Microbial Growth Lecture Notes - Medical Microbiology (BIO 219) PDF

Chapter 3: Microbial Growth Medical Microbiology (BIO 219) Dr. Mohammad Fayyad-Kazan Spring 2023  Microbial Growth  Learning Outcome  By the end of this lecture, you should be able to: 1. Underst...

Chapter 3: Microbial Growth Medical Microbiology (BIO 219) Dr. Mohammad Fayyad-Kazan Spring 2023  Microbial Growth  Learning Outcome  By the end of this lecture, you should be able to: 1. Understand the physical and chemical requirements for microbial growth 2. Understand the types of microbial growth. 3. Understand the phases of microbial growth  Microbial Growth Provided with the right conditions (food, correct temperature, etc) microbes can grow very quickly. Depending on the situation, this could be a good thing for humans (yeast growing in wort to make beer) or a bad thing (bacteria growing in your throat causing strep throat). Multicellular organisms' growth is typically measured in terms of the increase in size of a single organism, Microbial growth is measured by measuring the increase in cell number Cell division leads to the growth of cells in the population.  Microbial Growth Two Types of Asexual Reproduction in Microbes: 1. Binary Fission - Bacterial reproduction occurs through fission. The bacterial cell doubles in size and replicates its chromosome. Following DNA replication, the two chromosomes attach to separate sites on the plasma membrane, and the cell wall is laid down between them, producing two daughter cells. 2. Budding - A few bacteria and some eukaryotes (including yeasts) may also replicate by budding, forming a bubble-like growth that enlarges and separates from the parent cell.  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature 2-pH 3-Osmotic pressure  Chemical requirements: 1-Carbon 2-Nitrogen, sulfur, phosphorus 3-Trace elements 4-Oxygen 5-Growth factors  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature  Microbes are loosely classified into several groups based on their preferred temperature ranges. 1. Psychrophiles: “Cold-loving”. Can grow at 0 oC. Two groups: A. True Psychrophiles:  Sensitive to temperatures over 20 oC.  Optimum growth at 15 oC or below.  Found in very cold environments (North pole, ocean depths).  Seldom cause of disease or food spoilage. B. Psychrotrophs: o Optimum growth at 20 to 30 oC. o Responsible for most low temperature food spoilage.  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature  Microbes are loosely classified into several groups based on their preferred temperature ranges. 2. Mesophiles: “Middle loving”. Most bacteria. Include most pathogens and common spoilage organisms. Best growth between 25 to 40oC. Optimum temperature commonly 37oC. Many have adapted to live in the bodies of animals. 3. Thermophiles: “Heat loving”. o Optimum growth between 50 to 60oC. o Many cannot grow below 45oC. o Adapted to live in sunlit soil, compost piles, and hot springs. o Some thermophiles form extremely heat resistant endospores. 4. Extreme Thermophiles (Hyperthermophiles):  Optimum growth at 80oC or higher.  Archaebacteria.  Most live in volcanic and ocean vents.  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature 2-pH Most bacteria prefer neutral pH (6.5-7.5). Molds and yeast grow in wider pH range, but prefer pH between 5 and 6.4 Acidity inhibits most microbial growth and is used frequently for food preservation (e.g.: pickling). Alkalinity inhibits microbial growth, but not commonly used for food preservation. Acidic products of bacterial metabolism interfere with growth. Buffers can be used to stabilize pH.  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature 2-pH  Organisms can be classified as: 1. Acidophiles: “Acid loving”. Grow at very low pH (0.1 to 5.4) Lactobacillus produces lactic acid, tolerates mild acidity. 2. Neutrophiles: Grow at pH 5.4 to 8.5. Includes most human pathogens. 3. Alkaliphiles: “Alkali loving”. Grow at alkaline or high pH (7 to 12 or higher) Vibrio cholera optimal pH 9. Soil bacterium Agrobacterium grows at pH 12.  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature 2-pH 3- Osmotic Pressure If a solution with less concentration is placed on one side of the semi permeable membrane and more concentrated solution on the other side, the solvent molecules move from the less concentrated solution to the more concentrated solution and try to make the concentration on both sides equal. This action continues until the concentration on both sides is equal. “The spontaneous flow of solvent through a semipermeable membrane from a solution of lower concentration towards a solution of higher concentration is known as Osmosis.”  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature 2-pH 3- Osmotic Pressure  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature 2-pH 3- Osmotic Pressure  Cells are 80 to 90% water. 1. Hypertonic solutions:  High osmotic pressure removes water from cell, causing shrinkage of cell membrane (plasmolysis).  Used to control spoilage and microbial growth. Sugar in jelly. Salt on meat. 2. Hypotonic solutions:  Low osmotic pressure causes water to enter the cell.  In most cases cell wall prevents excessive entry of water.  Microbe may lyse or burst if cell wall is weak.  Microbial Growth  The requirements for growth  Physical requirements : 1-Temperature 2-pH 3- Osmotic Pressure 1. Halophiles: Require moderate to large salt concentrations. Ocean water contains 3.5% salt. Most bacteria in oceans. 2. Extreme or Obligate Halophiles: Require very high salt concentrations (20 to 30%). Bacteria in Dead Sea. 3. Facultative Halophiles: Do not require high salt concentrations for growth, but tolerate 2% salt or more.  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 1.Carbon: Makes up 50% of dry weight of cell. Structural backbone of all organic compounds! Chemoheterotrophs: Obtain carbon from their energy source: lipids, proteins, and carbohydrates. Chemoautotrophs and Photoautotrophs: Obtain carbon from carbon dioxide.  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 2.Nitrogen, Sulfur, and Phosphorus: A. Nitrogen: Makes up 14% of dry cell weight. Used to form amino acids, DNA, and RNA!  