9.1 How Microbes Grow
5 min read•june 18, 2024
Microbes multiply rapidly through , doubling their population in a specific . This process follows distinct growth phases: lag, log, stationary, and death. Understanding these phases is crucial for controlling microbial growth in various settings.
Microbes can form , communicate through , and adapt to diverse environments. These abilities impact their survival, virulence, and resistance to control methods. Knowing how to manipulate environmental factors and apply control strategies is essential for managing microbial populations effectively.
Microbial Growth and Reproduction
Generation time in binary fission
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- Time required for a microbial population to double in size during
- Calculated using the formula:
- represents the
- represents the elapsed time
- represents the number of generations that occurred during the elapsed time
- Number of generations can be calculated using:
- represents the final population size after the elapsed time
- represents the initial population size at the beginning of the elapsed time
- Logarithms are used to simplify the calculation (base 2 due to binary fission)
Phases of microbial growth curves
- : slow growth as microbes adapt to new environment conditions
- Log (exponential) phase: rapid, exponential growth as microbes divide at maximum rate
- Cells double at regular intervals, leading to a logarithmic increase in population size
- : population growth slows and stops due to nutrient depletion and waste accumulation
- Rate of cell division equals rate of cell death, resulting in a stable population size
- : population declines as rate of cell death exceeds rate of new cell production
- Caused by depletion of essential nutrients and accumulation of toxic waste products
Methods for cell count determination
- Total cell count includes both living and dead cells in a microbial population
- Direct microscopic count using a counting chamber () to visually count cells
- Spectrophotometric methods measure the (cloudiness) of a culture to estimate total cells
- cell count includes only living cells capable of reproduction and colony formation
- method involves plating dilutions of a culture on nutrient agar and counting colonies
- Colonies represent (CFUs) that developed from single viable cells
- provides a statistical estimation of viable cells in a sample
- Based on the pattern of positive growth in a series of dilutions inoculated into liquid media
- method involves plating dilutions of a culture on nutrient agar and counting colonies
Binary fission vs other reproduction
- Binary fission is the most common form of bacterial reproduction
- Single parent cell divides into two genetically identical daughter cells
- Occurs rapidly, allowing bacterial populations to grow exponentially
- is an asymmetrical division where a small bud forms and separates from the parent cell
- Characteristic of some yeasts (Saccharomyces) and bacteria (Planctomycetes)
- involves the breaking of hyphae into smaller pieces, each capable of growing
- Seen in some fungi (molds) and filamentous bacteria ()
- Spore formation produces specialized reproductive structures resistant to environmental stresses
- are produced by some bacteria (Bacillus, Clostridium)
- Fungal spores are produced by fungi (mushrooms, molds)
Biofilms: formation and implications
- are communities of microorganisms attached to surfaces and embedded in
- EPS matrix provides protection and facilitates nutrient exchange
- Biofilm formation involves several stages:
- Attachment of cells to a surface
- Microcolony formation as cells divide and produce EPS
- Maturation of the biofilm with complex architecture and channels
- of cells from the biofilm to colonize new surfaces
- Biofilms exhibit increased resistance to antimicrobials and host defenses compared to planktonic cells
- EPS matrix acts as a barrier, limiting penetration of antimicrobial agents
- Biofilms are involved in various health problems:
- Chronic infections associated with medical devices (, implants)
- Dental plaque formation leading to tooth decay and gum disease
- Biofouling of industrial equipment and water distribution systems
Quorum sensing in microbial behavior
- is a form of cell-to-cell communication using signaling molecules called
- Allows bacteria to coordinate gene expression based on population density
- As the bacterial population grows, accumulate in the environment
- At a critical threshold concentration, autoinducers bind to receptors and trigger changes in gene expression
- Quorum sensing regulates various microbial behaviors:
- Bioluminescence in marine bacteria () for symbiosis with squid
- Virulence factor production in pathogens () for coordinated attack on host
- Biofilm formation and dispersal in response to environmental cues and population density
Strategies for microbial growth control
- Physical methods aim to remove or inactivate microbes:
- Heat sterilization using high temperatures ( at 121°C, incineration)
