Receptors of Salmonella enterica serovar Entritidis bacteriophages isolated in Belgium

Bacteriophage therapy is potentially an interesting tool for the control of bacterial infections. For this aim, bacteriophages infecting a Salmonella enterica serovar Enteritidis (S. Enteritidis) phage type 4 strain were isolated from different surface waters in the Brussels area.
All 12 phages isolated initially, by plating of the water samples on the wild type S. Enteritidis host, were unable to infect a rough mutant, carrying an rfaI mutation, of the same strain. Therefore these phages most likely use the bacterial lipopolysaccharide (LPS) as a receptor. Among these, at least 9 different phage isolates could be distinguished, by plating the bacteriophages on different phage-resistant mutants.
Phages that do not bind on an LPS receptor were isolated using the rfaI mutant as a host. Some of these are unable to grow on the wild type smooth strain. By plating on different phage-resistant mutants, the remaining phages were shown to belong to four different groups. In order to identify the receptors of these bacteriophages, a phage-resistant mutant was obtained by transposon mutagenesis. This mutant was resistant to bacteriophages belonging to the four groups. The genetic linkage between the resistance and the transposon insertion was proved by transduction, mediated by bacteriophage P22. Amplification of the Salmonella nucleotide sequence adjacent to the transposon, by the inverse polymerase chain reaction (Ochman et al., 1988), followed by nucleotide sequence analysis, revealed that the transposon inserted in the btuB gene, encoding an outer membrane cyanocobalamin (vitamin B12) transporter, that is also the receptor of bacteriophage BF23 (Rioux and Kadner, 1989).
These data show that most of the isolated Salmonella bacteriophages recognize receptors on the bacterial LPS. LPS also seems to mask the protein receptor of other phages. Several bacteriophages recognizing an accessible pro-tein receptor were shown to bind to the same protein, the vitamin B12 trans-porter. It will be interesting to confirm these findings with samples collected in different areas, in particular on or near of poultry farms.