R. J. DUBOS 



Ehrlich first stated clearly the possibility of describing these reactions 

 in terms of cellular structure. He postulated that the living cell 

 possesses a number of chemically reactive groups which he called 

 "receptors" and with which dyes, bactericidal substances, and immune 

 antibodies react selectively. Ehrlich regarded these "receptors" as 

 definite chemical entities capable of entering into union with dyes, 

 antiseptics, and antibodies. According to his theory, characteristic 

 staining reactions, diflferential susceptibilities to toxic substances, and 

 specific reactions with corresponding antibodies could all be explained 

 by assuming the existence of a sufficient number of receptors in the 

 bacterial cell. These phenomena can consequendy serve as tests to 

 facilitate the recognition of the receptors and their isolation in pure 

 form. During the past decades, immunochemists and students of the 

 theory of chemotherapy have gone far toward identifying, and in 

 several cases separating in a purified state, several of the cellular 

 components with which antibodies and antibacterial agents react 

 selectively. 



The specific chemical relationships involved in the chemo- 

 therapeutic reactions are discussed in other essays of this volume and 

 need not be considered here. There are certain aspects of the problem, 

 however, which are too ill-defined to warrant discussion in terms of a 

 chemical theory, but which deserve mention at this time since they bid 

 fair to help in the elucidation of some interesting details of cellular 

 structure. Empirical staining reactions led very early to the division 

 of the bacterial world into three broad groups, the Gram-positive, the 

 Gram-negative, and the acid-fast, which are defined by their behavior 

 toward two staining techniques (the Gram and Ziehl-Neelsen methods) . 

 These three bacterial groups not only differ in their staining properties, 

 but also exhibit striking differential susceptibilities to the different 

 types of antiseptics and antibacterial agents. By way of illustration, 

 most Gram-negative bacilli grow readily in the presence of basic dyes, 

 penicillin, or gramicidin, whereas these organisms are extremely 

 susceptible to the bactericidal and lytic effect of immune serum. On 

 the contrary, many Gram-positive species are completely resistant to 

 lysis by immune serum, but are extremely susceptible to the bacterio- 

 static and bactericidal effect of small concentrations of dyes, penicillin, 

 and gramicidin. As for the tubercle bacilli (acid-fast), they are re- 

 markably resistant to all the classical antisejitics and to a gi'eat variety 



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