- Filtration to remove microbes from liquids (membrane filters) or gases ()
- Radiation using UV light to damage microbial DNA or gamma rays for sterilization
- Chemical methods use antimicrobial agents to kill or inhibit microbial growth:
- are applied to inanimate objects (alcohol, chlorine, quaternary ammonium compounds)
- are applied to living tissue (iodine, hydrogen peroxide, chlorhexidine)
- target specific microbial processes (cell wall synthesis, protein synthesis, DNA replication)
- Environmental control strategies manipulate conditions to prevent microbial growth:
- control (refrigeration, freezing) to slow microbial metabolism
- control (acidification, alkalinization) to create unfavorable conditions
- Moisture control (desiccation) to limit water availability for microbial growth ()
- Competitive exclusion using beneficial microbes () to outcompete pathogens
Environmental factors affecting microbial growth
- : Microbes need various nutrients for growth and reproduction
- Essential elements (carbon, nitrogen, phosphorus) and trace elements
- : organic compounds required by some microbes that cannot synthesize them
- Temperature: Different microbes have optimal growth temperatures
- Psychrophiles (cold-loving), mesophiles (moderate temperatures), thermophiles (heat-loving)
- pH: Most microbes grow best at neutral pH, but some tolerate acidic or alkaline conditions
- : Microbes can be classified based on their oxygen needs
- Aerobes (require oxygen), anaerobes (grow without oxygen), facultative anaerobes (can grow with or without oxygen)
Key Terms to Review (95)
Actinomycetes: Actinomycetes are a group of Gram-positive bacteria known for their filamentous growth and soil-dwelling nature. They are notable for producing numerous antibiotics.
Aerial filaments: Aerial filaments are filamentous structures that extend above the surface of a substrate, often produced by certain bacteria and fungi. These structures aid in the dispersal of spores or other reproductive units.
Antibiotics: Antibiotics are a class of antimicrobial drugs used to treat and prevent bacterial infections. They work by selectively targeting and disrupting essential processes in bacteria, effectively killing or inhibiting their growth, while generally having minimal impact on human cells.
Antiseptics: Antiseptics are chemical agents that inhibit or destroy the growth and development of microorganisms, particularly on living tissue. They are used to prevent infection and promote healing by reducing the risk of pathogenic microbial contamination in various medical and personal care applications.
Autoclaving: Autoclaving is a sterilization method that uses high-pressure steam to kill microorganisms, including spores, through the application of moist heat. This process is commonly used in laboratories, healthcare facilities, and industrial settings to ensure the sterility of equipment, materials, and solutions.
Autoinducers: Autoinducers are small signaling molecules produced by bacteria to communicate with each other in a process called quorum sensing. They regulate gene expression based on the density of the bacterial population.
Autoinducers: Autoinducers are signaling molecules produced and released by microorganisms that allow them to communicate and coordinate their behavior in a process known as quorum sensing. These molecules play a crucial role in regulating various microbial processes, including growth, virulence, and biofilm formation, in response to changes in population density.
Batch culture: Batch culture is a closed-system microbial growth method where nutrients are not added and waste products are not removed. Growth occurs in phases: lag, exponential, stationary, and death.
Binary fission: Binary fission is a form of asexual reproduction in which a single microbial cell divides into two genetically identical daughter cells. It is the primary method of reproduction for prokaryotic organisms such as bacteria.
Binary Fission: Binary fission is a type of asexual reproduction in which a single parent cell divides into two genetically identical daughter cells. This process is a fundamental characteristic of prokaryotic cells and a key mechanism by which microbes grow and proliferate.
Biofilms: Biofilms are complex communities of microorganisms that adhere to surfaces and are embedded within a self-produced matrix of extracellular polymeric substances. They provide protection to the bacteria from environmental stresses and antimicrobial agents.
Biofilms: Biofilms are complex communities of microorganisms, primarily bacteria, that adhere to surfaces and are encased in a self-produced extracellular matrix. These microbial communities exhibit unique characteristics and behaviors that distinguish them from free-floating, planktonic microbes.
Budding: Budding is a form of asexual reproduction in which a new individual organism develops from an outgrowth or bud on the parent. It is commonly observed in yeast and some bacteria.
