ENZYMES, TOXINS, PTOMAINS 47 
4. Chemotaxis. — Bacteria respond to various chemical stimuli. 
Substances which can be used by them for nutritional purposes, as 
various constituents of laboratory media, appear to attract bacteria. 
Harmful substances, as acids or alkalis, may act in the reverse manner. 
Oxygen is a powerful chemotactic agent for many aerobic bacteria, 
while many anaerobes are repelled by it. The mutual chemotactic 
relations of bacteria and leukocytes, and the well-defined tendency 
of certain invasive bacteria to localize in definite tissues or organs 
of the animal body are interesting fields for speculation. Nothing 
conclusive is known about these relations. 
K. ENZYMES, TOXINS, PTOMAINS. 
Enzymes.— The phenomena of chemical interchange between 
bacteria and their environment indicate that enzyme activity plays 
an important part in bacterial metabolism. ^ 
Enzymes may be defined as substances of unknown composition 
produced by living cells which incite specific chemical reactions with- 
out permanently combining with the products of reaction. Accord- 
ing to Beatty' the whole of the chemical action of enzymes may be 
reduced in the last analysis to the action of hydrogen and hydroxyl. 
Any enzyme, according to this hypothesis, possesses a general non- 
specific power of attracting one or other of the H and OH radicals, 
and a specific power of absorbing a specific substrate. The use of 
enzymes in biological processes is to control the rate and sphere of 
naturally occurring reactions. The former is secured by increasing 
the number of the active H and OH radicals; the latter by limiting 
them to definite points to which the substrate of the reaction is also 
attracted. 
A small amount of enzyme acting under favorable conditions will 
cause a relatively extensive transformation of substance without 
itself being used up or inactivated. There is, however, a limit to 
the amount of transformation which a given amount of enzyme can 
accomplish, for the accumulation of reaction products tends to restrict 
enzyme action; the removal of reaction products as they are formed, 
however, favors an extension of the action of the enzyme. All bac- 
teria cells appear to produce or to possess enzymes, probably several, 
which may be divided somewhat arbitrarily into tw^o classes, the 
extracellular or exo-enzymes, and the intracellular or endo-cnzymes. 
Exo-enzymes. — Exo-enzymes are those which are excreted from the 
organism. They may be obtained in an active state from filtrates of 
cultures of bacteria. Their difi'usion from the bacterial cell and their 
filterability suggests that they may be relatively simple in molecular 
aggregation. Their function is essentially a "preparatory" one, for 
they transform potential nutritional substances, as proteins, car- 
1 See Waksman and Davison: Enzymes, 1926. 
2 The Method of Enzyme Action, P. Blakiston's Son & Co., Philadelphia, 1917. 
