A research on the isolation and identification of Listeria spp. in ice-cream samples

Listeria species are able to grow up in cold enviroments and cause harm in ice cream production facilities, they are also difficult to isolate due to their existance in small amounts in the enviroment.16 samples from 4 patisseries, 40 samples from 10 small plants and 4 packs ice-creams from 1 firm in Adana provience; in total of 30 plain and 30 flavor ice-cream samples have been used in this research. FDA and ISO 11290-1 classical isolation methods and selective enrichment plates Oxford Agar and PALCAM Agar have been used in isolation process.Listeria spp. suspected colonies have been found in 8 plates (3 for plain ice cream samples, 5 for fruit ice cream samples) and 13 plates (7 for plain ice cream samples, 6 for fruit ice cream samples) by FDA and ISO isolation methods, respectively.The suspected colonies in 7 of 13 plates which were found with ISO 11290-1 method were isolated with PALCAM Agar and 6 of them were isolated with Oxford Agar. The suspected colonies in 5 of 8 plates which were found with FDA method were isolated with PALCAM Agar and 3 of them were isolated with Oxford Agar. Totally, we came across with 12 plates on which that have suspected Listeria spp. colonies were found with PALCAM Agar and 9 plates on which that have suspected Listeria spp. colonies were found with Oxford Agar.It was found that none of samples were Listeria spp. after biochemical identifications and Microbact 12L identification test.False positive results occur frequently in Listeria spp. isolation and so does in this research. Despite the selective ingridents that had been used for inhibition, grown of competitive flora which look like typical Listeria spp. image in PALCAM Agar and Oxford Agar couldn?t be inhibited. Moreover the possibility of obtaining fake positive results because of grown of large colonies in some plates has also been considered.