Studies regarding the incidence of bacteria from Listeria genus in fish meat

Abstract

Listeria is a bacterial genus whose species are Gram positive bacilli and it contains seven representatives. Joseph Lister was an English surgeon who was one of the first in the field of sterile surgery and the name Listeria is given in his honor. Sources of Listeria are soil, contaminated water, animals and vegetables. Because bacteria from Listeria monocytogenes specie are very dangerous for human health, with a mortality rate of 20% for infected patients, a serious attention has to be accorded in consumption of those foods able to transmit microorganisms. This article presents a study about the incidence of Listeria in two species of fish which were tested to prove presence or absence of Listeria monocytogenes, because fish meat is one of the foods susceptible to transmit bacteria and to induce grave intoxication. From fresh water fish Carp was selected and as a representative of ocean fish, Mackerel respectively. Different methods for detection were described and discussed. Those methods are legally accepted in European and Romanian normative for food consumption security as Food Microbiology and Specific Rules for Microbiological Analyzes. OXFORD, PALCAM and CAMP are test used for detection and confirmation of Listeria species of bacteria. Also API-Listeria tests were used for confirmation the presence or absence of different Listeria species in tested samples. Agar with sheep blood medium was inseminated with samples of fish meat because this is the proper medium for bacterial colonies development. The reason is the fact that bacteria colonies formed dark colored rings rounded by a black haloes, because in their metabolic process bacteria produce hydrolysis of aesculin (a glycoside contained in agar medium). The results of analyzes for those two species of fish meat do not confirm the presence of Listeria monocytogenes, but other species of Listeria (Listeria welshimeri and Listeria inocua), fortunately not pathogenic, were present.