by Michael Yarmolinsky
Patrons of upscale seafood restaurants are given the opportunity to see that the unfortunate creatures destined for the lobster pot are waving their antennae about. Savvy customers at downscale seafood markets evaluate questionable claims of freshness by smell. A fastidious bacteriophage would welcome the opportunity to gauge the quality of a potential meal, if only it could make that assessment. I was recently reminded, in the course of disposing of old reprints, that a bacteriophage named Chi can do so. It attacks only motile strains of bacteria, and then only if the flagella are active. How is this circumspect appraisal accomplished?
Phage Chi X 220,000. Source.
It has been several years since the publication of convincing support for a mechanical model that can account for the discrimination exhibited by Chi. Why revisit it now? First, for the benefit of those who have overlooked the remarkable story of Chi’s fastidious behavior and second because I suspect that we have learned only the half of it.
What sort of phage is Chi?
Chi is a virulent, double-stranded DNA phage with a long, uneven history and a long, tapered tail ending in a single, long, kinky tail fiber. It was first characterized in 1936 in the laboratory of Félix d’Herelle, the adventurer-scientist who is jointly credited with the discovery of the viruses he designated “bacteriophages.” Chi’s genome (60 kb) has recently been sequenced (Andrew Kropinski, Kelly Hughes and Roger Hendrix, in preparation).
The initial report showed that Chi attacks only flagellated bacteria. Growth of sensitive strains on agar containing phenol, at a concentration known to prevent the development of flagella, rendered the bacteria Chi-resistant. Chi-resistance could be used to select mutants defective in flagellation. Mutants altered in several of the 44 known flagellar genes of Salmonella were subsequently selected in this way.
The flagellar organelle. Source.
What sort of organelle is a flagellum?
Flagella are long, thin, helical filaments commonly much longer than the bacterial body from which they emerge at various sites. Their somewhat flexible, sinusoidal appearance was interpreted, until the early 70s, as evidence that they act like whips, but their helicity is intrinsic and their action that of a propeller rotated by a motor embedded in the cell body. Normal rotation speeds for Escherichia coli flagella are around 6000 rpm, but a record speed, set by a vigorous Vibrio, is 100,000 rpm. Each flagellum is both a reversible motor organelle and a protein export and assembly apparatus that fabricates the external filament by extruding flagellin monomers through a central channel and adding them to the growing flagellum at its very tip.