SUMMARY: The F-pilus retraction theory suggests that F-pili retract on contact with a recipient bacterium or pilus-specific bacteriophage, but there has been no direct demonstration of any form of pilus retraction. Electron microscopy is used here to study average length changes in RNA-phage pili after the adsorption of RNA-phage virions to the sides. The theory predicts that, if one phage per pilus is adsorbed, there should be a reduction of about 50% in the mean length due to the adsorption of virions at random points; retraction would be stopped at the point of adsorption when the virus reached the bacterial surface. A phage-resistant, pilus-bearing strain (PAO68), used as a control, showed no such change. The phage-sensitive strain PAOI showed an average reduction in pilus length of about 42.5% relative to its length before adsorption, or 50% relative to the PAO68 average. This strongly suggests pilus retraction in PAOI but not in PAO68. In electron micrographs, phage virions are seen almost always at the bases of PAOI pili, whereas virions adsorbed to PAO68 are randomly distributed along the pili.


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