Field of Science

P1 (vir?)

I have some phage P1, courtesy of a colleague. But (as usual) complications have arisen. The colleague is out of town, and his P1 collection consisted of five different lysate tubes, of unknown ages as none of the tubes had a date. So I tested them all for viable phage by streaking a drop onto a plate that had a lawn of host cells on it, and looked this morning for the plaques (spots of lysis) that phage would make.

Two of the lysates gave no plaques, so they were probably very old. Two others (#1 and #4) gave lots of conspicuous plaques, and #2 gave tiny plaques. The colleague tells me (by email) that #1 is his newest stock, but the big plaques are not what I expected.

The phage should be not normal (wildtype) P1 but a mutant called P1vir (vir = virulent). Wildtype P1 can form lysogens, becoming dormant in cells that are then immune to being killed by other P1. The large plaques I see look like plaques made by wildtype P1, because they have cloudy centers (the plaques are "turbid"), where surviving cells are growing; these are likely to be lysogens. The virulent mutant should not form lysogens, so its plaques should be clear. Our manual of bacterial genetics methods says that P1vir plaques are tiny, so I think lysate #2 is more likely to be genuine P1vir. Lysates #1 and #4 would then have been grown from P1vir that had mutated, reverting to wildtype. (Perhaps the person who made the lysate mistakenly picked a "nice big plaque" instead of the more common tiny plaques.)

Wildtype P1 is not very suitable for doing transductions because many of the survivors are likely to be lysogens, whereas we usually want to work with phage-free cells. I'll do test transductions with lysates #1 and #2; if #2 works well I won't bother doing experiments on #1 to sort out the virulence issue, but just make a good fresh lysate from a single plaque of #2. Part of this lysate will be given to the colleague who supplied the original lysates.

Before making proper lysates and testing transduction I need to grow up appropriate E. coli host strains. I did the preliminary checking on the strain carrying the ppdD::lacZ fusion. Unfortunately the computer that we keep our strain list on is in the shop, but one of the post-docs has a recent backup.


  1. All P1(vir and wt) make small, crappy-looking plaques. I would suspect that any large plaques you see are a contaminant (one of the T-even phages, more than likely).

  2. The big(ish) plaques are turbid so they're definitely not a T-even phage. They're probably small enough to be temperate P1, but I've discarded the lysates containing them and am making a fresh lysate from a single plaque.


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