Listeria monocytogenes

Question 1

Diseases

Answer 1

• meningitis & sepsis in
  newborns, pregnant women, & immunosuppressed
  adults.
• outbreaks of febrile gastroenteritis-> concern for food industry.

Question 2

Important Properties

Answer 2

• small gram-positive rod
  arranged in V- or L-shaped formations.
• tumbling movement.
• Colonies on blood agar plate produce narrow zone of β-hemolysis resembles
  hemolysis of streptococci.
• cold enhancement (grows well at cold temperatures) -> storage of
  contaminated food in fridge increases risk
  of gastroenteritis.

Question 3

Pathogenesis: 2 clinical settings disease occurs

Answer 3

(1) fetus or newborn-> transmission across placenta or during delivery

(2) pregnant women (reduced cell mediated immunity during 3rd trimester) & immunosuppressed adults (renal transplant
patients).

Question 4

Pathogenesis

Answer 4

• in animals, plants, & soil-> transmitted to humans by ingestion of unpasteurized milk products, undercooked meat, & raw vegetables.
• Contact with farm animals & feces
• Following ingestion, bacteria appear in colon & colonize female genital
  tract-> infect fetus if
  membranes rupture or neonate during passage
  through birth canal.
• ability to invade & survive within cells.
• Invasion of cells mediated by internalin made by Listeria & E-cadherin
  on cell surface
• pass placenta & enter meninges-> invade gastrointestinal tract depending on interaction of internalin & E-cadherin on tissues.
• Upon entering cell, listeriolysin produced-> escape from phagosome into cytoplasm escaping destruction in phagosome.
• grows intracellularly,
  cell-mediated immunity-> host defense.
• Suppression of cell-mediated immunity predisposes to infections.
• move from cell to cell by
  actin rockets—filaments of actin polymerize &
  propel bacteria through membrane of one human
  cell into another.

Question 5

Clinical Findings

Answer 5

• During pregnancy-> abortion, premature
  delivery, or sepsis during peripartum period.
• Newborns-> acute meningitis 1 to 4 weeks later.
• bacteria reach meninges via bloodstream (bacteremia).
• infected mother
  asymptomatic or influenzalike illness.
• immunocompromised adults-> sepsis or meningitis.
• Gastroenteritis-> watery diarrhea, fever, headache, myalgias, & abdominal cramps but little vomiting.
• contaminated dairy products, undercooked chicken & hot dogs & ready-to-eat
  foods (coleslaw).

Question 6

Laboratory Diagnosis

Answer 6

• Gram stain &
  culture.
• gram-positive rods resembling diphtheroids & small, gray colonies
  with narrow zone of β-hemolysis on a blood agar plate
• isolation confirmed-> motile organisms

Identification-> sugar fermentation tests.

Question 7

Treatment

Answer 7

• trimethoprim-sulfamethoxazole.
• Combinations,
  ampicillin & gentamicin or ampicillin & trimethoprim-sulfamethoxazole
• Resistant strains are rare.
• gastroenteritis not require treatment.

Question 8

Prevention

Answer 8

• difficult; no vaccine.
• Limiting exposure of pregnant women & immunosuppressed
  patients to sources (farm animals, unpasteurized milk products, & raw vegetables.
• Trimethoprim-sulfamethoxazole given to
  immunocompromised patients to prevent Pneumocystis pneumonia also prevent listeriosis.