Discipline: quality; Key words: producer-distributor milk, Enterobacteriaceae, E. coli, shigatoxin, antibiotic resistance

Members of the Enterobacteriaceae family have been associated with food safety and spoilage problems. Enterobacteriaceae can enter an inadequately cleaned dairy chain and cause enzymatic breakdown of proteins or lipids, instigating spoilage which contributes to substantial economic losses and wastage. Some Enterobacteriaceae have also emerged as potential opportunistic pathogens due to acquisition of virulence and antibiotic resistance and some, including E. coli (STEC), Salmonella spp., and Yersinia enterocolitica, have been detected frequently in milk borne disease outbreaks. The occurrence of such milk-borne disease outbreaks has been mostly associated with consumption of raw milk at farms or milk which is sold directly from producer-distributor (PD), or from farm to consumer in bulk tanks and less so from pasteurised milk to the commercial market. As we experience a growing number of PDs selling milk directly from producers to consumers and this milk is mostly consumed by poor and vulnerable members of the population, it is important to establish the risk of contamination. At the same time it is also advisable to monitor pasteurized milk sold in the commercial market.  

Therefore, the study by the authors cited below was undertaken to characterize Escherichia coli and other Enterobacteriaceae in raw and pasteurised producer-distributor bulk milk (PDBM).  A total of 258 samples were collected from purchase points in eight of the nine provinces. The samples were tested for antibiotic residues, phosphatase, total aerobic bacteria, coliforms and E. coli counts. Isolates were identified by MALDI-TOF-MS and E. coli isolates were tested for virulence factors, antimicrobial resistance, serotypes and presumptive E. coli O157:H7 isolates. 

Antibiotic residues and alkaline phosphatase were detected in 2 % and 21 % of the samples respectively. A total of 729 isolates belonging to 21 genera and 59 species were identified. E. coli, Enterobacter cloacae, Klebsiella oxytoca and Raoultella ornithinolytica were the most abundant species. SpoilageEnterobacteriaceae species exceeded 50% of the total isolates. E. coli was detected and isolated from 36 % of the milk samples. Thirty one E. coli isolates harboured virulence genes and 38% were presumptive O157:H7. The prevalence of samples with presumptive shigatoxin producing E. coli (STEC) was 10 %. Antimicrobial resistant E. coli isolates were detected in 70 % of the milk samples with 36 % showing multi-drug resistance. 

In conclusion, the study showed poor microbiological quality characterized by the occurrence of high microbial counts which may reduce shelf life and can be a reservoir of pathogenic bacteria that potentially pose public health risks in PDBM. A diversity of Enterobacteriaceae species belonging mostly to spoilage microbiota was detected in the PDBM. Considerable genetic diversity was found among the E. coli, S. liquefaciens and K.  oxytoca strains. Multidrug resistant pathogenic E. coli was detected in both raw and pasteurised retail PDBM, although much lower in pasteurised milk. This is cause for concern. These findings call for further assessment of the potential risk posed to public health through quantitative microbial risk assessment as a prerequisite to formulation of situation specific mitigation strategies.  


V. Ntuli, P.M.K Njage & E.M. Buys, 2016. Characterisation of E. coli and other Enterobacteriaceae in producer-distributor bulk milk. Final Report to Milk SA of Research Project.