Survival potential of food pathogens in river water after UV-C irradiation treatment

Thesis (MScFoodSc)--Stellenbosch University, 2024.

ENGLISH ABSTRACT: To investigate Ultraviolet (UV) irradiation as an effective disinfection method for irrigation water irrespective of microbiological recovery, several studies were conducted. These studies monitored the recovery of bacteria following various UV doses, two UV sources and diverse water matrixes. From these studies it was found that river water in the Western Cape used for irrigation have higher Escherichia coli (E. coli) counts than recommended in the South African irrigation water guidelines. This could lead to fresh produce contamination if not addressed at farm level before use. Although UV has been identified as a potential disinfection method to be applied to irrigation water before use, its constant efficacy is still being questioned in practice. The aim of this study was to investigate the survival potential of food pathogens in river water with varying physico-chemical characteristics after UV-C irradiation treatment to improve the safety of fresh produce. The first research chapter (chapter 3), concentrated on optimising the recovery experimental design and determining the influence of water matrix on the survival of L. monocytogenes, Salmonella spp. and E. coli strains. This was done by measuring the low-pressure UV dose response and recovery under light and dark conditions by way of colony count comparisons. Findings indicate that the physico-chemical characteristics (Ultraviolet transmission percentage (UVT%), total suspended solids (TSS), total dissolved solids (TDS) and turbidity) that influence light penetration negatively, influence UV disinfection efficacy and recovery potential of bacteria negatively. S. enterica indicated a higher recovery potential than E. coli and therefore E. coli should not be used as the only indicator of UV disinfection efficacy. Applying a higher UV dose (60 mJ.cmˉ²) did however prove the ability to overcome the challenges posed by bacterium types with higher UV resistance and changing physico-chemical characteristics of the water matrix to an extent when utilising a low-pressure UV collimated beam device. The second research chapter (chapter 4) employed a deoxyribonucleic acid (DNA) dye molecular investigation to distinguish whether the increase in colony counts ensuing the recovery period was due to active cells multiplying, viable but non-culturable (VBNC) cells resuscitating and reproducing, or DNA damage being repaired and consequent cell replication taking place. DNA recovery could not be confirmed via Hoechst dye staining and fluorescence intensity measurements by means of spectrofluorometry. The colony counts observed for these same bacterial strains succeeding the recovery period, could possibly be assigned to VBNC cells that resuscitate once placed in the ideal growth environment as is the case with plate count methods. The third research chapter (chapter 5) focussed on practically implementing UV irradiation on larger volumes (1 000L) of river water used for irrigation purposes by treatment in a medium-pressure UV disinfection pilot plant. The results pointed toward medium pressure UV being more effective at reducing bacteria in raw river water, but allows more recovery to take place, than low pressure UV. By increasing the UV dose administered by the medium pressure UV source, the recovery of bacteria was decreased substantially. Surviving bacteria were identified to be opportunistic pathogens which could still pose a health risk. Overall, it was recognized from this study that a higher UV dose (60 mJ.cmˉ²) can overcome the challenges posed by low water quality. Salmonella spp. indicated a higher potential for UV resistance and opportunistic pathogens found in bacterial communities are more likely to survive UV-C treatment. Medium pressure UV decreased the bacterial populations significantly and therefore could still be utilised as part of a hurdle strategy to improve the safety of irrigation water from contaminated river sources.

