Conditions That Affect Microbial Growth

What type of bacteria thrives in low temperatures?

Psychrophile

What type of bacteria thrives in midrange temperatures?

Mesophile

What type of bacteria thrives in high temperatures?

Thermophile

What type of bacteria thrives in extremely high temperatures?

Hyperthermophile

What is the optimum temperature for psychrophiles?

15°C or lower

What is the optimum temperature for mesophiles?

39-45°C

What is the optimum temperature for thermophiles?

45-80°C

What are the optimum temperatures for hyperthermophiles?

80-106°C

Where are mesophiles found? Tell me about mesophiles.

Mesophiles are bacteria that thrive in midrange temperatures (39-45°C). Due to their midrange favorability, mesophiles can be found in:

• Warm-blooded animals (favorable body temperature)

• Terrestrial and aquatic environments, e.g., freshwater lakes/rivers, soil

• Temperature and tropical climates

What are extremophiles?

Extremophiles are organisms that grow under incredibly extreme conditions, i.e., very hot or very cold climates.

• Tardigrades!

What are psychrotolerant bacteria?

Psychrotolerant bacteria are organisms that have an optima range of 20-40°C, but can still grow at 0°C.

• Psychrotolerant microorganisms are widely distributed in nature.

• This means that psychrotolerant bacteria can grow in the fridge.

• Listeria is psychrotolerant. It’s what poisons milk!

What can you tell me about the structural adaptations of psychrophiles?

• Under colder conditions, cells unfit for the climate would have an incredibly slow metabolic rate due to a stiff membrane and a rigid, unusable enzyme. Psychrophiles have structurally adapted to cold climates by:

  • Producing enzymes that have more α-helices than β-pleated sheets

  • Having more polar, hydrophilic amino acids and less hydrophobic amino acids

  • Enzymes with less thermostability but greater enzymatic activity

  • Because the enzymes are incredibly unstable, they denature quickly at moderate temperatures.

  • Has modified cytoplasmic membranes with a high unsaturated fatty acid content

Above what temperature do only prokaryotic life forms exist?

65°C. It is important to note that Archaea are the only organisms with the highest temperature optima.

What can you tell me about hyperthermophiles?

• Hyperthermophiles are organisms that thrive above 80°C.

• Hyperthermophiles are found in boiling hot springs, where both chemoorganotrophic and chemolithotrophic species can be found.

What can you tell me about the structural adaptations of thermophiles and hyperthermophiles?

Cells that are unfit for high temperatures will undergo two things: (1) denaturation of enzymes, and (2) thermal lysis. Thermophiles and hyperthermophiles are more structurally thermostable because of:

• Critical amino acid substitutions in select locations = more heat-tolerant protein folds

• More ionic bonds between basic and acidic amino acids = less protein denaturing in the cytoplasm

• Up-regulation of thermostabilizing protein solutes

The cytoplasmic membrane adaptation to thermophily is different between bacteria and archaea:

• Bacteria have a cytoplasmic lipid bilayer membrane rich in saturated fatty acids

• Archaea have a cytoplasmic lipid monolayer membrane rich in saturated fatty acids

What type of bacteria thrives under low pH? Can you tell me a little bit about it?

Acidophiles. Some acidophiles are obligate acidophiles, meaning that the stability of the cytoplasmic membrane is contingent on acidity.

What type of bacteria thrives under high pH?

Alkaliphiles

What type of bacteria thrives under neutral pH?

Neutrophiles

What is the optimal pH of acidophiles?

>6

What is the optimal pH of neutrophiles?

6-8

What is the optimal pH of alkaliphiles?

>9

Can you tell me about the internal and external pH of a cell?

Typically, the internal pH of any cell—acidophile, neutrophile, and alkaliphile—likes to hover around neutral. Every protein in a cell only works at a particular pH. If this pH is disrupted, proteins can denature and DNA could be damaged. Therefore, a pH balance is typically formed through a semipermeable membrane and proton pumps.

However, in extreme conditions, pumping protons in/out of the membrane costs more energy than it is worth. Therefore, extremophiles can have an internal pH as low as 4.6 and as high as 9.5.

What is a buffer?

Buffers are added to microbial culture medias to keep the broth at a constant pH.

