6.2: Temperature, pH, and Osmotic Requirements
Learning Outcomes
- Recognize the cardinal temperatures of growth for bacteria
- State the conditions for classification of psychrophiles, mesophiles, thermophiles, and hyperthermophiles
- Recognize the pH requirements for bacteria
- Recognize the osmotic requirements for bacteria
Environmental Requirements: Temperature
How does temperature affect bacterial growth?
Organisms grow best over a certain temperature range, and this range has restrictions. The cardinal temperatures are the range of temperatures over which an organism can grow. Every organism has evolved to live at a particular optimum temperature.
- Minimum: lowest temp where reproduction occurs
- Maximum: highest temp where reproduction occurs
- Optimum: highest rate of reproduction
Organisms are classified based on the temperature ranges they live in:
- Psychrophiles: less than zero
- Psychrotrophs: 0-30°C
- Mesophiles: middle temperatures 15-45°C
- Thermophiles: 40-80°C
- Hyperthermophiles: above 65°C
Microorganisms require a temperature growth range dictated by the heat sensitivity of its cellular components. As a result, microbial growth has a characteristic temperature dependence with distinct cardinal temperatures ---the minimum, optimum, and maximum, temperatures at which it can grow. The optimum temperature is usually correlated to its natural habitat.
Image 1 and 2: General temperature ranges compared to growth rate
Environmental Requirements: pH
How does pH affect bacterial growth?
Hydrogen ions in a solution = pH. Organisms grow best at a specific pH range based, in part, on the environment they have evolved to live in. If bacteria are outside their optimal pH range their proteins can become denatured. Ranges of pH over which an organism can live place them in groups:
- Acidophiles: below pH 5.5
- Neutrophiles: pH 5.5 -8.5
- Alkaliphiles: pH above 8.5
- The pH is another environmental condition that dictates microbial growth. pH impacts the activities of enzymes and each microbial species has a pH growth range. Acidophiles have a growth range between pH 0.0-5.5, neutrophiles grow btween 5.5 and 8.5, while alkalophiles do best between 8.5-11.5 (or higher). Generally, different microbial genera have characteristic pH optima ranges. The majority of bacteria are neutrophiles while molds and yeasts tend to prefer slightly acidic environments with a pH range of 4-6. Many bacteria produce acids as part of their metabolism, and this can lower the pH of their environment. One excellent example of this are the Lactic Acid Bacteria (LAB); a large and diverse group of Gram-positive bacteria that produce lactic acid as the major end product of the fermentation of carbohydrates. The lactic acid can inhibit the growth of pathogenic and food spoilage microorganisms in food. Thus the LAB play a significant role in food fermentation, contributing to a wide variety of fermented products (ie. cheese, yogurt, meat, fish, fruit, vegetable and cereal products). Their breakdown of various carbohydrates, proteins, and lipids contribute to the flavor, texture and nutritional value of the fermented foods.
Image 3 : pH Classification for microorganisms.
Environmental Requirements: Salinity
How does osmotic pressure affect bacterial growth?
Water is essential to all organisms. The ability to control the movement of water across a membrane is necessary for the survival of all cells. Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semi-permeable membrane. The movement of water is controlled by the concentration of solutes contained within the water (usually salt). Bacteria can be classified based upon the salinity they can tolerate:
- Halophiles (prefer NaCl concentrations of 3% or higher)
- Extreme halophiles (prefer NaCl concentrations of 15%-25%)
- Xerophile (prefer low salt concentrations)
Image 4: Impact of various salt concentrations on movement of water into/out of a cell.
Quantifying Bacterial Growth:
How do we quantify bacterial growth?
Often in microbiology, we need to determine the number of bacterial cells in a broth. We can do this directly through spread plates (Chapter 5: Enumeration of bacteria) or indirectly by assessing the turbidity (cloudiness) of broth tubes. We measure turbidity using a spectrophotometer which gives us a reading of the light absorbance:
- more bacteria = more cloudy = higher absorbance
- less bacteria = less cloudy = lower absorbance