Molecular screening for the presence of Streptococcus agalactiae, Escherichia coli and Listeria monocytogenes in samples of milk from dairy herds

This study aims to verify the predominance of contamination with pathogenic microorganisms in dairy herds. In order to validate the initially used methodology, an artificial contamination was conducted in commercially acquired whole UHT milk, with strains of Listeria monocytogenes, Streptococcus agalactiae and Escherichia coli, in final concentrations from 2.10-7 to 2.100 CFU/mL, which were submitted to a DNA extraction protocol and to a later amplification using the polymerase chain reaction (PCR) technique. The 702 bp fragments were identified, 884 and 524 bp corresponding, respectively, to L. monocytogenes, E. coli, S. agalactiae. In order to verify the presence of these pathogens in in natura milk, the samples were obtained directly from the teats of 125 cows from the dairy herds of four producers, and from the cooling tanks of eight producers, being submitted to DNA extraction, and posterior PCR analysis. The data were analyzed with the Chi-squared test (c2) and different sensibility and specificity values were obtained for each microorganism. In cooling tanks, a prevalence of 37.5% of contamination with S. agalactiae and of 31.25% by E. coli was found. Regarding samples obtained from cow teats, we observed the presence of S. agalactiae and E. coli in 16.2 and 47.5% of the samples. No sample tested positive for L. monocytogenes. The results obtained indicate that the isolation protocol of bacterial DNA directly from the milk, and the PCR technique were efficient to detect the analyzed microorganisms, and may be incorporated as part of routine tests. Moreover, PCR may be an important mechanism to evaluate the quality of milk to be consumed.