[Listeria--a challenge for diagnosis]

The isolation of Listeria spp. from polymicrobially contaminated materials from the environment of human beings is rather difficult. Suppression of the concomitant bacterial flora requires selective growth conditions. Different recipes have been reported to achieve this goal, whereby various antibiotics, among which cephalosporins and nalidixic acid, and dyes, such as acriflavin, have been proposed as supplements to suppress gram-positive as well as gram-negative bacteria. Cycloheximide may help to stop the growth of molds. The diversity of recommendations indicates, however, that an optimal selective medium has not yet been found. A possibility for selective enrichment is the cultivation in the cold, i.e. at +4 degrees C. In the future, genetic as well as immunologic methods will ameliorate the detection of Listeria spp. in environmental specimen. Actually, the offered test assays are still too insensitive so that they need a preceding enrichment process. The isolation of Listeria spp. from specimen of patients when they are present in pure culture is rather easy since these bacteria are not fastidious, so that they grow on the commonly used culture media. Sometimes it is difficult to differentiate Listeria spp. from other gram-positive rods. For the sake of the evaluation of a cultural result it is advisable to characterize further the isolate by simple biochemical tests, since the seven known Listeria spp. among the genus Listeria differ quite definitely from each other in respect to their pathogenic potency. Only bacteria belonging to Listeria monocytogenes represent a potential risk for human health.