In-vitro/in-vivo characterization of phage resistant E. coli K1 and impact on Bacterial Fitness and Biofilm Production

Understanding bacterial phage resistance mechanisms has implications for developing phage-based therapies. This study aimed to explore human and avian Escherichia coli K1 isolates’ resistance to K1_ULINTec4, a K1-dependent bacteriophage. Resistant colonies were isolated from two different strains (APEC 45 and C5), both previously exposed to K1_ULINTec4. Genome analysis and several parameters were evaluated, including growth capacity, phage adsorption, phenotypic impact at capsular level, biofilm production and virulence in the in-vivo Galleria mellonella model. One of the resistant isolates exhibited a significantly slower growth rate suggesting the presence of a resistance mechanism altering its fitness. Comparative genomic analysis revealed insertion sequences at various capsular gene sites. In addition, antigenic tests targeting the K1 capsule showed a very low positive reaction compared to the control. Nevertheless, microscopic images of resistant strains revealed the presence of capsules with a clustered organization and biofilm assessment showed increased biofilm production compared to the original strains. In the G. mellonella model, larvae infected with phage-resistant strains showed better survival rates than larvae infected with phage-sensitive strains. In conclusion, a phage resistance mechanism was detected at the genomic level, had an impact on capsular expression, biofilm production and was able to decrease the virulence of E. coli K1 in-vivo.