As bacterial cultures grow, they generally release waste products, i.e., organic acids or ammonia, that would affect the pH of the growing medium. If we let the bacteria simmer in its own waste, the pH would shift, and the bacteria would either stop growing or die all together.

In short: buffers prevent bacteria from making the culture medium uninhabitable.

Can you explain the osmotic effects on microbial growth?

• Water activity (a_w) is the amount of water that is actually usable in an environment.

  • It is defined as the ratio of the vapor pressure of air in equilibrium with the substance to the vapor pressure of pure water, mathematically defined as a_w=P/P_0

  • The scale is from (dehydrated) 0.00-1.00 (hydrated)

  • Adding sugar or salt to water “traps” the water molecules around the solute particles. This reduces the volume of “free water” available to the microbes.

• Typically, the cytoplasm has a higher solute concentration than the external environment, thus making the cell slightly hypotonic (positive water balance).

• When a microbe is in a hypertonic environment, water will flow out and kill the cell unless it has a mechanism to prevent this.

What is a halophile?

Halophiles are organisms that grow best under reduced water potential, meaning that a dehydrated environment is the best environment.

• Typically, different halophiles have specific NaCl concentration requirements

• There are organisms that are considered “extreme halophiles”, meaning that they require NaCl concentrations of 15-30% for optimal growth. For context, ocean water has a NaCl concentration of 3.5%.

What are halotolerant organisms?

Halotolerant organisms are organisms that grow the best in the absence of solute, but can tolerate some reduction in water potential.

What organisms thrive in an environment with high sugar concentrations?

Osmophiles

What organisms thrive in incredibly dry environments?

Xerophiles

How have halophiles structurally adapted to dry environments?

By increasing the internal solute concentration. This can be done by:

• Pumping inorganic ions from the environment into the cell

• By synthesizing compatible solutes: compounds used by the cell to counteract low water activity in the surrounding environment.

What type of organism requires oxygen to live?

Obligate aerobes

What type of organism can not survive in oxygen?

Obligate anaerobes

What type of organism can live with or without oxygen, but tends to prefer oxygen when it is available?

Facultative organisms

What type of organism can tolerate oxygen, even if it doesn’t use it?

Aerotolerant anaerobes

What type or organism can only use oxygen when it is present at tiny levels (levels that are lower than air)?

Microaerophiles

What techniques are required to culture obligate aerobes and anaerobes?

• Obligate aerobes require extensive aeration.

• Obligate anaerobes require reducing agents: chemicals that may be added to the culture media to reduce oxygen concentration (e.g., thyoglycholate)

What are the four primary factors that affect microbial growth?

• Temperature

• pH

• Water availability (Osmolarity)

• Oxygen concentration

What are chemoorganotrophs?

Chemoorganotrophs are organisms that obtain their energy from eating, i.e., breaking down organic substances (anything with carbon) like sugars, fats, proteins, and dead organisms.

• “Chemo-” = chemistry

• “-organo-” = organic, anything containing carbon

• “-troph” = nourishment

The nutrients are metabolized via cellular respiration in one of two ways:

1. Aerobes ⟶ terminal electron acceptor is oxygen (O2)

2. Anaerobes ⟶ terminal electron acceptor is nitrate, sulfate, CO2, and other inorganic compounds

What are chemolithotrophs?

Chemolithotrophs are organisms that obtain energy from minerals, i.e., inorganic compounds like H2, hydrogen sulfide, ammonia, and iron.

• “Chemo-” = chemistry

• “-litho-” = rock

• “-troph” = nourishment

This process is exclusive to prokaryotes.

What are phototrophs?

Phototrophs are organisms containing specialized pigments, i.e., chlorophyll, that allow them to use light as an energy source.

• Oxygenic photosynthesis produces oxygen as a by-product.

• Anoxygenic photosynthesis does not produce oxygen.

What do all living organisms require?

Carbon. If it’s alive, it needs carbon to survive.

What is an autotroph?

Autotrophs can “make their own food” by intaking carbon dioxide (CO2).

• Autotrophs are also referred to as primary producers.

What are heterotrophs?

Heterotrophs are organisms that can not produce their own food. Heterotrophs rely on one or more organic molecules for their carbon source, and feed directly on autotrophs or live off produces produced by autotrophs.