Characterising the microbial profiles of various river sources and investigating the efficacy of UV radiation to reduce microbial loads for improved crop safety

Thesis (MScFoodSc)--Stellenbosch University, 2021.

ENGLISH ABSTRACT: The rivers used for the irrigation of fresh produce in the Western Cape have been under frequent investigation in recent years. Results have frequently shown that in rivers used for irrigation, the faecal coliform concentrations (Escherichia coli) frequently exceed the guideline limit of 1 000 colony forming units per 100 mL. These findings present a health risk for consumers of fresh produce. Ultraviolet (UV) radiation treatment has proven to offer some advantages for water disinfection over conventional treatment methods such as filtration and chemical treatments. However, this is not yet a common practice in South Africa. Knowledge gaps exist with regard to the efficacy of UV radiation on environmental strains of pathogenic microorganisms such as Salmonella species and Listeria monocytogenes. The aim of this study was to investigate the effect of low-pressure (LP) UV radiation on water obtained from various river water sources, in order to disinfect water used for irrigation purposes to ultimately reduce the risk of causing foodborne disease outbreaks from the consumption of contaminated fresh produce. Four rivers in the Western Cape were sampled five times each between the wet winter and dry summer seasons, to establish the microbial and physico-chemical profiles of the rivers. These results were compared to the guideline limits. The samples were exposed to three doses (20, 40 and 60 mJ.cm-2) of LP UV radiation at laboratory-scale. It was established that LP UV radiation was effective at reducing the microbial loads to non-detectable levels. Pathogenic microorganisms were successfully inactivated after a dose of 20 mJ.cm-2. Heterotrophic Plate Count colony numbers were lowered more steadily, and therefore, showed greater resistance to treatment. Thirteen strains were isolated and stored for future experiments. It was suggested that a pre-treatment step be implemented to improve the physical quality of the river water prior to treatment. Isolated strains of E. coli (n = 3), Salmonella species (n = 2) and L. monocytogenes (n = 8) were stored for further testing. The L. monocytogenes isolates (n = 8) were subjected to lineage typing experiments, where it was established that all isolates were lineage I. This lineage is most frequently associated with listeriosis. Extended-spectrum beta-lactamase (ESBL) testing indicated that none of the Enterobacteriaceae isolates (n=5) were ESBL-producers. All Enterobacteriaceae isolates showed resistance to tetracycline, ampicillin and trimethoprim-sulfamethoxazole. Resistance of L. monocytogenes isolates (n=5) was observed against trimethoprim-sulfamethoxazole, while four L. monocytogenes isolates showed resistance to ampicillin, penicillin and erythromycin. Multidrug resistance was reported for 90% of river water isolates (n=9). Four different bag filter pore sizes (5, 20, 50 and 100 μm) were investigated to determine the most effective pre-treatment step to improve the UV transmission (UVT %) of the water. This experiment was performed on the ‘worst case scenario’ river, the Mosselbank River. Improvements in the total suspended solids, chemical oxygen demand and turbidity were reported, however, the extremely high total dissolved solids content (728.67 mg.L-1) prevented a larger improvement in the UVT %. It was established that the 5 μm bag filter was the most effective pore size. In the current study, LP UV radiation was successfully able to produce water of an acceptable standard for the irrigation of fresh produce. The physical quality of the water did not prevent a successful disinfection, but rather increased the exposure time required to deliver a specific dose and therefore, decreased efficiency. It was established that LP UV radiation is able to reduce pathogenic microorganisms to non-detectable levels. This method of disinfection, therefore, shows promise for full-scale application of irrigation water treatment.

