Listeria monocytogenes: Morphology and Identification, Culture and Growth Characteristics, Antigenic Classification, Pathogenesis and Immunity, Clinical Findings, Treatment.

Question 1

Morphology and identification

Answer 1

1. Listeria monocytogenes is a small, gram-positive, non-spore-forming rod that is motile with tumbling motility at 22°C but not at 37°C.
2. It is catalase positive, oxidase negative, and shows weak beta-hemolysis on blood agar. 3. Under the microscope, it may appear singly, in short chains, or in palisades.
3. It can survive and multiply at refrigeration temperatures (psychrophilic), which makes contaminated refrigerated foods a common source.

Question 2

Culture and growth characteristics

Answer 2

1. L. monocytogenes grows well on blood agar, showing weak beta-hemolysis similar to Streptococcus agalactiae.
2. Cold enrichment (refrigerated culture over several days) enhances isolation.
3. It is facultatively anaerobic and grows over a wide temperature range (1°C to 45°C), including at 4°C.
4. It shows tumbling motility in liquid media at room temperature and umbrella-shaped growth in semisolid agar.
5. CAMP test is positive when Listeria is streaked perpendicular to Staphylococcus aureus.

Question 3

Antigenic classification

Answer 3

1. L. monocytogenes has somatic (O) and flagellar (H) antigens.
2. Thirteen serotypes are identified based on combinations of these antigens.
3. Most human infections are caused by serotypes 1/2a, 1/2b, and 4b.
4. These serotypes are associated with foodborne outbreaks and invasive disease.

Question 4

Pathogenesis and immunity

Answer 4

1. Listeria is a facultative intracellular pathogen that invades epithelial cells, M cells, and macrophages.
2. Internalin helps the bacterium adhere and enter host cells.
3. Once inside, listeriolysin O (a pore-forming toxin) allows escape from the phagosome into the cytoplasm.
4. ActA protein mediates actin polymerization, propelling the bacterium within and between cells (actin rockets).
5. Cell-to-cell spread helps evade humoral immunity.
6. Immunity is primarily cell-mediated; thus, immunocompromised individuals (e.g., neonates, elderly, pregnant women, transplant patients) are highly susceptible.

Question 5

Clinical findings

Answer 5

1. Listeriosis can cause: 1) Gastroenteritis: self-limited febrile diarrhea from contaminated food.
   2) Invasive disease: in immunocompromised, elderly, and neonates—sepsis and meningoencephalitis.
   3) Neonatal listeriosis: early-onset (in utero) causes granulomatosis infantiseptica (disseminated abscesses and high mortality); late-onset (from birth canal) presents as meningitis.
   4) Pregnancy: causes flu-like illness in mother but can lead to fetal death, miscarriage, or neonatal sepsis. Listeria crosses the placenta and the blood-brain barrier.

Question 6

Treatment

Answer 6

1. First-line treatment is ampicillin, often combined with gentamicin for synergistic effect in severe cases (e.g., meningitis, sepsis).
2. Trimethoprim-sulfamethoxazole is an alternative for penicillin-allergic patients.
3. Cephalosporins are ineffective.
4. Early diagnosis and treatment are crucial in high-risk patients.