Budding: Budding is a type of asexual reproduction in which a new individual organism develops from an outgrowth or bud on the parent organism. This process is commonly observed in certain microbes, such as yeast, where a daughter cell forms from the parent cell.
Calibration curve: A calibration curve is a graphical representation that shows the relationship between the concentration of a substance and its corresponding measurement signal. It is used to determine the concentration of an unknown sample by comparing it to a set of standard samples with known concentrations.
Candida albicans: Candida albicans is a dimorphic fungus that can exist in both yeast and hyphal forms. It is a common opportunistic pathogen in humans, causing infections primarily when the immune system is compromised.
Catheters: Catheters are medical devices inserted into the body to drain fluids or administer treatments. They can become a site for microbial growth, potentially leading to infections.
Chemostat: A chemostat is a bioreactor where the chemical environment is kept constant by continuously adding fresh medium and removing culture fluid. This allows for the study of microbial growth under steady-state conditions.
Closed culture: A closed culture is a microbial growth environment where no new nutrients are added, and waste products are not removed. This results in distinct growth phases including lag, log, stationary, and death phases.
Coliforms: Coliforms are a group of Gram-negative, non-spore-forming bacteria that are commonly found in the environment and in the feces of warm-blooded animals. They are often used as indicator organisms to assess water quality and potential contamination.
Colony-forming unit: Colony-forming unit (CFU) is a measure used to estimate the number of viable microorganisms in a sample. It represents individual cells or groups of cells that can form a colony when cultured under suitable conditions.
Colony-Forming Units: Colony-forming units (CFUs) are a measure of the number of viable bacteria or other microorganisms in a sample. They are used to quantify the concentration of microorganisms in a given volume or area, providing a way to assess the growth and proliferation of microbial populations.
Confocal microscopy: Confocal microscopy is an advanced optical imaging technique that increases optical resolution and contrast by using a spatial pinhole to block out-of-focus light. It enables detailed visualization of microbial structures in three dimensions.
Contact lenses: Contact lenses are thin, curved lenses placed directly on the surface of the eyes to correct vision or for cosmetic reasons. They can create a favorable environment for microbial growth if not properly maintained.
Coulter counter: A Coulter counter is an instrument used to count and size cells by detecting changes in electrical resistance. It is commonly used to measure microbial populations.
Culture density: Culture density refers to the number of microbial cells in a given volume of culture medium. It is commonly expressed as cells per milliliter (cells/mL).
Culture medium: A culture medium is a substance that provides the essential nutrients for microorganisms to grow. It can be solid, liquid, or semi-solid depending on its physical state.
Cyanobacteria: Cyanobacteria are a group of photosynthetic, gram-negative bacteria found in various aquatic environments. They play a significant role in oxygen production and nitrogen fixation.
Cystic fibrosis: Cystic fibrosis (CF) is a genetic disorder that affects the respiratory and digestive systems, caused by mutations in the CFTR gene. It leads to thick, sticky mucus production, which obstructs airways and promotes bacterial infections.
Death phase: The death phase is the final stage of microbial growth where the number of dying cells exceeds the number of new cells being produced. This results in a net decrease in the overall population of microbes.
Death Phase: The death phase, also known as the stationary phase, is the final stage of the microbial growth curve where the population of cells declines due to the depletion of nutrients and the accumulation of waste products. During this phase, the rate of cell division is equal to the rate of cell death, resulting in a stable or declining population.
Direct microscopic cell count: Direct microscopic cell count is a method to estimate the number of cells in a sample by observing them directly under a microscope. This technique often involves using a counting chamber or specialized slides.
Disinfectants: Disinfectants are chemical agents that are used to destroy or inhibit the growth of harmful microorganisms, such as bacteria, viruses, and fungi, on inanimate surfaces or objects. They play a crucial role in controlling microbial growth and preventing the spread of infectious diseases in various settings, including healthcare facilities, food processing plants, and households.
Dispersal: Dispersal is the process by which microorganisms spread from one location to another. This can occur through various mechanisms such as air, water, or host organisms.
Divisome: The divisome is a protein complex in bacterial cells responsible for cell division. It orchestrates the formation of the septum that separates the dividing cells.
