THIS WEEK IN ENVIRONMENTAL HEALTH




WEEK 3: SEPTEMBER 15 - 19, 1998



OBJECTIVES


At the conclusion of today’s session, you should understand:

1. Several types of interactions microorganisms have with other microbes and other hosts.

2. What "normal flora" is, and why it is important to our health and well-being.

3. What Koch’s postulates are, and why they are important

4. The fundamentals of transmission of microorganisms

5. Differences between direct and indirect transmission



Vocabulary -- Lecture # 3


normal flora --- microorganisms that live in a harmonious relationship with their host

pathogen --- a disease-producing microorganism

opportunistic pathogen--- microorganisms which usually do not produce disease in a healthy host

Koch’s postulates --- a set of procedures by which a specific organism can be related to a specific disease.

epidemiology --- the science of studying when and where diseases occur and how they are transmitted.

infectious disease --- a disease in which pathogens invade a susceptible host and carry out at least part of their life cycle in that host.

communicable disease --- a disease which can be transferred between two hosts.

contagious disease --- a disease that is easily spread from person to person (or animal to animal).

direct transmission (of a disease) --- a method of spreading infection from one host to another through some kind of close association of hosts.

indirect transmission (of a disease) --- the spread of pathogens by fomites (non-living objects)

host --- organism infected by a pathogen

reservoir(of infection) --- a human or animal that retains disease organisms in the body but has not experienced disease and shows no signs of illness



STUDY QUESTIONS
Lecture # 3


1. Match up the word with its proper combination of relationship interactions

_____ antagonism

_____ commensalism

_____ mutualism

a. Both "partners" benefit
b. One "partner" benefits; the other one is harmed
c. One "partner" benefits; the other is unaffected


2. Which of the following is not a biofilm?


3. Normal flora is:

a. Found in the mouth, throat, and large intestines of mammals
b. Protects against the invasion of pathogenic microorganisms
c. Can become pathogenic under certain conditions
d. All of the above
e. (a) and (b)


4. Louis Pasteur developed the process we know as "pasteurization" to save which industry first?

a. The dairy industry
b. The wine industry
c. The beer industry
d. The baking industry


5. The set of tests used to identify most microorganisms as the causative agent of a particular disease is called ________________ _________________________.


6. T or F: An infectious disease is always communicable. (Why?)


7. T or F: A communicable disease is always infectious. (Why?)


8. Select the statement below which best completes the following statement: Direct transmission of a disease...


9. Identify each of the following as a fomite or vector of disease transmission:


10. List two ways we can make microbes "visible" enough to study them.




THE GOOD, THE BAD, AND THE UGLY:
TRANSMISSION AND HOST RELATIONSHIPS OF MICROBES
Lectures # 3 - ENV 103


1. Microbial Interactions

Microbes do not like the single life... in fact, they operate better in the company of others. It is rare to find a single type or species or microbe alone in nature; in fact, in order to survive, they must be in close association with other microbes or plants or animals. These relationships are called symbiosis, and can be beneficial, harmful or neutral for the organisms involved. To become permanent member of any community, a microbial species interacts with and influences other organisms. To survive, it must compete for nutrients and overcome any competition. Sometimes its presence changes the environment for other organisms.

(overhead)

Neutral interactions are likely only when one organism is in a dormant state or when one organism is solely a support on which the second one grows. An example of this would be a mold spore, which is a dormant phase of mold, found on another organism.

Commensalism --- One species benefits and the other is not affected by the relationship. An example of this in the microbial world can be found in the human intestine, where billions of microorganisms are necessary for the complete digestion of food. Some of the beneficial bacteria here are anaerobes, bacteria which are killed in the presence of oxygen. Other bacteria present can tolerate both aerobic (with oxygen) and anaerobic environments and utilize the oxygen, thus protecting the anaerobic bacteria.

Mutualism --- Both partners in the relationship benefit here. An excellent example of this type of relationship is lichens. (Remember last week, one of our grab bag items was a lichen... you were stumped by it, and I said it was a combination of algae and a fungus.) Lichens are usually found in habitats that don’t have much in the way of nutrients or water. Thus, the algae, being photosynthetic, provides the fungus with oxygen and nutrients, while the fungus, may use its filaments to act like roots and protect its partner from dehydration and supply it with essential minerals. In this way, the two organisms can live in less than ideal environments. Lichens can be found on granite boulders, in deserts, and in Arctic areas.

Biofilms are another example of microbial mutualism. Many species join together and form complex aggregate communities, forming thin sheets of slime on the surface of objects. This can be ponds or the surface of your teeth. Or, they can be found in sewage ditches or contaminating surgical instruments.

