2. Describe the offending organism of this case, its morphologic, cultural characteristics  and their normal habitat.

Listeria monocytogenes is a zoonoses. Carried by some animals.

In many foods (cabbage, milk products, soft cheese, pasteurized milk, sliced dale meats and cheese). Mexican style soft cheese has been implicated in a large out break of listeriosis.

Can  grow at refrigerator temperatures. 

• Gram positive rod
• Beta hemolytic
• Catalase positive
• Motile at 25 degree centigrade (tumbling motility/umbrella motility)
• Non-spore forming rod

3. What kinds of hosts are susceptible?

• Newborns
• Elderly 
• Immunocompromised.
• Malignancy

4. How do these organisms gain access, invade and spread in humans?

Exposure to contaminated food.

Transplacental infection (mother to infant)

• Invasion of bacteria into intestinal crypt cells, macrophages, M cells mediated by internalin protein.
• Bacteria enter cell in a vacuole and escape by producing listerolyisn, a cytolysin similar to streptolysin O
• Listeriolysin mediates escape from the membrane bound vacuole into the cytosol. Hemolysins hydrolyze host cell membranes. 
• Listeria is able to manipulate host cell action (actin) to propel it into pseudopods that extend to adjacent host cells. Thus, listeria can spread from cell to cell with minimal contact with the host immune system. 

5. What defenses humans have against these organisms?

• Requires cellular immunity with activated macrophages to resolve infection with Listeria
  • Macrophages ingest and kill these intracellular pathogens.

Suppression of cell mediated immunity due to cyclosporine, azathioprine and prednisone. Azathioprine, cyclosporine and glucocorticoids suppress the ability of macrophages to ingest and kill intracellular pathogens in this patient.

6. How do these organisms able to overcome human defenses?

Listeria is able to manipulate host cell action (actin) to propel it into pseudopods that extend to adjacent host cells. Thus, listeria can spread from cell to cell with minimal contact with the host immune system. 

7. What is the end result of this battle between organisms and humans?

Causes bacteremia and meningitis.

8. How do you diagnose this infection?

• Culture: Isolate typical colonies from blood, CSF, wounds or stool cultures
• CSF findings in this patient: The mononuclear pleocytosis, elevated protein and low sugar are consistent with meningitis due to Listeria. 

Other useful evaluations

• The CT suggests an encephalitis which Listeria has a propensity for causing in an immunosuppressed patient.

9. What will be your therapeutic strategy?

• Ampicillin or penicillin are ideal. 
  • Short courses may be associated with relapse hence at least 4 weeks of treatment is recommended. 
• The ability of Trimethoprim/Sulfamethoxazole to penetrate intra cellularly may offer an advantage in the treatment of Listeriosis.

10. How can you prevent it from spreading to others? Prevent its occurrence?

Only by not being exposed to contaminated food?

11. What are other clinical infections with these organisms

• Neonatal infections
• Puerperal infections
• Granulomatosis infantiseptica