Updated: 10/31/96
Updated: 10/31/96
DISEASE
= Any change from a state of good health. Types of diseases include hereditary
diseases; dietary diseases; environmental diseases; and INFECTIOUS
DISEASES. In this section we will be concerned only with the
fourth type.
ETIOLOGICAL
AGENT = This is the organism (species
or strain) responsible for producing a disease.
SYMBIOSIS
= An association between two or more different organisms that has evolved
to a state where they are generally found in association with each other.
NORMAL
FLORA = The organisms normally found
on or in a healthy host MOST OF THE TIME.
For example, if we were to culture organisms in the mouths of every healthy
person in this audience, we would probably find that all of them had a
large percentage of the same microbes inhabiting their mouths.
PARASITE
= A type of symbiosis where an organism lives on or in a host & OBTAINS
NUTRIENTS from that host. Note that a parasite DOES
NOT NECESSARILY harm the host and in some cases their presence
is even beneficial to the host. However, in most of these cases ONLY
ONE of the associates TRUELY BENEFITS
from the association.
For example, older people, like yours truly, often have tiny mites that live in our ever expanding eyebrows. These insects apparently live off the dead skin cells, oil and excretions from our sweat glands and they do us no apparent harm or good. They are just one more inhabitant of the planet HURLBERT.
PATHOGEN
= A parasite that causes a disease in its host.
INFECTION
= Growth on or in a host. Again this may or may not cause harm to the host.
INFESTATION
= An infection with a larger (than single cell) organisms. Click
here for a really gross example of an infestation (I dare you!). Click
here for a picture of the pubic louse.
MUTUALISTIC
SYMBIOSIS = A situation where two or
more species live together in a situation where
BOTH
BENEFIT from the association. In some cases one or more the
symbiotic species are totally dependent on the other. Consider the following
when you are FRENCH KISSING your "significant other"; many microbial species
inhabit our mouths. Some of these species do us harm by contributing to
the production of cavities in our teeth, but many of them are thought to
be beneficial because they OUT-COMPETE harmful microbes that otherwise
would live in our oral cavity.
PATHOGENICITY
= The ability to CAUSE
A
DISEASE. Many organisms are pathogenic
under certain conditions, but some are almost always pathogenic.
For example, chicken pox and the flu are almost always pathogenic. However,
photosynthetic and nitrogen-fixing bacteria are never pathogenic.
VIRULENCE
= The DEGREE of pathogenicity. A virulent
organism is one that usually produces a violent and severe disease, often
leading to death. For example, chickenpox, while clearly a pathogen, is
not considered to be virulent. Whereas Yersinia pestis, the etiological
agent of bubonic plague, is virulent.
These two terms are sometimes used almost interchangeably which can lead to confusion. A practical way of differentiating them is consider how worried you would be if you were a non immune person exposed to a potential pathogen. If you find upon having been exposed to organism "X", you yawn, and think "I'll keep my eye on what happens, it is a pathogen, but one unlikely to do me serious harm". Whereas if you get so scared you are unable to poop, it is probably a VIRULENT PATHOGEN.
AVIRULENT
ORGANISM = An avirulent organism is the term used to describe
a virulent pathogen that has LOST its
ability to produce a disease.
OPPORTUNISTIC
PATHOGEN = A normal flora organism that NORMALLY
is not a pathogen, but that becomes pathogenic when inoculated into the
WRONG PLACE or when the host becomes
WEAKENED and IMMUNOLOGICALLY
COMPROMISED. An immunologically compromised host is one whose
immune system is NOT WORKING to its
full capacity and thus the host's defenses are weak. Generally people who
are undergoing chemotherapy for cancer treatment are immunologically compromised
because the drugs harm the immune system. However, people under stress,
such as STUDENTS AT EXAM TIME, often have a lowered immunity and are more
susceptible to catching infections. Do you tend to gets COLDS around exam
time?
I would enjoy taking you through some of the intricate
and extraordinary symbioses that have been discovered, but sadly there
is to little time. However, the next time you look at a cow munching grass
in a field producing your milk or next hamburger, consider that it is a
prime example of a mutualistic symbiosis between a mammal and microbes,
neither of which could exist on earth without the other. So ponder this
question; "Is the cow here to preserve the
many microbial species that require it as a host or are the microbes here
to maintain the cow?"
Figure 1. Location of major non-specific defense mechanisms.
