Microbial Responses to Temperature

Microbial Responses to Unfavorable Environments

• Environmental conditions like temperature, pH, osmolarity, and oxygen availability dictate microbial growth and distribution.
• Ideal growth conditions for microbes include a temperature where enzymes function optimally, ample food supply, proper respiratory atmosphere, and available water.
• Temperature significantly impacts chemical and enzymatic reactions within microbes.
• Extreme temperatures (too hot or too cold) can prevent microbial growth.
• Cardinal temperatures (minimum, optimum, maximum) define the temperature range for microbial growth.
• Different microbes have evolved to thrive at vastly different temperatures.
• Enzymes and proteins denature above certain temperatures, cells may not function properly below certain temperatures, the cell membrane's structure and its function in nutrient transport and energy production may be altered depending on growth media
• Psychrophiles are adapted to cold temperatures, have optimal growth at 15°C or below, are found in constantly cold environments, often die at room temp., difficult to isolate and are commonly denatured/inactivated by moderate temperatures.. Cold-active enzymes have different structures from standard enzymes; psychrophiles have modified cell membranes to aid in nutrient transport at lower temperatures.
• Thermophiles thrive in high temperatures; Examples: Bacillus stearothermophilus, optimums above 45°C. Hyperthermophiles tolerate extremely high temperatures; examples: Thermococcus celer, and Pyrodictium brockii, optimums are above 80°C. High temperatures are commonly associated with volcanic phenomena and hot spring environments. Archaea are more thermophilic than bacteria.
• Proteins and enzymes have different amino acid sequences, leading to heat stability.
• Cell membranes are also specially adapted in thermophiles to keep their integrity in extreme temperatures, while maintaining normal functions.
• DNA stability in hyperthermophiles is ensured via an enzyme called reverse DNA gyrase, which introduces positive supercoils that stabilize it against high-temperature separation.

Freezing

• Water is essential for microbial growth.
• Freezing prevents microbial growth, but does not always kill cells.
• Freezing's effects include dehydration and ice crystal formation.
• Water-miscible liquids (e.g., glycerol, DMSO) at low concentrations can protect cells from freezing damage.
• These additives are routinely used for preserving bacterial cultures at -20°C and -80°C.

High Temperatures

• Microbial life exists up to and including the boiling point of water.
• Only prokaryotic organisms (bacteria and archaea) survive above 65°C.
• Thermophiles have optimum growth temperatures exceeding 45°C. High-Temperature environments are commonly associated with volcanic activity (hot springs, vents in deep oceans), examples: Bacillus stearothermophilus.
• Hyperthermophiles have optimum growth temperatures exceeding 80°C. Examples: Thermococcus celer, Pyrodictium brockii

Protein/Enzyme/Membrane/DNA Stability at High Temperatures

• Critical amino acid substitutions in thermophiles are found in proteins to facilitate stable folding.
• Alternative membrane compositions are found in thermophiles or hyperthermophiles to maintain membrane structure and function at high temperatures.
• Double-stranded DNA molecules typically separate at high temperatures. Reverse DNA gyrase, present in hyperthermophiles, stabilizes DNA by introducing positive supercoils, preventing this separation.

Bacterial DNA

• Heat-labile DNA can be distinguished from stable DNA in hyperthermophiles by the presence of an enzyme called gyrase.

pH

• pH relates to the concentration of hydrogen ions (H+) in a solution.
• The pH scale is logarithmic to express large variations in H+ concentration.
• Low pH corresponds to high H+ concentration, and high pH corresponds to low H+ concentration.
• Most microorganisms thrive at pH 6-8 (neutrophiles).
• Acidophiles thrive in acidic environments, whereas alkaliphiles thrive in alkaline environments.
• Extreme pH levels can damage cell macromolecules (enzymes, proteins, nucleic acids).
• Internal cell pH must remain close to neutral (pH 5-9) even with highly acidic or alkaline external environments.

Helicobacter pylori

• Describes how Helicobacter withstands stomach acid.

Osmolarity

• Water is necessary for growth in all cells.
• Water availability is not only dependent on moisture level but also on solute concentration.
• Dissolved solutes (e.g. salt, sugar) have an affinity for water, making it unavailable to the cells to absorb it.
• Osmosis is the diffusion of water from high to low water concentration, controlled by the cell membrane.
• Microorganisms obtain water by increasing their intracellular solute concentration. Compatible solutes are highly water-soluble substances that increase water absorption into a cell.
• Halophiles thrive in high salt environments, while osmophiles thrive in high sugar environments.

Oxygen

• Oxygen is only weakly soluble in water, which affects microbial growth in aquatic habitats.
• Aerobes require full oxygen concentrations. Microaerophiles require reduced O2 concentrations. Anaerobes cannot use oxygen for respiration or metabolism. Facultative anaerobes can grow in the presence or absence of oxygen.
• Oxygen can destroy microbes via ROS (reactive oxygen species) and killing mechanisms like NADPH oxidase.

Bacterial Sporulation

• Some Gram-positive bacteria produce spores that provide protection against adverse conditions (heat, drying, radiation, freezing, toxic chemicals, and antibiotics).
• Spores can remain dormant for extended periods.
• Spores can germinate and cause infection if they enter a favorable environment.
• Gram-negative bacteria cannot form spores under similar conditions.
• Spore formation is triggered by adverse environmental conditions. The spore contains a peptidoglycan-rich cortex layer and a keratin-like spore coat surrounding a DNA-containing core.

Spores and Diseases

• Spores, while often harmless until germination, contribute to the transmission of diseases to humans, such as anthrax, tetanus, botulism, and gas gangrene. Examples: caused by Bacillus anthracis, Clostridium tetani, Clostridium botulinum, and Clostridium perfringens.

Anthrax

• Spores of Bacillus anthracis persist in soil and animal products, leading to human infections via those sources (e.g. wool-sorters disease). Symptoms include high fever, massive swelling, bacteremia/systemic symptoms and often, death. This is treatable with antibiotics like Penicillin, and ciprofloxacin. Anthrax spores are also used as potential bioterrorism agents.

Bacterial Biofilms

• In natural environments, microbes form complex communities called biofilms for protection from adverse conditions in numbers/safety.
• Biofilms are widely seen in bacterial infections, including infections of cystic fibrosis patients (Pseudomaonas aeruginosa infections) and catheter related infections (Staphylococcus epidermidis infections).
• Biofilms form when bacteria attach to surfaces and secrete slimy substances to anchor themselves to each other.
• Biofilm infections are hard to eliminate compared to planktonic bacteria due to penetration issues.

Biofilm Resistance to Antimicrobial Agents

• Microbial biofilms are resistant to antibiotics due to slower penetration into the film structure compared to planktonic cells, the inherent nature of the structure (dense bacterial cells and matrix material) resists antibiotic penetration and dispersal, and the adaptation of the cells involved.

Explore how various environmental factors such as temperature, pH, and osmolarity affect microbial growth and distribution. This quiz covers key concepts like cardinal temperatures and the adaptations of different microbes to extreme environments. Test your understanding of microbial physiology in relation to their environments.