ABSTRACT
L. monocytogenes is a Gram-positive, facultatively anaerobic, non-spore-forming, short, rodshaped bacterium. It is one of six species in the genus Listeria, and the vast majority of human illnesses are caused by the species L. monocytogenes. Relatively few cases of human listeriosis are caused by L. ivanovii, and at least one case attributed to L. seeligeri has been described.4 Most strains of L. monocytogenes, L. ivanovii, and L. seeligeri demonstrate β-hemolysis-the lysing of blood cells visualized by a zone of clearing on blood agar. Hemolysis is not seen, except in rare cases, in the nonpathogenic L. innocua, L. welshimeri, and L. grayi. All of the Listeria spp. are easy to grow in the laboratory on any common complex medium. Sugars are the primary carbon sources; however, one can use amino acids and peptides as carbon sources as well. Generation times for
5.1 Introduction ......................................................................................................................... 139 5.2 Detection Targets ................................................................................................................ 142
5.2.1 Shared Targets.......................................................................................................... 142 5.2.1.1 Inhibitor Resistance .................................................................................... 142 5.2.1.2 Esculinase ................................................................................................... 144 5.2.1.3 Temperature Tolerance ............................................................................... 145 5.2.1.4 Somatic and Flagellar Antigens ................................................................. 145
5.2.2 Specic Targets ........................................................................................................ 146 5.2.2.1 Phosphatidylinositol Phospholipase C ........................................................ 147 5.2.2.2 Listeriolysin O ............................................................................................ 149
5.3 Identication Methods ........................................................................................................ 150 5.3.1 Nonspecic Methods ............................................................................................... 150 5.3.2 Biochemical Methods .............................................................................................. 150
5.3.2.1 Hemolysis ................................................................................................... 151 5.3.2.2 Carbohydrate Utilization ............................................................................ 152
5.3.3 Immunological Methods .......................................................................................... 153 5.4 Practical Considerations ...................................................................................................... 156
5.4.1 Selecting an Enrichment and Isolation Protocol ...................................................... 156 5.4.2 Selecting an Identication Protocol ......................................................................... 161 5.4.3 Enumeration ............................................................................................................. 162
5.5 Conclusions and Perspectives ............................................................................................. 163 References ...................................................................................................................................... 163
L. monocytogenes in rich media at generally used growth temperatures (22-37°C) are in the range of 1-2 h. It is a very hardy bacterium with a growth temperature range reported from –0.4 to 50°C. The pH range for growth is from pH 4.3 to pH 9; however, that range is dependent on the growth temperature.5 The organism can also withstand high osmotic pressure and has been demonstrated to grow at almost 2 M NaCl. For these reasons, it is adapted to grow well in foods, particularly processed foods, which might use salt, pH, and/or temperature to control the growth of spoilage bacteria. Pasteurization is effective in killing the organism, but foods can become contaminated after pasteurization.
