Factors Affecting Microbial Growth PDF

FACTORS AFFECTING MICROBIAL GROWTH T. Falconer/G.Mignot  Prokaryotes reproduce asexually by cell division called binary fission Many bacteria divide using binary fission. BINARY Binary Fission: Results in the separation of a single cell i...

FACTORS AFFECTING MICROBIAL GROWTH T. Falconer/G.Mignot  Prokaryotes reproduce asexually by cell division called binary fission Many bacteria divide using binary fission. BINARY Binary Fission: Results in the separation of a single cell into two identical daughter cells each containing at least one FISSION copy of the parental DNA (clone) Binary fission is the evolutionary precursor to mitosis Steps involved IN BINARY FISSION: Elongation  Doubling Time: (or Generation time) is the period required for cells in a microbial population to enlarge, divide and produce two daughter cells for each one that existed before. Growth of bacterial populations is seen by geometric progression, or exponential, 2n, where n= number of generations. Binary Fission  A low doubling time indicates rapid growth Exponential growth will continue if adequate nutrients are available and toxic waste products are at tolerable levels When bacterial growth is monitored in freshly inoculated culture medium over a period of time, the population goes through different phases of growth.  Bacterial variants will grow in environments unfavourable to parent strain. Bacterial variants 2 conflicting hypotheses : the variants arise spontaneously the environment produces the during growth under normal observed change (adaptation ) conditions, (spontaneous mutation)  Variations - change in bacterial genotype or phenotype. Bacterial variations 1. Transformation 2. Transduction 3. Conjugation. BACTERIAL GROWTH CURVE Phases of growth Bacteria take time to adjust to the new environmental Lag phase- population conditions to which they remains temporarily have been introduced (pH, unchanged. temperature, nutrients, etc.). Exponential growth phase- cells are dividing by binary fission and are growing by geometric progression. Conditions are optimal for growth.  Stationary phase- limited population growth. With the exhaustion of nutrients and build-up of waste products, the growth rate has slowed to the point where the growth rate equals the death rate. This phase may be a metabolically active one, as cells begin to synthesize proteins they may need when they can no longer grow. E.g. Sporulation Death phase- number of viable cells in the population declines due to a lack of nutrients and toxic metabolic by-products. Dynamic state- some cells die releasing nutrients that support the growth of other cells in the population The typical bacterial growth curve. When bacteria are grown in a closed system (also called a batch culture), like a test tube, the population of cells almost always exhibits these growth dynamics  Growth Growth rate is dependent largely requirements of upon availability of nutrients microorganisms  Carbon Nutritional requirements: Autotrophs obtain CO2 from Major atmosphere, heterotrophs from organic compounds such as elements sugars, organic acids  Not needed by anaerobes; aerobes obtain it from the same molecule that serves as their Carbon source, or from H2O. Oxygen Aerobes use O2 to generate ATP, thus cannot grow in its absence. H2O2 is a toxic by-product of aerobic respiration; catalase reduces it to O2+2H2O Nitrogen Important constituent of protein, nucleic acids. Taken into cell as nitrates, amino acids Phosphates Constituent of nucleic acids, phospholipids; taken into cell as phosphates Nutritional requirements: Trace elements K, Fe, Mn, Mg, Ca Small quantities required for microbial growth Act as co-factors & as parts of co-enzymes Nutritional requirements: Growth factors Amino acids, vitamins If microbe is unable to synthesize them then they are usually acquired from the environment Environmental requirements: Temperature Extrinsic factor  Every spp. grows well over a range of temperatures. Optimum temperature is the one at which the organism grows most rapidly.  Above the optimum temp, growth rate declines because cellular proteins are inactivated.  Thermophiles thrive at high temperatures because their enzymes are heat- stable. Psychrophiles can only grow at low temperatures because their enzymes are very heat-sensitive. Environmental requirements: Hydrostatic pressure  Pressure applied to a liquid, unit atm.  Many prokaryotes can thrive at high pressures- Barophiles  High pressure does not crush microbial cells as H2O passes through membranes  High pressure tends to inhibit chemical reactions that undergo an increase in molecular volume. Environmental requirements: pH Intrinsic factor Prokaryotes tend to grow best at slightly alkaline pH, fungi slightly acidic pH, protozoa & algae at neutral pH. Alkaliphiles & acidophiles can adjust intracellular pH by pumping Hydrogen ions into or out of the cell. E.g. E.coli Environmental requirements: Osmotic pressure Measure of how much water is available. If concentration of solutes in external environment increases, the bacterium reacts to maintain turgor pressure. It can pump Potassium ions into cell to alter osmotic pressure inside. This increase in solutes tends to decrease available water however and slow growth.  Halophiles can withstand high salt concentrations by maintaining a high intracellular concentration of salt. Measuring microbial growth 1. Direct determination of dry weight of cells in a culture. ◦ Cells are filtered or centrifuged from medium and dried before weighing. Tedious.  Turbidity- a spectrophotometer is used to measure how much light a liquid culture of microbial cells transmits. As mass increases, turbidity increases. Use of a standard curve relates spec readings to cell mass 2. Counting the number of cells in a population- total & viable cell counts. Total cell counts-Both dead & live cells are counted using counting chamber Viable cell counts- Only live cells are counted; counts are dependent on colony formation:  Viable cell counts: a) Plate counts (spread, pour plates) are sensitive, but tedious Pour plate method b) MPN Involves making a series of tenfold dilutions of the culture sample in a liquid medium. Samples from the tubes are incubated then examined. Dilutions that received cells from the original sample show turbidity, those that did not remain clear. A statistical MPN table is used to determine the MPN of organisms per unit volume in the original sample. 3. Measuring metabolic activity- Indirectly estimates the quantity of microbial cells. Rates of utilization of substrates (eg. Glucose, Oxygen) & rates of formation of metabolic products (acids) by microbial cultures.  A number of situations Microbial make it more likely that an infection will develop. growth: Host People are more at risk factors of infection if they have:  recently taken a course of antibiotics an immune system weakened by cancer neutropaenia or HIV infection A depressed immune system less effectively fights off all types of infection. Thus an organism that normally would be controlled by the immune system may begin to grow.  been taking oral steroids Corticosteroids are a group of drugs that reduce inflammation. They effectively treat many skin disorders. Unfortunately, these drugs also can reduce our immune response and improve conditions for microbial growth  persons with extensive burns, trauma have lung disease (emphysema) Other Factors Nutritional status Age Sex Infectious dose Infectious dose is the number of organisms that must be consumed in order to cause illness Infectious dose The infectious dose varies from one organism to another. Relatively small numbers of Campylobacter (as few as 500) bacteria can cause infection; whereas larger numbers of Salmonella (100,000) are required to cause infection. References Brock Biology of Microorganisms- Madigan, Martinko & Parker Medical Microbiology- Jawetz, Melnick & Adelberg Prescott, Harley & Klein’s Microbiology- Willey, Sherwood & Woolverton