Antagonistic activity against Listeria monocytogenes and Escherichia coli from lactic acid bacteria isolated from raw milk

Resumo

Lactic Acid Bacteria (LAB) are naturally detected in several foods and can be important on pathogens control. Through production of numerous antimicrobial substances, LAB can inhibit microorganisms such as Listeria monocytogenes and Escherichia coli. Aiming to verify the presence of L. monocytogenes, enumerate E. coli and LAB, and verify the antagonistic activity of isolated LAB against L. monocytogenes and E. coli, samples of raw milk were collected from 45 dairy farms from Agreste region of Pernambuco. For L. monocytogenes detection VIDAS Listeria (bioMeriex) system was used while E. coli was enumerated by PetrifilmTM EC (3M) after incubation at 35oC for 48 hours. For LAB enumeration, samples were diluted in MRS broth, distributed in PetrifilmTM AC (3M) plates and incubated at 30°C for 72 hours in microaerophilic conditions. Antagonism of 671 LAB isolates was determined through spot-on-the-lawn modified methodology using L. monocytogenes ATCC 7644 and E. coli ATCC 25922 as targets. L. monocytogenes was not detected in any milk sample. E. coli counts varied from < 103 to 1.3 x 105 CFU/mL. LAB counts varied from 11.1 x 104 to 9.9 x 106 CFU/mL. When LAB isolates were tested for antagonistic activity against L. monocytogenes, 549 (81.8%) samples were positive, from which 410 (61.1%) showed total inhibition and 139 (20.7%) partial inhibition. Concerning E.coli, 258 (38.5%) isolates showed antagonistic activity, all with partial inhibition. High counts of E. coli and total coliforms found indicate poor hygienic conditions during obtainment of milk. In addition the elevated frequency of LAB inhibitory to L. monocytogenes could explain the absence of this pathogen in milk samples studied.