[Isolation and germ count of Listeria monocytogenes in raw and biologically treated waste water]

The concentration of Listeria monocytogenes was determined in 66 samples of waste water and of the effluent after the biological step in the waste water pretreatment plant of Braunschweig. 697 strains of Listeria were isolated, 84 per cent of them were L. monocytogenes. The concentration of Listeria varied between 10(3)-2.4 X 10(5) cells/l. Neglecting some exceptions, there were about ten times more Listeria in the sludge than in the clearly filtered waste water. In general, Listeria seem to be not reduced by the biological oxidation during the waste water treatment. On the contrary, we observed a multiplication in 45 percent of all cases. The propagation of Listeria within amoebae or other protozoa is discussed. Finally, the hygienic role of sludge is discussed because Listeria are concentrated in it and they are able to survive many months up to some years.