Sources of nitrogen: 1. Protein or amino acids : Most bacteria 2. Ammonium: Found in organic matter 3. Nitrogen gas (N2): Obtain N directly from atmosphere. Important nitrogen fixing bacteria, live free in soil or associated with legumes (peas, beans, alfalfa, clover, etc.). 4. Nitrates: Salts that dissociate to give NO3-.  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 2.Nitrogen, Sulfur, and Phosphorus: B. Sulfur Used to form proteins and some vitamins (thiamin and biotin).  Sources of sulfur: Protein: Most bacteria Hydrogen sulphide Sulfates: Salts that dissociate to give SO42-. C. Phosphorus: Used to form DNA, RNA, ATP, and phospholipids. Sources: Mainly inorganic phosphate salts and buffers.  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 3. Other Elements: Potassium, magnesium, and calcium are often required as enzyme cofactors. Calcium is required for cell wall synthesis in Gram positive bacteria. 4.Trace Elements: Many are used as enzyme cofactors. Commonly found in tap water. o Iron o Copper o Molybdenum o Zinc  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 5. Oxygen Organisms that use molecular oxygen (O2), produce more energy from nutrients than anaerobes. Can classify microorganism based on their oxygen requirements: 1. Obligate Aerobes: Require oxygen to live. Example: Pseudomonas, common nosocomial pathogen. 2. Facultative Anaerobes: Can use oxygen, but can grow in its absence. Have complex set of enzymes. Examples: E. coli, Staphylococcus, yeasts, and many intestinal bacteria. 3. Obligate Anaerobes: Cannot use oxygen and are harmed by the presence of toxic forms of oxygen. Examples: Clostridiumbacteria that cause tetanus and botulism.  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 5. Oxygen 4. Aerotolerant Anaerobes: Can’t use oxygen, but tolerate its presence. Can break down toxic forms of oxygen. Example: Lactobacillus carries out fermentation regardless of oxygen presence. 5. Microaerophiles: Require oxygen, but at low concentrations. Sensitive to toxic forms of oxygen. Example: Campylobacter  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 5. Oxygen  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 5. Oxygen  Toxic Forms of Oxygen: 1. Singlet Oxygen: Extremely reactive form of oxygen, present in phagocytic cells. 2. Superoxide Free Radicals (O2-.): Extremely toxic and reactive form of oxygen. All organisms growing in atmospheric oxygen must produce an enzyme superoxide dismutase(SOD), to get rid of them. SOD is made by aerobes, facultative anaerobes, and aerotolerantanaerobes, but not by anaerobes or microaerophiles  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 5. Oxygen  Toxic Forms of Oxygen: 3. Hydrogen Peroxide (H2O2): Peroxide ion is toxic and the active ingredient of several antimicrobials(e.g.:benzoylperoxide). There are two different enzymes that break down hydrogen peroxide:  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 5. Oxygen  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements : 6. Growth factors They are essential substances that the organism is unable to synthesize from available nutrients. Growth factors are organized into three categories. 1. Purines and pyrimidines: required for synthesis of nucleic acids (DNA and RNA) 2. Amino acids: required for the synthesis of proteins 3. Vitamins: needed as coenzymes and functional groups of certain enzymes Mutant strains of bacteria that require some growth factor not needed by the wild type (parent) strain are referred to as auxotrophs. Thus, a strain of E. coli that requires the amino acid tryptophan in order to grow would be called a tryptophan auxotroph and would be designated E. coli trp-.  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements  Growth of Bacterial Cultures  Bacterial Division: Occurs mainly by binary fission.  Generation Time: Time required for a cell to divide, and its population to double.  Generation time varies considerably: 1. E. coli divides every 20 minutes. 2. Most bacteria divide every 1 to 3 hours. 3. Some bacteria require over 24 hours to divide  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements  Culture media  Obtaining Pure Cultures  Growth of Bacterial Cultures  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements  Culture media  Obtaining Pure Cultures  Growth of Bacterial Cultures Four phases of Bacterial Growth in a liquid growth medium : 1. Lag Phase: Period of adjustment to new conditions. Little or no cell division occurs, population size doesn’t increase. Phase of intense metabolic activity, in which individual organisms grow in size. May last from one hour to several days.  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements  Culture media  Obtaining Pure Cultures  Growth of Bacterial Cultures Four phases of Bacterial Growth in a liquid growth medium : 2. Log Phase: Cells begin to divide and generation time reaches a constant minimum. Period of most rapid growth. Number of cells produced > Number of cells dying Cells are at highest metabolic activity.  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements  Culture media  Obtaining Pure Cultures  Growth of Bacterial Cultures Four phases of Bacterial Growth in a liquid growth medium : 3. Stationary Phase: Population size begins to stabilize. Number of cells produced = Number of cells dying Overall cell number does not increase. Cell division begins to slow down. Factors that slow down microbial growth: 1. Accumulation of toxic waste materials 2. Acidic pH of media 3. Limited nutrients 4. Insufficient oxygen supply  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements  Culture media  Obtaining Pure Cultures  Growth of Bacterial Cultures Four phases of Bacterial Growth in a liquid growth medium : 4. Death or Decline Phase: Population size begins to decrease. Number of cells dying > Number of cells produced Cells lose their ability to divide. A few cells may remain alive for a long period of time  Microbial Growth  The requirements for growth  Physical requirements  Chemical requirements  Culture media  Obtaining Pure Cultures  Growth of Bacterial Cultures