EXOTOXINS 189 



is more marked in some cases than in others. The two best 

 examples of bacteria thus producing soluble toxins are the 

 diphtheria and tetanus bacilli. In these and similar cases when 

 bouillon cultures are filtered bacterium-free by means of a 

 porcelain filter, toxic fluids are obtained, which on injection into 

 animals reproduce the highly characteristic symptoms of the 

 corresponding diseases. This contrasts with such cases as those 

 of the pneumococcus or of b. anthracis, filtered cultures of 

 which are usually non-toxic. Poisons appearing in culture 

 media have been called extracellular toxins or exotoxins, 

 but we cannot as yet say whether they are excreted by the 

 bacteria or whether they are produced by the bacteria acting 

 on the constituents of the media. The exotoxins are easily 

 obtainable in large quantities, but no method has been dis- 

 covered of obtaining them in a pure form, and our knowledge 

 of their properties is exclusively derived from the study of 

 the toxic filtrates of bouillon cultures — these filtrates being 

 usually referred to simply as the toxins. These toxins differ 

 in their effects from the endotoxins in that specific actions on 

 certain tissues are often manifested. Thus the toxins of the 

 diphtheria, the tetanus, • and the botulismus bacilli all act on 

 the nervous system ; with some of the pyogenic bacteria, on the 

 other hand, poisons, probably of similar nature, produce solution 

 of red blood corpuscles (this last might be thought to explain 

 the anaemias so common in the associated diseases, but here 

 further work is still necessary). In the action of many of these 

 toxins the occurrence of a period of incubation between the 

 introduction of the poison into the animal tissues and the ap- 

 pearance of symptoms is often a feature. 



Amongst the properties of the exotoxins are the following : 

 They are apparently all uncrystallisable ; they are soluble in 

 water and they are dialysable ; they are precipitated along 

 with proteids by concentrated alcohol, and also by ammonium 

 sulphate; if they are proteids they are either albumoses or 

 allied to the albumoses; they are often relatively unstable, 

 having their toxicity diminished or destroyed by heat (the 

 degree of heat which is destructive varies much in different 

 cases), light, and by certain chemical agents. Their potency is 

 often altered in the precipitations practised to obtain them in 

 a pure or concentrated condition, but among the precipitants 

 ammonium sulphate has little if any harmful effect. Regarding 

 the endotoxins we know much less, but it is probable that, 

 chemically, their nature is similar, though some of them at least 

 are not so easily injured by heat, e.g., those of the tubercle 



