Alternative mechanism for bacteriophage adsorption to the motile bacterium Caulobacter crescentus

2D and 3D cryo-electron microscopy, together with adsorption kinetics assays of ϕCb13 and ϕCbK phage-infected Caulobacter crescentus, provides insight into the mechanisms of infection. ϕCb13 and ϕCbK actively interact with the flagellum and subsequently attach to receptors on the cell pole. We present evidence that the first interaction of the phage with the bacterial flagellum takes place through a filament on the phage head. This contact with the flagellum facilitates concentration of phage particles around the receptor (i.e., the pilus portals) on the bacterial cell surface, thereby increasing the likelihood of infection. Phage head filaments have not been well characterized and their function is described here. Phage head filaments may systematically underlie the initial interactions of phages with their hosts in other systems and possibly represent a widespread mechanism of efficient phage propagation

The mechanism of a host-dependent abortive bacteriophage infection in <i>Caulobacter</i>

Several Caulobacter phages inhibit or kill host bacteria without phage production. Host-range studies showed that many of the Caulobacter strains that were sensitive to a particular phage were not productive hosts for that phage. Phage adsorbed to both productive and non-productive sensitive hosts but not to non-sensitive controls, nor to host cells treated with NaCN, or cells killed by ultraviolet (UV), heat, formaldehyde, or desiccation. Lysis from without could not be demonstrated. Ultraviolet-inactivated phage showed the same inhibitory effect on the non-productive hosts as did viable phage; however, purified tails of phage ø6 did not exhibit the capacity to inhibit the sensitive hosts. The UV adsorbance of the suspending medium of cell–phage mixtures increased with time in the case of the non-productive strain but not the productive strain. This increase indicates that leakage of intracellular material into the medium occurred as a result of phage infection of the non-productive strain but not the productive strain. </jats:p

Pilus-dependent, double-stranded DNA bacteriophage for Caulobacter

Caulobacter phage phi 6, previously reported to adsorb specifically to bacterial flagella, was shown here to attach to pili more frequently than to flagella. Phage phi 6 was shown to contain double-stranded DNA by circular dichroism spectroscopy and thermal denaturation accompanied by a hyperchromic shift at 260 nm. Morphologically, phage phi 6 fits group B2 (H.-W. Ackermann, in A. I. Laskin and H. A. Lechevalier, ed., Handbook of Microbiology, vol. 1, p. 638-643, 1973) with a long, noncontractile tail and an elongate head. Pilus-less mutants of the host Caulobacter vibrioides CV6 are phage phi 6 resistant, whereas flagellum-less mutants, which produce pili, are phage susceptible. Treatments of susceptible cells which remove or immobilize pili and flagella, e.g., blending or cyanide, inhibited phage phi 6 infection. Our evidence suggests that phage of phi 6 initiates infection in a manner similar to the pilus-specific phages for Pseudomonas described previously (D. E. Bradley, Virology 51:489-492, 1973; D. E. Bradley and T. L. Pitt, J. Gen. Virol. 24:1-15, 1974).</jats:p