Phage-infected bacteria rotate (top row) before abruptly stopping (middle) and lysing (bottom).

Young/NAS

Just before they lyse, bacteria infected with lambda phage have no idea what is about to hit them, according to Angelika Gründling, Mike Manson, and Ry Young of Texas A&M University (College Station, TX). Gründling watched bugs spin around their single tethered flagella as an indicator that their membranes were still energized. The spinning stopped abruptly a few seconds before the cells lysed, thus contradicting the assumptions of one model for lysis timing.

That model was born from the observation that membrane energy poisons induce lysis prematurely. Energy poisons, some suggested, were merely accelerating a normal process of gradual proton leakage. In this model, phage-produced holin proteins would cause the leak, until a critical reduction in proton motive force (pmf) across the membrane triggered any remaining holins to form into pores and finish off the bug.

Gründling's new observations suggest that there may indeed be a critical level of pmf, but that there is no significant leakage early on that would gradually lead the cell to this critical level. Instead, holins increase in concentration until a pore spontaneously precipitates in the membrane. The catastrophic proton leak that results initiates the formation of many other pores, leading to the bug's rapid demise.

This triggering mechanism leads not only to rapid and complete lysis, but may also reduce competition from other phage. As new phage attach to a cell, they cause a transient drop in pmf. This drop may trigger holin aggregation, thus killing the cell before the invader has time to start competing with the resident phage. ▪

Reference:

Gründling, A., et al. 2001. Proc. Natl. Acad. Sci. USA. 10.1073/pnas.151247598 http://www.pnas.org/cgi/content/full/151247598