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Unformatted text preview: 1. What is the causative agent, how does it enter the body and how does it spread a) within the body and b) from person to person? Causative agent Listeria monocytogenes is the cause of human disease. It is a gram-positive nonsporing motile bacillus (Figure 2). There are six species of Listeria : L. monocytogenes , L. ivanovii , L . welshimeri , L. innocua , L . seeligeri , and L . grayi . The most common cause of human disease is L. monocytogenes , although L. ivanovii can rarely cause disease. Listeria species grow on blood agar (see Figure 1) and L. monocytogenes produces a narrow zone of b-hemolysis often only seen beneath the colonies. The organism grows at 37 C but can also grow slowly at 4 C and this can be used as an enrich- ment technique when examining foodstuff. The organism shows a charac- teristic tumbling (end over end) movement at 25 C, which is diagnostic. L. monocytogenes has several serotypes based on cell wall (O) and flagellar (H) antigens. The majority of disease is caused by serotypes 1/2a, 1/2b, and 4b. Several molecular subtyping techniques ( multilocus enzyme electrophoresis , pulse field gel electrophoresis , and ribotyping ) have been found useful in epidemiological investigations. Source of infection L. monocytogenes has been isolated from a variety of natural sources includ- ing soil, water, animals, and vegetables and is widely distributed in the animal kingdom, with over 40 species of wild and food-source animals including birds, crustaceans, and fish being colonized. The organism can Case 21 Listeria monocytogenes A young woman was admitted to hospital with fever, headache, myalgia , and joint pains of 48 hours duration. The previous day she had attended a lunch where she had eaten ham and a soft cheese. She was 26 weeks pregnant. The admitting physician thought the patient may have listeriosis and a blood culture was taken in addition to a full blood count. Gram-positive cocci were reported on the Gram stain of the blood culture and the patient was started empirically on teicoplanin. The following day small hemolytic colonies were present on the blood agar (Figure 1) and a Gram stain revealed gram-positive rods. Further testing showed the organism was motile with tumbling motility and it was biochemically identified as Listeria monocytogenes . A diagnosis of listeriosis was made and the patients treatment was changed to ampicillin and gentamicin. Figure 1. Colonies of Listeria monocytogenes on blood agar. Figure 2. Scanning electron microscopic (EM) image of Listeria monocytogenes showing flagella. Lydyard, Peter; Cole, Michael; Holton, John; Irving, Will; Porakishvili, Nino; Venkatesan, Pradhib; Ward, Kate, Jan 01, 201 Garland Science, Hoboken, ISBN: 9780203856871 also be carried in the intestine of about 5% of the human population with- out any symptoms of disease. Infection is acquired by consumption of con- taminated food (Figure 3) such as fish, salad, pate, soft cheeses, salami, ham, and coleslaw, where contamination rates as high as 70% may occur.ham, and coleslaw, where contamination rates as high as 70% may occur....
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This note was uploaded on 09/14/2011 for the course PHARM mb taught by Professor Staff during the Spring '11 term at UCSD.
- Spring '11