38 FREDERICK EBERSON 



that we are dealing with distinct biotypes side by side in a culture of bacteria. 

 When conditions are optimum for a particular biotype, variations do not occur 

 readily. A new type which may have been brought out in company with the 

 predominating type by a change to a less favorable environment can be sup- 

 pressed as conditions are restored to normal. It is to be noted, however, that 

 prolonged cultivation under what were at the outset unfavorable conditions, 

 often, if not as a rule, leads to adaptation, and it becomes impossible to obtain 

 a single biotype. 



This hypothesis is supported strongly by the following observation : A sub- 

 culture on plain agar made from the 4th generation on sheep blood medium 

 (bacillary forms predominating) developed a peculiar surface growth of very 

 heavy opaque masses lying in a more or less translucent delicate growth which 

 composed the greater part of the transplant. Microscopic examination of the 

 heavy growth showed purely bacillary forms with isolated cocci, whereas the 

 translucent area gave cocci only. Touching a platinum needle lightly to the 

 heavy mass a second tube of plain agar was inoculated and exactly the same 

 picture was obtained (Figs. 17, 18, and 19, etc.). This was observed on blood 

 medium also (Chart 1). As to the microscopic appearance of the culture it 

 was found that no hard and fast rule can be laid down regarding the character 

 of the bacillary or coccus growth. On any medium the former developed moist 

 and opaque while the latter grew more delicately and translucent or trans- 

 parent. These characteristics were observed with the agar strain of C. enzy- 

 micus and are diametrically opposed to Mellon's description. The strain 

 cultured on blood medium conformed somewhat to his statement, yet transfers 

 to plain agar gave heavy, moist growth as well as the more delicate and trans- 

 lucent. On fresh Loeffler's medium or blood agar the coccus type could not be 

 distinguished macroscopically from the bacillary form. 



To settle the question whether the coccus form is a mutant it is necessary 

 to isolate in pure culture the two biotypes which cultural experiments seem to 

 reveal. A true mutation must appear in a pure line which has been observed 

 over a definite period and shown to consist of a single biotype. As soon as 

 reversion occurs when environmental conditions are restored to normal, we are 

 no longer dealing with a mutation. Different types, to be genuine mutants, 

 must have developed when conditions remained constant. According to Mellon 

 the diplococcus form of C. enzymicus when obtained from the barred, long 

 bacillary type, remains quite constant, and although it is possible, as he states, 

 to cause it to assume a bacillary form again, this is accomplished with great 

 difficulty. It is obvious that these characteristics are not those of a true mutant, 

 since the coccus, according to the author, is not obtained until conditions are 

 changed and since it may be transformed to the bacillary type although this is 

 attended with difficulty. Mellon's view is that the organism is extremely pleo- 

 morphic and he disregards the likelihood of the coccus being a mutant or a 

 distinct biotype in his original pure culture. It is difficult, admittedly, to prove 

 that the coccus is an entity in the original culture, yet it can be done, especially 

 since there are so many factors in favor of this conception. Mellon goes on 

 to describe the source of his culture and says in part "the rabbit had received 

 intravenous injections of the strain in bacillary form and was recovered from 

 the gallbladder in long chains of streptococci." Is pleomorphism the only pos- 

 sible explanation for this strange transformation? It is just as plausible and 

 simpler to account for this by selective action on the part of the organs for 

 the coccus biotype. 



