Characterizing Listeria Species of Dairy Environments: Phenotypic Insights
Authors: S Chaithra, , C Kempanna, , R Prabha. 
Published: December 2024

Aim: Genus Listeria include food borne pathogens. They contaminate the milk through soil, sewage and other sources in the dairy farm.  In the present study, attempt was made to collect dairy environmental samples and carried out enumeration, isolation as well as phenotypic characterization of the Listeria isolates obtained.
Study Design: The dairy environmental samples were collected  from ILFC, Veterinary college, KVAFSU, Bengaluru, Karnataka and subjected for enumeration of Listeria and toatal bacterial counts. The Listeria isoates obtained were characterized for morphology, Gram’s reaction and other biochemical tests. Identification key for Listeria isolates were prepared based on recent edition of Bergey’s namual of systematic Bacteriology and based on the activities they were identified up to species level.
Place and Duration of Study: The work was carried out in the Department of Dairy Microbiology, Dairy Science College, KVAFSU, Hebbal, Bengaluru, Karnataka from June.2021 to June 2022.
Methodology: The dairy environmental samples were collected aseptically and enumerated for Listeria and total bacterial counts using PALCAM agar and Plate Count agar, respectively. The Listeria isolates were coded and subjected for Gram’s staining, catalase test, oxidase test, motility test, methyl red and nitrate reduction to confirm the genus and specific biochemical tests like growth at 10% sodium chloride, sugar fermentation tests, CAMP test, coagulase, haemolysin and lecithinase activities on particular media. The results obtained for the tests conducted were compared with the standard identification key and declared the species of Listeria.
Results: "Listeria count in soil, fodder, dung and feed samples ranged from 2.30 to 3.14 log10cfu/g, in which fodder showed more followed by feed. Udder swabs, pail rinse, pail milk and can milk had Listeria count of 0.30 log count while 0.60 log count was observed in chilled milk whereas water, swabs of hand, udder and equipment, pail and can rinses, air from dairy farm, aseptic milk and pasteurized milk did not show the presence of Listeria.  The phenotypic identity of the 23 isolates of Listeria obtained from dairy environmental samples revealed that Listeria monocytogenes predominated (56.50) followed by L. innocua (17.39), L. ivanovii (8.70); L. seeligeri  (4.35) and 13.06 percent of unidentified  Listeria spp. All the 23 Listeria isolates, except one isolate of L.monocytogenes (L16) grew well at 50C indicating psychrotrophic nature. At deep freeze temperature (-180C), only 9 out of 23 Listeria isolates showed growth specifying their psychrophilic nature. Among 23 Listeria isolates, 14 showed haemolysis, phospholipase and coagulase activities indicating their pathogenic nature. "
Conclusion: Dairy fodder, soil, dung, feed showed high Listeria count among the dairy environmental samples. Among the twenty-three species of Listeria, Listeria monocytogenes predominanted,  majority were psychrotrophs, while only a few were psychrophiles and pathogens indicating the entry of pathogen from dairy farm samples to milk. Indroducing strict hygenic practices at dairy farm level during milking and stirage as well in priocessing of milk may reduce the entry of pathogenic species of Listeria.