The lambda bacteriophage which infects E. coli was first discovered by Esther Lederberg in 1950.

However, in the earliest paper on the lambda phage that I could find, I was unable to find the reason behind why the phage was named after the Greek letter λ.

Is there a source (by Lederberg or previous scientists) that mentions the etymology of this phage?


The original paper from Lederberg from January 1953 (see reference 1) indeed doesn't mention the origin of the name, but the paper in reference 2 does. It says:

The isolation of λ was first reported in 1951 by Esther Lederberg (119), then a Ph.D. student at the University of Wisconsin, and later was described, in greater detail, in a 1953 Genetics paper by Esther and Joshua Lederberg (120). The discovery was accidental, when a λ-sensitive strain of Escherichia coli K-12 (W518, obtained after UV irradiation) was crossed with its parent. The mixture yielded plaques, and the source of the virus was the K-12 parent. W518 cells that survived infection became stable lysogens which, like the K-12 parent, were immune to superinfection and which released unaltered phage. Although the Lederbergs were initially opposed to the notion, crosses between lysogens and sensitive cells led them to suggest that λ prophage was chromosomal and linked to gal. Joshua Lederberg recalls that he was convinced, from earlier work of Burnet and Lush (21), that lysogeny was a real phenomenon but that he “fully expected lambda to be a [plasmid]—in fact the term lambda was modeled after Sonneborn's kappa [in Paramecium; see reference 156 for a recent review], so it was quite a shock to discover the contrary.” (In fact, years later Hideo Ikeda and Jun-ichi Tomizawa [98] showed that prophage P1, unlike λ, is a plasmid and not part of the host chromosome!)

The mentioned kappa particle in Paramecium is an inheritable cytoplasmatic symbiont and the Lederbergs thought of something similar for their discovery. My assumption here is that lambda was the next greek letter, hence the name. The publication in which Sonneborn describes Kappa can be found in reference 3, a short historical overview on the discovery of Kappa in the review in reference 4.


  1. Genetic Studies of Lysogenicity in Escherichia Coli.
  2. Little lambda, who made thee?
  3. Mating Types in Paramecium Aurelia: Diverse Conditions for Mating in Different Stocks; Occurrence, Number and Interrelations of the Types
  4. R-body-producing bacteria.
  • 1
    $\begingroup$ Thanks for the well-researched answer! However, I didn't manage to find anywhere in the second paper mentioning that the phage was named lambda because it was the next letter after kappa. $\endgroup$
    – March Ho
    Sep 7 '16 at 0:24
  • $\begingroup$ @MarchHo There is indeed no direct source for that - this is my assumption. But I think it is logical, that's why I wrote it. I will edit the question and make that clearer. $\endgroup$
    – Chris
    Sep 7 '16 at 6:18
  • $\begingroup$ @MattDMo This answer did not answer my original question, and therefore I will wait for further answers. $\endgroup$
    – March Ho
    Sep 7 '16 at 18:16
  • $\begingroup$ @MarchHo Obviously greek letters where quite commonly used for cytoplasmatic factors, see this review - the relevant piece is on page 114, right column in the upper part of the page. $\endgroup$
    – Chris
    Sep 7 '16 at 20:30
  • $\begingroup$ My assumption is that a Lederberg discovered the Lysogen, and lambda in the Greek alphabet translates as the letter 'L' so it could be a Greek 'L' for Lederberg, or a Greek 'L' for Lysogen, or better yet a Greek 'L' for both. All of the people who were involved are deceased, so if they didn't write it down . . . . Look at all the various explanations for the origin of the name of the amber stop codons. It happened so long ago that the people who selected it don't even remember the circumstances. Now the origin of the name of the lambda cohesive end sites as cos, that we know. $\endgroup$
    – mdperry
    Sep 8 '16 at 1:20

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