VARIATION OF SALMONELLA PULLORUM 3 



with regard to variation of 5. pullorum is, indeed, very limited, as has been shown 

 by Mailman (1932). It was deemed advisable to determine to what extent 

 variants of this organism occur in nature, as well as to what extent they may be 

 present under certain laboratory conditions; and also to study some of the morpho- 

 logical, cultural, pathogenic, and serological properties of the variants, with the 

 purpose of offering an explanation for problems encountered in our present 

 diagnostic methods. 



HISTORICAL REVIEW 



The field of bacterial variation has attracted considerable interest and caused 

 much controversy among investigators since the early days of bacteriology. 

 Apparently two strongly opposing schools on the subject of variability and 

 mutation existed. The one subscribed to the monomorphic view, while the other 

 supported the pleomorphic concept that microorganisms are extremely variable 

 in their characteristics, especially when subjected to changed external conditions. 

 The monomorphists maintained that bacterial species preserve their morphological 

 and other characters with great constancy, and that all deviations from the 

 normal type are to be considered as contaminations or so-called involution forms. 



Since the literature on all phases of variation, dissociation and mutation has 

 been so voluminous, the reader is referred to the reviews published by Lohnis 

 (1922), Hadley (1927) and Arkwright (1930). In the resume of the literature 

 cited here it is deemed advisable and expedient to present some of the outstand- 

 ing observations of general interest, as well as those contributions which are 

 related to the phases included in the problem under investigation. 



The extreme views of Nageli (1877) concerning variability are of interest. 

 He believed that all species of bacteria are highly variable and capable of passing 

 from one morphological state to another, as well as undergoing profound physio- 

 logical changes. Furthermore, he claimed that one species can be the etiological 

 agent in various forms of disease. 



For a decade or so after the views of Nageli were presented very little was 

 published on the subject of variation. 



Chamberlain and Roux (1883) observed that anthrax cultures became asporo- 

 genic when subjected to phenol and other antiseptics. This characteristic was 

 retained even upon passage through guinea pigs, rabbits, and sheep. Later 

 Roux (1890) reported that the virulence of the asporogenic type was reduced, 

 but that it could be raised by passage through susceptible animals. In cultural 

 and physiological properties this type resembled the sporogenic form. The 

 author suggested that the loss and gain in virulence may occur under natural 

 conditions, and that the so-called saprophytes may progressively adapt them- 

 selves to a parasitic mode of living. 



Bacteria cultivated in the presence of small amounts of agglutinating serum 

 were reported by Durham (1898) to exhibit departures from their normal habit 

 of growth. Morphologically the organisms revealed a tendency to grow in chains 

 or threads, which were normally absent. 



Nicolle (1898) observed spontaneous clumping of broth cultures of Bad. 

 typhosum to the extent that they were unsuitable for the agglutination test. 

 The phenomenon was similar to that observed when the culture was placed in 

 contact with immune serum. Cultures exhibiting spontaneous agglutination 

 possessed slight sensitivity toward serum agglutination. Repeated inoculation 

 into serum-broth medium failed to eliminate the property described. 



Horton-Smith (1900) encountered considerable variation in the agglutinability 