AFRIKAANSE OPSOMMING: Ten einde UV-bestraling as 'n effektiewe ontsmettingsmetode vir besproeiingswater te ondersoek, ongeag mikrobiologiese herstel, is verskeie studies uitgevoer. Hierdie studies het die herstel van bakterieë gemonitor na verskeie UV-doserings, twee UV-bronne en diverse watermatrikse. Uit hierdie studies is gevind dat rivierwater in die Wes-Kaap wat vir besproeiing gebruik word, hoër Escherichia coli (E. coli) tellings het as wat in die Suid-Afrikaanse besproeiingswaterriglyne aanbeveel word. Dit kan lei tot besmetting van vars produkte indien dit nie voor gebruik op die plaas aangespreek word nie. Alhoewel UV geïdentifiseer is as 'n potensiële ontsmettingsmetode om voor gebruik op besproeiingswater toegepas te word, word die konstante doeltreffendheid daarvan steeds in die praktyk bevraagteken. Die doel van hierdie studie was om die oorlewingspotensiaal van voedselpatogene in rivierwater met wisselende fisies-chemiese eienskappe na UV- C bestralingsbehandeling te ondersoek om die veiligheid van vars produkte te verbeter. Die eerste navorsingshoofstuk (hoofstuk 3), het gekonsentreer op die optimalisering van die herstel eksperimentele ontwerp en die bepaling van die invloed van watermatriks op die oorlewing van Listeria monocytogenes, Salmonella spp. en E. coli-spesies deur die lae-druk UV dosis reaksie en herstel onder lig en donker toestande te meet by wyse van kolonietelling vergelykings. Dit het bevind dat die fisies-chemiese eienskappe (UVT%, TSS, TDS en troebelheid) wat ligpenetrasie negatief beïnvloed, UV- ontsmettingseffektiwiteit en herstelpotensiaal van bakterieë negatief beïnvloed. Salmonella enterica het die hoogste herstelpotensiaal aangedui en is nie akkuraat voorgestel deur die E. coli resultate nie. Die toediening van 'n hoër UV-dosis het egter die vermoë bewys om die uitdaging van bakterieëtipe en watermatriks tot 'n mate te oorkom wanneer 'n laedruk UV-gekollimeerde straaltoestel gebruik word. Die tweede navorsingshoofstuk het 'n molekulêre ondersoek gebruik om te onderskei of die toename in kolonietellings wat die herstelperiode gevolg het, te wyte was aan aktiewe selle wat vermenigvuldig het, VBNC-selle wat resussiteer en reproduseer, of DNA-skade wat herstel is en gevolglike selverplisering wat plaasvind. DNS-herwinning kon nie deur middel van Hoechst-kleurstofkleuring en fluoressensie- intensiteitmetings deur middel van spektrofluorometrie bevestig word nie. Die kolonietellings wat waargeneem is vir dieselfde bakteriese stamme wat die herstelperiode opvolg, kan moontlik toegeken word aan VBNC-selle wat resussiteer sodra dit in die ideale groeiomgewing geplaas is, soos die geval is met plaattellingmetodes. Die derde navorsingshoofstuk het gefokus op die praktiese implementering van UV-bestraling op groter volumes (1 000L) rivierwater wat vir besproeiingsdoeleindes gebruik word deur behandeling in 'n mediumdruk UV-ontsmettingsproefaanleg. Die resultate het daarop gewys dat mediumdruk UV meer effektief is om bakterieë in rou rivierwater te verminder, maar laat meer herstel plaasvind as laedruk UV. Deur die UV-dosis toegedien deur die mediumdruk UV-bron te verhoog, is die herstel van bakterieë aansienlik verminder. Oorlewende bakterieë is geïdentifiseer as opportunistiese patogene wat steeds 'n gesondheidsrisiko kan inhou. Oor die algemeen is uit hierdie studie erken dat 'n hoër UV-dosis (60 mJ.cmˉ²) die uitdagings wat deur lae waterkwaliteit gestel word, kan oorkom. Salmonella spp. het 'n hoër potensiaal vir UV-weerstand aangedui en opportunistiese patogene wat in bakteriese gemeenskappe gevind word, is meer geneig om UV- C-behandeling te oorleef. Mediumdruk UV het die vermoë om die bakteriese populasies aansienlik te verminder en kan dus steeds gebruik word as deel van 'n hekkiestrategie om die veiligheid van besproeiingswater uit besmette rivierbronne te verbeter.

Masters