Unit 4: Microbial Growth and Reproduction - Subjective Questions

Microbial growth refers to an increase in the number of microbial cells in a population, rather than merely an increase in the size of an individual cell. In bacteria, growth usually occurs through binary fission, in which one parent cell divides to form two genetically similar daughter cells. Therefore, bacterial population growth is generally exponential under favorable conditions. In contrast, multicellular organisms grow mainly through an increase in cell size and cell number within an organized body. Important features of microbial growth include nutrient uptake, synthesis of cellular components, DNA replication, cell division, and an increase in the total population.

The bacterial growth curve in a batch culture typically contains four major phases:

• Lag phase: Cells adapt to the new environment, synthesize enzymes and cellular components, and repair damage. There is little or no increase in cell number.
• Log or exponential phase: Cells divide at a constant and maximal rate. The population doubles at regular intervals, and cells are metabolically highly active.
• Stationary phase: Nutrients become depleted and toxic metabolic products accumulate. The rate of cell division becomes equal to the rate of cell death, so the total population remains nearly constant.
• Death or decline phase: Viable cell numbers decrease because unfavorable conditions, nutrient exhaustion, and toxic waste accumulation cause cell death.

The length and characteristics of each phase depend on the organism, medium composition, temperature, pH, oxygen availability, and inoculum size.

During exponential growth, the number of cells increases by binary fission. If is the initial number of cells and is the number of cells after generations, then:

Taking logarithms:

Since , the number of generations is:

The generation time, , is the time required for one generation. If the total time is , then:

Thus, a shorter generation time indicates faster bacterial growth. For example, if a culture increases from to cells in one hour, then , and the generation time is approximately minutes.

Generation time is the time required for a bacterial population to double in number or for one cell to divide into two daughter cells. It varies considerably among bacterial species and environmental conditions.

Factors influencing generation time include:

• Nutrient availability: Rich media generally support faster growth.
• Temperature: Growth is fastest near the organism's optimum temperature.
• pH: Each bacterium has a preferred pH range.
• Oxygen availability: Aerobic, anaerobic, and facultative organisms respond differently to oxygen.
• Water activity and osmotic pressure: Excessive salt or sugar can slow growth.
• Accumulation of toxic products: Metabolic wastes inhibit cell division.
• Age and physiological state of the inoculum: Actively growing cells usually divide faster than old or damaged cells.
• Population density: Competition for nutrients and accumulation of inhibitory compounds can increase generation time.

Synchronous growth occurs when most cells in a microbial population divide at approximately the same time and pass through the cell cycle together. It is useful for studying DNA replication, cell division, gene expression, and cell-cycle events.

Methods for obtaining synchronous cultures include:

• Selection by cell size: Cells of a similar size or age are separated by filtration or density-gradient centrifugation.
• Mechanical selection: Cells are collected immediately after division, when newly formed daughter cells are released.
• Induction by environmental change: A sudden change in temperature or nutrient availability can synchronize cell-cycle events.
• Chemical inhibition and release: A substance temporarily blocks a particular stage of the cell cycle; removal of the inhibitor allows cells to proceed together.

Synchronization is usually temporary because individual cells gradually lose synchrony due to differences in metabolism and division timing.

Batch culture is simple and widely used, whereas continuous culture provides greater control but requires careful regulation to prevent contamination and washout.

A chemostat is a continuous-culture system in which fresh medium enters a vessel at a constant rate and an equal volume of culture leaves, maintaining a constant culture volume. A limiting nutrient in the incoming medium controls microbial growth.

The dilution rate is defined as:

where is the flow rate of fresh medium and is the culture volume. At steady state, the specific growth rate of the organism is approximately equal to the dilution rate:

If the dilution rate is increased beyond the organism's maximum specific growth rate, cells cannot reproduce rapidly enough to replace the cells leaving the vessel. This condition is called washout, and the cell concentration falls greatly or approaches zero. Chemostats are used to study growth kinetics, nutrient limitation, and industrial microbial production.

The turbidity method estimates microbial growth by measuring the cloudiness produced by cells suspended in a liquid medium. A spectrophotometer measures optical density, commonly at a wavelength such as .