Doubling time: Doubling time is the period required for a population of microorganisms to double in number. It is a critical parameter in understanding microbial growth dynamics.
Dry weight of a culture sample: Dry weight of a culture sample is the mass of microbial cells after removing all water content. It provides an accurate measure of cell biomass in a culture.
Electronic cell counting device: An electronic cell counting device, also known as an automated cell counter, is a laboratory instrument used to count and analyze the characteristics of cells in a sample. It uses electrical impedance or optical methods for precise measurements.
Endospores: Endospores are specialized, dormant structures formed by certain Gram-positive bacteria, such as Bacillus and Clostridium species, in response to environmental stressors. These resilient structures allow the bacteria to survive harsh conditions and remain viable for extended periods, even in the absence of nutrients or water.
Epulopiscium: Epulopiscium is a large, cigar-shaped bacterium commonly found in the intestines of certain fish. It is notable for its unique reproductive method and significant size compared to other bacteria.
Exponential growth phase: The exponential growth phase is a period in the microbial life cycle where the population size doubles at a constant rate. This phase occurs after the lag phase and before the stationary phase.
Exponential Phase: The exponential phase, also known as the log phase, is a critical stage in the growth cycle of microbes where they experience rapid and accelerated growth. This phase is characterized by an exponential increase in the number of microbial cells as they divide and multiply at their maximum rate, making it a crucial period for understanding microbial growth dynamics.
Extracellular Polymeric Substances: Extracellular polymeric substances (EPS) are high-molecular-weight compounds secreted by microorganisms into their surrounding environment. These substances play a crucial role in the growth and survival of microbial communities, particularly in the context of how microbes grow.
Extracellular polymeric substances (EPS): Extracellular polymeric substances (EPS) are a complex mixture of polymers secreted by microorganisms into their environment. EPS form a protective and adhesive matrix that supports biofilm structure and function.
Extrachromosomal DNA: Extrachromosomal DNA is DNA that exists outside the chromosomal DNA within a cell. It is commonly found in plasmids and can replicate independently of chromosomal DNA.
Fimbriae: Fimbriae are hair-like appendages present on the surface of many bacteria, primarily involved in adhesion to surfaces and other cells. They play a significant role in bacterial colonization and virulence.
Flagella: Flagella are long, whip-like structures that protrude from the cell body of certain prokaryotic and eukaryotic cells. They are primarily used for locomotion and can also serve sensory functions.
Fragmentation: Fragmentation is a form of asexual reproduction where a parent organism splits into fragments, each capable of growing independently into a new organism. It is common among certain bacteria and fungi.
Fragmentation: Fragmentation is the process by which a larger entity is broken down into smaller, distinct parts or fragments. In the context of microbial growth, fragmentation refers to the division or separation of a single microbial cell or organism into multiple independent cells or fragments, allowing for rapid reproduction and dispersal.
FtsZ: FtsZ is a protein that assembles into a ring at the future site of the septum in bacterial cell division. It is functionally analogous to tubulin in eukaryotes and is essential for cytokinesis.
Generation time: Generation time is the period it takes for a microbial population to double in number. It is a critical metric in understanding microbial growth and replication rates.
Generation Time: Generation time, also known as doubling time, is the time required for a population of microorganisms to double in number under optimal growth conditions. It is a fundamental concept in microbiology that describes the rapid reproductive capabilities of microbes and their ability to quickly adapt and proliferate in various environments.
Growth curve: A growth curve is a graphical representation of the change in the number of microbes in a population over time. It typically includes distinct phases: lag, log (exponential), stationary, and death.
Growth Factors: Growth factors are signaling molecules that stimulate cell growth, proliferation, and differentiation. They play a crucial role in the growth and development of microorganisms, as well as in the preparation of culture media for their cultivation.
Hansen’s disease: Hansen’s disease, also known as leprosy, is a chronic infectious disease caused by the bacterium Mycobacterium leprae. It primarily affects the skin, peripheral nerves, upper respiratory tract, and eyes.
Hemocytometer: A hemocytometer is a specialized counting chamber used to count cells in a given volume of liquid. It is commonly used in microbiology to estimate the concentration of microbes in a sample.
