# Bacteriophage Mu contamination impacts interbacterial competition

## TL;DR

A commonly used bacterial strain was found to be contaminated with bacteriophage Mu, which affected competition outcomes in experiments.

## Contribution

The study identifies Mu contamination in a bacterial strain and shows its impact on interbacterial competition.

## Key findings

- Mu contamination was found in a Citrobacter rodentium ∆cpxRA mutant strain.
- Strains carrying Mu outcompeted Mu-free competitors in interbacterial assays.
- The contamination likely occurred during conjugation with a Mu-containing donor strain.

## Abstract

Here, we report the identification of bacteriophage Mu contamination in a commonly used Citrobacter rodentium DBS100 ∆cpxRA mutant strain. After re-constructing a new Mu-free ∆cpxRA strain, we independently replicated the results of a recent study by A. Gilliland, C. Gavino, S. Gruenheid, and T. Raivio (Infect Immun 90:e00314-22, 2022, https://doi.org/10.1128/iai.00314-22). The only result from Gilliland et al. that was impacted by the presence of Mu was the outcome of interbacterial competition assays with the ∆cpxRA strain, as strains carrying Mu consistently outcompeted susceptible Mu-free competitors. These results are important for the field, as the contaminated DBS100 ∆cpxRA mutant strain has been used in six different studies. We believe that the Mu contamination occurred during the construction of the ∆cpxRA allele, during the conjugation of DBS100 with a popular Mu-containing donor strain. Our results highlight the importance of using Mu-free conjugal donor strains and how phage contamination can impact bacterial physiology and experimental results.