Unit 8: Effect of physical and chemical environment on growth - Subjective Questions

Bacterial growth is the increase in the number of bacterial cells in a population, rather than merely an increase in cell size. It usually occurs through binary fission.\n\nMethods for measuring growth include:\n- Direct microscopic count: Cells are counted using a counting chamber, although living and dead cells may not be distinguished.\n- Viable plate count: A diluted culture is plated on solid medium, and colonies are counted. Results are expressed as colony-forming units per millilitre, or .\n- Turbidity measurement: Optical density is measured with a colorimeter or spectrophotometer. Increased turbidity generally indicates increased cell mass.\n- Determination of dry weight: Cells are harvested, dried, and weighed.\n- Metabolic measurements: Changes in oxygen consumption, acid production, or other metabolic products can indicate growth.\n\nThe selected method depends on whether total cell number, viable cell number, or biomass is required.

A typical bacterial growth curve contains four major phases:\n\n- Lag phase: Cells adapt to the new environment, synthesize enzymes and cellular components, and show little or no increase in number.\n- Log or exponential phase: Cells divide at a constant maximum rate. The population increases geometrically, and cells are usually most metabolically active.\n- Stationary phase: Nutrients become limited and toxic waste products accumulate. The rate of cell division becomes approximately equal to the rate of cell death.\n- Death or decline phase: Viable cell numbers decrease because unfavorable conditions become severe.\n\nTemperature and carbon source can alter the length and slope of these phases. A suitable temperature and usable carbon source generally shorten the lag phase and increase the rate of exponential growth.

Temperature affects bacterial growth by influencing enzyme activity, membrane fluidity, transport processes, and the stability of cellular structures.\n\n- At low temperatures, enzymatic reactions slow down and membrane transport becomes less efficient, so growth is reduced.\n- At the optimum temperature, enzymes function efficiently, membranes have suitable fluidity, and growth occurs at the maximum rate.\n- At high temperatures, proteins denature, membranes become damaged, and essential cellular components lose their function. Growth eventually stops and cells may die.\n\nEach bacterial species has a minimum, optimum, and maximum growth temperature. The difference between these temperatures reflects the organism's physiological adaptation to its environment.

The three cardinal temperatures are defined as follows:\n\n- Minimum growth temperature: The lowest temperature at which a bacterium can reproduce. Below this temperature, metabolic activity is too slow for growth.\n- Optimum growth temperature: The temperature at which the bacterium grows at its highest rate.\n- Maximum growth temperature: The highest temperature at which growth can occur. Above this temperature, irreversible damage to proteins, membranes, or nucleic acids usually occurs.\n\nThese temperatures are characteristic of a bacterial species but can vary with pH, nutrient availability, oxygen concentration, and incubation conditions.

Bacteria can be grouped according to their preferred growth temperatures:\n\n- Psychrophiles: Prefer very low temperatures, commonly around to . They occur in polar regions and cold waters.\n- Psychrotrophs: Can grow at refrigeration temperatures but have an optimum generally above that of psychrophiles. They are important in food spoilage.\n- Mesophiles: Grow best at moderate temperatures, approximately to . Many human pathogens, including Escherichia coli, are mesophiles.\n- Thermophiles: Prefer high temperatures, approximately to .\n- Hyperthermophiles: Grow optimally at temperatures above approximately .\n\nThe exact temperature ranges may vary among textbooks and species.

Procedure:\n1. Prepare sterile nutrient broth or another suitable growth medium.\n2. Inoculate equal volumes of the same bacterial suspension into several sterile tubes.\n3. Incubate the tubes at different temperatures, such as , , , and .\n4. Keep inoculum size, medium composition, volume, incubation time, and aeration constant.\n5. Measure growth at regular intervals using turbidity, optical density, or viable plate counts.\n6. Include an uninoculated medium as a sterility control.\n7. Plot growth against temperature and identify the temperature giving the greatest growth rate.\n\nInterpretation: Little growth at low temperature indicates reduced metabolic activity, while little or no growth at excessive temperature may indicate enzyme denaturation or cell death. The temperature producing maximum growth is the experimental optimum.