Hemocytometer: A hemocytometer is a specialized microscope slide used to count and determine the concentration of cells, such as blood cells or microorganisms, in a liquid sample. It consists of a thick glass slide with a defined counting grid etched into the surface, allowing for accurate cell enumeration.
HEPA Filters: HEPA (High-Efficiency Particulate Air) filters are a type of air filtration system designed to remove a high percentage of small airborne particles, including microbes, from the air. These filters are widely used in various settings, including hospitals, laboratories, and industrial facilities, to maintain clean and sterile environments.
Inoculum: Inoculum is the material used to initiate a microbial culture. It typically consists of cells or spores introduced into a growth medium.
Intrinsic growth rate: Intrinsic growth rate is the maximum rate at which a microbial population increases in size under optimal environmental conditions. It reflects the inherent capacity of the microbe to reproduce.
Lag phase: The lag phase is the initial period of microbial growth where cells adapt to their new environment before starting to divide. During this phase, there is little to no increase in cell number.
Lag Phase: The lag phase is the initial period of time when a microorganism is adapting to a new environment or growth conditions. During this phase, the cells are not actively dividing, but are instead preparing for rapid growth by synthesizing enzymes, building up cellular components, and repairing any damage that may have occurred during the previous growth conditions.
Logarithmic (log) growth phase: The logarithmic (log) growth phase is a period during which the population of microbes doubles at a constant rate. This phase follows the lag phase and is characterized by exponential growth.
Membrane filtration technique: Membrane filtration technique is a microbiological method used to separate and count microorganisms in a liquid sample by passing it through a membrane filter. The retained microbes are then cultivated on appropriate media for identification and enumeration.
Most Probable Number: The most probable number (MPN) is a statistical method used to estimate the number of microorganisms in a sample, particularly when the microbes are present in low numbers and cannot be easily counted directly. It is a crucial technique in the field of microbiology, especially in the context of understanding microbial growth and population dynamics.
Most probable number (MPN) method: The Most Probable Number (MPN) method is a statistical technique used to estimate the number of viable microorganisms in a sample. It is particularly useful for samples with low concentrations of bacteria.
Mycobacterium tuberculosis: Mycobacterium tuberculosis is a pathogenic bacterial species in the Mycobacteriaceae family, responsible for causing tuberculosis (TB). It primarily affects the lungs but can also infect other parts of the body.
Nutrient Requirements: Nutrient requirements refer to the essential nutrients that microbes need to grow and thrive. These include various organic and inorganic compounds that provide the building blocks and energy for microbial cellular processes and metabolism.
Oxygen Requirements: Oxygen requirements refer to the varying needs for molecular oxygen (O₂) among different microorganisms, which is essential for their growth and metabolic processes. This term is particularly important in the context of how microbes grow, as oxygen availability can significantly impact their ability to thrive and reproduce.
Persisters: Persisters are a subset of bacterial cells that can survive antibiotic treatment by entering a dormant state. They are not resistant to antibiotics but can resume growth once the treatment is stopped.
Petroff-Hausser chamber: A Petroff-Hausser chamber is a specialized microscope slide used for counting cells in a liquid sample. It features a grid etched into the surface to facilitate precise cell enumeration.
PH: pH, or potential of hydrogen, is a measure of the acidity or basicity of a solution. It is a scale that ranges from 0 to 14, with 7 being neutral. Solutions with a pH less than 7 are considered acidic, while solutions with a pH greater than 7 are considered basic or alkaline. The pH of a solution is a critical factor in the growth and survival of microorganisms, as it can significantly impact their metabolic processes and overall functioning.
Pili: Pili are hair-like appendages found on the surface of many bacteria that facilitate attachment to surfaces and genetic exchange. They are essential for processes like conjugation and biofilm formation.
Planktonic: Planktonic refers to microorganisms that are free-floating in a liquid environment, as opposed to being attached to a surface. These microbes can include bacteria, algae, protozoa, and other single-celled organisms.
Plate count: Plate count is a method used to estimate the number of viable microorganisms in a sample by counting the number of colonies that grow on an agar plate. It is commonly used to assess microbial load in various substances.
