Characterization and preclinical in silico safety assessment of three virulent bacteriophages targeting carbapenem-resistant uropathogenic Escherichia coli
Authors: Dhungana Gunaraj , Nepal Roshan , Houtak Ghais , Bouras George , Vreugde Sarah , Malla Rajani 
Published: January 2024

Escherichia coli causes several infections like intestinal diarrhea, urinary tract infections, and pneumonia, which are difficult to treat due to the emergence of antimicrobial resistance. The World Health Organisation (WHO) declared carbapenem-resistant E. coli a critical pathogen and called for the development of new antimicrobial agents. Phage therapy, a century-old technique, has been revitalised recently with many successful applications against multi-drug resistant infections. As such, phage therapy is considered one of the most effective alternatives or adjuvants to antibiotics and is also effective in treating multidrug-resistant (MDR) bacterial infections. However, few phages that can kill pathogenic E. coli strains from Southeast Asia have been studied at a molecular level, although the burden of MDR, particularly carbapenem resistance, is high in this region. In this study, three closely related obligately virulent Escherichia phages (øEc_Makalu_001, øEc_Makalu_002, and øEc_Makalu_003) that could infect uropathogenic E. coli were isolated from sewage samples and characterized using morphological as well as sequence-based analysis. Further, a comparative genomic and phylogenetic analysis was done to predict the functional genes and evolutionary relationships. Electron microscopy and genome-based classification revealed that all three phages belonged Krischvirus genus of Straboviridae family. One-step growth curve showed that the latent period of the phages ranged from 15–20 min, the outbreak period was about 50 min, and the burst size ranged between 74–127 PFU/bacterium. Moreover, the phages could tolerate a pH range of 6 to 9 and a temperature range of 25–37°C for up to 180 minutes without significant loss of phage viability. Spotting the purified phages in different clinical isolates showed a broad host-range of isolated Escherichia phages and could lyse up to 30% of the tested pathogenic clinical isolates, including an intra-genus isolate of Klebsiella pneumoniae. The genome organization of all three phages was almost identical and similar to T4-like phage with a full-length genome of ~ 163 kb. Around 284 coding sequences (CDS) were predicted in each phage, of which only 40% were functional with an average gene length of 951 bp. The coding density in all phages was approximately 95%, while the gene density was 1.73 gene/kbp. Putative lysin, holin, endolysin, and spanin genes were found in the genomes of all three phages, resembling that of T4-like phages implying their phylogenetic relatedness. Furthermore, various in silico sequence analysis tools confirmed that all three phages were strictly virulent with functional lysis modules but devoid of any known virulence or toxin genes, integron-associated genes and antimicrobial-resistance (AMR) genes. In summary, pre-clinical experimental analysis and exploration of genomic features suggest that all three phages may be suitable candidates for therapeutic applications against uropathogenic E. coli.