Lecture 5: The Effect of Physical and Chemical Conditions on Microbial Growth

Cardinal temperatures

The minimum, optimum, and maximum temperatures at which an organism grows;

What happens at the minimum temperature?

Membrane gels and transport is too slow for growth;

What happens at the optimum temperature?

Enzymatic reactions run at their maximal possible rate;

What happens at the maximum temperature?

Protein denaturation, collapse of the cytoplasmic membrane, and thermal lysis;

Psychrophile

Organism with a cold temperature optimum (<20°C) that lives in permanently cold environments;

Psychrotolerant

Organism that can grow at 0°C but has an optimum of 20-40°C, more widespread than true psychrophiles;

Mesophile

Organism with a midrange temperature optimum, found in warm-blooded animals and temperate/tropical environments;

Thermophile

Organism with a growth optimum between 45°C and 80°C, found in hot springs and active compost;

Hyperthermophile

Organism with a growth optimum above 80°C, found in boiling springs and hydrothermal vents;

Extremophile

Organism that grows under very hot or very cold conditions;

Molecular adaptations to cold

Enzymes that work in the cold and membranes with high unsaturated fatty acid content;

Molecular adaptations to heat

Thermally stable enzymes and proteins plus heat-stable membranes;

How do Bacteria and Archaea stabilize their membranes at high temperatures?

Bacteria use lipids rich in saturated fatty acids and Archaea use a lipid monolayer instead of a bilayer;

Highest temperature above which only prokaryotes exist

About 65°C;

Highest temperature for phototrophy

About 70°C (73°C on the graph);

Which organism holds the temperature record for growth?

Methanopyrus kandleri, an archaeon that grows at 122°C;

Taq polymerase

Heat-stable enzyme from a thermophile used to automate the repeated steps of PCR;

Why are thermophile enzymes useful industrially?

They are more stable and tend to have higher activity than mesophilic counterparts (e.g. proteases, cellulases, lipases);

Why may hyperthermophiles be close to ancient microbes?

They sit on the deepest, shortest branches of the phylogenetic tree;

Which metabolism may have been the first energy-yielding one?

Oxidation of H₂, which is common in hyperthermophiles;

Neutrophile

Organism that grows best between pH 6 and 8;

Acidophile

Organism that grows best at low pH (<6);

Alkaliphile

Organism that grows best at high pH (>9);

Why do culture media contain buffers?

To maintain a constant pH;

How do microbes change the pH of their environment?

Acid production (acetic, lactic, sulfuric) lowers pH and amino acid breakdown releases ammonia, which raises pH;

Water activity (aw)

Vapor pressure of a solution divided by vapor pressure of pure water, reflecting how much water is available;

Why does water tend to move into a cell?

The cytoplasm usually has a higher solute concentration than the surroundings, creating turgor pressure;

Halophile

Organism that grows best at reduced water potential and specifically requires NaCl (many marine microbes);

Extreme halophile

Organism that requires 15-30% NaCl, such as microbes from the Great Salt Lake or Dead Sea;

Halotolerant

Organism that tolerates reduced water activity but grows best at low solute concentrations (e.g. Staphylococcus aureus, up to 17.5% NaCl);

Osmophile

Organism that grows in high sugar concentrations;

Xerophile

Organism able to grow in very dry environments;

Why do honey, jam, jerky, and salted cod have few microbes?

Their low water activity limits growth;

Why is salt used in sauerkraut and pickle fermentation?

High salt selects for acid-producing microbes and the combination of high salt and low pH prevents growth of most pathogens;

Compatible solutes

Organic compounds the cell makes or concentrates to counteract low water activity outside;

How do cells combat low external water activity?

Raise internal solute concentration by pumping in inorganic ions or by making or concentrating organic solutes;

Obligate aerobe

Requires oxygen to live and grows only in the oxic zone at the top of a thioglycolate tube;

Strict anaerobe

Does not require oxygen and may be killed by it, growing only in the anoxic zone at the bottom of the tube;

Facultative aerobe (facultative anaerobe)

Grows with or without oxygen but grows better with it, so growth occurs throughout the tube and is best at the top;

Microaerophile

Needs O₂ for respiration but is killed by atmospheric levels, so it grows in a narrow band between the oxic and anoxic zones;

Aerotolerant anaerobe

Cannot use oxygen but is not harmed by it, growing evenly throughout the tube;

Thioglycolate broth

Complex medium that separates microbes by oxygen requirement, with thioglycolate reacting with O₂ so only the top is oxic;

What does the dye in thioglycolate broth show?

Pink where oxygen is present and colorless where oxygen is low or absent;

How are anaerobes grown in the lab?

Add reducing agents (thioglycolate, cysteine, H₂S) or replace air with an inert gas like nitrogen or argon;

Toxic forms of oxygen

Superoxide anion, hydrogen peroxide, and hydroxyl radical;

Catalase reaction

2 H₂O₂ → 2 H₂O + O₂;

Peroxidase reaction

H₂O₂ + NADH + H⁺ → 2 H₂O + NAD⁺;

Superoxide dismutase reaction

2 O₂⁻ + 2 H⁺ → H₂O₂ + O₂;

Superoxide reductase reaction

O₂⁻ + 2 H⁺ + reduced rubredoxin → H₂O₂ + oxidized rubredoxin;

Why can't most obligate anaerobes tolerate oxygen?

They lack some or all of the enzymes that neutralize toxic oxygen species, so they can't combat oxidative stress;