Flow cytometric microbial monitoring for safeguarding drinking water quality

Jorien Favere
Tot op de dag van vandaag blijft het een wereldwijde uitdaging om de microbiële drinkwaterkwaliteit te waarborgen. De huidige monitoringstechnieken schieten tekort op vlak van accuraatheid en/of snelheid. Flow cytometrie heeft deze beperkingen niet en is daarom een veelbelovende monitoringstechniek. In deze studie werd de waarde van flow cytometrie als microbiële monitoringstechniek in drinkwater geëvalueerd.

Bacteriën in drinkwater: nieuwe techniek onthult hun geheimen

Veilig en zuiver drinkwater… een evidentie?

Drinkwater? Het lijkt voor u vanzelfsprekend, het water dat dagelijks in gulle hoeveelheden uit uw kraan stroomt. Het is dan ook een basisrecht om toegang te hebben tot veilig en zuiver drinkwater. Echter, tot op de dag van vandaag blijft het een wereldwijde uitdaging om de drinkwaterkwaliteit te waarborgen. Dit geldt voor zowel ontwikkelingslanden als ontwikkelde landen, denk bijvoorbeeld aan de droogte van deze zomer die zelfs in Vlaanderen voor grote onrust zorgde. In de toekomst zullen de toenemende verstedelijking, de bevolkingsgroei en de opwarming van de aarde grote uitdagingen met zich meebrengen. Drinkwater is essentieel, maar hoe kunnen we de kwaliteit van het water van nu en van de toekomst blijven garanderen?  

Bacteriën zijn de boosdoeners

Om te kunnen anticiperen op drinkwaterproblematieken, moeten we eerst de mogelijke oorzaken bestuderen. Met andere woorden, wat kan ons drinkwater onveilig maken? Drinkwater bevat weinig voedingsstoffen, maar desalniettemin kunnen er veel micro-organismen in overleven en zelfs groeien, al is die microbiële gemeenschap voornamelijk onschadelijk. Het grootste gezondheidsrisico is de besmetting van het drinkwater met ziekteverwekkende micro-organismen. Uitbraken van door drinkwater overgedragen ziektes zijn ook in Vlaanderen niet zeldzaam. Het is daarom van essentieel belang om de drinkwaterkwaliteit, en in het bijzonder de microbiële kwaliteit, op te volgen om uitbraken van ziektes te voorkomen.

Meten is weten

In de voorbije tweehonderd jaar werden verschillende technieken ontwikkeld om de microbiële drinkwaterkwaliteit op te volgen. Plaattellingen, een van de oudste methoden, wordt vandaag nog steeds als de gouden standaard beschouwd. Hierbij worden de bacteriën in petrischaaltjes opgegroeid, en na één à twee dagen worden de gevormde kolonies geteld. Er zijn echter twee cruciale beperkingen; ten eerste is er dus een lange wachttijd tussen de staalname en het resultaat. Ten tweede is er sprake van de “Great Plate Count Anomaly”, die zegt dat slechts minder dan 1% van de aanwezige bacteriën in het drinkwater ook effectief in de petrischaaltjes kunnen groeien. Er is dus nood aan een accurate en snelle techniek voor routinematige opvolging van de microbiële drinkwaterkwaliteit.

Flow cytometrie, een techniek overgewaaid uit de medische sector, heeft deze beperkingen niet en is daarom een veelbelovende techniek voor de monitoring van de microbiële drinkwaterkwaliteit. Flow cytometrie is een optische techniek die de interactie van partikels, in dit geval bacteriële cellen, met een lichtbundel meet. Zo krijgt men een beeld van het aantal cellen en hun uiterlijke kenmerken. In combinatie met geavanceerde data-analyse kan flow cytometrie gebruikt worden om een vingerafdruk van de microbiële gemeenschap te maken. Deze vingerafdruk is uniek voor een microbiële gemeenschap, net zoals een menselijke vingerafdruk uniek is. Flow cytometrie heeft dus veel potentieel, maar wat is nu de waarde van deze techniek voor microbiële monitoring van drinkwater?

