OCCURRENCE OF LISTERIA-MONOCYTOGENES IN SEWAGE-SLUDGE

The search for salmonella is the only bacteriological index included i n Italian guidelines (in accordance with EU regulations) for the use o f sewage sludge in agriculture. As a result, information regarding the presence of Listeria monocytogenes is rather limited. We therefore de cided to carry out an investigation of Listeria in the sludge produced by the Bologna (Italy) treatment plant during the various phases of t reatment. Five different types of sludge were analysed (primary raw, a ctivated, thickened, digested and dewatered) in a total of 66 samples. The highest frequency and concentrations of Listeria species (100 % a nd 2,743 MPN/g dry matter) and the lowest (63 % and 6 MPN/g dry matter ) were found in the activated and digested sludge respectively. These bacteria were mostly present in spring and autumn and positively corre lated only with fecal streptococci. Four species were isolated: Lister ia monocytogenes, Listeria innocua, Listeria welshimeri and Listeria g rayi. Listeria monocytogenes (prevalent serotype 4b) was seen to be re sistent to the biological oxidation but sensitive to anaerobic conditi ons during thickening and digestion. The dewatering process led to an increase in contamination. Since the sludge is used to fertilize land destined for vegetable farming our results show that it may represent a potential health risk.