🔬 Phage Takes

Bacteriophage Ecology Group News, or BEG News, was published mostly quarterly as an online newsletter for a total of 26 issues, starting July 1, 1999 and continuing through December 31, 2007. As follows is a reprint of the editorial from the newsletter. Also included in issues were lists of new members to the Bacteriophage Ecology Group, an introduction to new website features, a list of upcoming meetings, phage images found on the web (remember, this was 2000, so effectively pre-Google), etc., but most of all, a listing of new phage ecology-related publications. The newsletter was modelled after T4 News, which was a printed newsletter distributed earlier in the 1990s. The newsletter's successors are the ongoing Phage.org website, phage-therapy.org, and the Bacteriophage Ecology Group Facebook page.

"The power to lysogenize is the property of temperate phages, as opposed to virulent ones."
— André Lwoff, 1953 (Lysogeny. Bacteriological Reviews 17:269-337), p. 273.

Welcome to the wonderful world of bacteriophage classification based on phage-host interaction. It turns out that there is some confusion as to how one classifies these interactions. Here, then, are a few definitions, which I follow with a proposition that we stop describing lytic, non-temperate phages as "virulent." I find all of these terms useful (except virulence as typically defined), but only when properly employed. Thus, there is no such thing as a "lysogenic phage" and temperate phages typically are perfectly capable of lysing their bacteria hosts.

Common practice has been to differentiate phages into at least two types, temperate versus virulent or lytic, to which a third type, chronic or continuous, should be habitually included. Temperate phages can produce reductive (lysogenic), productive, or abortive infections while non-temperate phages can give rise only to productive or abortive infections. Chronically infecting phages extrude their progeny from infected cells without lysing their hosts, while both temperate and lytic (or virulent) phages lyse their hosts to release progeny phages. Unfortunately, this contrast between chronic release and lytic release gives rise to an ambiguity: Temperate phages, in practice, are lytic, but, strictly speaking, are not "Lytic phages." Less ambiguous, temperate phages are not virulent phages, but as I will consider, this latter term, too, is problematic.

The term "Virulence" dates from early phage characterization in which it was noted that some phages more readily lyse bacterial cultures than others. For instance:

The question of virulence has been mentioned and emphasis placed upon the necessity of utilizing a race of maximum virulence. By this is meant a race of bacteriophage which will cause a complete and permanent dissolution of the organisms actually present in the infectious process. (p. 178, F. d'Hérelle as translated by G. H. Smith, 1930. The Bacteriophage and its Clinical Applications. Charles C. Thomas, Publisher. Springfield, Illinois)

Only subsequently did the term "Virulent" come to designate those phages that fail to display lysogeny in the modern sense of that term:

Phages have been classified in two categories, temperate and virulent according to the presence or absence of the power to lysogenize. (p. 319, Lwoff, 1953)

Regardless, I am of the opinion that it is time to stop employing the term virulent as a synonym for not temperate. Why? First, it is very likely that by doing so we do not use the term in its original sense. Second, especially as we ponder the use of phages as antibacterial agents, it is probably useful (taking d'Hérelle's lead) to have a term that distinguishes those phages that more readily lyse bacterial cultures from those that less readily lyse bacterial cultures. Virulent is the obvious and perhaps original term used for this purpose. Third, without extensive molecular characterization there is always significant uncertainty surrounding our declarations of phage virulence in the Lwoff sense:

Unfortunately, the definition of the character virulent is purely negative. If, after the action of a temperate phage, most survivors are nonlysogenic, the rare lysogenic survivors, because of their low proportion, may be practically impossible to find. Thus, as a result of the study of a system with a low lysogenization quotient, a temperate phage could be considered as virulent... and one may conceive of a phage behaving as a strong virulent in one bacterium and able to be reduced to a prophage into another. (p. 319, Lwoff, 1953)

Lastly, as pointed out by Michael DuBow during this past June meeting [June 2000, at the Millennial Phage Meeting, hosted by Michael DuBow], the term VIRULENT has some serious PR baggage as phage therapists ponder injecting what we hope are harmless little viruses into people's arms.

What's the alternative? Except for the various vir derivatives of temperate phages (the description of which is sufficiently ingrained that there is no going back), I propose using "Obligately lytic" to describe those phages that obligately initiate their lytic cycle upon successful adsorption, especially if one has demonstrated to some reasonable degree that a "Lytic phage" truly is not capable of inducing lysogeny on any host. I reject "Non-temperate" as a synonym because chronically infecting phages are also non-temperate but are not obligately lytic.

We should reserve virulent as a description of the ability of a phage to kill some (large) proportion of the cells found within a bacterial culture.

See Also

Hobbs, Z. & Abedon, S.T. (2016). Diversity of phage infection types and associated terminology: the problem with 'Lytic or lysogenic'. FEMS Microbiology Letters 363(7):fnw047. 10.1093/femsle/fnw047

Abedon, S.T. (2026). Broth optical density-based assessment for phage therapy: turbidity reduction, antibacterial virulence, and time-kill. Viruses 18(1):97. 10.3390/v18010097