Listeria monocytogenes Case File

Listeria monocytogenes Case File

Eugene C.Toy, MD, Cynthia Debord, PHD, Audrey Wanger, PHD, Gilbert Castro, PHD, James D. Kettering, PHD, Donald Briscoe, MD

CASE 13

An 18-day-old female infant is brought to the pediatric emergency room by her panicked mother. The child has developed a fever and has been crying nonstop for the past 4 hours. She has fed only once today and vomited all of the ingested formula. The baby was born by vaginal delivery after an uncomplicated, full-term pregnancy to a healthy 22-year-old gravida1 para1 (one pregnancy, one delivery) woman. The mother has no history of any infectious diseases and tested negative for group B Streptococcus prior to delivery. The immediate postpartum course was routine. The baby had a routine check-up in the pediatrician’s office 3 days ago, and no problems were identified. On examination, the child has a temperature of 38.3°C (100.9°F), pulse of 140 beats per minute, and respiratory rate of 32 breaths per minute. She is not crying at the moment. She has poor muscle tone, will not regard your face or respond to loud stimuli. Her anterior fontanelle is bulging. Her mucous membranes are moist, and her skin is without rash. Her heart is tachycardic but regular, and her lungs are clear. Her white blood count is elevated, a urinalysis is normal, and a chest x-ray is clear. A Gram stain of her cerebrospinal fluid (CSF) from a lumbar puncture shows gram-positive coccobacilli.

◆ What organism is responsible for this infection?

◆ How does this organism avoid antibody-mediated host defenses?

ANSWERS TO CASE 13: Listeria monocytogenes

Summary: An 18-day-old infant presents with meningitis and Gram stain of the CSF reveals gram-positive coccobacilli.

◆ Organism responsible for this infection: Listeria monocytogenes

◆ Mechanism of avoidance of antibody-mediated defenses: Intracellular replication and spread from cell to cell by phagocytosis

CLINICAL CORRELATION

Listeria is found in the environment but is not normal fecal flora in humans. Infection with Listeria is more common in the summer months. Disease is often the result of ingestion of the organism from infected foods such as milk, soft cheese, undercooked meat, or unwashed vegetables. Listeria monocytogenes causes asymptomatic or mild gastrointestinal infections in persons with intact immune systems and severe disease, most commonly meningitis, in those with impaired cellular immunity, such as pregnant women, neonates, AIDS patients, and posttransplant patients on immunosuppressive medications. Two types of neonatal disease have been described. Early-onset disease,

which occurs with in the first 2 days of life, is the result of transplacental infection. Initial signs and symptoms include difficulty breathing and pneumonia. This infection is also called granulomatosis infantiseptica, because severe disease can be associated with a granulomatous rash with abscesses. Late-onset disease, which usually occurs 2–3 weeks after birth, is thought to result from exposure to Listeria either during or shortly after delivery. This infection most commonly presents as meningitis. Clinically these syndromes can be difficult to distinguish from that seen with group B streptococci.

APPROACH TO SUSPECTED Listeria monocytogenes

Objectives

1. Know the structure and physiology of L. monocytogenes.
2. Know the life cycle, virulence factors, and diseases associated with L. monocytogenes.

Definitions

Cold enrichment: Used to enhance growth of Listeria, particularly from food.

Granulomatosis infantiseptica: Severe form of Listeria infection of neonates in which granulomatous skin lesions are evident.

Anterior fontanelle: An opening of the neonatal skull between the sutures.

Gravida: Number of total pregnancies.

Para: Number of deliveries (usually after 20-week gestation); a pregnancy that ends prior to 20-week gestation is an abortus.

DISCUSSION

Characteristics of Listeria

Listeria monocytogenes is a small, facultative anaerobic, gram-positive bacillus. It may appear as coccobacilli in pairs or chains, so it can be mistaken for Streptococcus pneumoniae or Enterococcus on Gram stain. Listeria monocytogenes is an intracellular pathogen, which allows it to avoid antibodymediated defenses of the host. It initially enters host cells via the action of a protein, internalin, which induces phagocytosis of the bacteria. Listeria produces a toxin, which then lyses the phagosome, releasing the bacteria into the host cell’s cytoplasm. It replicates in the host cytoplasm and moves to the host membrane. By pushing against the membrane, a protrusion, known as a filopod, is produced, which can be phagocytized by adjacent cells. 

