Microbial Responses to Unfavorable Environments (MICR20010 Lecture 8 2024) PDF

MICR20010 Lecture 8 Microbial responses to unfavourable environments Dr. Jennifer Mitchell Microbiology School of Biomolecular and Biomedical Science  Learning Outcomes Environmental conditions Temperature pH Osmolarity Oxygen Spo...

MICR20010 Lecture 8 Microbial responses to unfavourable environments Dr. Jennifer Mitchell Microbiology School of Biomolecular and Biomedical Science  Learning Outcomes Environmental conditions Temperature pH Osmolarity Oxygen Spore formation Bacterial Biofilms What are ideal growth conditions for a microbe? A temperature where all their enzymes are folded properly and working at the optimum rate Plenty of food The correct atmosphere for their own type of respiration Available water Environmental conditions dictate microbial growth and the distribution and habitats of microbes Temperature pH Osmolarity (water concentration/availability) Oxygen  Effect of temperature on growth Perhaps the most important environmental factor controlling microbial growth Too hot or too cold can prevent growth However different microbes have evolved to growth at very different temperatures Cardinal temperatures Minimum Optimum Maximum Temperature controls chemical and enzymatic reactions Above the maximum temperature, enzymes and proteins are denatured Below the minimum temperature, the cell membrane may no longer function Cell Membrane is required for nutrient transport and for energy production. Cell Membrane composition is altered depending on growth media – Maximum and minimum temperatures supporting growth are different in rich media versus minimal media  Growth at cold temperatures Organisms adapted for growth at cold temperatures do better when the temperature is constant e.g. deep ocean (approx 2oC, arctic/antarctic waters. Environments with high summer temperatures and cold winter temperatures are less suited to growth. Psychrophiles have an optimal growth temperature of 15oC or lower Found in environments that are constantly cold and rapidly die at room temperature – difficult to isolate and grow in laboratory Enzymes in psychrophiles are denatured/inactivated at even moderate temperature Cold-active enzymes are structurally different to normal enzymes Membrane structure is different in psychrophiles – allows normal nutrient transport at cold temps.  Frozen but not dead Liquid water is required for microbial growth Freezing prevents microbial growth but does not always cause cell death Effects of freezing: 1. Dehydration 2. Ice crystal formation Water-miscible liquids (e.g. glycerol, DMSO – liquids which mix well with water) at a low concentration (10%) are protective: These penetrate the cell and reduce the severity of the effects if freezing Routinely used for storing bacterial cultures at -20oC and -80oC. Can occur to varying degrees in natural environments  Growth at high temperatures Microbial life flourishes at high temperatures up to and including boiling point of water Above 65oC only prokaryotic life (bacteria and archaea) exists Thermophiles – optimum growth temperature >45oC Hyperthermophiles – optimum growth temperature >80oC High temperature environments found in nature are associated with volcanic phenomena – hot springs, hydrothermal vents in deep oceans Archaea are more thermophilic than bacteria Protein/enzyme stability at high temperature Critical amino acid substitutions facilitate heat stable folding Membrane stability at high temperature Alternative membrane composition maintains structure and function DNA stability at high temperature Double stranded DNA molecule usually separates at high temperature In hyperthermophiles, an enzyme called reverse DNA gyrase prevents this from happening. This enzyme is absent in organisms that grow below 80oC. Introduces positive supercoils into DNA, resulting in increased stability  Bacterial DNA Gyrase Heat labile Heat stable  Effect of pH on growth pH refers to the concentration of hydrogen ions (H+) in a solution and is commonly expressed in terms of the pH scale, which is a log scale. A log scale is used because the large variations in H+ ion concentration in different solutions. Low pH corresponds to high hydrogen ion concentration High pH corresponds to low hydrogen ion concentration. pH values are calculated as (-) the log of the H+ concentration (- log is used to get positive values for the pH scale). pH = -log [H+]  Most microorganisms grow best at pH 6 – pH 8 (Neutrophiles) Acidity and alkalinity can greatly affect growth Acidophiles – acid loving Alkaliphiles – alkaline loving Tolerance of extreme pH may depend in part on altered membrane stability Important: Internal cell pH must be close to neutral (between pH 5 – pH 9) even if external pH is very acidic or alkaline At pH extremes – the cell macromolecules (enzymes, proteins, nucleic acid) are destroyed How does Helicobacter withstand Stomach acid?  