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

Abstract

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 Petrifilm TM EC (3M) after incubation at 35 o C for 48 hours. For LAB enumeration, samples were diluted in MRS broth, distributed in Petrifilm TM 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 < 10 3 to 1.3 x 10 5 CFU/mL. LAB counts varied from 11.1 x 10 4 to 9.9 x 10 6 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. 45 propriedades leiteiras da região agreste de Pernambuco. Para detecção de L. monocytogenes foi utilizado o sistema VIDAS Listeria (bioMeriex) enquanto E. coli foi enumerada pelo Petrifilm TM EC (3M), após incubação a 35°C por 48 horas. Para contagem de BAL as amostras foram diluídas em caldo MRS, semeadas em placas de Petrifilm TM AC (3M) e incubadas a 30°C por 72 horas em microaerofilia. O antagonismo dos 671 isolados de BAL foi determinado através da metodologia spot-on-the-lawn modificada, utilizando a cepa L. monocytogenes ATCC 7644 e E. coli ATCC 25922 como alvos. Em nenhuma amostra de leite foi detectada a presença de L. monocytogenes. As contagens de E. coli variaram de < 1, 0 x 103 UFC/mL a 1,3 x 105 UFC/mL. As contagens de BAL variaram de 11,1 x 104 UFC/mL a 9,9 x 106 UFC/mL. Quando os isolados de BAL foram testados para capacidade antagonista contra L. monocytogenes, 549 (81,8%) foram positivos, sendo que 410 (61,2%) com inibição total e 139 (20,7%) com inibição parcial. Quanto à E. coli, 258 (38,5%) dos isolados apresentaram atividade antagonista, sendo todas com inibição parcial. As altas contagens de E. coli e coliformes totais encontradas indicam condições higiênicas inadequadas durante a obtenção do leite. Além disso, a elevada freqüência de BAL com capacidade antagonista a L. monocytogenes, poderia e...

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