Exploring the Antibacterial Properties of a Newly Isolated Microviridae Phage Against Multidrug-Resistant Escherichia coli

In response to the alarming rise in antimicrobial resistance, bacteriophages have re-emerged as a promising alternative to conventional antibiotic therapy. The main objective of this paper was to characterize a newly isolated phage (vB_SEC_3) in the context of its suitability for phage therapy against MDR <i>E. coli</i>, which is considered a priority pathogen. The phage was characterized at the morphological, genomic, and biological levels relevant to phage therapy. TEM analyses revealed a non-enveloped icosahedral capsid lacking tail structure. Phylogenetic and tANI analyses placed the phage within the α3 phages (genus <i>Alphatrevirus</i>) of the less-studied family <i>Microviridae</i> and revealed <95% similarity to its closest relatives, suggesting vB_SEC_3 is a putative novel species within this genus. The genome (6085 bp, GC content 45.3%) displayed the conserved organization typical for these phages, including overlapping genes. No known genes associated with lysogeny, antibiotic resistance, or virulence were detected. Briefly, vB_SEC_3 was able to effectively lyse two MDR strains of <i>E. coli</i> (S1 and B5, EOP 0.735 and 0.961, respectively). Tolerance to a wide pH range (4-10.5) and to temperatures up to 80 °C was established. Six-month storage of the crude lysate at 4 °C resulted in a slight decrease (<0.16 log₁₀ PFU/mL) in phage titer. This study provides additional insights into the biology and diversity of <i>Microviridae</i> phages and offers a basis for future investigations into their potential relevance in the context of combating MDR bacteria.