Isolation, characterization and evaluation of virulent bacteriophages against Listeria monocytogenes

Bacteriophages are considered a potential antimicrobial food intervention due to their ability to inactivate target bacteria in different environments. This study was undertaken to isolate, characterize, and identify a mixture of phages that could be used for control of Listeria monocytogenes. A total of 37 phages were isolated from raw sewage sludge using L. monocytogenes serotypes 4b, 1/2a and 1/2b strains. The host range of these phages was determined by spot-testing. Ten lytic phages were selected for further evaluation with efficiency of plating (EOP) analysis and characterization by pulsed field gel electrophoresis (PFGE). According to the spot tests and EOP results, the isolated phages against L. monocytogenes 4b strains were found to be capable of lysing a wider range of ribotypes. Also, three phages specific for 1/2a and 1/2b serotypes had similar lysis capabilities over a range of target Listeria with high EOP values between 0.5 and 1.0. PFGE analysis identified 5 distinct restriction profiles of the 10 selected phages whose DNA was digested with either EcoRI or HinDIII. Transmission electron microscopy (TEM) pictures revealed that five selected phages belonged to the Myoviridae and one phage to the Podoviridae family. Based on screening results, a phage cocktail (LP1) comprised of six lytic phages was used to inactivate a mixed culture of five L. monocytogenes strains. LP1 was able to lyse bacterial cells within 30–60 min at multiplicity of infection values (MOI, PFU/CFU) of 1000 and maintained the target cells under detection levels for 24 to 48 h at 4, 10, and 30 °C. These results suggested the LP1 phage collection contained effective and strongly lytic phages that may serve as antibacterial preparation to control L. monocytogenes of different serotypes.