Moselio Schaechter

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January 13, 2011

Talmudic Question #70

Do you think that all bacteria are lysogens?


How many cryptic prophage (i.e. not a true lysogen since they cannot be activated under Paul's definition) are actually Integrative and Conjugative Elements (ICE)? So would we count ICE elements as lysogens since they are technically active?

I think if you restrict your definition of lysogen sufficiently (to only functional phages that can be activated, the typical textbook definition) then there are plainly bacteria that don't have phages in the known genomes. Obviously, the things we don't know we, er, don't know. I suppose that the mycoplasmas might have genomes so small that lysogeny puts an unbearable metabolic burden on them, such that mycoplasma phages (are there any? I would think so) would evolve toward obligate lysis, since there would be no value in lysogeny. I imagine this argument would work in other small genome bacteria, and maybe in very slow growers (deep cave bacteria and the like) where lysogeny would be a dead end of sorts).

As to the mega-plasmid/chromosome story, I think that given proper information you can make a good prediction, but there seems to be a "real world" grey area - what about a second circular DNA that contains lots of critical functions, but a cell can limp along without it. I think under the strict definition this is a plasmid, but in practice, it is indispensable. I think it is worth challenging students with these definitions to illustrate the continuity of life (although it can confuse them, you have to be careful!)

I agree entirely about Mark's comments on prophages, but I think the plasmid and megaplasmid vs chromosome issue is much simpler (in fact, I often use that as a thought question at the beginning of microbial genetics classes, AND students figure it out very quickly). A chromosome is required under ALL growth conditions, independent of size of the DNA "fragment" -- for example because the DNA fragment carries genes involved in central dogma that are not duplicated elsewhere in the genome. may be that prophages can lend unanticipated advantages to their "host" cell!

I love this kind of question! Certainly, there are genomes in which there exists no evidence of prophages. Others that are rife with them. But I wonder if, with sufficient time, the prophages are "sandblasted" by evolution sufficiently to be difficult to identify. Certainly an organism that interests me, Bdellovibrio, does not appear to have clear-cut prophages, yet there are many lytic phages for that beastie!

In a way, this reminds me of definitions of the term "plasmid." Large plasmids, sometimes still called "megaplasmids" elide into a definition for chromosome!

So the question may well be definitional rather than objective?

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