Lesson 10:
Physical and Chemical Control of Microbes




In this lesson we will learn the following:

  • Types of physical and chemical control for microbes.
  • How to determine the death rate of various microbes.



Reading Assignment

In addition to the online lecture, read chapter 11 in Foundations in Microbiology.




The wastewater operator's goal is to provide the optimal environmental conditions so that beneficial microorganisms will grow quickly while harmful microorganisms die.  In this lesson, we will learn several ways to control microbial growth.


Controlling Microbial Growth

It is very important to control microbial growth in surgical and hospital settings, as well as in industrial and food preparation facilities. There are many terms used to describe the fight to control microorganisms.

Sterilization is the destruction of all microorganisms and viruses, as well as endospores. Sterilization is used in preparing cultured media and canned foods.

Aseptic means to be free of pathogenic contaminants. Examples include proper hand washing, flame sterilization of equipment, and preparing surgical environments and instruments.

Disinfection is the destruction or killing of microorganisms and viruses on nonliving tissue by the use of chemical or physical agents. Examples of these chemical agents are phenols, alcohols, aldehydes, and surfactants.

Sanitation is the treatment to remove or lower microbial counts on objects such and eating and drinking utensils to meet public health standards. This is usually accomplished by washing the utensils in high temperatures or scalding water and disinfectant baths.



Microbial Death Rates

Microbial death is the term used to describe the permanent loss of a microorganism's ability to reproduce under normal environmental conditions. A technique for the evaluation of an antimicrobial agent is to calculate the microbial death rate. When populations of particular organisms are treated with heat or antimicrobial chemicals, they usually die at a constant rate.

The effectiveness of antimicrobial treatments is influenced by the number of microbes that are present. The larger the population, the longer it takes to destroy it. The different variations of certain microorganisms influence death rate because, for example, endospores are difficult to kill.

Environmental influences, such as the presence of blood, saliva, or fecal matter, inhibits the action of chemical antimicrobials. Time of exposure to heat or radiation is also important. Many chemical antimicrobials need longer exposure times to be effective in the death of more resistant microorganisms or endospores.



Action of Antimicrobial Agents

There are two categories that chemical and physical antimicrobial agents fall into: those that affect the cell walls or cytoplasmic membranes of the microorganism and those that affect cellular metabolism and reproduction. As mentioned earlier in this course, the cell wall is located outside the microorganism's plasma membrane. The cell's plasma membrane regulates substances that enter and exit the cell during its life. Nutrients enter the cell as waste products exit the cell. Damage to the plasma membrane proteins or phospholipids by physical or chemical agents allow the contents of the cell to leak out. This causes the death of the cell.

Proteins act as regulators in cellular metabolisms, function as enzymes and form structural components in cell membranes and cytoplasm. The function of a protein depends on its three-dimensional shape. The hydrogen and disulfide bonds between the amino acids that make up the protein maintain its shape. Extreme heat, certain chemicals and very high or low pH can easily break some of these hydrogen bonds. This breakage is referred to as the denaturing of the protein. The protein's shape is changed, thus affecting the function of the protein and ultimately bringing death to the cell.

Certain chemicals, radiation, and heat can damage nucleic acids. The nuclear acids, DNA and RNA, carry the cell's genetic information. If these are damaged, the cell can no longer replicate or synthesize enzymes.

Electrolyte Concentration

As you will remember, water passes into or out of a cell through osmosis, with the direction and amount of water movement depending on the difference in electrolyte concentration between the cell and its environment.  Cells which are well adapted to their environment have internal electrolyte concentrations very similar to the electrolyte concentrations found in the surrounding environment.  The state in which a cell is in equilibrium with its environment is known as isotonic.  In an isotonic environment, very little water passes into or out of the cell. 

Isotonic and hypotonic environments.

When a cell is moved from an isotonic environment into an environment with a different electrolyte concentration, the cell can either burst or deflate.  An example of the former situation occurs when a microorganism adapted to living in saltwater is placed in freshwater, as is shown in the picture above.  In the freshwater, the electrolyte concentration is suddenly much greater inside the cell than outside the cell - a situation known as hypotonic.  In a hypotonic environment, osmosis forces water into the cell so quickly that the cell can actually burst out of its membrane. 