Temperature changes the kinetic energy of molecules and therefore affects the frequency and effectiveness of molecular collisions. As temperature rises within the tolerated range, enzyme-catalyzed reactions become faster because substrates collide more frequently with enzymes. This increases metabolic activity and growth.\n\nBeyond the optimum temperature, weak bonds maintaining protein structure are disrupted. Enzymes may denature, so their active sites no longer bind substrates correctly. High temperature can also damage ribosomes, nucleic acids, and membrane proteins. At low temperatures, enzymes are not usually denatured, but their reaction rates decrease and membrane transport becomes slower. Therefore, growth follows a temperature-dependent pattern with minimum, optimum, and maximum values.

Low temperature:\n- Slows enzyme-catalyzed reactions.\n- Reduces nutrient transport across the membrane.\n- Decreases metabolic rate and cell division.\n- May cause temporary dormancy, but usually does not kill all cells.\n\nHigh temperature:\n- Increases reaction rates initially.\n- Causes protein denaturation above the tolerance limit.\n- Damages membranes and increases leakage of cellular contents.\n- May damage nucleic acids and ribosomes.\n- Can cause irreversible cell death.\n\nThus, low temperature mainly produces reversible inhibition of metabolism, whereas excessive high temperature commonly causes structural and functional damage.

A carbon source supplies carbon atoms required for the synthesis of cellular materials. Bacteria use carbon to produce:\n\n- Carbohydrates and cell-wall components.\n- Lipids and cell membranes.\n- Amino acids and proteins.\n- Nucleotides and nucleic acids.\n- Energy-producing metabolites.\n\nSome bacteria use organic carbon sources, such as glucose, lactose, acetate, or peptone. Others use carbon dioxide as their principal carbon source and are called autotrophs. The ability of a bacterium to use a particular compound depends on transport systems and the enzymes needed to metabolize it. A suitable carbon source generally supports faster growth and greater biomass production.

Autotrophic bacteria:\n- Use inorganic carbon, mainly carbon dioxide, as their principal carbon source.\n- Synthesize organic cellular compounds from carbon dioxide.\n- May obtain energy from light or from oxidation of inorganic substances.\n- Examples include photosynthetic and chemolithotrophic bacteria.\n\nHeterotrophic bacteria:\n- Obtain carbon from preformed organic compounds.\n- Use substances such as sugars, organic acids, amino acids, or proteins.\n- Include many decomposers, commensals, and pathogenic bacteria.\n\nThe distinction is based primarily on the source of cellular carbon, not on whether the organism requires oxygen.

Experimental design:\n1. Prepare a defined basal medium containing essential salts, nitrogen, minerals, and vitamins, but no carbon source.\n2. Divide the medium into sterile flasks or tubes.\n3. Add equal concentrations of different carbon sources, such as glucose, lactose, sucrose, acetate, and glycerol.\n4. Include a negative control without a carbon source.\n5. Inoculate every flask with the same amount of a pure bacterial culture.\n6. Incubate all cultures at the organism's optimum temperature for the same duration.\n7. Measure growth by optical density or viable plate count.\n8. Compare the results and identify the carbon source supporting maximum growth.\n\nAll other variables must remain constant. Growth in a medium indicates that the bacterium can use the supplied compound under the experimental conditions.

Glucose is often a preferred carbon and energy source because many bacteria possess efficient transport systems and metabolic pathways for its utilization. It can enter central metabolic pathways such as glycolysis and be converted into pyruvate, generating ATP and reducing power. Biosynthetic intermediates derived from glucose can also be used to synthesize cellular components.\n\nOther carbon sources may require additional enzymes, transport proteins, or metabolic steps before they can be used. Consequently, they may produce a longer lag phase or a lower growth rate. However, glucose is not universally superior; the result depends on the species, environmental conditions, and regulatory mechanisms controlling metabolism.

Diauxic growth is a two-stage growth pattern observed when bacteria are supplied with two carbon sources, usually one preferred and one less preferred source.\n\n- During the first phase, the bacterium consumes the preferred carbon source, such as glucose, and grows rapidly.\n- When the preferred source is exhausted, growth may temporarily stop, producing a diauxic lag.\n- The cells then synthesize or activate enzymes required to metabolize the second carbon source.\n- Growth resumes at a different rate during the second phase.\n\nThis phenomenon results from catabolite repression, in which the preferred carbon source suppresses expression of genes needed for utilization of alternative substrates.

