Cape Town — Stellenbosch University scientists are investigating the levels of harmful bacteria on locally grown fruit and vegetables, hoping to establish a link between the bugs on the food and contaminated river water used for irrigating crops.

The research is important because it is likely to increase pressure on local municipalities to improve their water-treatment management. Fruit- and vegetable-growing businesses risk losing key customers if there are unacceptably high levels of dangerous bacteria on their crops, but had limited scope to clean up the water they obtained from local rivers since setting up their own water treatment plants would be impractical and unaffordable, said Jo Barnes, an epidemiologist from the university's community health department.

Most of the dangerous bugs came from improperly treated sewage discharged into rivers by municipalities, but a small amount came directly from communities living along river banks, she said.

"With our massively polluted rivers, all producers will have a hard time producing clean fruit and vegetables."

Barnes' research into river contamination in Western Cape has already shown that the water is laden with germs such as E coli, salmonella, and listeria, which cause potentially fatal diarrhoea and food poisoning. She also found high levels of adenovirus, which causes respiratory tract infections, and several strains of streptococcal bacteria that can damage the heart or reproductive system. All these bugs exact a greater toll among poor communities, where there is already a higher burden of diseases. Young children, pregnant women and the elderly or malnourished are extra vulnerable.

One of the rivers Barnes has analysed closely is the Plankenburg River, a tributary of the Eerste River, which farmers use to irrigate crops. She measured the levels of E coli to gauge the extent of pollution. She found the levels of E coli were thousands of times higher than SA's recommended safety levels of 2000 bacteria per 100ml of water, a threshold that is already twice that of the European Union.

In January 2006, for example, the E coli count was 9,2-million per 100ml, a slight drop on April 2005, when she recorded 10,8-million E coli per 100ml.

Bugs such as E coli accumulate on the surface of fruit and vegetables irrigated with contaminated water and can survive there for weeks after harvest, said Barnes. The concentration of bacteria rises with each successive watering.

She and her colleagues are now using sophisticated molecular analysis to assess whether the E coli found in the river water matches the bugs found on the vegetables irrigated from this source. They hope that by proving a direct link between what is found on the food and in the irrigation water, municipalities will be forced to clean up their act.

Since cooking destroys most bacteria and viruses, the researchers are focusing on food eaten raw or lightly cooked. Barnes said anxious consumers could reduce their risk of infection by rinsing fruit and vegetables in a bowl of water containing a bit of household bleach.