AFRIKAANSE OPSOMMING: Die riviere wat vir die besproeiing van vars produkte in die Wes-Kaap gebruik word, is die afgelope paar jaar gereeld ondersoek. Resultate het getoon dat in sommige riviere die fekale koliforme konsentrasies (Escherichia coli) gereeld die riglynlimiet van 1 000 kolonievormende eenhede per 100 ml oorskry. Hierdie bevindings dui op 'n gesondheidsrisiko vir die verbruikers van vars produkte. Ultraviolet (UV) bestralingsbehandeling het bewys dat dit voordele bied vir die ontsmetting van water bo konvensionele behandelingsmetodes soos filtrasie en chemiese behandelings. Dit is egter nog nie 'n algemene gebruik in Suid-Afrika nie. Daar is kennisgapings met betrekking tot die doeltreffendheid van UV-bestraling op patogene mikroörganismes soos Salmonella-spesies en Listeria monocytogenes. Die doel van hierdie studie was om die effek van lae druk (LP) UV-bestraling op water, verkry uit verskillende rivierwaterbronne, te ondersoek, met die doel om water wat vir besproeiingsdoeleindes gebruik word, te ontsmet om uiteindelik die risiko van kontaminasie vir die verbruikers van vars produkte te verminder. Monsters is vyf keer tussen die nat winter- en droë somerseisoene uit elkeen van vier riviere in die Wes-Kaap geneem om die mikrobiese en fisies-chemiese profiele van die riviere vas te stel. Hierdie resultate is vergelyk met die riglyn-limiete. Die monsters is op laboratoriumskaal aan drie dosisse (20, 40 en 60 mJ.cm-2) LP UV-bestraling blootgestel. Daar is vasgestel dat LP UV-bestraling effektief was om die mikrobiese lading tot nie-waarneembare vlakke te verminder. Patogene mikroörganismes is suksesvol geïnaktiveer na 'n dosis van 20 mJ.cm-2. Die heterotrofiese kolonie-getalle het meer geleidelik verlaag en het dus groter weerstand teen behandeling getoon. Dertien isolate is verkry en geberg vir toekomstige eksperimente. Daar is voorgestel dat 'n voorafbehandelingsstap geïmplementeer word om die fisiese kwaliteit van die rivierwater te verbeter voor UV behandeling. Isolate van E.c oli (n = 3), Salmonella species (n = 2) en L. monocytogenes (n = 8) is gestoor vir latere toestsing. Die gebergde L. monocytogenes-isolate is onderwerp aan stam tiperingseksperimente, waar vasgestel is dat al die isolate aan stam I behoort. Hierdie stam word meestal met listeriose geassosieer. Uitgebreide-spektrum beta-laktamase (ESBL) toetse het aangedui dat geen van die Enterobacteriaceae-isolate ESBL-produseerders was nie. Alle Enterobacteriaceae-isolate (n = 5) het weerstand teen tetrasiklien, ampisillien en trimetopriem-sulfametoksasool getoon. Weerstand van L. monocytogenes isolate (n = 5) is waargeneem teen trimetopriem-sulfametoksasool, terwyl vier L. monocytogenes isolate weerstand getoon het teen ampisillien, penisillien en eritromisien. Veelvuldige middelweerstand is gerapporteer vir 90% van die rivierwater-isolate (n = 9). Vier verskillende sakfilter poriegroottes (5, 20, 50 en 100 μm) is ondersoek om die mees effektiewe voorbehandelingstap te bepaal om die UV-transmissie (UVT %) van die water te verbeter. Hierdie eksperiment is uitgevoer op die ‘slegste geval scenario'-rivier, die Mosselbankrivier. Verbeterings in die totale gesuspendeerde vastestofinhoud, chemiese suurstofbehoefte en troebelheid is gerapporteer, maar die uiters hoë totale opgeloste vastestofinhoud (728.67 mg L-1) het 'n groter verbetering in die UVT % verhoed. Daar is vasgestel dat die 5 μm sakfilter die doeltreffendste poriegrootte was. In die huidige studie kon LP UV-bestraling suksesvol aangewend word om water van 'n aanvaarbare standaard vir die besproeiing van vars produkte te produseer. Die fisiese kwaliteit van die water het nie 'n suksesvolle ontsmetting verhoed nie, maar het die blootstellingstyd wat nodig was om 'n spesifieke dosis te lewer, verhoog en dus die doeltreffendheid verminder. Daar is vasgestel dat LP UV-bestraling patogene mikroörganismes kan verminder tot nie-waarneembare vlakke. Hierdie ontsmettingsmetode toon dus belofte vir volskaalse toepassing op besproeiingswater.

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