Bacteriophage Mu contamination impacts interbacterial competition

Here, we report the identification of bacteriophage Mu contamination in a commonly used <i>Citrobacter rodentium</i> DBS100 ∆<i>cpxRA</i> mutant strain. After re-constructing a new Mu-free ∆<i>cpxRA</i> 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 ∆<i>cpxRA</i> strain, as strains carrying Mu consistently outcompeted susceptible Mu-free competitors. These results are important for the field, as the contaminated DBS100 ∆<i>cpxRA</i> mutant strain has been used in six different studies. We believe that the Mu contamination occurred during the construction of the ∆<i>cpxRA</i> 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.