34 THE MORPHOLOGY OF BACTERIA 



H. MUTATION: CONSTANCY OF TYPES. 1 



True mutation or discontinuous variation is rarely observed among 

 bacteria, although a few instances are on record which have been sub- 

 jected to satisfactory scrutiny. Mutation must be carefully differen- 

 tiated from the loss of one or more characteristics of bacteria during 

 cultivation; the loss or suppression of one or more characteristics is 

 fairly commonly observed among bacteria. Pigment production, and 

 proteolytic activity as for example the ability to liquefy gelatin 

 are frequently lost to cultures of bacteria during prolonged cultiva- 

 tion, but these properties may be regained when the organisms are 

 placed once more in a suitable environment. Similarly, strains of 

 fermenting bacteria may temporarily, or even permanently, become 

 unable to decompose certain carbohydrates. Change in virulence, 

 or loss of virulence is rather commonly noticed among pathogenic 

 bacteria grown outside the animal body. It is even possible to so 

 parasitize organisms by prolonged cultivation upon one medium that 

 they will develop not at all, or slowly at best, on other media. Thus, 

 a strain of B. proteus has been grown continuously upon agar with 

 frequent transfers for four years, and the organism will no longer 

 grow in broth. Similarly, B. bulgaricus is an obligate milk parasite. 

 Exposure to unfavorable environmental conditions may also suppress 

 important characters: Pasteur's celebrated experiment of growing 

 anthrax bacilli at 43 C. for some hours and establishing an asporeless 

 variety is a familiar example. The suppression of characters as out- 

 lined above is frequently important as the starting point for new 

 adjustments between pathogenic bacteria and their hosts. 



Turning to the production of disease in man, it is certain that at 

 least some organisms produce the same reaction today they did years 

 ago : tuberculosis appears to be the same disease today it was centuries 

 ago, as is evidenced by the lesions found in Egyptian mummies. Clini- 

 cally, the observations of Hippocrates would be a fair exposition of the 

 phenomena seen in tuberculous patients at the present time. Leprosy 

 also appears to be the same entity now it was during the middle ages, 

 although the geographical distribution is much more restricted. With 

 respect to more acute diseases, which require more careful examination 

 to differentiate them, the evidence is less certain, although typhoid 

 bacilli do not appear to have changed since they were first isolated 



1 Eisenberg, Ueber Mutationen bei Bakterien und anderen Mikroorganismen in 

 Ergebnisse d. Immunitatsforsch. experimentellen Therapie, Bakteriologie und Hygiene, 

 Berlin, 1914, pp. 28-142, for summary. 