Microbial mutualism can also occur between microbes and plants... nodules on clover roots for nitrogen fixation; fungi with plant roots, to increase ability to absorb from the soil.

With animals, microbes provide protection in the form of normal flora... microorganisms that live on body surfaces and protect a host from harm. Ex--- intestinal flora provide vitamins for host; aid in digestion of foods; autotrophic bacteria in marine animals (algae in jellyfish); luminescent bacteria (nocturnal or deep water fish)

Antagonism --- occurs when one organism harms the other one. Sometimes end products of microbial metabolism can prove toxic to its host. Antagonism between microbes can help a host however, as in the case of normal flora -- which compete with harmful bacteria and rob the bad guys of nutrients and space by growing faster or producing products (bacteriocins) that harm disease-causing organisms, called pathogens.

When a much larger host is invaded by a smaller one, the antagonistic relationship is call parasitism.

Many plants are destroyed by antagonistic relationships with microbes. (Think about a vegetable garden; rose mildew, etc.)

And then... the classic microbe/host antagonistic response: disease.

2. Normal flora and a delicate balance

(Ask one person to take off a shoe and hold it up) This shoe, which probably weighs between 2-3 pounds, represents the total weight of microbes found on your skin and in your body. If you went by the total number of microorganisms, there are 10 times the number of microbes as you have cells in your body. (Remember --- they are very small!) They are there all the time... and if they were not, you would be sick all the time. Why? Because these microbes act as your first line of defense against harmful microbes. These microbes are called normal flora, microorganisms that live in a harmonious relationship with their host, you. (What type of relationship is this? Mutualism)

(Overhead - normal flora sites)

These microorganisms "set up home" on different surfaces and provide a wide range of beneficial services. Not all surfaces upon which microbes reside are external; they are found inside your body also, for very good reasons. These surfaces include your nose and throat (the upper respiratory tract); your mouth; the large intestine and the urogenital system.

I have always sort of thought of all the surfaces with normal flora as having all once been on the outside... when the embryo begins to differentiate itself, there is a sphere of cells which sort of folds into itself. Three different cell layers are formed at this time: the ectoderm, the mesoderm, and the endoderm. To keep this simple, the ectoderm then becomes all surfaces within your body that contain epithelial cells: the skin; the mouth; the intestines; the mucus membranes. It’s like they have always been the outer layer, and as such, now welcome microbes as part of their make-up.

(Overhead & handout --- representative normal flora and where they live...)

So... how do these microbes protect you? The microbes that are resident on your skin produce byproducts that are antagonistic to potential pathogens.. Secretions from oil and sweat glands have antimicrobial properties also, and prevent potentially pathogenic bacteria from becoming infectious... usually! (We will look at some exceptions later...) Normal flora microorganisms protect against invading organisms by competing for nutrients; excreting byproducts that are toxic to these invaders (for instance, by creating an environment that is too acid for the invaders, as the lactobacilli do in the vagina) When this acidic balance is disrupted, it allows organisms, such as Candida albicans, a yeast, to overgrow the normal flora and cause an infection of the vagina.

We can see these relationships in the mouth, where the predominant normal flora of streptococci produce byproducts that prevent the growth of most other cocci. In the intestine, E. coli produce bacteriocins, proteins which inhibit the growth of certain pathogenic bacteria.

Now, normal flora isn’t always friendly; in fact, at times it can become a real problem. But this is usually when these normally friendly microbes find themselves in a part of the body which is normally sterile... a seemingly hostile environment for them, if you will... or if the host suffers from immunosuppression (as in AIDS or diabetes), a condition in which almost any microbe can become a problem. Thus, a normally friendly streptococcus, found as part of the normal flora of the throat and mouth, can find itself suddenly in the sterile blood stream due to a mouth injury, and cause bacteremia, an infection of the blood. Such microorganisms are called opportunistic pathogens, or organisms that ordinarily do not cause disease in their normal habitat in healthy individuals.

Throughout this course, we will be talking about environmental factors that can cause individuals to become more susceptible to pathogenic microorganisms... and also what we can do to prevent these invasions of the systems of our bodies.

3. Transmission of microorganisms

How many of you are going into early childhood education? (Show of hands...) Okay, folks... this next topic should be of utmost importance to you... because it will be the #1 reason for absences from your schools; absences among your staff; and absences due to family members of your charges.