• Principle: More cells scatter more light, producing a higher optical density.
• Procedure: A blank containing uninoculated medium is used to set the instrument, and the optical density of the culture is then measured.
• Advantages: It is rapid, nondestructive, and suitable for repeated measurements during a growth experiment.
• Limitations: It does not distinguish living cells from dead cells, and very dense cultures may require dilution. The method is less sensitive at very low cell concentrations and requires a calibration curve to convert optical density into cell number or biomass.

Thus, turbidity is especially useful for monitoring changes in population density over time.

In a direct microscopic count, a measured volume of microbial suspension is placed on a counting chamber such as a Petroff–Hausser chamber or a hemocytometer. Cells in a known grid volume are counted under a microscope, and the number of cells per milliliter is calculated.

If cells are counted in a chamber volume from a dilution factor , then:

Advantages:

• Rapid and relatively simple.
• Does not require incubation.
• Useful for organisms that grow poorly on artificial media.

Disadvantages:

• Living and dead cells are counted together unless a viability stain is used.
• Small cells may be difficult to observe.
• Motile cells can complicate counting.
• It is less accurate at low cell concentrations and when cell clumping occurs.

The viable plate count estimates the number of living microorganisms capable of forming colonies. A sample is serially diluted, a measured volume is spread or poured onto suitable agar, and the plates are incubated until colonies develop.

The count is expressed as colony-forming units per milliliter (CFU/mL):

For example, if 85 colonies grow from of a dilution:

Only plates containing a countable number of colonies, commonly about 30–300, should be used. The method measures viable cells, but one colony may arise from a cluster of cells, so the result is reported as CFU rather than exact cell number.

• Viable plate count: Measures living cells capable of forming colonies. It is useful for determining CFU/mL but requires incubation and may underestimate cells that are viable but nonculturable.
• Direct microscopic count: Measures cells directly in a counting chamber. It is rapid but generally counts living and dead cells together.
• Turbidity measurement: Estimates cell concentration from light scattering. It is rapid and nondestructive but requires calibration and cannot distinguish viable from nonviable cells.
• Cellular dry weight: Cells are harvested, washed, dried, and weighed. It provides a direct estimate of biomass and is useful for dense cultures, but it is time-consuming and insensitive for dilute cultures.

The best method depends on whether the objective is to measure viable cell number, total cell number, optical density, or total biomass.

Temperature affects bacterial growth by influencing enzyme activity, membrane fluidity, nutrient transport, and the stability of cellular macromolecules. Each species has a minimum, optimum, and maximum growth temperature.

• Psychrophiles: Prefer low temperatures, usually with an optimum near or below .
• Psychrotrophs: Can grow at refrigeration temperatures but usually have an optimum above .
• Mesophiles: Grow best at moderate temperatures, commonly between approximately and . Many human pathogens are mesophiles.
• Thermophiles: Prefer elevated temperatures, approximately –.
• Hyperthermophiles: Grow optimally at temperatures above approximately .

Low temperatures slow enzymatic reactions, whereas excessively high temperatures denature proteins and damage membranes.

pH affects bacterial growth by influencing enzyme activity, membrane transport, proton gradients, and the stability of cellular structures. Most bacteria grow best near neutral pH, although some are adapted to acidic or alkaline environments.

• Acidophiles: Grow optimally at low pH, commonly below . They maintain an internal pH that is higher than the surrounding environment.
• Neutrophiles: Prefer approximately neutral conditions, generally around pH –.
• Alkaliphiles: Grow best at alkaline pH, often above pH .

Bacteria maintain internal pH through proton pumps, ion-exchange systems, buffering compounds, and changes in membrane permeability. Extreme pH can denature proteins, disrupt transport systems, and damage nucleic acids.

Oxygen affects bacteria because it can serve as a terminal electron acceptor but can also generate toxic reactive oxygen species. Bacteria differ in their ability to use oxygen and detoxify compounds such as superoxide and hydrogen peroxide.

• Obligate aerobes: Require oxygen for growth and use aerobic respiration.
• Obligate anaerobes: Are harmed or killed by oxygen because they lack sufficient detoxifying enzymes.
• Facultative anaerobes: Use oxygen when available but can grow by fermentation or anaerobic respiration in its absence.
• Aerotolerant anaerobes: Do not use oxygen but tolerate its presence.
• Microaerophiles: Require oxygen at concentrations lower than atmospheric levels.

