This Module focuses on what microbes need to survive and grow, how we measure their growth, and how we control them. Microorganisms have nutritional requirements for growth as any other living being does in this world. Organisms use different sources of energy to grow and reproduce, maintain their structures, and respond to their environments. Their ability to convert energy that they obtain from their environment into useable forms is called metabolism. There are many metabolic pathways, or chemical reactions, that organisms use to convert food into the molecule ATP, which can be used anywhere in the cell to drive other reactions.
There are many ways in which an organism can metabolize the resources they acquire. Many simple microbes such as bacteria and fungi accomplish fermentation, which does not require oxygen. This is an example of anaerobic respiration. Many organisms, including ourselves, can accomplish aerobic respiration, which requires oxygen and yields large amounts of ATP. One important difference between anaerobic and aerobic respiration, is the presence of an advanced cell membrane transport system called the electron transport chain. It is like a factory with a turbine that produces ATP.
Microorganisms are also distinguished by the manner in which they react to atmospheric oxygen. Aerobic organisms obtain their oxygen from the atmosphere. They live in the presence of atmospheric oxygen. Anaerobic organisms are inhibited and even die when exposed to atmospheric oxygen. Facultative anaerobes are microorganisms that can grow either in the presence or absence of atmospheric oxygen.
We use this information about a given microbe to identify it based on its metabolic pathways, cellular composition, growth patterns, and sensitivity to chemicals (including oxygen). These characteristics are considered in phenotypic systems of classification.
Controlling Pathogenic Microorganisms
If we know what an organism needs to survive, and what chemicals it is sensitive to we can control it! One aspect of controlling microorganisms that is very important in public health is the control of pathogenic microorganisms. Pathogens are important because they spread disease among populations and cause significant death within the world today. As we have witnessed with the SARS epidemic, for example, the failure to control acute infectious diseases caused panic and extreme concern among virtually everyone with knowledge and at risk for possible infection.
In public health we are not trying to control all microorganisms. We focus our efforts on controlling the pathogens. One aspect of pathogenic microorganisms that you should keep in mind as you review this course is that pathogens thrive under conditions which are conducive for growth in the human body. If we can control the various modes of transmission, that is the route that infectious disease takes to enter our bodies, then we may be able to minimize our risk to infectious disease.
To illustrate this point, let’s look at the controls for lowering the risk of a foodborne pathogen causing foodborne illness to persons exposed to the pathogenic bacteria or virus. The basic strategy for controlling foodborne illness is based on controlling the environmental factors that the foodborne pathogens require for growth. These factors are as follows:
Foodborne pathogens require a source of nutrients. Specifically, they need foods rich in nutrient value such as proteins and carbohydrates. The principle relating to this requirement is that foods rich in proteins and carbohydrates must be protected and managed with a higher degree of safety than other foods.
pH Value – Although most bacteria are relatively insensitive to concentrations of hydrogen and hydroxyl ions, the pathogenic bacteria do not develop well at unfavorable pH values. In fact, pH values beyond specified limits are toxic to these bacteria. Most bacteria can tolerate pH values between 6.0 and 9.0. What does this mean when we are attempting to control foodborne pathogens? For those foodborne pathogens that can be controlled by adjusting the pH in a food substance we can eliminate/control that particular pathogen. Botulism is controlled by this method.
Temperature – Bacteria are found to exist and at times even thrive within a wide range of environmental temperatures. However, pathogenic bacteria usually propagate within a much narrower range of temperatures. These bacteria are controlled and eliminated in a food substance by maintaining unfavorable temperatures for growth by freezing or refrigerating. Most foodborne microorganisms grow well between the temperatures of 35 degrees F to approximately 110 degrees F. Of course, they develop and quickly thrive at temperatures close to the average temperature of the human body. After all they are pathogens!
Growth Time – Bacteria grow at a constant rate in a favorable environment. The key to controlling pathogenic bacteria in food is to prevent the bacteria from reaching the exponential or logarithmic phase of growth. Once the pathogen reaches the exponential growth phase the numbers of bacteria in the food substance will quickly reach an infectious dose.
Oxygen requirements – Different pathogens have differing oxygen requirements for growth. Depending on the specific pathogen targeted for control, the atmospheric oxygen may be managed. For instance, if the targeted pathogen is Salmonella then it is important to store the food source in an oxygen-free environment, since Salmonella are generally aerobic bacteria.
Water activity (aw) – Since most foodborne pathogens require some water for growth, the infectivity of the pathogen can be reduced if the water activity level is below 0.85. Dehydration is an example of controlling microbes by reducing water availability. This is why dehydrated fruit can be safely eaten for longer periods of time, but fresh fruit must be eaten right away.
The above is an example of how knowledge of the nutrients and environmental factors which pathogenic bacteria need for growth can be used to control their growth and protect the public’s health. In your Case Assignment you will continue to examine how we measure growth and chemical methods of pathogen control. There are biologic controls that humans have for protection against pathogens as well.
In the Module 3 Case Assignment you will explore bacterial growth and methods used to measure and control the growth of microbes. After you have read the information on the Home page, answer the following questions:
- From the video on metabolism, where do they say metabolism is accomplished in the human cell? Be specific and include a brief description of the organelle they describe. Do we accomplish aerobic or anaerobic metabolism?
- What does the video describe as the final electron acceptor in the electron transport chain?
- Using the dichotomous key linked to the term “identify” on the Home page, which organisms in the first frame of the video have been described in previous module materials?
Now choose a pathogenic bacteria to discuss and find the antibiotic that can be used to control it. Use this link to Antibiotic Modes of Action to learn more about how antibiotics work and to identify the appropriate antibiotic for your chosen pathogen. Use the following format to write this section of your Case Assignment:
Include the growth requirements and phenotypic characteristics of your pathogenic bacteria. Refer to the dichotomous keys, growth patterns, and the CDC website to properly discuss these phenotypic descriptions of your pathogen.
Body Paragraph 1
Is this bacteria ONLY pathogenic or does it reside as a normal part of the Human Microbiome? If it is also a nonpathogenic resident microbe, what conditions create the pathogenic state? (We use the term “opportunistic pathogen” to describe this type of pathogen).
Body Paragraph 2
Describe the mode of action of the antibiotic class used to control this bacteria.
Summarize the topics you described and explain how the characteristics of your pathogen can be used to control it.
Please include a cover sheet for your Case Assignment. Use headings to separate each section’s questions, and answer each question using complete sentences. Use your own words and build on the ideas of others. Quoted material should not exceed 10% of the total paper (since the focus of these assignments is critical thinking). When material is copied verbatim from external sources, it MUST be enclosed in quotes. The references should be cited within the text and also listed at the end of the assignment in the References section (preferably in APA format).
References: List the references used to address the second part of this assignment. The references should be listed at the end of the assignment in a References section (preferably in APA format).
Organization: Subheadings should be used to organize your paper according to question.
The following items will be assessed:
Assignment-Driven Criteria – Did you address each requirement?
Critical Thinking – How well did you synthesize and evaluate the topics addressed?
Scholarly Writing – Is everything explained in complete sentences?
Quality of References and Assignment Organization – Did you organize your paper with headings?
Citing Sources – Did you list your references and cite information where necessary?