Microbiology Lab

Exercise 26: Temperature: Effects on Growth

Exercise 27: Temperature: Lethal Effects

Exercise 28: pH and Microbial Growth

Exercise 29: Water Activity and Osmotic Pressure

Ex. 26: Temperature: Effects on Growth

Enzymes (catalysts for metabolic reactions) have a minimum, optimum, and maximum temperature for activity.

• Maximum enzyme activity will occur at the optimum temperature.
• At temperatures above the maximum, the enzymes will denature and lose activity.

Based on their temperature requirements, bacteria are divided into four groups…

• Psychrophiles – optimal growth between -5˚C and 20˚C
• These bacteria are found in Arctic and Antarctic waters
• Mesophiles – optimal growth between 20˚C and 50˚C
• Most bacteria are mesophiles
• Normal body temperature is 37˚C
• Most pathogens grow between 35˚C and 40˚C
• Psychotrophs – mesophiles that can grow at 4˚C, refrigerator temperature, and cause food spoilage
• e.g. Proteus, Pseudomonas, Campylobacter, Leuconostoc
• Thermophiles – optimal growth between 50˚C and 80˚C
• These bacteria are found in soils and compost piles
• Hyperthermophiles – optimal growth above 80˚C
• These bacteria are found near thermal vents in the ocean
• Which of these is most likely to cause disease in humans? ____________________

Ex. 27: Temperature: Lethal Effects

This allows us to compare the susceptibility of different bacteria to elevated temperatures. Two methods of comparison are used…

• Thermal death point (TDP) – the temperature at which an organism is killed in 10 minutes.
• Thermal death time (TDT) – the time required to kill a suspension of cells or spores at a given temperature.

Ex. 28: pH and Microbial Growth

The concentration of hydrogen ions is commonly expressed in terms of the pH scale.

· Low pH corresponds to high hydrogen ion concentration.

o We call such solutions acidic.

· High pH corresponds to low hydrogen ion concentration.

o We call such solutions basic or alkaline.

· The pH scale is a negative logarithmic scale with a base number of ten.

o Each decrease in pH by one pH unit means a tenfold increase in the concentration of hydrogen ions.

o Thus, a solution with a pH of 2 is 10 times more acidic than a solution with pH of 3.

o How much stronger of a base is a solution with pH of 10.3 than a solution with pH of 8.3? __________________

· Some substances enable solutions to resist pH changes when an acid or base is added. Such substances are called buffers.

o Buffers are very important in helping organisms maintain a relatively constant pH.

Below is a pH chart that will give you some understanding of pH values. (You are NOT expected to memorize these pH values.)

The hydrogen ion concentration affects proteins and other charged molecules in the cell. Based on their temperature requirements, bacteria are divided into four groups…

· Most bacteria are neutrophiles, which live within the pH range of 5 to 8.

· Acidophiles grow optimally at a pH below 5.5.

· Alkaliphiles grow optimally at a pH above 8.5.

o Which of these is most likely to cause disease in humans? _______________________________

o Helicobacter pylori is a bacterium that is capable of growing in human stomachs. Which term would best describe this bacterium? _______________________________

Ex. 29: Water Activity and Osmotic Pressure

The availability of water in an environment can profoundly affect the growth of bacteria.

Osmosis – the diffusion of water from an area of high concentration, across a semi-permeable membrane, to an area of low concentration.

Microorganisms can be grouped based upon their ability to cope with extreme osmotic pressure into the following groups…

• Halophiles – require high concentrations of salt to grow. They are found in salt lakes.
• Halotolerant – capable of growing in moderate concentrations of salt.
• Given the following bacteria, S. aureus, E. coli, and H. salinarium, rank them from least to most salt tolerant.
1. Least tolerant = ____________________
2. Mid-tolerance = ____________________
3. Most tolerant = ____________________
• Osmophiles – able to grow in environments where sugar concentrations are excessive.
• These organisms can ruin maple syrup, jams, and jellies. They are typically fungi (such as yeast).

Why does water move into and out of cells?

A. It depends upon the type of solution the cell is in.

1. Water is a solvent. Anything dissolved in water is a solute.

2. As the solute concentration in a solution increases, the water

concentration decreases & vice versa.

B. We are going to look at an imaginary bacterial cell.

1. Our cell will be 50% water (we will use o’s to represent water).

2. Our cell will be 50% solutes (we will use x’s to represent the solutes).

C. First, let’s put our cell into a hypertonic solution – one that has a

lower water concentration than the cell.

1. We will put the cell into a solution that is 75% solutes and 25%

water.

a. Water moves from higher to lower concentration.

b. The cell is 50% water, the solution is 25% water.

c. There will be a net movement of water out of the cell into

the solution.

2. In hypertonic solutions, water will exit the cell.

a. The cell’s plasma membrane will shrink away from the

cell wall.

b. This is known as plasmolysis. The cell will die.

D. Next, let’s put our cell into an isotonic solution – one that has the

same concentration of water as the cell.

1. We will put the cell into a solution that is 50% solutes and 50%

water (just like the cell).

a. Water moves from higher to lower concentration.

b. However, the cell is 50% water and the solution is 50%

water. Therefore, there is no area of higher or lower water

concentration.

c. There will NOT be a net movement of water.

2. In this solution, the plasma membrane of the bacterial cell will be

pushed up against the cell wall.

E. Finally, let’s put our cell in a hypotonic solution – one that has a

higher water concentration than the cell.

1. We will put the cell into a solution that is 25% solutes and 75%

water.

a. Water moves from higher to lower concentration.

b. The cell is 50% water, the solution is 75% water.

c. There will be a net movement of water into the cell

from the solution.

2. In these solutions, animal cells swell & burst.

3. However, bacterial cells have a rigid cell wall that prevents this.