Bacteria of the genus Listeria are widely distributed in the environment; they are isolated from soil and water ecosystems, food products, environmental objects, and circulate in vivo. L.monocytogenes are pathogenic for animals and humans. The ecological plasticity, stress resistance and tolerance of Listeria determines their ability to switch from a saprophytic to a parasitic life cycle and survive under various environmental conditions. After thawing and subsequent cultivation of Listeria on fresh nutrient medium, a pronounced populational heteromorphism is noted: formation of protoplast-type cells, L-forms and convoluted revertant cells, which requires the use PCR and ELISA for bacteria detection.
It is known that non-pathogenic Listeria, as well as pathogenic microorganisms forming a biocenosis with L.monocytogenes, can serve as a reservoir of pathogenicity and resistance determinants and be transferred to pathogenic Listeria by horizontal transfer, which leads to the emergence of new, more virulent and antibiotic-resistant strains. In addition, the most important adaptation mechanism of L.monocytogenes to adverse environmental factors is their ability to form biofilms markedly enhancing survival and disinfectant resistance. The relatively high genomic similarity between L.monocytogenes and some non-pathogenic Listeria species and often their coexistence in similar ecological niches, may provide an opportunity for the transfer of resistance or virulence genes. At the same time, the transmission of pathogenicity genes from L.monocytogenes to L.innocua is also possible, which predetermines the appearance of atypical hemolytic pathogenic strains, which, given the greater environmental prevalence of the latter can pose a great danger to humans and animals.
The increasing role of Listeria in the pattern of human and animal infectious pathologies, the variability of their morphological, cultivable and biochemical properties, as well as the constant modification of the surface Listeria antigens underlies a need to improve listeriosis diagnostics and requires creation of new immunobiological preparations and modern regimens for isolation and identification of various Listeria types.
This review discusses current views on Listeria spp. prevalence and biological qualities, virulence and pathogenicity factors of L.monocytogenes, as well as methods for identifying different Listeria species.