THE BACTERIAL C:i;i,I, 



and the biological reagents, be they antiseptics, antibodies, enzymes, 

 bacteriophages, on the other. 



One of the most intriguing applications of the indirect cyto- 

 logical methods discussed in the preceding pages has been the analysis 

 of the phenomena of bacteria variability. It has long been known 

 that most bacterial cultures — even those arising from single cells — 

 often undergo profound transmissible modifications of their morpho- 

 logical, biochemical, and physiological properties. Immunochemical 

 studies have revealed, in particular, a type of variation, now recognized 

 in practically all bacterial species, which involves the loss of the specific 

 surface components of the cell (the capsular polysaccharides of pncu- 

 mococci, the M proteins of streptococci, the capsular polypeptide of 

 anthrax, the lipid-protein-polysaccharide complexes of the dysentery 

 and typhoid bacilli, etc.). These transmissible modifications of the 

 surface of the bacterial cell have attracted particular attention because 

 they are in many cases correlated with alteration of the virulence of 

 the organism concerned. They constitute, however, only a very nar- 

 row aspect of the total problem of bacterial variability. One can 

 observe within one given bacterial culture transmissible modifications 

 of many unrelated properties: ability to attack sugars or proteins, to 

 synthesize amino acids or pigments, to resist antiseptics or other 

 injurious procedures, to produce flagella, spores, capsules, and so on. 

 All these variations occur independently of each other, thus giving to 

 each bacterium the possibility of manifesting its existence under a 

 great diversity of forms and properties. The production of these large 

 numbers of variant forms deficient in one or another of the cellular 

 components has greatly helped in the analysis of many immuno- 

 chemical problems. 



From a more general point of view, it is of the greatest interest 

 that a given organism can successfully continue to exist and to multiply 

 as an independent living object after having lost a great variety of 

 structures and functions which had appeared to constitute important 

 components and attributes of the "normal" parent form. As already 

 stated, these structures and functions can be lost and regained inde- 

 pendently of each other, without altering the essential nature of the 

 germ or the potentialities of the cell. Even more striking is the fact 

 that it is possible to substitute experimentally one character for an- 

 other. Thus, by adding to a strain of pneumococcus which has lost 



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