Effect of osmolarity on growth Water is required for growth of all cells – water is the solvent of life Water availability is dictated not only by how moist or dry an environment is but is also dependent on the concentration of solutes (e.g. NaCl or sugar) in the water Why? Because dissolved solutes have an affinity for water and make it unavailable to the microbial cell Osmosis is the diffusion of water from high water concentration (low solute concentration) to low water concentration (high solute concentration. Controlled in cells by the cytoplasmic membrane  Osmosis Diffusion of water Water molecules diffuse easily across the CM Water molecules associated with other molecules in solution do not Dissolved sugar limits that availability of water to the cell In nature, absence of water inhibits life and thus biologically relevant water availability linked to solute (usually NaCl) concentration Halophiles – NaCl loving Osmophiles – can grow high sugar concentrations Food preservatives: Salt and sugar are commonly used as preservatives to inhibit microbial growth How do microbes grow under conditions of low water availability? NaCl NaCl NaCl NaCl H2O NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl H 2O NaCl NaCl NaCl NaCl Compatible solute H2O NaCl H2O NaCl Compatible solute H2O NaCl NaCl NaCl H2O Cell NaCl Cell NaCl NaCl NaCl H2O Compatible solute H2O NaCl H2O NaCl NaCl Compatible solute H 2O H2 O NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl NaCl H2O NaCl NaCl NaCl H2O Increase in internal solute concentration High NaCl, low H2O availability - Increased uptake of H2O Compatible Solutes Microbes in high solute, low water environments Obtain water by increasing intracellular solute concentration Synthesising or accumulating an organic solute This solute must be non-toxic to cellular metabolism – hence called compatible solute Compatible solutes are highly water soluble – thus they “attract” water into cell  Effect of oxygen on growth Because most higher animals require oxygen – doesn’t mean the same is true for microbes O2 is only weakly water soluble – hence many aquatic habitats are anoxic Aerobes – grow at full oxygen tensions (air is 21% O2) Microaerophiles – grow at reduced O2 concentrations (may have oxygen sensitive enzymes Anaerobes - cannot use oxygen for respiration – strict and aerotolerant Facultative anaerobes – can grow in presence or absence of oxygen Oxygen killing bacteria in neutrophils ROS  Bacterial Sporulation Some Gram-positive bacteria can form Spores which provide protection from adverse conditions Spores introduced into a wound site can germinate and cause infection Gram-negative bacteria cannot form spores  Spore Formation Adverse environmental conditions trigger spore formation. The spore is surrounded by a peptidoglycan-rich cortex layer and a keratin-like spore coat.  Spores are resistant to: Heat, Drying, Radiation, Freezing, Toxic chemicals Antibiotics & Can be difficult to eradicate with standard disinfectants. The existence of bacterial spores highlights the need for proper sterilisation - 121oC, 15psi Spores can persist for hundreds and possibly thousands of years, before germinating under the right conditions Although harmless themselves until they germinate, they are involved in the transmission of some diseases to humans including: *anthrax, caused by Bacillus anthracis; *tetanus, caused by Clostridium tetani; *botulism, caused by Clostridium botulinum; and *gas gangrene, caused by Clostridium perfringens  B. anthracis - Anthrax Spores persist in soil - animals and animal products are usual source of human infection Woolsorters were often infected - woolsorters’ disease Disease: Treatment: High fever, bacteraemia Penicillin, ciprofloxacin Massive swelling (oedema) Systemic affects Death Anthrax endospores as bioterrorism agents ? Bacterial Biofilms In all natural environments, microbes grow in complex communities called biofilms. Biofilms offer safety in numbers and provide increased resistance to adverse environmental conditions The majority of bacterial infections treated by clinicians involve biofilms Pseudomonas aeruginasa infections in cystic fibrosis patients Staphylococcus epidermidis catheter related infections Coagulase-negative staphylococci form biofilms or slime on implanted biomaterials and catheters Biofilms form when bacteria adhere to surfaces and excrete slimy glue-like substances which anchor the cells  Why are biofilm infections difficult to treat? Antibiotic Doses 1 1000 Planktonic Biofilm cells cells  Further Reading Brock Biology of Microorganisms Chapter 6 “Microbial Growth”