Prevalence, characterisation and potential origin of Escherichia coli found in surface and ground waters utilized for irrigation of fresh produce

[摘要] ENGLISH ABSTRACT: Over the past two decades, there has been an increase in the use of water sources forirrigation, as well as an increase in Escherichia coli outbreaks linked to fresh produce. The fullextent and type of E. coli contamination present in natural water sources is unknown and thecontamination sources have also not been confirmed. The aim of this study was to enumerateand characterise E. coli from both irrigation water and potential contamination source sites.Total coliform and E. coli counts found in contamination source sites wereas high as log 7.114 and log 6.912 MPN.100 mL-1, respectively. Total coliform and E. coli countsfor irrigation sites were lower, with maximum counts of log 5.788 andlog 5.768 MPN.100 mL-1, respectively. It was found that more than one third (5/14 = 35.71%) ofthe irrigation sites had E. coli counts exceeding the guidelines (<1 000 counts.100 mL-1) for 'safe'irrigation water for fresh produce (<1 000 counts.100 mL-1) as set by the Department of WaterAffairs (DWA) and World Health Organisation (WHO), making the water unsuitable for theirrigation of fresh produce.Phylogenetic subgroups (A0, A1, B1, B22, B23, D1 and D2) and the MALDI Biotyper system(PCA dendrogram) were used to create a fingerprint of each E. coli isolated from the environment.These were then used to link E. coli strains from irrigation water to their most probablecontamination origin. Escherichia coli population structure was found in this study, to be bettersuited for linking E. coli strains from irrigation water to their most likely source, than just applyingthe phylogenetic grouping. The MALDI Biotyper data in combination with the phylogeneticsubgroup assignment was then used to group similar strains and link E. coli from irrigation waterto their contamination sources by comparing population structures. Strains isolated from surfaceand groundwater showed similar distribution patterns, but groundwater strains showed apopulation structure more indicative of porcine and bovine origin, while surface water showedpopulation characteristics which could not be used to make conclusive links between theirrigation water and suspected contamination sources.When investigating the population structures of individual sample sites, it was found thatphylogenetic subgroups A0, A1 and B1 frequently made up the bulk of the E. coli population. Itwas also found that linking individual irrigation sites to contamination sources was successful, asirrigation site Berg-2 was found to have a similar population structure to contamination sourcesite Plank-1 which represents human pollution from an informal settlement. This led to the conclusion that Berg-2 was being contaminated by human pollution, most probably from aninformal settlement. Upon further investigation it was found that Berg-2 is downstream of aninformal settlement, proving that E. coli population structure is a successful means of microbialsource tracking (MST).Virulence factors of the 153 E. coli isolated during the study were identified and thepotential risk associated with using the investigated irrigation water for irrigation of freshproduce, was determined. Two enteropathogenic E. coli (EPEC) strains were isolated from theirrigation water, one from the Plankenburg River water, and the other from a borehole in theDrakenstein area. The latter indicates that borehole water is not as safe as was once thought, andthat there are bacterial contaminants finding their way into groundwater. The occurrence of anEPEC strain in river water shows that neither ground nor surface water is guaranteed to be safe,and that treatment of water being used for the irrigation of fresh produce should beimplemented.

[发布机构] Stellenbosch University