SKIN: The skin is tough, dry, salty, oily, rich in fatty acids, low in nutrients (lots of dead, empty cells) & thick. The sweat glands secrete a mixture of salt, & fatty acids that inhibit many microbes. It also is home for a host of NORMAL FLORA organisms that are ANTAGONISTIC to potential pathogens. Also, the normal flora scarf up any potential nutrients that make their way to the skin surface. We are taught by our mothers from an early age to keep our skin clean, usually to forestall a serious antisocial ODOR PROBLEM. However, the real benefit of this training is that it serves to PROTECT US from potential pathogens. This was amply illustrated during the 1960s when some "HIPPIES" decided to forgo cleanliness as a protest against the establishment (talk about shooting yourself-in-the-foot). The only problem was that they were not very MICROBIOLOGICALLY SMART (in contrast to you), since the only thing this protestation helped was the education of the physicians that saw some skin diseases that hadn't been seen in the western world since the MIDDLE AGES. It seems that by not washing, the balance on the skin was disturbed and pathogens flourished in the rich nutrient provided by a protester's smelly, dirty bodies (kind of takes the bloom off the hippie idea of "free love", doesn't it?). Remember, MOTHER IS USUALLY RIGHT, even for the wrong reasons.
A
FAQ is "Can I wash too much?"
ANSWER: Of course you can--one can do anything too much (and many of us do). However, outside of taking 20 showers per day and using sandpaper or a scouring pad to clean with, it is unlikely that any of us are going to overdo cleanliness. My observations of 63 years indicate that the trend is clearly in the opposite direction.
In the early submarines there were no washing amenities, so crews wore the same clothes and did not bath throughout the entire time at sea. It must have been impressive when they returned to base & opened the hatch to visitors.
MOUTH AND GASTRO INTESTINAL TRACT: Basically we are designed as an inverted tube. The skin on the outside forms a long tube which makes up our DIGESTIVE SYSTEM (Fig. 1). This means that the cells lining this tube are a form of skin cells. As discussed above, the mouth harbors a host of microbes that live more or less permanently, on the surface and in the nooks and crannies of the mouth tissues. These microbes are symbiotic and usually do us little harm as long as we remain healthy. Regardless of how thoroughly we brush our teeth this NATURAL FLORA remains literally attached to us. As you might guess from this description, mouth microbes have evolved elaborate systems for "sticking" to things: remember pili? By preventing other, potential pathogens from establishing themselves, they protect us. In addition, there is a continuous flow of fluid (saliva) through the mouth which FLUSHES loose microbes into the stomach.
The stomach (Fig. 1), as anyone who has barfed, or belched strongly, knows, contains a strong (hydrochloric) acid. Many microbes are killed by this acidic environment and digested by the proteolytic enzymes in the digestive system. Until fairly recently it was even suggested that the stomach was essentially sterile due to the low pH. However, with the discovery of the bacterium Helicobacter pylori that lives happily in the stomach and causes dreadful things like ulcers and stomach cancer, we know this is not the case. This knowledge is not being applied as rapidly as necessary to the treatment of ulcers.
The small intestine (Fig. 1) is full of DIGESTIVE ENZYMES and DETERGENTS (bile) that agreeably digests microbes as well as hamburger & pizza. Further, even though the small intestine may be full of nutrients, the adsorption system of the body is so efficient that these nutrients are ABSORBED from the intestine so rapidly that residential microbes have little to live on. Further, the intestines are anaerobic, so obligate aerobes are unable to grow there even if they should survive the trip through the stomach.
The large intestine (Fig. 1) is a different story as it collects and processes undigested material that passes through the small intestine. Bacteria, including some very nasty pathogens, grow robustly on this debris and yet they rarely manage to invade the body. The wall of the large intestine is coated with a PROTECTIVE MUCOUS LAYER that separates the contents from direct contact with the cells lining the large intestine. The normal flora of the large intestine evolved to live on the available food supply in the anaerobic conditions found there. Feces are approximately 40% bacteria by weight.
FAQ: "Are the intestinal gases inflammable?"
ANSWER: Yes, the anal-gases we vent are indeed highly flammable, so I wouldn't recommend demonstrating it to your friends unless you have a fire extinguisher available. For more information on this interesting subject see Microbes in the News.
One problem with antibiotics taken by mouth is that they upset the natural microbial balance and allow unusual microbes to establish them selves in our bowel. This often results in intestinal problems (e.g. excess gas and diarrhea) until the original mix of microbes is again established.
GENITOURINARY TRACT: This region of the body is a rich source of infection for obvious reasons. The urine is a good nutrient for many microbes (Fig. 1). In addition the kidneys provides numerous nooks and crannies where microbes can hide from the body's defense systems. Sexual activity SIGNIFICANTLY INCREASES the exposure to potential pathogens. The efficiency of our NDS is shown by the fact that we don't suffer more urogenital infections than we do. Microbes are prevented from reaching the bladder and kidneys mainly due to the vigorous flushing of urine and mucous out of the body through the urethra. However, because the urethra is shorter in women than men, bladder infections are more common in women. Microbial pathogens that infect this region have adhesive pili that attach them to the cells lining the urethra and bladder.
