Specific biological properties of Listeria innocua spp. isolated in Primorye Territory

Rationale: Most cases of listeriosis are caused by the pathogenic Listeria monocytogenes. Some cases of isolation of L.  innocua with pathogenicity factors from foods have been published, as well as on the cases of the disease in humans caused by this species. Aim: To assess biological properties including potential pathogenicity of L.  innocua, isolated from food and environmental objects. Materials and methods: We performed microbiological study of L. innocua cultures isolated from foods (n = 35) and environmental objects (n = 15) on the territory of Primorye Territory (Russian Federation), as well as assessment of their sensitivity to antibiotics. Results: The studied L.  innocua cultures showed stable phenotypic features of their biological properties, such as morphology, typical colony growth on the medium with characteristic odor of fermented milk, blue or blue-green luminescence induced by inclined light, presence of catalase activity and absence of the oxidase activity. Only 38 ± 6.9% of L.  innocua demonstrated movements at T 22 °С. L.  innocua cultures did not ferment mannitol (100% of cultures); they degraded ramnose to its acid without gas (70 ± 6.5%) and degraded xylose (42.8 ± 7%). Listeria isolated from vegetables and environmental objects could ferment ramnose (92.8 ± 7.2% of the studied cultures) and xylose (28.5 ± 12.5%) more frequently than L. innocua isolated from meat and fish foods. L.  innocua demonstrated variable biochemical activities towards mannose (92 ± 3.8%), saccharose (85.7 ± 7.8%) and melesitose (76.2 ± 9.5%). L. innocua cultures with hemolytic activity (34 ± 6.7%) (α or β  type) were isolated, more commonly from fish products. All Listeria irrespective of their isolation source showed lipase activity. L.  innocua cultures from foods and environmental objects were highly sensitive to antimicrobials from the following classes: penicillins (ampicillin, carbenicillin, combined amoxicillin and clavulanic acid), aminoglycosides (gentamycin, amikacin), carbapenems (meropenem), and fluoroquinolones (ofloxacin). Conclusion: Some biological properties of L. innocua were variable depending on the source of isolation. Isolation of Listeria with atypical properties in theterritory of Primorye Territory requires that these microorganisms should be studied in more detail.