Abstract

There is increasing evidence that urban grown vegetables in developing countries can be contaminated with pathogenic microorganisms. This is particularly true when wastewater is used in irrigation. The microbiological quality of wastewater grown lettuce on farms, markets and at street food vendor sites were evaluated for thermotolerant coliforms, enterococci and Salmonella using standard methods. Farm irrigation water and market refreshing water (water used in keeping the lettuce fresh) samples were also analysed. Thermotolerant coliforms on lettuce varied from 2.3 × 10³ to 9.3 × 10⁸ on farm, 6.0 × 10¹ to 2.3 × 10⁸ on market and 2.3 x 10⁶ to 2.4 x 10⁹ at street food vendor sites. Indicator bacterial numbers on farm lettuce were higher compared to the irrigation water (1.5 × 10³ to 4.3 × 10⁶) used on the farms. Thermotolerant coliform numbers in market refreshing water(9.0 × 10³ to 4.3 × 10¹⁰) were higher compared to that on the market lettuce.Enterococci numbers on lettuce were lower and ranged from 3.9 × 10¹ to 1.0 × 10⁶ on farm, 6.0 × 10¹ to 9.0 × 10⁴ on market and 5.1 x 10³ to 2.5 x 10⁶ at street food vendor sites. Salmonella numbers recorded at food vendor sites ranged from 1.5 × 10¹ to 9.3 × 10². In general, thermotolerant coliforms numbers increased by 18% while enterococci numbers reduced by 64% from the farms to the street foods. Bacterial counts on farm lettuce and irrigation water, market lettuce and refreshing water and street foods all exceeded the recommended World Health Organization (WHO) and International Commission on Microbiological Specifications for Food (ICMSF) standards of 10³. Wastewater use on farms and refreshing water in markets could be the main contributors to lettuce contamination and that education on use of effective de-contamination or washing methods before eating will contribute to reducing the risk associated with the consumption of such contaminated foods.

Key words: Salmonella, irrigation water, thermotolerant coliforms, enterococci, lettuce.