Pour plate method: The pour plate method is a microbiological technique used to isolate and quantify bacteria by diluting a sample and mixing it with molten agar before pouring it into a Petri dish. This allows colonies to grow within the medium, making it easier to count and identify individual colonies.
Probiotics: Probiotics are live microorganisms, typically bacteria, that are beneficial to human health, especially the digestive system. They are often referred to as the 'good' bacteria that can help maintain a healthy gut microbiome, support immune function, and promote overall wellbeing.
Pseudomonas aeruginosa: Pseudomonas aeruginosa is a Gram-negative, opportunistic pathogen that can cause serious infections in humans. It is known for its resistance to antibiotics and ability to thrive in various environments.
Quorum sensing: Quorum sensing is a cell-to-cell communication mechanism used by bacteria to coordinate group behaviors based on population density. It involves the production, release, and detection of chemical signal molecules called autoinducers.
Quorum Sensing: Quorum sensing is a communication mechanism used by bacteria to coordinate gene expression and behavior in response to changes in cell population density. It allows bacteria to sense and respond to the presence of other bacteria, enabling them to act in a coordinated manner as a community rather than as individual cells.
Septum: A septum is a partition or wall that forms in a cell, typically in the context of microbial cell division. It plays a crucial role in separating the two daughter cells during binary fission.
Serial dilution: Serial dilution is a stepwise dilution of a substance in solution, often used to reduce a dense culture of cells to a more usable concentration. It is frequently employed in microbiology to estimate the number of organisms present in a sample.
Sessile: Sessile refers to microorganisms that are attached to a surface and do not move. These microbes form biofilms and are often involved in chronic infections.
Spectrophotometer: A spectrophotometer is an analytical instrument used to measure the amount of light that a sample absorbs. It quantifies concentrations of compounds in solutions by measuring absorbance at specific wavelengths.
Spread plate method: The spread plate method is a microbiological technique used to isolate and count microorganisms by spreading a liquid sample evenly across an agar plate. This method allows for the growth and enumeration of individual colonies.
Stationary phase: The stationary phase is a period in the microbial growth curve where the rate of cell growth equals the rate of cell death. This results in a plateau in the number of viable cells.
Stationary Phase: The stationary phase is a crucial stage in the growth cycle of microorganisms, where the rate of cell division equals the rate of cell death, resulting in a stable population size. This phase is characterized by a plateau in the growth curve, indicating that the microbial population has reached its maximum capacity within the given environment.
Temperature: Temperature is a measure of the average kinetic energy of the particles in a substance, which determines the degree of hotness or coldness. It is a fundamental parameter that significantly influences the growth and behavior of microorganisms.
Turbidity: Turbidity is a measure of the cloudiness or haziness of a fluid caused by large numbers of individual particles. In microbiology, it is often used as an indicator of microbial growth in liquid culture media.
Turbidity: Turbidity is a measure of the cloudiness or haziness of a liquid, caused by the presence of suspended particles that scatter light. It is an important parameter in the context of microbial growth, as it can provide insights into the density and activity of microbial populations.
Viable: A microbe is considered viable if it can grow and reproduce under suitable environmental conditions. Viability indicates the ability to form colonies or increase in number.
Viable plate count: Viable plate count is a microbiological technique used to estimate the number of living bacteria in a sample by counting the number of colony-forming units (CFUs) that grow on an agar plate. It provides a quantitative measure of microbial population size.
Vibrio fischeri: Vibrio fischeri is a bioluminescent marine bacterium that engages in symbiotic relationships with various marine organisms, including the Hawaiian bobtail squid. It is a Gram-negative, curved rod-shaped bacterium that is part of the Proteobacteria phylum and is known for its ability to produce light through a process called quorum sensing.
Water Activity: Water activity is a measure of the availability of water in a system, which is a critical factor in determining the growth and survival of microorganisms. It is a dimensionless quantity that ranges from 0 to 1, with 0 representing a completely dry environment and 1 representing pure water.
Z ring: The Z ring is a protein structure composed mainly of FtsZ that forms at the future site of bacterial cell division. It plays a crucial role in cytokinesis by recruiting other proteins involved in cell wall synthesis and constriction.