The influence of the microbial quality of wastewater, lettuce cultivars and enumeration technique when estimating the microbial contamination of wastewater- irrigated lettuce P. Makkaew, M. Miller, N. J. Cromar and H. J. Fallowfield ABSTRACT This study investigated the volume of wastewater retained on the surface of three different varieties of lettuce, Iceberg, Cos, and Oak leaf, following submersion in wastewater of different microbial qualities (10, 10 2 , 10 3 , and 10 4 E. coli MPN/100 mL) as a surrogate method for estimation of contamination of spray-irrigated lettuce. Uniquely, Escherichia coli was enumerated, after submersion, on both the outer and inner leaves and in a composite sample of lettuce. E. coli were enumerated using two techniques. Firstly, from samples of leaves – the direct method. Secondly, using an indirect method, where the E. coli concentrations were estimated from the volume of wastewater retained by the lettuce and the E. coli concentration of the wastewater. The results showed that different varieties of lettuce retained significantly different volumes of wastewater ( p < 0.01). No statistical differences ( p > 0.01) were detected between E. coli counts obtained from different parts of lettuce, nor between the direct and indirect enumeration methods. Statistically significant linear relationships were derived relating the E. coli concentration of the wastewater in which the lettuces were submerged to the subsequent E. coli count on each variety the lettuce. P. Makkaew (corresponding author) N. J. Cromar H. J. Fallowfield Health and Environment Group, School of the Environment, Flinders University, GPO Box 2100, Adelaide 5001, South Australia, Australia E-mail: patrick.makkaew@flinders.edu.au M. Miller Department of Health and Ageing, OzFoodNet, Communicable Disease Control Branch, P.O. Box 6, Rundle Mall, Adelaide 5000, South Australia, Australia Key words | health risk, leaf morphology, lettuce, microbial risk, wastewater irrigation INTRODUCTION Worldwide, wastewater irrigation of crops is being applied increasingly in agriculture due to pressures associated with population growth and the shortage of freshwater resources. One of the major public health concerns from this practice is the human health risk from exposure to pathogens associ- ated with the consumption of wastewater-irrigated crops, particularly salad crops, which are generally consumed raw. The consumption of lettuce has been linked to several outbreaks of foodborne diseases (Brandl & Amundson ; Barker-Reid et al. ). In an effort to minimise the adverse health effects from human exposure to pathogens associated with wastewater reuse in agriculture, the World Health Organization (WHO) published the third edition of the guidelines for the safe use of wastewater, excreta and greywater in 2006 (WHO ). The guidelines offer mul- tiple approaches to risk management to meet the health- based target for the burden of waterborne disease, 10 6 disability adjusted life years, associated with working in wastewater-irrigated farms, or consuming wastewater- irrigated crops. In this guideline, a quantitative microbial risk assessment (QMRA) approach (Haas et al. ) was used to estimate the health risk from wastewater irrigation. In brief, QMRA translates the exposure of consumers to pathogens under a specific set of conditions (exposure scen- arios) to the probabilities of infection by applying four steps, namely: hazard identification, dose-response assessment, exposure assessment and risk characterisation. Although QMRA could be an effective tool for health risk estimation, 228 © IWA Publishing 2017 Journal of Water and Health | 15.2 | 2017 doi: 10.2166/wh.2016.145 Downloaded from https://iwaponline.com/jwh/article-pdf/15/2/228/393655/jwh0150228.pdf by guest on 18 June 2020