Listeriosis laboratory tests
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: João André Alves Silva, M.D. [2]
Overview
For symptomatic patients, diagnosis is confirmed only after isolation of Listeria monocytogenes from a normally sterile site, such as blood, spinal fluid (in the setting of nervous system involvement), or amniotic fluid/placenta (in the setting of pregnancy). Stool samples are of limited use and are not recommended. Listeria monocytogenes can be isolated readily on routine media, but care must be taken to distinguish this organism from other Gram-positive rods, particularly diphtheroids. Selective enrichment media improve rates of isolation from contaminated specimens. The cultures take 1-2 days for growth. Importantly, a negative culture does not rule out infection in the presence of strong clinical suspicion. Serological tests are unreliable, and not recommended at the present time. There is no clinical value in performing laboratory testing on asymptomatic patients, even if higher risk.[1]
Laboratory Tests
Cell Culture
- Listeria grows on media such as Mueller-Hinton agar.[2]
- Identification is enhanced if the primary cultures are done on agar containing sheep blood because the characteristic small zone of hemolysis can be observed around, and under the colonies.
- Isolation can be enhanced if the tissue is kept at 4°C for some days before inoculation into bacteriologic media.
- The motility at room temperature and hemolysin production are primary findings that help differentiate listeria from coryneform bacteria.
Stool Cultures
Stool cultures are not indicated in systemic listeriosis patients because routine culture media for enteric pathogens are not appropriate for the growth of Listeria. However, in cases of outbreaks of listeriosis or individual patients with suspected listerial gastroenteritis, special selected media can be used. The special culture media may be suggested by a local microbiology laboratory, a state health department or the CDC.
Anton Test
- Anton test can be used in the identification of Listeria monocytogenes.
References
- ↑ "Listeria".
- ↑ Chapter 13. Non-Spore-Forming Gram-Positive Bacilli: Corynebacterium, Propionibacterium, Listeria, Erysipelothrix, Actinomycetes, & Related Pathogens ,Jawetz, Melnick, & Adelberg's Medical Microbiology, 24th Edition ,The McGraw-Hill Companies