Determining the Potential Link between Irrigation Water Quality and the Microbiological Quality of Onions by Phenotypic and Genotypic Characterization of Escherichia coli Isolates

Abstract

The potential transfer of human pathogenic bacteria present in irrigation water onto fresh produce was investigated, because surface water sources used for irrigation purposes in South Africa have increasingly been reported to be contaminated with enteric bacterial pathogens. A microbiological analysis was performed of a selected river in Limpopo Province, South Africa, that is often contaminated with raw sewage from municipal sewage works and overhead irrigated onions produced on a commercial farm. Counts of Escherichia coli, coliforms, aerobic bacteria, fungi, and yeasts and the prevalence of E. coli O157:H7, Salmonella, and Listeria monocytogenes were determined. Identities of bacterial isolates from irrigation water and onions were confirmed using matrix-assisted laser desorption ionization-time of flight mass spectrometry, PCR, and biochemical tests. To establish a potential link between the microbiological quality of the irrigation source and the onions, the E. coli isolates from both were subjected to antibiotic resistance, virulence gene, and enterobacterial repetitive intergenic consensus PCR analyses. River water E. coli counts exceeded South African Department of Water Affairs and World Health Organization irrigation water guidelines. Counts of aerobic bacteria, coliforms, fungi, and yeasts of onions from the market were acceptable according to Department of Health Directorate, Food Control, South Africa, microbiological guidelines for ready-to-eat fresh fruits and vegetables. E. coli O157:H7, Salmonella, and L. monocytogenes were not detected in onions, whereas only Salmonella was detected in 22% of water samples. Matrix-assisted laser desorption ionization-time of flight mass spectrometry and PCR identification of E. coli isolates from water and onions correlated. Of the 45 E. coli isolates from water and onions, 42.2% were resistant to multiple antibiotics. Virulence genes eae, stx1, and stx2 were detected in 2.2, 6.6, and 2.2% of the E. coli isolates, respectively. Phenotypic (antimicrobial) and genotypic (virulence gene prevalence, DNA fingerprinting) analyses showed a link between river, dam, irrigation pivot point, and onion E. coli isolates.

Occurrence of enteric viruses in reclaimed and surface irrigation water: relationship with microbiological and physicochemical indicators

“…Irrigation water is one of the vehicles identified for the contamination of produce during primary production with pathogenic micro‐organisms including viruses (Du Plessis et al . ). Enteric viruses can get on edible parts of the plants not only by direct contact with irrigation water but also by internalizing through the roots (DiCaprio et al .…”

Bacterial biomes and potential human pathogens in irrigation water and leafy greens from different production systems described using pyrosequencing

“…The link between microbial quality of irrigation water and safety of leafy green vegetables has been established in recent studies (Du Plessis et al . ).…”

“…; Du Plessis et al . ), while little attention has been paid to small‐scale farming systems or homestead gardens (Speelman et al . ; Erickson et al .…”

“…Water sources become contaminated when exposed to improperly treated municipal sewerage discharged into the main water ways or, untreated animal or human faeces being washed into rivers during rain events. The link between microbial quality of irrigation water and safety of leafy green vegetables has been established in recent studies (Du Plessis et al 2015).…”

“…Leafy green vegetables are often referred to as lettuce, spinach and cabbages commonly grown in commercial and small-scale farming systems or in homestead gardens. However, studies on the microbial quality of irrigation water and fresh produce have mostly been focused on commercial production systems (Ceuppens et al 2014;Du Plessis et al 2015), while little attention has been paid to small-scale farming systems or homestead gardens (Speelman et al 2008;Erickson et al 2013). A previous study by Jongman and Korsten (2016a) explored the microbial quality of leafy greens from different production systems using culture based techniques.…”