The vaginal area (Fig. 1) is another region where infections are easily established. However, the vagina is normally acidic due to the growth of lactobacilli that produce lactic and acetic acid. Further, there is a continuous outward flow of mucous that expels microbes from the vagina. Finally the entry to the reproductive organs is blocked by a mucous plug much of the time. However, the vaginal lining is THIN AND EASILY DAMAGED by unsuitable physical activity, including the improper use of sanitary napkins, and its rich blood supply makes it an easy entry point for pathogens. The ease of damage to the vaginal lining is a major reason why women are more likely to become infected with the HIV from fewer exposures.
The anal area is another potential source of infection if the natural defenses are not sustained. The anal region is continually exposed to fecal microbes, some like the Clostridium perfringens can produce serious or even fatal diseases if they are introduced into the blood or tissue. The mucous covering, while protective as long as it is maintained, is easily trespassed as is the delicate anal membranes lining. Thus anal penetration by foreign objects are very dangerous and explains why STDs are so easily contracted by anal intercourse. Regardless of any moral perspective, anal intercourse is not biologically safe as evolution simply did not design it for that purpose.
RESPIRATORY TRACT: The lungs offer a rich source of nutrients and a great potential for concealing pathogens (Fig. 1). Everyday we breath in hundreds of liters of air contaminated with dust, pollen and microbes, yet rarely do we get lung infections. First, the nose is designed so that the TURBULENT FLOW of air throws particulate matter onto the sticky mucous lining where much of it adheres. In addition, this circuitous route through the nasal passages WARMS cold air and COOLS hot air as it contacts the tissues along the way. The particles trapped in the mucous are moved by the BEATING OF CILIA that line the air passages into the throat where they are swallowed. Similarly, particles trapped in the lung are carried up and out to the throat also by the BEATING OF CILIA lining the lung. Further, sneezing and coughing expel material out of these passages. Again, mother was biologically cool when she told you to "close your mouth and breath through your nose".
Consider the importance to spreading for an AIRBORNE RESPIRATORY VIRUS to induce sneezing and coughing in its host; a nice evolutionary touch don't you think?
MISCELLANEOUS NDSs: A variety of other NDSs exist. These include the enzymeLYSOZYME in tears and other secretions. Lysozyme is an enzyme that LYSES many prokaryotic cells by digesting the peptidoglycan in their cell walls. Most pathogens require IRON in order to survive, but the blood contains a substance, TRANSFERRIN, that binds free iron so TIGHTLY that many microbes can not obtain enough. The blood also contains various WHITE BLOOD CELLS (WBC-about which we will learn more shortly) that are programmed to attack anything they don't immediately recognize as being "SELF"; in a nasty world "NONSELF" is likely to be bad, so they "SHOOT FIRST AND ASK QUESTIONS LATER". FEVER seems to inhibit the growth of many pathogens and perhaps speeds up the activities of other defense systems. Finally, there is the series of reactions that contribute to inflammation; including the DILATION of blood vessels, and the FORMATION OF CLOTS, which result in localized pain, heat and swelling. These reactions attract WBC & antibodies to any sites of damage and the clots tend to LOCALIZE INFECTIONS so they can be cleaned up by the body's defenses before they spread.
SUMMARY OF NDSs: In summary, the combination of mechanical barriers, chemical inhibitors, enzymes and flushing actions all serve to keep us healthy most of the time? We have been superbly designed for survival and we are a very tough species, but our defenses are all too easily breached if we "choose poorly" (such as the villain in "Indiana Jones and the Last Crusade") our life-style.
When viewed from an evolutionary standpoint, the NDS systems are awesome, taking into account the fact that for over 5 million years we humans survived in filth and squalor, only being washed when we accidentally got caught in a rain storm or fell into water. Remember, that, not all that long ago, ladies strolled the streets of London holding umbrellas over their heads, not to keep the rain off, but to protect them from the contents of the chamber pots (honey pots) routinely emptied from the upper stories into the streets below. Also perfume was invented to cover up the fact that most humans stank-to-high-heaven for lack of bathing.
SCIENCE HALL, ROOM 440CA
PHONE: 509-335-5108
FAX: 509-335-1907
E-MAIL: hurlbert@mail.wsu.edu or hurlbert@wsu.edu hurlbert@pullman.com
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