CHAPTER V 



FILTRABLE MICROORGANISMS* 



The terms "filtrable microorganisms' 3 and 'filtrable viruses' 1 

 are used to designate a group of disease-producing microorganisms that 

 are characterized by their ability to pass through ordinary "bacteria- 

 proof" filters. In the past it has been customary to speak of these filter 

 passers as invisible or ultramicroscopic because of the fact that, besides 

 being filtrable, they were, with the single exception of the virus of 

 bovine pleuro-pneumonia, invisible under the microscope and incapable 

 of multiplying in vitro on any of the usual culture media. Recent 

 discoveries indicate that the terms " in visible" and "ultramicroscopic' 3 

 are incorrect, at least with respect to some members of the group. 

 So long as clear, filtered fluids that gave no visible evidence of life were 

 capable of setting up infectious disease in men or in animals there was 

 some reason for the use of those terms and even for Beijerinck's fanciful 

 conception of a "living fluid contagion." However, the brilliant 

 researches of Noguchi have offered a technique by means of which some 

 of these hitherto invisible viruses have been cultivated outside of the 

 animal body and made visible under the microscope. While some 

 members of this group may indeed be of ultramicroscopic size, there is 

 reason to believe that many of them will eventually be rendered visible 

 through improvements in bacteriological technique. 



The characteristics of the filtrable viruses may be best understood 

 by consideration of a typical example, the virus of foot-and-mouth 

 disease. In this disease vesicles form in the mouths and on the feet 

 of infected cattle. The virus is known to be present in the lymph 

 which forms in these vesicles because this lymph will produce typical 

 attacks of foot-and-mouth disease when inoculated into susceptible 

 animals. If now this infectious lymph be diluted with water and passed 

 through a Berkefeld filter the resulting filtrate will be found to be free 

 from all visible microorganisms and in addition the usual culture tests 

 will give negative results. Notwithstanding this apparent sterility, 



* Prepared by M. Dorset. 



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