A defined medium has a precisely known chemical composition and contains measured amounts of each nutrient. It is useful for carbon-source experiments because:\n\n- The test carbon source is the only major variable being changed.\n- Hidden carbon sources present in peptone, yeast extract, or complex ingredients are avoided.\n- Growth differences can be attributed more confidently to the ability to use the tested compound.\n- Reproducibility between experiments is improved.\n- The concentration of carbon can be controlled accurately.\n\nA basal medium should provide nitrogen, minerals, salts, water, and any required growth factors, while the experimental carbon source is added separately.

Turbidity measurement:\n- Measures light scattering by cells using a colorimeter or spectrophotometer.\n- Rapid and non-destructive.\n- Suitable for repeated measurements during a growth experiment.\n- Measures both living and dead cells.\n- Requires a calibration relationship between optical density and cell concentration.\n\nViable plate count:\n- Measures cells capable of forming colonies.\n- Results are reported as .\n- Distinguishes viable reproductive cells from nonviable cells more effectively.\n- Requires serial dilution, plating, incubation, and colony counting.\n- Is slower and may underestimate cells that are alive but unable to grow under the selected conditions.\n\nThe two methods may produce different results because turbidity measures total particulate biomass, whereas plate counts estimate viable culturable cells.

During exponential growth, the number of cells follows:\n\n\n\nwhere is the initial cell number, is the cell number after time , and is the number of generations. Taking logarithms gives:\n\n\n\nIf is the generation time, then the number of generations in time is:\n\n\n\nTherefore,\n\n\n\nThe specific growth rate is commonly written as:\n\n\n\nSince , a shorter generation time corresponds to a higher growth rate. Temperature and carbon source can change both and .

Temperature and carbon source are not completely independent factors. Temperature determines the activity of enzymes and transport proteins required to obtain and metabolize the carbon source. A bacterium may grow well on a particular carbon source at its optimum temperature but poorly at a lower temperature because transport and enzymatic reactions are slowed. At excessively high temperature, the enzymes required for carbon metabolism may be damaged.\n\nThe carbon source can also influence the observed temperature response because different substrates require different enzymes and pathways. Therefore, a complete experiment should test several temperatures with each carbon source and compare growth rates using a matrix or factorial design. The best condition is the combination that produces the greatest reproducible growth under controlled conditions.

For reliable results, the following variables should be controlled:\n\n- Bacterial species or strain and purity of the inoculum.\n- Initial cell concentration and inoculum volume.\n- Composition, pH, and volume of the growth medium.\n- Concentration and chemical form of the carbon source.\n- Incubation time and aeration or shaking rate.\n- Culture vessel, light exposure, and humidity when relevant.\n- Sterilization conditions and aseptic technique.\n- Method and time of growth measurement.\n- Number of replicates.\n- Presence of positive and negative controls.\n\nControlling these factors ensures that differences in growth can be attributed mainly to temperature or carbon source.

Positive control: A culture grown under conditions already known to support growth. It confirms that the organism is viable, the medium is suitable, and the incubation procedure works.\n\nNegative control: An uninoculated medium or a medium lacking an essential nutrient, depending on the experiment. It detects contamination or non-biological changes such as precipitation and color changes.\n\nFor a carbon-source experiment, a medium containing a known usable carbon source can serve as a positive control, while basal medium without any carbon source can serve as a negative control. Controls make the experimental findings more reliable and help identify technical errors.

The results suggest that the organism is adapted to moderate temperatures and is probably a mesophile.\n\n- Highest growth at indicates that this temperature is close to its optimum under the experimental conditions.\n- Moderate growth at indicates that metabolic reactions continue but occur more slowly.\n- Absence of growth at suggests that the temperature is above the organism's maximum growth temperature and may have caused protein denaturation or membrane damage.\n\nThe result does not by itself prove that all cells were killed at . A subculture into fresh medium could determine whether the effect was bacteriostatic or bactericidal.