Serological identification, virulence factors, antibiogram and plasmid profile of Escherichia coli serotypes in raw milk and pasteurized milk products

Abstract

Milk is an excellent medium that supports microbial growth with resultant spoilage of the products. Thus, this study determined the antibiogram, presumptive location of antibiotic resistance markers (ARM), and plasmid DNA molecular weight of pathogenic Escherichia coli serotypes in raw milk (n = 76) and pasteurized milk products (n = 77). Quality of raw milk and pasteurized milk products was evaluated using Methylene Blue Reduction Test (MBRT). Occurrence of E. coli in the samples was determined by Cefixime tellurite-sorbitol MacConkey agar and latex agglutination kits. The virulence factors, antibiotic susceptibility, location of ARM, and plasmid DNA molecular weight of E. coli serotypes were assayed using bacteriological media, disc diffusion, acridine orange and agarose gel electrophoresis, respectively. Based on the MBRT, 2.6% samples were graded as excellent, 9.8% samples were of good quality, while 22.2% samples were of a very poor quality as decolourization of methylene blue occurred in ˂ 20 min. E. coli 0157 and E. coli 0145 had the highest and lowest percentage of occurrence, respectively; ≤ 29 E. coli serotypes were caseinase, amylase and lecithinase producers. E. coli serotypes were highly sensitive to Ciprofloxacin and Gentamycin with percentage sensitivity ranging from 74.6 % to 81.4 %; ≤ 50.0 % E. coli 0125 were sensitive to Ofloxacin; between 33.3 % and 50.0 % E. coli 0103 were resistant to Nalidixic Acid and Ofloxacin; ≥ 53.7 % E. coli serotypes were multi-drug resistant; 19.0 % had their entire ARM located on the plasmids, while 50.0 % had their ARM residing both on the plasmids and chromosomes with plasmids sizes ranging from 0.54 kb to 43.5 kb. The occurrences of pathogenic E. coli in the milk samples indicated the need to enlighten milkers on the hygienic practices to adopt during milking and as well showed that post-pasteurization contamination of milk products should be avoided.