Effects of receptor destruction by Salmonella bacteriophages ϵ15 and c341

Abstract

O-Antigen-specific bacteriophages ϵ15 and c341 cleave the lipopolysaccharide receptor of Salmonella anatum enzymatically before they undergo the final steps of infection, which end with the release of the viral DNA. Employing the electron microscope, we observed a number of effects which phage adsorption and subsequent desorption exert on host cell and virus: (1) After adsorption of high multiplicities of virus particle at 0–4°, the host bacteria can be covered with more than 10 3 virions of either phage ϵ15 or phage c341. (2) Temperature shift-up to 35° caused the majority of the ϵ15 virions to desorb. However, several hundred virus particles per cell remained attached, and most of these showed empty heads. (3) The heads of the desorbed virus particles appeared to remain filled. Cells from which the phages had desorbed failed to adsorb newly added phage. (4) When adsorption of high m.o.i. of ϵ15 was carried out right away at 35°, the number of adsorbed phages was comparable to the number of virions observed at (2) above. In contrast to phage ϵ15, phage c341 virions stayed attached to the host cell at 35° under both conditions (that is, after shift-up or primary adsorption at 35), and most of their heads remained full. When S. anatum was pretreated with the receptor-hydrolyzing enzyme from isolated adsorption organelles of phage E15, the subsequent adsorption of either type of phages was abolished. However, the enzyme treatment failed to release already adsorbed phage e341. The electron microscope data were supported by measurements of the distribution of radioactively labeled E15 before and after desorption. Desorption was accompanied by a 50 to 70% loss of infectivity. DNase treatment reduced to about half the infectious titer of the desorbed virions. We hypothesize that the striking difference in the desorption behavior of the two phages is caused by the differences in the substrate and binding site of the LPS receptors: phage ϵ15, hydrolyzing glycosidic linkages in the backbone of the oligosaccharide, will dislodge adjacent receptor strands with virions attached. In contrast, phage c341 hydrolyzes the O-acetyl side group in the O-antigen subunit, but maintains its binding to the backbone of the LPS.