Bacteriophage-Insensitive Mutants of Antimicrobial-Resistant Salmonella Enterica are Altered in their Tetracycline Resistance and Virulence in Caco-2 Intestinal Cells

Abstract

Bacteriophages have shown promise as therapeutic alternatives to antibiotics for the control of infectious bacteria, including the human pathogen Salmonella. However, the development of effective phage-based applications requires the elucidation of key interactions between phages and target hosts, particularly since host resistance to phage is inevitable. Little is known about the alteration of host phenotypes following the development of resistance to phage. The aim of this study is to evaluate the antibiotic susceptibility and virulence of a Salmonella isolate following the development of resistance to bacteriophage SI1. We observed enhanced susceptibility to tetracycline and decreased invasion capacity in a differentiated Caco-2 intestinal cell line. Whole genome sequence analysis revealed an array of mutations, most notably, truncations in vgrG1_2, a core gene involved in Type VI secretion and mutations in the lipopolysaccharide, thereby indicating the plausible attachment site of phage SI1. These findings shed light on understanding the underlying mechanism for phage immunity within the host. Importantly, we reveal an associated genetic cost to the bacterial host with developing resistance to phages. Taken together, these results will aid in advancing strategies to delay or eliminate the development of host resistance when designing informed phage-based antimicrobials.

Keywords: Salmonella; antibiotic resistance; phage resistance; virulence.

Figure 1

Caco-2 cell invasion capacity by the S. Agona strains. Error bars indicate the standard deviations of the means of three biological replicates. Means of the BIMs were compared to that of the WT. Asterisks located above mean values indicate significance below α = 0.05 (one-way ANOVA).

Figure 2

Schematic representation of the T6SS gene cluster in S. Agona FSL S5-517. Arrows indicate direction of transcription. Black arrows indicate gene products involved in the T6SS. Grey arrows represent gene products of unknown function but believed to play a role in the T6SS. White arrow indicates a hypothetical protein.