S o6 JOSHUA LEDERBERG 



The detection of fermentation mutants is readily accomplished by the use of 

 indicator media. The medium "EMB-lactose" used in routine bacteriological 

 work was found to be highly useful. It consists of the following (in g/1) : pep- 

 tone (or "N-Z-Case") 10, yeast extract i, lactose 10, agar 15, eosin Y 0.4, 

 methylene blue 0.06, sodium chloride 5, dipotassium phosphate 2. On this 

 medium, colonies of bacteria which can ferment lactose (or any other sugar 

 added in its place) rapidly turn a deep purple color, while colonies of non- 

 fermenting organisms remain white or pink but may slowly turn light blue. 



Lac mutations have been recovered in two instances. Among 15,000 colonie s 



Table i 

 Symbols used for various loci. 



1. Nutritional requirements. Allele for requirement of a given substance is designated by the 

 superscript "~"; independence by " + ". E.G., B~ is biotinless; B + is biotin-independent. 



B biotin L leucine Pa phenylalanine (<£ was used previously, 



Bi thiamin M methionine but has been modified for typographical 



C cystine P proline reasons) 



T threonine 



2. "Sugar" fermentations. The ability to ferment is designated " + "; the inability "~". 

 Lac lactose Gly glycerol 



3. Bacteriophage resistance. Resistance is designated by the superscript " r "; sensitivity by "•". 

 E.G. Vf. 



Vi resistant to Ti, T5 



Via resistant to Ti; sensitive to T5 



V lb resistant to Ti; mucoid colonies 



V t resistant to T6. 



4. Resistance to chemical agents. Resistance and sensitivity V and u s" respectively, as Cla*. 

 Cla sodium chloroacetate 



A sodium azide 



of strain Y-10 (T~L~Bi~) obtained by spreading a culture previously treated 

 with ultraviolet light on EMB-lactose agar, a single pink colony was noted. It 

 proved to be the same, nutritionally, as Y-10 and was therefore regarded as a 

 Lac~ mutant; this stock is labelled Y-53. Among 30,000 colonies of Y-40 

 (B~M~Vi r ) a single Lac~ was recovered following treatment with nitrogen- 

 mustard (Tatum, 1946), and was designated as Y-87. Tests showing that these 

 independent mutations are probably allelic will be described in a later section 

 (see table 5). Strains Y-53 an d Y-87 differ m the rate at which the Lac~ char- 

 acter reverts to the Lac+ condition, but whether this is due to different allelic 

 states or to differences at other loci, cannot be definitively asserted. 



Attempts to obtain maltose, mannitol, and galactose-negative mutants 

 were not successful, presumably because the populations tested were too small. 

 A glycerol-negative strain has been obtained, but the wild type ferments this 

 polyalcohol so poorly to begin with that accurate scoring is difficult; studies 

 on this character will not be further reported here. 



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