Isotonic and hypertonic environments

In contrast, when a freshwater microorganism is placed in saltwater, it is in a hypertonic environment.  In a hypertonic environment, the concentration of electrolytes is greater outside the cell than inside the cell.  As a result, water rushes out of the cell and the cell collapses in on itself like a deflated balloon in a process known as plasmolysis

Although rapid changes in environmental electrolyte concentration typically kill the cell, many microorganisms are able to adapt to slow changes in electrolyte concentration.  In the wastewater treatment plant, rapid changes in electrolyte concentration seldom occur, so electrolyte concentration is usually not a very important consideration.  Humans have learned to take advantage of plasmolysis, however, by pickling meats and vegetables in a strong salt solution, preserving the food by killing all of the bacteria present. 


Another environmental variable which affects the growth of microorganisms is pH.  The environment's pH will influence the speed with which chemical reactions take place in the cell.  pH also influences the ability of the cell to transport objects through its membrane.  Finally, and perhaps most importantly, pH influences the ability of enzymes to function. 

Normal and denatured enzymes

Enzymes are proteins which help the cell perform chemical reactions by holding two or more chemicals together in close proximity.  The animation above illustrates the action of a normal enzyme.  Enzymes are essential to many of the chemical reactions taking place in a typical cell, including the reactions involved in respiration. 

Animation of a denatured enzyme.

When the pH of the environment changes, enzymes can become denatured, meaning that the enzymes change shape and can no longer hold chemicals together.  Denatured enzymes are not able to aid in chemical reactions, so the microorganism will be unable to make energy and will die. 

The optimum pH for biological wastewater treatment systems is between 6.8 and 7.4.  Changes in pH of the wastewater will significantly inhibit the growth of microorganisms and will reduce plant efficiency.  In addition, a change in pH can temporarily favor the growth of other microorganisms which may be less beneficial to the treatment process.  For example, more acidic conditions promote the growth of fungi which do not settle very well and thus cause serious operational problems at the plant.  

Changes in pH of wastewater can have a variety of causes, ranging from the influx of industrial waste to natural processes within the wastewater.  When sugars are present in high concentrations in wastewater, some microorganisms will create organic acids during respiration, thus lowering the pH of the water.  However, nature also has a buffer system to prevent changes in the water's pH.  The carbonic acid/bicarbonate buffer system is able to resist a change in pH if the water has a high enough alkalinity.  New wastewater treatment systems often have a low buffer capacity which is not able to deal with changes in pH, but this buffer system will build up over time. 


Temperature, like pH, affects microorganisms by affecting the chemical reactions taking place within the cell.  Both high and low temperatures can be problematic to microorganisms, and each species has a temperature range in which growth is optimized. 

Cold temperatures affect microorganisms by slowing down chemical reactions.  Most microorganisms will not be killed by low temperatures, but their growth and reproduction will slow down or cease completely when temperatures drop below a certain point.  When microorganisms are moved from a cold environment to an optimal environment, they are able to resume their normal life cycle. 

Heat can be more problematic to microorganisms than cold, though the rise in temperature is initially favorable.  As the environment is heated, the chemical reactions within each microorganism accelerate, allowing the microorganism to grow and reproduce more quickly.  For example, it is estimated that every
18°F (10°C) rise in temperature causes the speed of chemical reactions to double. 

Beyond a certain temperature, however, heat can be fatal to microorganisms.  At temperatures higher than about 1
67°F (75°C), there is no longer an increase in the rate of chemical reactions, and the extreme heat may denature enzymes or harm membranes.  Heat can be used to disinfect instruments in the lab because most cells are killed at high temperatures unless the microorganisms are able to enter a spore state.  Since moist heat is able to kill microorganisms at a lower temperature than dry heat, steam is often used to sterilize laboratory equipment. 

As with the other environmental variables, there is no single temperature at which all microorganisms thrive.  Scientists place bacteria into three groups based on their tolerance for heat and cold.  Many bacteria are called mesophilic because they live at moderate temperatures, primarily between
68°F and 113°F (20°C - 45°C).   Mesophilic bacteria can be pathogens which live within other organisms or can live in the soil or water.  Other bacteria, known as psychrophilic bacteria, prefer cold temperatures  below 68°F (20°C) and can be found in the snow or in refrigerators.  Finally, thermophilic bacteria prefer high temperatures above 113°F (45°C) and are found in hot springs, volcanoes, and ocean vents. 

