Eagle phenomenon

The paradoxical effect or Eagle phenomenon (named after J. Eagle who first described it) refers to an observation of an increase in survivors, seen when testing the activity of an antimicrobial agent. Initially when an antibiotic agent is added to a culture media, the number of bacteria that survive drops, as you would expect. But after increasing the concentration beyond a certain point, the number of bacteria that survive, paradoxically, increases.

One of the explanations could be that as the concentration is too high, the agent might be self-antagonising the receptor with which it binds. (Penicillin binding proteins, for example, in the case of a penicillin.) This self antagonism, is only a possible explanation for the phenomenon. ["Pharmaceutical Microbiology", pg188, 7th Edition, Denyer, Hodges, Gorman] Another explanation could be that the antimicrobial agent precipitates out of solution, so activity is either not seen, or that the colorimeter is detecting crystals of antimicrobial.

Another example is in the area of drugs and reactions or effects totally contrary to the expected result. For example, there are serious complications occurring in conjunction with the use of sedatives creating a series of effects in some people, that create the total opposite effects as those expected. Malcolm Lader at the Institute of Psychiatry in London estimates the incidence of these adverse reactions at about 5%, even in short-term use of the drugs. The paradoxical reactions may consist of depression, with or without suicidal tendencies, phobias, aggressiveness, violent behavior and symptoms sometimes misdiagnosed as psychosis. [ [http://www.benzo.org.uk/paradox.htm Benzodiazepines: Paradoxical Reactions & Long-Term Side-Effects] ]

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