50 GENERAL PHYSIOLOGY OF BACTERIA 
red blood cells (hemolysins) and those which digest hemoglobin (hemo- 
digestins^) are intermediary in their general properties between enzymes 
and toxins, if indeed there is any tangible distinction between them. 
Vaughan^ has studied both enzymes and toxins extensively, and has 
summarized admirably the points of resemblance between exo-enzymes 
and exo-toxins as follows: 
"1. Both are destroyed by heat.^ 
"2. They act in very dilute solution. 
"3. When repeatedly injected into animals in non-fatal doses they 
cause the body cells to elaborate antibodies which neutralize the toxin 
(or the enzyme) both in vivo and in vitro. 
"4. In the development of their effects a period of incubation is 
required. 
"5. It has been shown (by Abderhalden) by optical methods that 
they have a cleavage effect upon proteins— they split complex proteins 
into simpler bodies; in other words, they have a proteolytic action. 
"6. They are specific in two senses: (a) they are specific according 
to the cell which produces them; (b) they are specific in the antibody 
elaborated in the animal body after repeated injections of non-fatal 
doses." 
Bacterial toxins are usually classified as exo- or soluble (extra- 
cellular) toxins, and endo- (intracellular) toxins. The former are 
soluble and diffuse out from the bacterial cell into the surrounding 
medium. Very few bacteria produce exo-toxins: the best known are 
those of the diphtheria, tetanus and botulismus bacilli. To these 
specific antitoxins are known. Endo-toxins are non-diffusible and 
are locked up in the bacterial cell; they are liberated only when the 
cell disintegrates. Xo specific antitoxin has been produced for an 
endo-toxin. 
Ptomains. — Ptomains are soluble, basic, nitrogen-containing sub- 
stances formed from proteins or protein derivatives by the action of 
microorganisms. They are non-specific in the sense that se^'eral 
microorganisms may produce the same substance, and they are rela- 
tively poor in oxygen content. Ptomains are almost certainly simpler 
in composition than either exo- or endo-toxins. No antibodies have 
been produced against them. Some are poisonous, many are not. 
The aromatic amines produced from the amino-acids— tyramine, phe- 
nylethylamine, beta-imidazolethylamine and indolethylamine — are 
physiologically active, even in very small amoimts (see page 74). 
Isoamylamine, probably derived from leucine, is also reactive physio- 
logically, but less so than tyramine.* 
1 Van Loghem: Centralbl. f. Bakteriol., orig., 1912-1913, 67, 410. 
2 Protein Split Products, Lea & Febiger, Philadelphia, 1913. 
3 Although they are somewhat more resistant to heat than the cells which produce 
them. 
* Dale and Dixon: Jour. Physiol., 1909, 39, 25- 
