# A commercial bacteriophage cocktail failed to decolonize Zophobas morio larvae and promoted overgrowth of an OXA-48-producing Salmonella enterica

## TL;DR

A commercial phage treatment failed to remove a harmful Salmonella strain from mealworms and instead caused it to grow more, showing the need for caution with phage therapies.

## Contribution

Demonstrates that a broad-spectrum bacteriophage cocktail can promote overgrowth of a carbapenem-resistant Salmonella strain in an in vivo model.

## Key findings

- INTESTIbc treatment led to significant regrowth of OXA-48-producing Salmonella enterica in Zophobas morio larvae.
- Phage administration reduced potential bacterial competitors of Salmonella, possibly contributing to its overgrowth.
- Zophobas morio larvae are a viable in vivo model for studying intestinal colonization by Salmonella enterica.

## Abstract

Effective decolonization strategies for intestinal carriers of carbapenem-resistant Enterobacterales are essential to prevent severe life-threatening infections. In this work, we established gut colonization in Zophobas morio larvae (ZmL) using an OXA-48-producing Salmonella enterica ST198 strain (Sk-1) and assessed the commercial INTESTI bacteriophage cocktail (INTESTIbc) for decolonization.

ZmL were fed with food contaminated with Sk-1 (INTESTIbc-susceptible) for 3 days and then maintained on a non-contaminated diet until day 14 (T14). At T3, ZmL were grouped in untreated, dPBS- or INTESTIbc-treated (oral force-feeding on T3 and T5). At specified intervals, ZmL were sampled for quantification and characterization of Sk-1 (antibiotic/INTESTIbc susceptibility and whole-genome sequencing). ZmL microbiota was also investigated by 16S rRNA amplicon sequencing.

ZmL were rapidly colonized by Sk-1 across all groups (T3: 4.3 × 106 CFU/mL). Untreated and dPBS-treated larvae remained consistently colonized (T10: 3.4–9.1 × 104 CFU/mL; T14: 2.9–5.9 × 104 CFU/mL), whereas INTESTIbc treatment induced a significant Sk-1 regrowth (T10: 4.0 × 106 CFU/mL; P < 0.05 vs. controls). Sk-1 strains recovered under different conditions between T7 and T14 did not show phenotypic and genotypic changes. Bacteriophages administration resulted in reduced relative abundance of potential bacterial competitors of Sk-1 (i.e., Pseudocitrobacter).

ZmL can be used as a new in vivo model of intestinal colonization with S. enterica. However, INTESTIbc administration failed to achieve decolonization and instead promoted hazardous overgrowth of the inoculated pathogen. These findings highlight the need for further investigations to clarify the therapeutic potential or possible risks of broad-spectrum bacteriophage cocktails against intestinal infections/colonization caused by hyperepidemic S. enterica clones.

The online version contains supplementary material available at 10.1007/s10096-025-05275-6.