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(Institute of Medicine, National Academy Press, Washington, D.C., 1992)
Summary
Pathogenic (disease-producing) "microbes have threatened human health for centuries" (presumably since the origin of the species), and it is believed that this "threat will continue" (in contrast to beliefs in preceding decades, when it was assumed that science would eliminate these threats) and "may even intensify in coming years."
Several factors, operating singularly and in combination, have contributed to the emergence of microbial pathogens. Like all species, infectious organisms are subject to genetic change and evolution. These changes are manifested in their ability to infect new hosts (e.g., pigs humans seals), alterations in their susceptibility to antimicrobial drugs (e.g., penicillin resistant Staphylococci), and changes in their response to host immunity (i.e., their lifetimes are measured in hours compared to those of humans). Alterations are occurring in the geographic ranges of some pathogenic organisms, due to the development of modern transportation (e.g., air travel can disperse an infected humans around the world within a day and ships can transport invader species to ports around the world). The human host is also changing (e.g., new types of personal behavior and new types of food-processing methods may enhance the transmission of some pathogens). New diseases (e.g., AIDS) and modern medical treatment (e.g., antibiotics) may result in immunosuppression and thus increase susceptibility to pathogenic microorganisms. Some infectious agents exist in other species (e.g., mosquitoes and rats) that have migrated and increased in numbers. The environment is being changed by both natural processes and human activities (e.g., deforestation, urbanization, industrialization, global warming), which increases the range and exposure to some pathogens. Finally, poverty, civil unrest and war cause breakdowns in sanitation, which allow microbes to flourish (e.g., cholera in Rwanda).
In summary, factors contributing to "emerging microbial threats to health" are:
1. Emergent Bacteria
|
Agent |
Related Diseases/Symptoms |
Mode of Transmission |
Cause(s) of Emergence |
|---|---|---|---|
|
Aeromonas species |
Aeromonad gastro enteritis, cellfllitis, wound infection, septicemia |
Ingestion of con taminated water or food; entry of organism through a break in the skin |
Immunosuppression; improved technology for detection and differentiation |
|
Campylobacter jejuni |
Campylobacter enteritis: abdominal pain diarrhea, fever |
Ingestion of contami nated food, water, or milk; fecal-oral spread from infected person or animal |
Increased recognition; consumption of uncooked poultry |
|
Clostridium difficile |
colitis: abdominal pain, watery diarrhea, bloody diarrhea |
fecal-oral route |
increased recognition, immunosuppression |
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Escehrichia coli 0157:H7 |
hemorrhagic colitis; thrombocytopenia; development of a new hemyltic uremic syndrome |
fecal-oral route (esp. undercooked and raw food) |
probably due to the pathogenic form of E. coli |
|
Helicobacter pylori |
Gastritis, peptic ulcer, possibly stomach cancer |
Ingestion of contaminated food or water, esp. unpasteur ized milk; contact with infected pets |
Increased recognition |
|
Legionella pneumonophila |
Legionnaires disease: malaise, myalgia, fever, headache, respiratory illness |
water supplies, air cooling systems |
recognition in an epidemic situation |
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Listeria monocytogenes |
listeriosis: meningoencephalitis and/or septicemia |
fecal-oral route: water and soil; inhalation of organism |
probably increased awareness, recognition, and reporting |
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Streptococcus pyogenes (Group A) |
scarlet fever, rheumatic fever, toxic shock |
direct contact with infected persons; fecal-oral route |
change in virulence of the bacteria; possibly mutation |
|
Cholera: severe diarrhea, rapid dehydration |
Ingestion of water contaminated with the feces of infected persons; ingestion of food exposed to contaminated water |
Poor sanitation/ hygiene; possibly introduced via bilge water from cargo ships | |
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Vibrio vulnificus |
Cellulitis; fatal bacteremia; diarrheal illness (occasionally) |
Contact of superficial wounds with seawater or with contaminated (raw or undercooked) seafood; ingestion (occasionally) |
Increased recognition |
2. Emergent Viruses
|
Agent |
Related Diseases/Symptoms |
Mode of Transmission |
Cause(s) of Emergence |
|---|---|---|---|
|
hepatitis E |
fever, abdominal pain, jaundice |
contaminated water |
newly recognized |
|
gastroenteritis; epidemic diarrhea |
most likely fecal-oral, (alleged vehicles of transmission are drinking & swimming water, and uncooked foods |
increased recognition | |
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enteritis; diarrhea, vomiting, dehydration, low grade fever |
fecal-oral route (primarily), fecal-respiratory |
increased recognition |
3. Emergent Helminth and Protozoans
Cholera, typhoid fever, dysentery, giardiasis, and hepatitis!