Let’s start out with a song that goes along with a game that you will probably be using with your children... one which most of us, I’m sure, played when we were small. Remember back when you formed a ring by holding hands with your classmates, and you skipped and hopped around in a ring while singing:

"Ring around the rosies,
Pocket full of posies,
Ashes, ashes,
All fall down."

This song dates back to the 16th-17th centuries, and that third line was really "achoo; achoo".... and it was a poem recited at funerals.... funerals for victims of bubonic plague. You see... the ring of rosies refers to the rose-shaped splotches on the torsos and armpits of plague victims. Pocket full of posies is in reference to herbs and flowers that people carried, hoping to ward off the disease. "Achoo, achoo"... the sneezes by which the plague bacteria were spread among the population... and all fall down... the almost inevitable deaths that occurred.

Yet the nursery rhyme still is true today in many instances, in the way that microorganisms are transmitted from person to person. Three-four hundred years ago, the people did not know anything about diseases being caused by microorganisms, but they had a pretty good grasp about the fact that disease could be transmitted.

Remember last time we talked about the "Golden Age of Microbiology" being that period during which massive advances were made in formulating modern microbiology. We spoke of Pasteur and his discoveries of the relationships between microorganisms and the wine industry. Another very important person of this era was Robert Koch, who developed a scientific system of identifying a particular organism as the causative agent of a disease. His initial study proved that the bacterium, Bacillus anthracis, was that causative agent of anthrax. He then used the same method to prove that Mycobacterium tuberculosis was the pathogen involved in tuberculosis, or consumption. A simplified version of his theory, now known as Koch’s postulates, states that:

(overhead & handout ---)

a. The same pathogen must be present in every case of the disease

b. The pathogen must be isolated from the diseased host and grown in the laboratory in a pure culture(no other organisms are present)

c. The pathogen from the pure culture must cause the original disease when it is inoculated into a healthy laboratory animal

d. The pathogen must be isolated from the inoculated animal and must be proven to be the original organisms.

Exceptions --- when organisms cannot be grown outside of the host.... T. pallidum and Myco. leprae or certain viruses.... can be seen in tissue and tissue inoculated causes disease.

Okay... once a disease is identified... how is it transmitted? Sometimes this is easy to determine; sometimes it’s pretty difficult. An entire branch in science is devoted to this subject: epidemiology, or the science of studying when and where diseases occur and how they are transmitted. To me, epidemiologists are truly detectives... being given a few unconnected clues, they slowly match all of the pieces and find the missing ones, to present a complete picture of a disease. Tracking the trail of microbes has been central to the plots of two of this year’s best sellers --- Patricia Cornwall’s "Unnatural Causes" (smallpox on Smith Island, VA) and Tony Hillerman’s current book, "The First Eagle" (bubonic plague in the SW US.)

(Story about hantavirus... epidemiologist who was also a Navaho... who figured out the source of hantavirus pulmonary syndrome)

In order to begin on any investigative journey, we have to understand the language of the epidemiologist. Here are some of the most important things to under stand:

(overhead)

INFECTIOUS DISEASE: A disease in which pathogens invade a susceptible host and carry out at least part of their life cycle in that host.

COMMUNICABLE DISEASE: A disease which can be transferred between two hosts.

CONTAGIOUS DISEASE: A disease that is easily spread from person to person (or animal to animal).

DIRECT TRANSMISSION (of a disease): A method of spreading infection from one host to another through some kind of close association of hosts. Examples:
· measles, influenza, strep throat (through droplets from sneezes or coughs)
· chicken pox, herpes (through contact with fluid from lesions)
· AIDS, gonorrhea, hepatitis B and C, rabies, leptosporosis (through contact with contaminated bodily fluids)

INDIRECT TRANSMISSION (of a disease): The spread of pathogens by fomites (non-living objects) Examples:
food poisoning from

· Salmonella. E.coli, Staph. aureus (infected host contaminates foods during processing or preparation)
malaria, yellow fever, dengue fever; equine encephalitis (transmitted from person to person by a insect · vector [living organism that transmits disease organisms])
· cryptosporosis (from water contaminated with infected human waste)
· AIDS, hepatitis B or C (through vectors as contaminated needles or instruments)

(overhead and handout: how diseases transmitted)

Summary: overhead and handout --- how diseases transmitted. --- which of these are direct; which are indirect?

Add questions: common cold through droplets on hands. Is this direct or indirect and why? Bubonic plague --- pneumonic plague explain differences, and then discuss direct vs. indirect.

Why should we know about such diseases in a course in environmental health?

Next week --- epidemics through the ages and how microbes can cause diseases.





Click here to go to Lecture #4!



Click here to fly back to the home page!