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Web Review of Todar's Online Textbook of Bacteriology. "The Good, the Bad, and the Deadly".

Tag words: Listeria, Listeria monocytogenes, listeriosis, food poisoning.

Listeria monocytogenes

Kingdom: Bacteria
Phylum: Firmicutes
Class: Bacilli
Order: Bacillales
Family: Listeriaceae
Genus: Listeria
Species: L. monocytogenes








Kenneth Todar currently teaches Microbiology 100 at the University of Wisconsin-Madison.  His main teaching interests include general microbiology, bacterial diversity, microbial ecology and pathogenic bacteriology.

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Listeria monocytogenes (page 3)

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Host Defenses

Because L. monocytogenes multiplies intracellularly, it is largely protected against circulating immune factors (AMI) such as antibodies and complement-mediated lysis. The effective host response is cell-mediated immunity (CMI), involving both lymphokines (especially interferon) produced by CD4+ (TH1) cells and direct lysis of infected cells by CD8+ (Tc) cells. Both of these defense mechanisms are expressed in the microenvironment of the infected foci, which are organized as granulomas, characterized by a central accumulation of macrophages with irregularly shaped nuclei, and by peripheral lymphocytes recognizable by rounded nuclei and a narrow border of intensely staining cytoplasm.

Treatment and Prevention

If diagnosed early enough, antibiotic treatment of pregnant women or immunocompromised individuals can prevent serious consequences of the disease. Antibiotics effective against Listeria species include ampicillin, vancomycin, ciprofloxacin, linezolid and azithromycin. However, early diagnosis is the exception rather than the rule, since the first signs of a case or an outbreak are reports of stillbirth or serious infections resembling listeriosis. By then, any cohorts who have become infected from eating the same food are likely recovered from an inapparent or flu-type infection, or they themselves may have developed serious disease. However, processed foods known to be the source of Listeria that may still be in the market place, restaurant or home should obviously not be used, and recalls should be imperative. It must also be constantly recognized that L. monocytogenes is able to grow at low temperatures.


Listeria monocytogenes Scanning EM

Summary

About 2500 cases of listeriosis occur each year in the United States. The initial symptoms are often fever, muscle aches, and sometimes gastrointestinal symptoms such as nausea or diarrhea. The illness may be mild and ill persons sometimes describe their illness as flu-like. If infection spreads to the nervous system, symptoms such as headache, stiff neck, confusion, loss of balance, or convulsions can occur. Most cases of listeriosis and most deaths occur in adults with weakened immune systems, the elderly, pregnant women, and newborns. However, infections can occur occasionally in otherwise healthy persons. Infections during pregnancy can lead to miscarriages, stillbirths, and infection of newborn infants. Outbreaks of listeriosis have been linked to a variety of foods especially processed meats (such as hot dogs, deli meats, and paté) and dairy products made from unpasteurized milk.

Because pregnant women, older adults, and people with weakened immune systems are at higher risk for listeriosis, CDC recommends the following measures for these persons.
  • Do not eat hot dogs and luncheon meats unless they are reheated until steaming hot.
  • Avoid cross-contaminating other foods, utensils, and food preparation surfaces with fluid from hot dog packages, and wash hands after handling hot dogs.
  • Do not eat soft cheeses such as feta, brie and camembert cheeses, blue-veined cheeses, and Mexican-style cheeses such as "queso blanco fresco." Cheeses that may be eaten include hard cheeses; semi-soft cheeses such as mozzarella; pasteurized processed cheeses such as slices and spreads; cream cheese; and cottage cheese.
  • Do not eat refrigerated pâtés or meat spreads. Canned or shelf-stable pâtés and meat spreads may be eaten.
  • Do not eat refrigerated smoked seafood, unless it is contained in a cooked dish, such as a casserole. Canned or shelf-stable smoked seafood may be eaten.
  • Do not drink raw (unpasteurized) milk or eat foods that contain unpasteurized milk.



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Kenneth Todar is an emeritus lecturer at University of Wisconsin-Madison. He has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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