SENSIBILIDAD DE Escherichia coli A DIFERENTES BACTERICIDAS DE USO AGRÍCOLA

Abstract

Fresh fruits and vegetables are an important part of a healthy diet, and from a microbiological perspective they are relatively low-risk foods compared to meat and dairy products. However, over the past few years, there has been in increase in the number of illness transmitted by fruits and vegetables. The available information shows that this is a growing problem, since they can serve as a vehicle for a number of bacteria, parasites, and viruses that are pathogenic to humans, which must be managed in the field and post-harvest through good agricultural and manufacturing practices. However, in most cases, the effects of agricultural bactericides on the disease-causing bacteria found in food are unknown. In order to generate this information, we evaluated in vitro the effects of commercial doses of the bactericides Agricultural Terramicin, Agrigent Plus, Gentamicin, Hidrobacter, Kasumin and copper sulfate on Eschericia coli strains O105ab, O157:H16 and O157:H7. The control treatment was sterile distilled water. We evaluated two different methods, the first by adding the antibiotic to the agar, and the second using nutrient agar plates inoculated with serotypes O157:H16, O105ab and O157:H7, avoiding leaving open zones to achieve a uniform plating, and incorporating the antimicrobial into discs of filter paper with a contact time of 10 minutes. The plates were incubated at 35°C. We used a completely randomized experimental design with three replicates for each method. For the statistical analysis, we used a split plot design for each method for each sampling time. In the first method, we evaluated the number of CFU/mL (Colony-forming units per milliliter) on each plate seeded after 24 and 72 hours. In this method, the analysis of variance indicated that there was a highly significant effect of bacterial strain, treatment, and their interaction. The E. coli strains O105ab (EcH), O157:H7 and O157:H16 (EcT) were 100% inhibited with the copper sulfate, Agri Gent Plus, agricultural terramicin, and Hidrobacter; that is, they were unable to grow at all on the surface of the medium. Kasumin inhibited only 95% of the CFU present, and the highest counts were obtained from the O157:H16 serotype. For the second method, we used a caliper to measure the diameter of the zone of inhibition (mm) that was formed around each disc. The analysis of variance showed that there was a significant effect of the factors treatment and date of evaluation using the sensidisc method. On the first evaluation date, the separation of means for the treatment factor indicated that agricultural terramicin presented the largest inhibition halo for the growth of all three bacterial strains, followed by Gentamicin and copper sulfate. On the contrary, Kasumin did not have an inhibitory effect on any of the three strains and its behavior was statistically similar to the control. On the second date, there were highly significant effects of strain, treatment, and their interaction; the separation of means for the strain factor showed that O157:H16 was the most strongly inhibited, followed by O157:H7. For the treatment factor, we found that all the bactericides had similar inhibitory effects in statistical terms, except for Kasumin. However, the strongest inhibitory effect was with the bactericide Gentamicin, followed by copper sulfate, results which contrast with the first evaluation date. On the contrary, the bactericide Kasumin did not have an inhibitory effect on any of the three strains, as occurred on the first date and with the control treatment. With respect to the strain factor, all the strains were statistically similar in their responses to the five bactericides, although the strain O157:H7 had more growth with the different bactericides