Combined Lytic Score as a new metric to quantify phage-host specificity, tested in a screening study on a collection of Salmonella strains.

Bacteriophages are promising alternatives for antibiotics. One challenge of developing bacteriophage-based preparation for practical application is to understand expected host range of such products. We evaluated the host range and lytic activity of BAFASAL(R), a four-phage cocktail targeting Salmonella in poultry production. For that purpose, we developed a new composite metric, the Combined Lytic Score (CLS), integrating results from two in vitro assays: serial dilutions spot test on semisolid medium and spectrophotometric growth inhibition in liquid culture. Using this approach phage cocktail was tested against collection of 72 Salmonella strains, including 55 S. Enteritidis isolates representing diverse geographic origins and genomic backgrounds. Spot test patterns were transformed into a continuous scale using the Most Probable Number (MPN) approach to estimate the number of phages required for visible lysis. In parallel, growth inhibition was quantified as the area-under-curve-based inhibition score (ANS). Both metrics were normalized and combined into CLS as a projection onto the regression line describing their correlation (R{superscript 2} {approx} 0.82). More than 65% of S. Enteritidis strains, reached normalized CLS values higher 75%, indicating high susceptibility to BAFASAL(R) in vitro. Phage susceptibility did not correlate with either phenotypic antibiotic resistance or the number of resistance and virulence genes. CLS provides a quantitative method to integrate different experimental methods of determination of bacterial susceptibility to bacteriophages and to rank bacterial strains by phage susceptibility. This approach supports robust host range determination and may facilitate regulatory evaluation and rational design of phage-based interventions in food safety and animal production. IMPORTANCEAssessment of bacteriophage host range is an important step in characterization of bacteriophage strains both in basic and translational research, yet it is still commonly based on qualitative or poorly standardized assays. This lack of harmonization limits reproducibility and complicates comparisons across studies, laboratories, and application contexts. In this work, we propose the Combined Lytic Score (CLS) as a quantitative framework that integrates outcomes from two widely used experimental approaches: serial-dilution spot assays and microtiter-based growth inhibition kinetics. By converting spot-test results into a continuous, concentration-dependent metric and combining them with normalized kinetic inhibition data, CLS enables more consistent interpretation of phage-host interaction outcomes. Application of CLS to a diverse collection of Salmonella enterica strains demonstrates how this approach can support systematic, scalable host range analyses. The CLS framework provides a practical step toward improved standardization of phage susceptibility testing, facilitating clearer data interpretation and comparison in both environmental and applied microbiology research.