Efficiency of four secondary enrichment protocols in differentiation and isolation of Listeria spp. and Listeria monocytogenes from smoked fish processing chains.
Four secondary enrichment protocols (conventional methods: UVM II, Fraser 24 h and Fraser 48 h: Impedimetric method: Listeria electrical detection medium) were studied for their ability to isolate Listeria spp. and Listeria monocytogenes from fish and environmental samples collected along the processing chain of cold-smoked fish. From all methods, Listeria spp. and L. monocytogenes were respectively present in 56 and 34 of 315 samples analysed. Fraser broth incubated for 48 h gave the fewest false negative Listeria spp. results [4/56; (7.1%)], but concurrently only 15/34 (44.1%) samples were correctly identified as containing L. monocytogenes, Listeria electrical detection (LED) medium detected only 36/56 (64.3%) Listeria spp. positive samples. Despite this lower isolation rate, LED identified 20/34 (58.8%) L. monocytogenes positive samples correctly and gave fewer false positive results. The overall conclusion was that more than one isolation method is needed to accurately estimate L. monocytogenes contamination rates.
Duarte
G
G
Escola Superior de Biotecnologia, Universidade Católica Portuguesa, Porto, Portugal. gabi@morango.esb.ucp.pt
Vaz-Velho
M
M
Capell
C
C
Gibbs
P
P
D000818:Animals; D004553:Electric Conductivity; D005188:False Negative Reactions; D005189:False Positive Reactions; D005396:Fish Products / Q000382:microbiology*; D005412:Flatfishes; D005516:Food Microbiology; D005525:Food-Processing Industry / Q000592:standards; D005517:Foodborne Diseases / Q000517:prevention & control*; D008087:Listeria / Q000302:isolation & purification*; D008089:Listeria monocytogenes / Q000145:classification / Q000254:growth & development / Q000302:isolation & purification*; D008088:Listeriosis / Q000517:prevention & control*; D012474:Salmon; D014413:Tuna