These are only a few of the more dreadful diseases caused by microorganisms that are easily transmitted from infected to uninfected individuals by the so-called "fecal-oral route," involving contaminated food or water. Probably the most important single source of such infections is drinking water contaminated by the feces of humans or other animals.
You may not worry much about the diseases mentioned above because you live in an industrialized area with a well developed and tax-funded public health system and sanitary infrastructure to protect your water suppliy from microbiological pollution. In many other parts of the world such public health infrastructure does not exist, and such diseases remain major causes of morbidity and mortality. Indeed, much of what the popular press refers to as "infant mortality" is a direct result of gastrointestinal diseases transmitted in contaminated food and water supplies.
A vitally important component of the public health measures which maintain the microbiological quality of our drinking water supplies is the regular testing, by local public health departments, of water for the presence of pathogens.
How can you tell if a water sample contains human pathogens? In practice, you can't. To test for each of the many different pathogenic organisms separately would be extremely time-consuming and costly. Instead, water is tested for the presence of non-pathogenic bacteria that commonly inhabit animal intestines. If water contains these "indicator" microorganisms then the presence of enteric pathogens is assumed and the water is not considered safe for use.
The indicator organisms used most often in water testing are referred to as "fecal coliform bacteria." The fecal coliform bacteria have several important characteristics that allow them to be detected, and distinguished from bacteria that naturally occurr in water. The major fecal coliforms are Escherichia coli and Klebsiella pneumoniae. E. coli and K. pneumoniae are adapted to life in the intestine, where they comprise some 10% or so of the intestinal bacterial flora. The presence of fecal coliforms in a water source almost certainly indicates fecal contamination.
It is important to reiterate that, although some strains of E. coli and K. pneumoniae may on occasion cause disease, they are usually not pathogenic and are used only as indicators that more serious human pathogens may also be present.
Fecal coliform tests of water supplies and treated sewage are performed dally in laboratories run by Santa Cruz County. Also, volunteers from the Santa Cruz Chapter of the Surfrider Foundation perform coliform analysis of coastal waters as a public service. Their hotline (423-POOP) provides water quality information and advisories for coastal recreation areas.
There are several officially accepted methods of fecal coliform analysis. We will use a membrane filter method because it is easy to demonstrate on a large scale in a short period of time.
Membrane-Filter Method
This method uses special filter membranes with pores that are so small (0.45 ~m dia.) that bacteria do not pass through, but are retained on the surface of the membrane when a water sample, say 500 ml, is passed through the filter by pulling a vacuum on the downstream side. The filters are then transferred to a special nutrient growth medium that is selective and differential. Selective means that the medium allows the growth of the indicator organisms but inhibits the growth of many, but not all, others. Differential means that the appearance of the bacterial colonies allows one to distinguish between the indicator organisms and others which have not been inhibited.
In our experiment we use a medium called "m FC Broth". The selectivity of m FC medium is due in part to the presence of "bile salts". Bile salts are the detergent-like substances secreted by your intestines to help break down fats and oils in the diet. Bile salts inhibit the growth of many free living bacteria but most intestinal bacteria are resistant. The medium also contains pH indicating dyes. Colonies of fecal coliforms are differentiated by their blue color, resulting when they produce acids by metabolizing the sugar lactose.
We will have several 500 ml water samples. The identity of the samples will be given at the lab. Each sample will be passed through a sterile filter membrane. Then the membranes will be placed on the surface of m FC Agar in a Petri dish and incubated at 44.5°C for 48 hours. The incubated plates will be stored in a refrigerator until the next meeting when they will be examined. Fecal coliform bacteria such as Escherichia and Klebsiella produce blue colonies. Other bacterial colonies, including the non-fecal coliform Enterobacter, produce gray or cream colored colonies. Most free-living bacteria are inhibited.
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