An Endolysin LysSE24 by Bacteriophage LPSE1 Confers Specific Bactericidal Activity against Multidrug-Resistant Salmonella Strains

Abstract

<i>Salmonella</i> is responsible for a wide range of infections and is a constant threat to public health, particularly in light of emerging antibiotic resistance. The use of bacteriophages and phage endolysins as specific antibacterial agents is a promising strategy to control this bacterial infection. Endolysins are important proteins during the process of bacteria lysis by bacteriophages. In this study, we identify a novel endolysin, named LysSE24. LysSE24 was predicted to possess <i>N</i>-acetylmuramidases activity, with a molecular mass of ca. 17.4 kDa and pI 9.44. His-tagged LysSE24 was heterologously expressed and purified by Ni-NTA chromatography. LysSE24 exhibited optimal bactericidal activity against <i>Salmonella</i> Enteritidis ATCC 13076 at a concentration of 0.1 μM. <i>Salmonella</i> population (measured by OD_{600 nm}) decreased significantly (<i>p</i> < 0.05) after 10 min of incubation in combination with the outer membrane permeabilizer in vitro. It also showed antibacterial activity against a panel of 23 tested multidrug-resistant <i>Salmonella</i> strains. Bactericidal activity of LysSE24 was evaluated in terms of pH, temperature, and ionic strength. It was very stable with different pH (4.0 to 10.0) at different temperatures (20 to 60 °C). Both K⁺ and Na⁺ at concentrations between 0.1 to 100 mM showed no effects on its bactericidal activity, while a high concentration of Ca²⁺ and Mg²⁺ showed efficacy. Transmission electron microscopy revealed that exposure to 0.1 μM LysSE24 for up to 5 min caused a remarkable modification of the cell shape of <i>Salmonella</i> Enteritidis ATCC 13076. These results indicate that recombinant LysSE24 represents a promising antimicrobial activity against <i>Salmonella</i>, especially several multidrug-resistant <i>Salmonella</i> strains. Further studies can be developed to improve its bactericidal activity without the need for pretreatment with outer membrane-destabilizing agents by synthetic biology methods.