module 5

• Temperature and Microbial Growth


  1. Cardinal temperatures: minimum, optimum, maximum

  2. Temperature is a major environmental factor controlling microbial growth
• pH and Microbial Growth

  1. pH is a measure of [H+]

  2. Each organism has a pH range and a pH optimum

  3. Different categories based on pH range: acidophiles, alkalophiles, lactic acid bacteria, Thiobacillus thiooxidans, fungi
• Nutrition is important for microbial growth as the chemical compounds inside a living cell are formed from nutrients
• Requirements for Microbial Growth

  • Temperature
  • pH
  • Osmotic pressure
  • Oxygen classes
• Halophiles and Related Organisms


  1. Halophiles: grow best at reduced water potential

  2. Halotolerant: can tolerate some reduction in water activity

  3. Xerophiles: able to grow in very dry environments
• Temperature Classes of Organisms

  1. Mesophiles (20 – 45C): Midrange temperature optima

  2. Psychrophiles (0-20C): Cold temperature optima

  3. Thermophiles (50-80C): Growth temperature optima between 45ºC and 80ºC

  4. Hyperthermophiles: Optima greater than 80°C
• Osmotic Effects on Microbial Growth

  1. Osmotic pressure depends on the surrounding solute concentration and water availability

  2. Water availability is generally expressed in physical terms such as water activity (aw)

  3. Different categories based on osmotic effects: osmophiles, osmotolerant, halophiles, halotolerant, barophiles, barotolerant
• Temperature

  • Minimum Temperature: Temperature below which growth ceases
  • Optimum Temperature: Temperature at which growth rate is the fastest
  • Maximum Temperature: Temperature above which growth ceases
• Most Bacteria can use Ammonia (NH3) and many can also use NO3-
• Macronutrients

  • Carbon (C)
  • Hydrogen (H)
  • Oxygen (O)
  • Nitrogen (N)
  • Phosphorus
  • Sulfur (S)
  • Potassium (K)
  • Magnesium (Mg)
  • Sodium (Na)
  • Calcium (Ca)
  • Iron (Fe)
• Nitrogen requirements


  1. Although many biological components within living organisms contain N, and N2 is the most abundant component of air, very few organisms can "fix" or utilize N2 by converting it to NH3

  2. N is often growth limiting as organisms must find source as NH4+ for biosynthesis

  3. Photosynthetic organisms and many microbes can reduce NO3- to NH4+
• Siderophores


  • Ferric enterobactin
  • Aquachelin
• Nitrogen fixers can utilize atmospheric nitrogen (N2)
• Autotrophs are able to build all of their cellular organic molecules from carbon dioxide
• Test for Oxygen Requirements of Microorganisms

  Thioglycolate broth contains a reducing agent and provides aerobic and anaerobic conditions
• Classification of organisms based on O2 utilization

  1. Utilization of O2 during metabolism yields toxic by-products including O2-, singlet oxygen (1O2) and/or H2O2

  2. Toxic O2 products can be converted to harmless substances if the organism has catalase (or peroxidase) and superoxide dismutase (SOD)

  3. SOD converts O2- into H2O2 and O2

  4. Catalase breaks down H2O2 into H2O and O2

  5. Any organism that can live in or requires O2 has SOD and catalase (peroxidase)
• Macronutrients


  Elements required in fairly large amounts
• Environmental factors and growth: anaerobes lack superoxide dismutase and/or catalase, anaerobes need high -, something to remove O2
• Classification of organisms based on O2 utilization

  1. Obligate (strict) aerobes require O2 in order to grow

  2. Obligate (strict) anaerobes cannot survive in O2

  3. Facultative anaerobes grow better in O2

  4. Aerotolerant organisms don't care about O2

  5. Microaerophiles require low levels of O2
• Micronutrients are critical to cell function and often used as enzyme cofactors
• Nitrogen is mainly incorporated in proteins and nucleic acids
• Iron


  • Plays a major role in cellular respiration, being a key component of cytochromes and iron-sulfur proteins involved in electron transport
• Minimum temperature


  membrane gelling; transport processes so slow that growth cannot occur
• between minimum and optimum temp

  enzymatic reactions occurring at increasingly high rates
• optimum temp


  enzymatic reactions occurring at maximal possible rate
• maximum temp


  protein denaturation; collapse of the cytoplasmic membrane; thermal lysis
• Temperature Classes of Organism

  • Mesophiles (20-45C)
  • Psychrophiles (0-20C)
  • Thermophiles (50- 80C)
  • Hyperthermophiles (>80C)
• pH – measure of

  [H+]
• acidophiles – optimum in pH range 1-4

  alkalophiles – optimum in pH range 8.5-11
• lactic acid bacteria – 4-7

  Thiobacillus thiooxidans – 2.2-2.8

  fungi – 4-6
• The internal pH is regulated by buffers and near-neutral conditions are maintained through the adjustment with ion pumps.
• Human blood and tissues has pH of 7.2-7.4
• Most organisms grow best between pH 6 and 8, but some organisms have evolved to grow best at low or high pH. The internal pH of a cell must stay relatively close to neutral even though the external pH is highly acidic or basic.
• Acidophiles : organisms that grow best at low pH ( Helicobacter pylori, Thiobacillus thiooxidans
• Alkaliphiles : organisms that grow best at high pH ( Vibrio cholera)
• Most of pathogenic bacteria are neutrophiles
• Osmotic pressure depends on the surrounding solute concentration and water availability
• Water availability is generally expressed in physical terms such as water activity (aw)
• Water activity is the ratio of the vapor pressure of the air in equilibrium with a substance or solution to the vapor pressure of pure water (aw 1.00)

  $aw=$$Psolu/Pwater$
• Osmotic Effect and water activity

  organisms which thrive in high solute – osmophiles

  organisms which tolerate high solute – osmotolerant

  organisms which thrive in high salt – halophiles

  organisms which tolerate high salt – halotolerant

  organisms which thrive in high pressure – barophiles

  organisms which tolerate high pressure