Important protective enzymes include superoxide dismutase, catalase, and peroxidase. The distribution of growth in a thioglycollate tube can help identify oxygen requirements.

Water is essential for microbial metabolism, transport, and biochemical reactions. The availability of water is described by water activity, represented as . When the surrounding environment has a high concentration of dissolved salts or sugars, water moves out of the bacterial cell by osmosis.

• Loss of water can cause plasmolysis, in which the cytoplasmic membrane pulls away from the cell wall.
• High salt or sugar concentrations reduce water activity and inhibit the growth of many bacteria.
• Halophiles require high salt concentrations for growth.
• Halotolerant organisms do not require salt but can tolerate relatively high salt concentrations.
• Some bacteria accumulate compatible solutes to maintain water balance without disrupting cellular reactions.

This principle explains why salting, sugaring, and drying can help preserve food by restricting bacterial growth.

Bacterial growth depends on the availability of carbon, nitrogen, sulfur, phosphorus, minerals, trace elements, water, and, in some organisms, growth factors. A shortage of any essential nutrient can limit biosynthesis and reduce the growth rate.

• Carbon and energy sources are required for cellular material and ATP production.
• Nitrogen, sulfur, and phosphorus are needed for proteins, nucleic acids, phospholipids, and other macromolecules.
• Trace elements act as enzyme cofactors.
• Growth factors such as vitamins, amino acids, or purines may be required by nutritionally demanding bacteria.

Inhibitory substances, including antibiotics, disinfectants, heavy metals, organic acids, and accumulated metabolic wastes, can interfere with enzymes, membranes, DNA, or protein synthesis. The effect may be bacteriostatic, stopping growth, or bactericidal, causing cell death.

Transformation is the uptake and incorporation of naked, extracellular DNA by a competent bacterial cell. It is a form of horizontal gene transfer.

Major steps include:

• DNA release: Donor cells may lyse and release DNA into the environment.
• Competence development: A recipient cell develops the ability to bind and take up extracellular DNA.
• DNA binding and uptake: DNA attaches to the cell surface and enters the cell, often as a single strand.
• Processing and recombination: The incoming DNA may integrate into the chromosome through homologous recombination or persist as a plasmid.
• Expression: If the acquired genes are functional, the recipient displays a new characteristic, such as antibiotic resistance.

Transformation may occur naturally in bacteria such as Streptococcus pneumoniae, Bacillus subtilis, and Neisseria species, or it may be induced artificially in the laboratory.

Transduction is the transfer of bacterial genetic material from one cell to another through a bacteriophage, which is a virus that infects bacteria.

• Generalized transduction: During the lytic cycle, a phage accidentally packages a random fragment of bacterial DNA instead of phage DNA. When this transducing particle infects another bacterium, the donor DNA may recombine with the recipient chromosome. Any bacterial gene may potentially be transferred.
• Specialized transduction: A temperate phage integrates into a specific site in the bacterial chromosome. If it later excises incorrectly, it may carry adjacent bacterial genes with it. These specific genes are transferred to a new recipient cell.

Thus, generalized transduction transfers random bacterial genes, whereas specialized transduction transfers genes located near the prophage integration site.

Conjugation is the direct transfer of DNA between bacterial cells through cell-to-cell contact. It commonly involves a donor cell carrying a conjugative plasmid and a recipient cell lacking that plasmid.

• An F donor contains the fertility factor, or F plasmid.
• The F plasmid carries genes for production of the sex pilus and proteins required for DNA transfer.
• The sex pilus attaches to an F recipient cell and brings the cells together.
• A nick is made in one strand of the F plasmid DNA.
• A single DNA strand is transferred while complementary strands are synthesized in both cells.
• After transfer, both cells generally become F.

Conjugation can also transfer antibiotic-resistance plasmids and other adaptive genes. It is an important mechanism for the rapid spread of resistance among bacterial populations.

All three processes increase genetic variation and can transfer genes for virulence, metabolism, or antimicrobial resistance. They differ mainly in the source of DNA and the mechanism of delivery.