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Listeria is a genus of small, motile gram-positive bacilli which infect many types of animals and contaminates numerous foods – including vegetables, chicken, milk and beef. The most important species of the genus, Listeria monocytogenes, is an important cause of foodborne illness which disproportionately affects pregnant women and their newborns.
Due to its high mortality rate, ongoing efforts are needed to further reduce the incidence of listeriosis. Proper storage and handling of refrigerated ready-to-eat foods can help reduce the risk of listeriosis; if the disease develops, assessing its severity is essential before antimicrobial therapy is instituted.
Febrile gastroenteritis due to Listeria monocytogenes is self-limited, with a typical duration of less than 2 days; hence in most instances antimicrobial treatment is not warranted. Nevertheless, antibiotics should be introduced as early as possible in the cases of bacteremia as disease can be severe with high mortality rates.
An antibiotic of choice should be able to enter the host cells, bind to the penicillin-binding protein 3 (PBP3) of Listeria and remain active in the hostile intracellular environment. Listeria strains are generally susceptible to the clinically important antibiotics, albeit usually resistant to cephalosporins.
Based on in vitro susceptibility testing, animal studies and case reports, ampicillin and penicillin (often in combination with gentamicin) represent first-line therapy of invasive listeriosis, especially in patients older than 50. In immunocompromised individuals the addition of rifampicin helps to eradicate the remaining bacteria and cure infection.
The adequate dose is also important. Knowing that Listeria monocytogenes has a high affinity for the central nervous system, the dosages of drugs intended for patients with listerial meningitis should be used for all individuals with listeriosis – even in the absence of proven neuroinvasive disease.
For patients intolerant or allergic to penicillins, trimethoprim-sulfamethoxazole is considered to be the best alternative single agent. Certain animal studies and in vitro data suggest a potential role for the newer fluoroquinolones, but clinical data are lacking.
Clinically significant antimicrobial resistance isolates of Listeria monocytogenes has not been encountered, but vigilance is warranted. Several research groups have identified putative genetic loci that may regulate tolerance to bacteriocins, ampicillin and penicillin, and transfer of resistance from enterococci has been documented.
Most effective strategies to control Listeria in high-risk foods include good manufacturing practices, time and temperature controls throughout the entire distribution and storage period, as well as adequate post packaging treatments to minimize environmental Listeria monocytogenes contamination and to destroy this microorganism on products.
High-risk individuals (i.e. pregnant women, the elderly and immunocompromised individuals) should be given thorough instructions on healthy eating, including specific information on high-risk foods that they should avoid, and strategies to reduce their risk (such as thorough cooking, avoidance of cross-contamination, and short-term refrigerated storage of cooked foods).
Science-based education and risk communication strategies aimed at vulnerable populations and focused on high-risk foods should be delivered through health care providers or other trustworthy sources of information. If pregnant women are unfamiliar of possible consequences of contracting Listeria, the mother and child are in danger of health complications.