Light can be either helpful or harmful to microorganisms.  Autotrophic microorganisms, such as algae, require light for survival.  However, ultraviolet light can kill many microorganisms by disrupting their DNA. 

Ultraviolet light, or UV light, is light with a wavelength less than 400 nm.  Although ultraviolet light is part of the light normally emitted by the sun, the amount of UV light in sunlight is usually too low to kill microorganisms.  However, some water and wastewater treatment plants are able to use a more intense form of UV light to disinfect water. 

Disinfection using UV light
Disinfection using UV light.

Disinfection using UV light has certain limitations based on the fact that the light rays must strike microorganisms directly in order to kill the cells.  Turbidity in water scatters UV light and makes the treatment ineffective.  In addition, glass and thick layers of water can prevent the movement of UV light, so UV light cannot be used to disinfect flasks or test tubes of water, nor can it be used to disinfect deep bodies of water. 

Chemical Agents

A toxin is any chemical which is harmful to an organism.  In the water and wastewater treatment fields, operators may use toxins known as disinfectants to purposely kill pathogenic organisms in the water.  Alternatively, pollutants in the water can act as toxins, shocking biological treatment systems in the wastewater treatment plant. 

The growth of a microorganism can be controlled through the use of a chemical agent. A chemical agent is a chemical that either inhibits or enhances the growth of a microorganism. A commonly used chemical agent is phenols.

Phenols are compounds derived from carbolic acid molecules and disrupt the plasma be denaturing proteins; they also disrupt the plasma membrane of the cell.

Halogens, a type of phenol, are nonmetallic, high resistive chemical elements. Halogens are effective against vegetative bacterial cells, fungal cells, fungal spores, protozoan cysts, and many viruses. The halogen we are concerned with, chlorine, is used to treat drinking water, swimming pools, and in sewage plants to treat wastewater.

Surfactants are chemicals that act on surfaces by decreasing the tension of water and distrupting cell membranes. Examples are household soaps and detergents.


Disinfectants are chemicals which kill disease-causing microorganisms.  There are two categories of disinfectants - germicides which kill microorganisms on contact and bacteriostatic agents which halt microbial reproduction without killing the microorganism.  In the water and wastewater treatment fields, most disinfectants are germicides.  The most commonly used germicide is chlorine, a chemical which kills most microorganisms but does not sterilize the water because it does not kill spores and viruses. 

Although operators purposely introduce disinfectants to water, a variety of other toxins make their way into natural waters and wastewater as pollutants.  Industrial wastes often have high concentrations of heavy metals such as zinc, mercury, lead, cadmium, copper, and chromium.  Other pollutants which can affect microorganisms include detergents, phenol, formaldehyde, and antibiotics.  Although microorganisms can adapt to the low levels of these pollutants which are usually found in wastewater, a sudden shock load of pollutants may overwhelm the microorganisms and adversely affect plant operation.  Once a biological treatment system has been upset, recovery is often very slow. 

Other Conditions

A host of other environmental conditions can also affect microbial growth.  As we mentioned in the last lesson, microorganisms can be divided into three groups depending on the level of oxygen they can tolerate in the environment. 

Another factor which can affect microbial growth is moisture.  Most microorganisms require a certain level of moisture in the environment to grow and reproduce.  Although some bacteria are able to form spores in a dry environment, they are not able to function normally until the water content of their environment returns to a more normal level.  We take advantage of this environmental variable when we preserve food by drying. 




It is very important to control microbial growth in surgical and hospital settings, as well as in industrial and food preparation facilities. Microbial death is the term used to describe the permanent loss of a microorganism's ability to reproduce under normal environmental conditions. There are various ways to control microbes ranging from temperature, to pH and chemicals. The effectiveness of antimicrobial treatments is influenced by the number of microbes that are present.




Betsy, Dr. Tom and Keogh, Jim. 2005. Microbiology Demystified . McGraw-Hill Publishing




Search the internet for possible answers to why high temperature destroys microbes but low temperature often does not. Submit your findings to the instructor via email, mail or fax.

In addition, the sixth Project paper is due this week.  Once you have completed the project either mail, fax, or email it to your instructor.




There is no lab associated with this lesson.




Answer the questions in the Lesson 10 quiz .  When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.