[Isolation of Listeria from the water of the Po River]

Abstract

Bacteria belonging to the genus Listeria were isolated from samples of water of the river Po, using the following methods: enrichment: 1000 ml of water were filtered through 0.45 micron membrane filters, which were in turn immersed in a tube containing 50 ml of tryptose phosphate broth and subsequently incubated at 4 degrees C up to 60 days. isolation: 1 ml from the above enrichment was subcultured fortnightly in 9 ml of nutrient broth supplemented with potassium thiocyanate (3.75% w/v) and nalidixic acid (100 mg/l). After a 48 h incubation at 37 degrees C, a loopful of this culture, diluted 1:10 with saline, was streaked on to a tryptose agar plate, which was then incubated at 37 degrees C for 48 h. Colonies showing a blue-green iridescence when observed with oblique transillumination were submitted to the usual identification tests. Organisms which proved to belong to the genus Listeria were sent to the Centre of Bacterial Ecology of the Pasteur Institute (Paris) and, in some cases, to the Institute of Hygiene, University of Würzburg, GFR, for further characterization (species, phagovar, serovar). This technique allowed us to isolate Listeria strains from 11 out of 50 water samples (22%). The organisms were distributed among the following species: L. monocytogenes (1 strain), L. innocua (7 strains), L. seeligeri (2 strains), L. welshimeri (1 strain). In spite of the limited number of isolates, the great variety of species identified has to be pointed out. Furthermore, one strain belonged to L. welshimeri, a new species which is, to our knowledge, rarely found in Europe, while it seems more common in U.S.A.