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The Scientific Method, Fish Diseases and Koch's Postulates

Our understanding of the situation is that fish have had lesions and that fish have died. Pfiesteria piscicida and Pfiesteria-like organisms have been cultured from water samples taken in the vicinity where fish with lesions have been observed or where fish kills have occurred. It was reported that Pfiesteria-like organisms have caused lesions and moralities in laboratory fish exposures.

OK. So why is it so difficult to conclude that fish lesions are caused by Pfiesteria or Pfiesteria-like organisms?

Well, as good scientists know, its not that easy. The very same water which was cultured for Pfiesteria could have also grown bacteria (you could bet that bacteria were in that water also, not to mention countless other microorganisms!). So, in order to properly study the cause of any disease (including Pfiesteria), modern day scientists generally follow Koch's postulates.

Who was Koch? Robert Koch (1843-1910) was a German physician and microbiologist who showed that "the key to the identification of bacterial pathogens was the isolation of pure cultures." Koch's powerful methodology led to the Golden Era of medicinal bacteriology, whereby a variety of pathogenic bacteria (including Bacillus anthracis, tubercle bacillus, cholera vibrio, typhoid bacillus, diphtheria bacillus, pneumococcus, staphylococcus, streptococcus, meningococcus, gonococcus and tenanus bacillus) were isolated. Subsequent studies followed to illuminate the mechanisms of pathogenicity of these organisms, host responses, and methods of prevention and treatment. [More about Dr. Robert Koch]

What are Koch's postulates?

In the course of his studies of anthrax and tuberculosis, Robert Koch formulated rules of procedure for proving that a certain microorganism is the cause of a particular disease. These rules, known as Koch's postulates, are still used today:

1. It must be shown that the microorganism in question is always present in diseased hosts.

2. The microorganism must be isolated from the diseased host and grown in pure culture (i.e., in a culture containing only that one species of microorganism).

3. Microorganisms obtained from the pure culture, when injected into a healthy, susceptible host, must produce the disease in that host.

4. Microorganisms must be isolated from the experimentally infected host, grown in pure culture, and compared with the microorganisms from the original culture.

Perhaps it is now more clear why it is difficult to establish a cause and effect relationship between fish lesions and Pfiesteria! For starters, Pfiesteria (or its toxin) has not yet been isolated from fish or fish lesions. Fish with lesions have been taken from waters where Pfiesteria-like organisms have not been identified (that's not to say that they were not there either). Also, the lesions on fish reported from some laboratory exposures are not identical to those seen in some field-collected specimens. Regardless, the lesions are considered non-specific in nature. That means that the lesions could have been caused by a number of different causative agents (bacteria, virus, etc.). In the end, we are unable to isolate the infectious organism from the host, culture it, and reinfect another host to be able to observe if the same lesions occur.

Now just because it is difficult to fulfill Koch's postulate does not mean that scientists cannot definitively correlate Pfiesteria exposure with fish (or human) lesions. Based on current information, scientists develop hypotheses (this is where we are at present). These hypotheses are repeatedly tested to attain greater confidence in the generalizations of the hypothesis.

For example, suppose you (a good scientist) OBSERVE that a great variety of insects including flies, beetles, grasshoppers and wasps all have three pairs of legs. And suppose that you arrive at the generalization that "all insects have three pairs of legs." You now reason deductively that if all insects have three pairs of legs, and if cockroaches, crickets, moths and bees are insects, they must also have three pairs of legs. In other words, you have used a generalization to form a HYPOTHESIS! When you find that all these insects indeed have three pairs of legs, and that this hypothesis was therefore a good one, you begin to feel more confident about your generalization. But you, the good scientist, do not stop there. You make new hypotheses to further test your generalization. Perhaps you might predict that immature flies and moths are insects too, and that they must also have three pairs of legs. However, observation shows you that some grubs (immature flies) and caterpillars (immature moths) have no legs at all. Your generalization must be modified to a more restricted form: "All adult insects have three pairs of legs."

So, let's summarize the steps that have led up to this point. You began, as all good scientists must, with observations. You used these observations to formulate a generalization. When a discrepancy was found between the generalization and an observation, the generalization was changed.

Eventually, after the generalizations (i.e., hypotheses) are tested, challenged, revised, retested and rechallanged (and re-revised), a THEORY is developed. This theory remains subject to debate by all who question it and can present evidence to suggest to the contrary. Ultimately, a theory may be accepted as FACT.

It must be remembered that all good scientists must be able to abandon their generalizations when new evidence contradicts them (just as Dr. Koch did). Further, hypotheses must be testable, or they are of no value. And testing must continue, and be altered when necessary, to conform with the evidence. The scientist's query must always be: "What is the evidence?" In the case of toxic dinoflagellate blooms in Chesapeake Bay, evidence is still being amassed by scientists from universities, state agencies and industry. Collaborative efforts from within the scientific community are soon expected to yield a "big picture" view of the situation. This viewpoint will incorporate insight and observations from the the many diverse fields of expertise including microbiology, molecular biology, pathology, ecology, and water quality and land management.

Portions were adopted from the writings of William T. Keeton.

 

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