Phage therapy has been used in the past as an alternative therapy against bacterial pathogens. However, phage-resistant bacterial strains can emerge. Some studies show that these phage-resistant strains are avirulent. In this study, we report that phage-resistant strains of Salmonella enterica
serovar Enteritidis (hereafter S. Enteritidis) were avirulent in the Caenorhabditis elegans
animal model. We isolated phage-resistant strains of S. Enteritidis ATCC 13076 by using three lytic phages (f2α
;SE and f18α
;SE). In these mutants, we explored different virulence factors like lipopolysaccharide (LPS), virulence plasmid (Pla), motility and type I fimbriae, all of which may have effects on virulence and could furthermore be related to phage resistance. The phage-resistant strains of S. Enteritidis showed loss of O-Polysaccharide (O-PS) and auto-agglutination, present a rough phenotype and consequently they are avirulent in the C. elegans
animal model. We speculate that the O-PS is necessary for phage attachment to the S. Enteritidis cell surface.