Queenie Maitem

• Bacterial growth curve


  Phases: Lag phase, Log phase, Stationary phase, Death phase
• Two reasons why bacteria die during the death phase
• Explain briefly why the use of antibiotics in animal feed and household products is controversial
• Introduction: In certain locations, such as within microbiology laboratories, the growth of microbes is encouraged; in other words, scientists want them to grow. In other locations-such as on hospital wards, in intensive care units, in operating rooms, in kitchens, bathrooms, and restaurants-it is necessary or desirable to inhibit the growth of microbes. Both concepts, encouraging and inhibiting the in vitro growth of microbes, are discussed in this chapter. Before discussing these concepts, various factors that affect the growth of microbes are examined.
• Factors that affect microbial growth: Availability of nutrients, Moisture, Temperature, pH, Osmotic pressure, Barometric pressure, Composition of the atmosphere
• Availability of Nutrients: All living organisms require nutrients to sustain life. Organisms obtain energy from these chemicals by breaking chemical bonds. Nutrients also serve as sources of carbon, oxygen, hydrogen, nitrogen, phosphorus, and sulfur as well as other elements that are usually required in lesser amounts.
• Moisture: Water is essential for life. Cells consist of anywhere between 70% and 95% water. All living organisms require water to carry out their normal metabolic processes, and most will die in environments containing too little moisture. Certain microbial stages (e.g., bacterial endospores and protozoan cysts) can survive desiccation.
• Temperature: Every microorganism has an optimum growth temperature, a minimum growth temperature, and a maximum growth temperature. The temperature range is crucial for the growth of microorganisms.
• The temperature range at which an organism grows can differ greatly from one microbe to another
• Enzymes


  Have optimum temperature and pH ranges at which they operate at peak efficiency
• If an organism's enzymes are operating at peak efficiency, the organism will be metabolizing and growing at its maximum rate
• Microorganisms based on temperature preference

  • Thermophiles
  • Hyperthermophiles
  • Mesophiles
  • Psychrophiles
• Thermophiles


  • Organisms that grow best at high temperatures
• Hyperthermophiles


  • Organisms that favor temperatures above 100°C
• Psychrophiles


  • Microbes that prefer cold temperatures
• The highest temperature at which a bacterium has been found living is around 113°C; it was an archaeon named Pyrolobus fumarii
• Most pathogens and members of the indigenous microbiota are mesophilic because they grow best at normal body temperature (37°C)
• Biologists studying microbial life in the Antarctic have reported finding organisms that thrive in cold ocean water and on snow
• Mesophilic


  Microbiota that grow best at normal body temperature (37°C)
• Psychrophiles


  Organisms that prefer cold temperatures
• Psychrophiles habitats


  • Cold ocean water, high altitudes with algae living on snow, Antarctic lake with -13°C temperature, 20% salinity, high concentrations of ammonia and sulfur
• Psychrotrophs


  Group of psychrophiles with an optimum growth temperature at refrigerator temperature (4°C)
• Psychroduric organisms


  • Microorganisms that prefer warmer temperatures but can tolerate very cold temperatures and be preserved in the frozen state
• pH preferences


  • Most microorganisms prefer neutral or slightly alkaline growth medium (pH 7.0-7.4), acidophiles prefer pH of 2 to 5, fungi prefer acidic environments, acidophiles thrive in highly acidic environments
• Osmotic pressure is the pressure exerted on a cell membrane by solutions inside and outside the cell
• When cells are suspended in a hypertonic solution, water leaves the cell by osmosis to equalize concentrations, causing crenation in human cells and plasmolysis in bacterial cells
• Osmosis is the movement of a solvent through a permeable membrane from a solution with lower solute concentration to a solution with higher solute concentration
• Plasmolysis inhibits bacterial cell growth and multiplication
• Shrinkage


  Called crenation, and the cell is said to be crenated
• If the cell is a bacterial cell with a rigid cell wall, it does not shrink. Instead, the cell membrane and cytoplasm shrink away from the cell wall, known as plasmolysis, inhibiting bacterial cell growth and multiplication
• Cells lose water and shrink when placed into a hypertonic solution
• Salts and sugars are added to certain foods as a way of preserving them. Bacteria that enter hypertonic environments will die due to loss of water and desiccation
• When the concentration of solutes outside a cell is less than inside, the solution is hypotonic. Water enters the cell to equalize concentrations. In human cells like erythrocytes, increased water causes the cell to swell
• If water enters a cell causing it to swell, it may burst. In erythrocytes, bursting is called hemolysis. In bacterial cells, bursting is referred to as plasmoptysis
• When solute concentrations outside a cell equal those inside, the solution is isotonic. In an isotonic environment, no plasmolysis or plasmoptysis occurs; the cell has normal turgor
• Sugar solutions for jellies and pickling brines preserve foods by inhibiting the exerted pressure on a cell membrane
• No change in pressure occurs within the cell in an isotonic solution. Internal pressure increases in a hypotonic solution
• Effects of changes in osmotic pressure

  1. No change in pressure occurs within the cell in an isotonic solution

  2. Internal pressure is increased in a hypotonic solution, resulting in swelling of the cell

  3. Internal pressure is decreased in a hypertonic solution, resulting in shrinking of the cell
• Some types of moulds and bacteria can survive and even grow in a salty environment
• Microbes that actually prefer salty environments are called halophilic organisms