www.morphostasis.org.uk

A proliferation of pathogens through the 20th century

Published In the August 2008 edition of the Scandinavian Journal of Immunology.)

It points out that the meaning of "pathogen" has shifted over the last 120-130 years.

Additional notes:

It should be an inescapable conclusion that the meaning of "pathogen" is still in transition. Its original meaning was "a specific causative agent of disease". Many people now define it as a "causative agent of disease - especially a bacterium or a virus" or something along those lines. However, there is a greater transition towards it becoming a synonym and replacement for a "pathogenic organism": this is now well under way - particularly in its extensive use in the bio-medical literature. Some dictionaries already define a pathogen as "a disease causing organism". I believe that there is an imperative to consider whether this drift should be allowed to continue and how far this drift should be allowed to proceed. In the published article I suggested, "Perhaps the prevailing theories of self/non-self-discrimination and of immune system function were fully compatible with this interpretation of pathogen - anomalies and anachronisms are only now coming into focus." This may be better expressed as "Perhaps the prevailing theories of self-/non-self-discrimination and of immune system function failed to highlight the anomalies and anachronisms that would have arrested this drift: they are only now coming into focus."

The term pathogenic bacterium may seem to be more or less the same as a bacterial pathogen. But these is a clear difference in emphasis. In the first it the bacterium that is the principal property; this has the expected properties of a bacterium with the additional property of being able to cause damage. In the second, it is the pathogen that is the principal property; this has the "expected" properties of a pathogen with the additional property of also being a bacterium. To me, it seems that there is far more emphasis on - and scope for - a variety of pathogenic mechanisms in the first than in the second. So, in addition to advocating a return of pathogen to mean any agent that causes disease or damage, I suggest that we should use pathogenic organism rather than an organismal pathogen (where organism is the generic proxy for bacterium, virus, fungus etc) unless there is a compelling reason to emphasise the second.

The article ends by suggesting that an alternative shorthand might be called for to replace pathogen where we really mean a pathogenic (micro-)organism.

Pathogerm could prove to be an appropriate substitute; this name is both "twee" and rather colloquial. Both these properties would act as appropriate reminders that this is nothing other than a convenient abbreviation that hides a wealth of deeper properties. It has the advantage of sounding similar to pathogen but remaining clearly distinct.

The perspective persiststhat aggressive adaptive immune responses as caused by the interaction of TLRs (toll like receptors - expressed on dendritic cells) with PAMPs (pathogen associated molecular patterns). However, as highlighted, PAMPs should really be regarded as MAMPs (microbe associated molecular patterns). Since several million different bacterial species are non-pathogenic to man and only about 50 are regularly pathogenic, why do we not become awash with aggressive adaptive immune responses to these non-pathogenic species (particularly the 500 or so different commensal species that live within and upon our bodies)? There is something not consistent here. Oral and gut bacteria are both encouraged to inhabit us and they are useful to us and play a protective role. If their MAMPs were to provoke the adaptive immune system into an aggressive response, the outcome could be a devastating inflammation of our mouths and gut (it looks as though Crohn's disease and ulcerative colitis may be unwelcome examples of such misdirected aggressive immune responses). This should indicate to us that these aggressive responses are channelled away from aggression when the encounter is with non-pathogenic, commensal organisms - which seem to be welcomed and encouraged into colonising our mouths and gut.

At the beginning of this article I have used the quotation "The repetition of a catchword can hold analysis in fetters for fifty years and more." (Benjamin N. Cardozo, 1870-1938). However, Francis Bacon (1561-1626) - perhaps the father of natural philosophy (science) - has much to say on this in his comments on "Idols of the Mind" in Novum Organum. "There are also idols formed by the intercourse and association of men with each other, which I call Idols of the Market-place, on account of the commerce and consort of men there. For it is by discourse that men associate; and words are imposed according to the apprehension of the vulgar. And therefore the ill and unfit choice of words wonderfully obstructs the understanding. Nor do the definitions or explanations wherewith in some things learned men are wont to guard and defend themselves, by any means set the matter right. But words plainly force and overrule the understanding, and throw all into confusion, and lead men away into numberless empty controversies and idle fancies."

Pathogenic probably came to us via the French from their pathogenique. They also use the term pathogene (both as a noun and as an adjective according to some dictionaries). They have been much more faithful to its original meaning as the following definitions (or pathogene) demonstrate: