MICROBIAL GROWTH AND CONTROL

• Microbial Growth


  Increase in number of cells (not the size of cells)
• Colonies


  Clumps of cells large enough to be seen without a microscope
• Biofilms


  Microbial communities that adhere to each other in nutrient poor environments
• Requirements for Microbial Growth


  • Physical Requirements
  • Chemical Requirements
• Physical Requirements - Temperature


  • Most microorganisms grow well at temperatures that humans favor, while some thrive at extremes of temperatures
  • Most bacteria grow only within a limited range of temperatures with their maximum and minimum growth temperatures only approx. 30°C apart
• Classification by preferred temperature


  • Psychrophiles - cold loving
  • Mesophiles - moderate temperature
  • Thermophiles - heat loving
• Minimum/optimum/maximum growth temperature


  The temperature range within which a microorganism can grow
• pH


  Acidity or alkalinity of a solution
• Most bacteria grow best in a narrow pH range near neutrality, between pH 6.5 and 7.5
• Osmotic pressure


  The pressure exerted by the movement of water molecules across a semipermeable membrane
• High osmotic pressure (hypertonic solution)


  Cellular water leaks out through the plasma membrane, causing plasmolysis and inhibiting cell growth
• Low osmotic pressure (hypotonic environment)


  Cellular water tends to enter the cell rather than leave it, causing the cell to burst (cytolysis)
• Obligate/extreme halophiles


  Thrive in high salt concentrations and even require salt for growth
• Facultative halophiles


  Able to grow at salt concentrations up to 2%
• Chemical Requirements - Carbon


  Structural backbone of living matter, needed for organic compounds and energy
• Chemical Requirements - Nitrogen


  Used primarily to form the amino group of amino acids in proteins, obtained through decomposition of proteins, acquisition of ammonium ions, or nitrogen fixation
• Chemical Requirements - Sulfur


  Used to synthesize sulfur-containing amino acids and vitamins, obtained from sulfate ion, hydrogen sulfide, or sulfur-containing amino acids
• Chemical Requirements - Phosphorus


  Essential for synthesis of nucleic acids and phospholipids, obtained from phosphate ion
• Chemical Requirements - Oxygen


  Many organisms require oxygen for aerobic respiration, with oxygen as the final electron acceptor
• Classification by preferred oxygen level


  • Obligate aerobes
  • Microaerophiles
  • Facultative anaerobes
  • Aerotolerant anaerobes
  • Obligate anaerobes
• Trace elements


  Elements such as iron, copper, molybdenum, and zinc, required in very small amounts, usually as cofactors for enzymes
• Biofilms


  Communities of bacteria adhering to each other in a matrix of polysaccharides, DNA, and proteins
• Culture medium


  A nutrient preparation used for the growth of microorganisms in the laboratory
• Inoculum


  Microbes introduced into a culture medium to initiate growth
• Culture


  The microbes that grow and multiply in or on a culture medium
• Agar


  A complex polysaccharide derived from marine alga added to culture media to solidify/thicken it
• Criteria for growing a culture


  • Culture medium must contain the right nutrients
  • Must contain sufficient moisture, proper pH, and suitable oxygen level
  • Culture medium must be sterile
  • Culture medium must be incubated at the proper temperature
• Chemically-defined medium


  Culture media whose exact chemical composition is known, usually reserved for experimental work or for the growth of autotrophic bacteria
• Complex media


  Culture media made up of nutrients including extracts from yeasts, meat, or plants, or digests of proteins, used for the culture of heterotrophic bacteria and fungi
• Anaerobic growth media & methods


  Special culture media and methods useful for cultivation of anaerobic bacteria that could be killed by exposure to oxygen, containing ingredients that chemically combine with and deplete dissolved oxygen
• Selective media


  Culture media designed to encourage the growth of desired microbes and suppress the growth of other unwanted bacteria
• Selective media


  • Bismuth sulfite agar - inhibits gram positives & most gram negative bacteria except for Salmonella typhii
  • Sabouraud's dextrose agar - used to isolate fungi that outgrow bacteria at its acidic pH of 5.6
• Enrichment culture


  A type of selective medium designed to increase very small numbers of the desired organism up to detectable levels, useful in contaminated samples
• Differential media


  Culture media that distinguish colonies of the desired organism from other colonies growing on the same plate through identifiable reactions
• Differential media

  • Blood agar - contains red blood cells, used to identify bacterial species that destroy red blood cells
  • Mannitol salt agar - contains 7.5% NaCl, mannitol, and a pH indicator that changes color if mannitol is fermented to acid
• Isolation streak plate method


  The most commonly used method to obtain pure cultures, involving streaking a mixed culture on a nutrient medium in a series of streaks to isolate individual colonies
• Short-term preservation


  Refrigeration can be used for the short-term storage of bacterial cultures
• Long-term preservation


  Freeze drying (lyophilization) is a common method for preserving microbial cultures for long-term storage
• Bacterial division


  Bacteria normally reproduce by binary fission, while some undergo budding
• Generation time


  The time required for a cell to divide (and its population to double) - time to form the "next generation"