nutrient depletion and waste accumulation.
Death Phase: Cell death exceeds division; population decreases logarithmically.
Plate Count: Counting colonies from diluted samples.
Filtration: Bacteria collected on a membrane filter, then cultured.
Direct Microscopic Count: Utilizes a Petroff-Hausser cell counter.
Turbidity: Measure cloudiness with a spectrophotometer.
Metabolic Activity: Measure production of metabolic products.
Dry Weight: Used for filamentous organisms after drying and weighing.