Antimicrobial activity of camel's milk against pathogenic strains of Escherichia coli and Listeria monocytogenes

Abstract

The inhibitory activity of camel's milk and colostrum at 4°C and 20°C was tested by the well diffusion assay against six pathogens. The activity was also studied in situ by monitoring the growth of a three-strain mixture of Listeria monocytogenes or Escherichia coli O78:K80 in camel's milk, colostrum or Tryptic Soy Broth (TSB) as function of time. The results of the well diffusion assay show that Bacillus cereus was resistant to the inhibitory activity present in camel's milk and to the colostrum, while L. monocytogenes LMG 13304 and E. coli O78:K80(JB2) were the most sensitive as judged by the diameters of the inhibition zones. The in situ test revealed a typical inhibition pattern of both pathogens in camel's milk samples during 48 h of incubation at both storage temperatures. The colostrum exerted a definite bactericidal activity against E. coli O78:K80 at ambient temperature, and the viable counts decreased to below the detectable level in a 1-mL sample at 48 h, while at the refrigeration temperature, the counts were reduced by 1 and 3 log units compared to the initial inoculum and to the positive control, respectively. The camel's milk and colostrum samples had a bacteriostatic effect against L. monocytogenes during the first 8 h of incubation; thereafter, a tendency to increase was noted in the colostrum at 20°C. Similar experiments were carried out on E. coli O78:K80 in heat-treated camel's or cow's milk and showed that the inhibitory effect of camel's milk was reduced by heat treatment.