Netwerken, netwerken, netwerken

Drinkwater wordt na de productie (meestal) gedesinfecteerd om de microbiële groei in het distributienetwerk te beperken. Deze netwerken bestaan uit leidingen, kranen, bekkens en watertorens. Afhankelijk van de opbouw van het drinkwaternetwerk, de watersamenstelling en de desinfectiemethode zal er zich een microbiële gemeenschap in het netwerk ontwikkelen. Die microbiële gemeenschap kan zelfs doorheen één netwerk sterk verschillen.

Om een beeld te krijgen van de dynamica van de microbiële gemeenschap in een drinkwaternetwerk werden metingen uitgevoerd in Washington DC (VS). Washington DC is een miljoenenstad, en wordt bijgevolg gekenmerkt door een uitgebreid drinkwaternetwerk. Dat brengt dus ook mogelijke obstakels met zich mee: lauw water dat lang stilstaat in de uithoeken van het netwerk vormt immers de ideale omgeving voor bacteriële groei. Om dit te vermijden worden de brandkranen van het drinkwaternetwerk, waar de brandweer haar slangen op aansluit, regelmatig gespoeld. Dergelijke operationele beslissingen worden momenteel voornamelijk op basis van de concentratie desinfectans (chloor) genomen. Concreet wil dit zeggen dat men een brandkraan zal spoelen wanneer een lage chloorconcentratie wordt gemeten, totdat de chloorconcentratie terug aanvaardbaar is. Zo wordt de waterkwaliteit op peil gehouden, en wordt verondersteld dat ook de microbiële kwaliteit terug hersteld is.

Om het gedrag van de microbiële gemeenschap in het drinkwaternetwerk op te volgen, werd gedurende drie maanden de drinkwaterkwaliteit tijdens het spoelen in enkele probleemzones opgevolgd, gecombineerd met flow cytometrische metingen. De vingerafdrukken van de microbiële gemeenschappen toonden aan dat de chloorconcentratie niet altijd een goede indicator is voor de microbiële drinkwaterkwaliteit. Met andere woorden, veranderingen in de microbiële gemeenschap kunnen tijdens het spoelen aangeven dat het water “ververst” is, iets wat niet altijd gereflecteerd werd in de chloorconcentratie. Beslissingen over het al dan niet spoelen van brandkranen zouden dus niet steeds hoofdzakelijk op basis van de chloorconcentratie moeten genomen worden.

Ook konden veranderingen in de waterkwaliteit aan veranderingen in de microbiële gemeenschap worden gelinkt: de microbiële gemeenschappen van het drinkwater veranderden met de tijd (seizoenaal) of waren afhankelijk van de spoelduur. Dit betekent dat men met behulp van “baseline” metingen gedurende een jaar, toekomstige afwijkende flow cytometrische metingen kan gebruiken als indicatie van mogelijke microbiële risico’s of uitbraken.

Ook in watertorens staat het drinkwater bij operationele problemen vaak lang stil, en kan er dus overmatige bacteriële groei plaatsvinden. Daarom werd flow cytometrie, in combinatie met een in-line staalnamerobot, ingezet als monitoringssysteem in watertorens van FARYS (Gent) en Pidpa (Antwerpen). Ook hier vertoonden de flow cytometrische vingerafdrukken een voldoende gevoeligheid om kleine operationele veranderingen, zoals aanpassingen van inkomende wateren of verandering in de grootte van het inkomend debiet, in de microbiële gemeenschap te detecteren.   

Flow cytometrie, de toekomst?

Zowel vanuit academisch als industrieel perspectief biedt flow cytometrie veel mogelijkheden. Niet alleen werd er veel additionele informatie over de microbiële gemeenschappen in drinkwaterdistributienetwerken verkregen, ook zouden deze gebruikt kunnen worden als basis voor operationele beslissingen. Door het slim inzetten van deze techniek, kan op lange termijn enorm veel tijd, geld en water bespaard worden, én de publieke gezondheid beschermd worden. Flow cytometrie lijkt dus de techniek van de toekomst voor de opvolging van de microbiële drinkwaterkwaliteit.

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Universiteit of Hogeschool
Master in de bio-ingenieurswetenschappen: chemie en bioprocestechnologie
Publicatiejaar
2018
Promotor(en)
Prof. Nico Boon, Prof. Bart De Gusseme
Kernwoorden
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