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Of all foodborne pathogens, Listeria monocytogenes may be the most deadly. Its primary victims are the immunocompromised and the elderly, along with pregnant women and neonates. Listeriosis initially presents with flu-like (persistent fever) or gastrointestinal (nausea, vomiting, diarrhea) symptoms; subsequent manifestations may include septicemia, meningitis, encephalitis, pneumonia, and urogenital tract infection. Its virulence derives from its abilities to survive and grow within host phagocytes and aggressively spread through tissues by inducing polymerization of host actin.
Although non-spore forming, L. monocytogenes is a hardy organism that can tolerate freezing, drying, and heat and hence persist and grow in diverse environments. Soil and silage rather than fecal contamination are the most likely sources of food contamination. These foods typically include raw milk products, soft cheeses, ice cream, raw fruits and vegetables, and many types of raw or smoked meats and fish. Post-harvest contamination is aided by its ability to grow at low temperatures.
Although most cases of food-borne listeriosis are sporadic, there have been multiple outbreaks in recent years, including one in 2011 responsible for 147 infections and 33 deaths across 28 states that was traced to contaminated cantaloupe (www.cdc.gov/listeria/outbreaks).
Tracking down the source of food-borne listeriosis is a challenging task that begins with laboratory culture (ca. 48 h each for enrichment and plating) and species identification (to distinguish L. monocytogenes from its non-pathogenic sister species). This may be followed by conventional serotyping, although its limitations including low resolution and high cost have largely led to its replacement with molecular typing systems. Among the latter, PFGE (pulsed-field gel electrophoresis) is the traditional gold-standard, but it too has limitations that have encouraged multiple laboratories to develop alternative methods (for review, see 1). The most promising and practical of these is MLVA (multilocus variable number of tandem repeats analysis), as recently optimized for 9 loci with size analysis by capillary electrophoresis (2).
Research at MicrobiType in collaboration with USDA researchers (3) has revealed that sequence analysis of selected tandem repeat loci in L. monocytogenes DNA is considerably more informative than size analysis. Sequence analysis is also immune from the reproducibility, subjectivity, and standardization issues of size-based methods.
Two cost-effective, complementary, polymorphic locus sequence typing services are offered, LmMT1 and LmMT2, which target repeat-containing loci within previously validated MLVA systems (2,3). As illustrated in the LmMT1 & LmMT2 dendrograms, excellent strain resolution is obtained with these typing services, while two sets of epidemiologically related isolates reproducibly cluster (red and green).
Results are reported in dendrogram and sequence alignment formats showing the relatedness of your submitted L. monocytogenes isolates to each other, to previously submitted isolates from your lab, and to GenBank database strains.
(1) S. Jadhav et al., 2012, J. Microbiol. Methods 88:327
(2) X. Li et al., 2013, Diag. Microbiol. Infect. Dis. 75:203
(3) T. Edlind and Y. Liu, 2015, J. Microb. Biochem. Tech. 7: 351 (http://dx.doi.org/10.4172/1948-5948.1000238)
(MicrobiType services are for research/investigational use only.)