Characterization and Source Tracking of E. coli in Bulk Milk

Introduction: Pathogenic strains of E. coli are among the common causes of foodborne disease outbreak in many parts of the world.  Understanding the characteristics of E. coli isolates and its pathogens in food is important for risk assessment and is also a critical step for diagnostic and epidemiological purposes. 

Purpose: The purpose of this study was to characterize E. coli in bulk milk for safety and microbial source tracking. 

Methods: A total of 258 bulk milk samples were collected from purchase points in 8 different geographical regions around South Africa. 3M E. coli petrifilm plates were used for detection, isolation and enumeration of E. coli. Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was used for rapid identification of the E. coli isolates. Virulence factors determination, antimicrobial agent susceptibility tests, detection of ESBL producing capacity and serotyping of the E. coli isolates was performed. 

Results: There was a significant difference (P < 0.05) in E. coli counts among the regions. E. coli (121) isolates were detected and isolated from 36.4% of the milk samples. More than 15% of the E. coli isolates were positive for virulence genes stx1/stx2 and 25.6% (n = 121) gave positive results for the O157:H7 serotype. Antimicrobial resistant E. coli isolates were detected in 70% of the milk samples, of which 36.8% of stx1/stx2 positive E. coli showed multi-drug resistance. Presence of ESBL capacity was observed in 20.6% of the isolates. Cluster analysis using GTG₅ finger printing revealed clustering of the isolates irrespective of origin. 

Significance: The information from the current research can be useful in modelling the risk from pathogenic E. coli in bulk milk sold in the informal market in South Africa.