Microbiology Sterilization Techniques Quiz

Microbial Growth and Control

• Picking Apart Microbial Consortia:
  • Microbial consortia work together in nature for metabolic processes like methane oxidation and sulfate reduction.
  • Methane oxidizer (ANME) and sulfate reducer work together.
  • Researchers used AQDS (artificial electron acceptor) to decouple the consortium, allowing pure culture growth of methane oxidizer.
  • Pure culture growth is the "gold standard" for studying a microbe's physiology.
• Cell Division and Population Growth:
  • Growth is the increase in the number of cells.
  • Binary fission:
    • Cells elongate and form a septum to divide into two daughter cells.
    • The septum forms from inward growth of the cytoplasmic membrane and cell wall.
    • Variations in septum formation exist in different bacteria (e.g., Bacillus subtilis).
  • Budding:
    • New daughter cells develop as cytoplasmic extensions.
    • Caulobacter and Hyphomicrobium are examples of bacteria that bud.
  • Biofilms:
    • Attached polysaccharide matrix containing embedded bacterial cells.
    • Form in stages, commencing with planktonic cell attachment.
    • Develop into multilayered sheets (microbial mats) in some cases.
    • Common in nature due to protection from chemicals and grazing.
• Quantitative Aspects of Microbial Growth:
  • Exponential growth:
    • Cell numbers double repeatedly at a constant time interval.
    • Plotted on arithmetic coordinates, it shows a rising slope.
    • Plotted on logarithmic coordinates (semilogarithmic), it produces a linear graph.
  • Generation time (g):
    • Time required for a population to double.
    • Determined by tracking cell numbers over time on a semilogarithmic graph.
    • Dependent on nutritional and genetic factors, along with temperature.
  • Microbial growth cycle:
    • Consists of lag phase (initial period of adjustment), exponential phase, stationary phase (growth rate stops), and death phase (decline in cell numbers).
    • Data from exponential phase is often used to study cells.
• Continuous Culture:
  • Chemostat:
    • Maintains cells in exponential phase by continuously adding fresh medium and removing spent medium.
    • Specific growth rate controlled by the dilution rate (F/V).
    • Cell density controlled by the concentration of limiting nutrient.
• Growth Media and Laboratory Culture:
  • Defined media:
    • Exact chemical composition known, useful for precise study.
  • Complex media:
    • Derived from digests of organisms, useful for culturing a broader range of organisms.
  • Selective media:
    • Inhibits growth of unwanted organisms.
  • Differential media:
    • Indicator compounds reveal metabolic activities.
• Microscopic Counts of Microbial Cell Numbers:
  • Counting cells, stained or unstained, via a microscope.
  • Counting chambers with etched grids provide a precise way to estimate cell density.
  • Limitations include that dead and live cells cannot be distinguished and motile cells must be immobilized.
• Viable Counting of Microbial Cell Numbers:
  • Counting viable cells (capable of reproduction), usually done via plate counts.
  • Spread-plate and pour-plate methods are used, usually with serial dilutions.
  • Error sources include non-uniform samples and inaccurate pipetting.
  • Plate counts generally underestimate true cell numbers in samples from natural environments (great plate count anomaly).
• Turbidimetric Measures of Microbial Cell Numbers:
  • Measuring turbidity allows for an estimation of cell number.
  • Density is proportional to cell number up to a point.
  • Spectrophotometer measures unscattered light.
• Environmental Effects on Growth: Temperature:
  • Temperature affects microbial growth rate and survival;
  • Cardinal temperatures:
    • Minimum temperature at which growth is not possible.
    • Optimal temperature at which growth is most rapid.
    • Maximum temperature at which growth is not possible.
  • Organisms are categorized as psychrophiles, mesophiles, thermophiles, and hyperthermophiles based on temperature optima.
• Environmental Effects on Growth: pH:
  • pH affects microbial growth and survival.
  • Acidophiles:
    • Grow in extremely acidic environments.
  • Alkaliphiles:
    • Grow in extremely alkaline environments.
• Environmental Effects on Growth: Osmolarity and Oxygen:
  • Different microorganisms tolerate different water activities (aW).
  • Halophiles:
    • Need high concentrations of salt (NaCl) to thrive.
  • Halo tolerant:
    • Can tolerate but not necessarily require high salt concentrations.
  • Obligate anaerobes:
  • Organisms that require no oxygen to grow.
  • Microaerophiles:
    • Grow better with reduced oxygen concentrations.
  • Aerotolerant anaerobes:
    • Grow better in the presence of oxygen, but do not require oxygen to grow.
• Controlling Microbial Growth:
  • Heat sterilization
    • Decimal reduction time (D): the time required for a 10-fold reduction in microbial population at a specific temperature.
  • Pasteurization
    • Method of reducing microbial load in liquids like milk, not a complete sterilization. -Other methods -Radiation and filtration -Chemical Methods
• Other Important Points:
  • Chemical antimicrobial agents like sanitizers, disinfectants, sterilants and antiseptics
  • Importance and techniques for maintaining PURE cultures