This cycle is then repeated in the new host cell, allowing Listeria to spread without being exposed to antibodies or other humoral immunity factors. For this reason, host cellular immunity factors protect against infection and those with impaired cellular immunity are vulnerable.

Diagnosis

Clinical diagnosis is difficult based on the nonspecific signs and symptoms. Clinically L. meningitis in neonates resembles group B streptococci, both are a significant cause of bacteria meningitis in that age group. Definitive diagnosis of Listeria is made by culture of the CSF and/or blood. Gram stain of the CSF would demonstrate small gram-positive bacilli, appearing similar to corynebacteria or S. pneumoniae.

Listeria will grow on routine agar media within 24–48 hours. On blood agar media Listeria demonstrate β-hemolysis, which differentiates it from Corynebacterium, but adds to the difficulty in distinguishing them from streptococci. Specific identification is made in part by observation of characteristic tumbling motility on a wet preparation after room temperature incubation. A reaction of catalase positive helps to distinguish Listeria from streptococci.

Culture of Listeria from food may require cold enrichment, which would enhance the growth of Listeria. Food samples would be sent to a public health laboratory where some of the food would be enriched in a selective broth media at room temperature or lower.

Treatment and Prevention

Treatment of Listeria septicemia or meningitis is with ampicillin plus or minus gentamicin. Of significance is the inherent resistance of Listeria to cephalosporins, which are commonly chosen as empiric therapy for meningitis in adults and would be appropriate for treatment of streptococcal meningitis in children. Prevention involves the avoidance of the consumption of under-cooked foods, especially in high-risk patients.

COMPREHENSION QUESTIONS

[13.1] A 22-year-old medical student suffers diarrhea for more than a week since his return from a short vacation in Mexico. While in Mexico, he consumed a large quantity of raw cheeses almost every day. Which of the following is the most likely organism causing his diarrhea?

A. Bacillus cereus

B. Escherichia coli

C. Listeria monocytogenes

D. Salmonella enteritidis

E. Shigella dysenteriae

[13.2] A premature neonate suffers pneumonia and sepsis. Sputum culture on blood agar plate yields pinpointed β-hemolytic colonies. Which of the following is a simple test to determine whether the organism is S. agalactiae or L. monocytogenes (these two organisms are important neonatal pathogens)?

A. Bacitracin test

B. Catalase test

C. Coagulase test

D. Polymerase chain reaction

E. Sugar fermentation test

[13.3] The most frequent source of infection with L. monocytogenes is through which of the following?

A. Human feces

B. Livestock

C. Raw milk

D. Soil

E. Ticks

Answers

[13.1] C. Outbreaks of gastroenteritis, as a consequence of L. monocytogenes, are related to the ingestion of unpasteurized milk products, for example, cheese. Bacillus cereus, causing food poisoning, is associated with spore survival and germination when rice is held at warm temperatures. Escherichia coli is usually associated with the enterohemorrhagic E. coli (EHEC) form (verotoxin) that causes a bloody diarrhea and is associated with improperly cooked hamburger. Salmonella causes a diarrhea associated with contaminated chicken consumption. Finally, Shigella is usually associated with enterocolitis outbreaks among children in mental institutions and day care centers.

[13.2] B. Streptococcus agalactiae (group B streptococci) is the leading cause of neonatal sepsis and meningitis. All streptococci (including S. agalactiae) are catalase-negative, whereas staphylococci are catalasepositive. Listeria monocytogenes is also catalase-positive.

[13.3] C. Unpasteurized milk is a common vector for transmission of Listeria. See answer to Question 13.1.

MICROBIOLOGY PEARLS ❖ Listeria meningitis clinically resembles group B streptococcal meningitis and needs to be distinguished because of the resistance of Listeria to cephalosporins. ❖ Listeria grown on blood agar media will be â-hemolytic and resemble streptococci; however, by Gram stain Listeria are small bacilli and not chains of cocci. ❖ Listeria infection is commonly associated with consumption of undercooked food or unpasteurized milk or cheese products.

REFERENCES

Bortolussi R, Schlech WF. Listeriosis. In: Infectious Diseases of the Fetus and Newborn infant, 4th ed. Philadelphia, PA: W.B. Saunders, 1995:1055–73. Murray PR, Rosenthal KS, Pfaller MA. Listeria and erysipelothrix. In: 

Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology, 5th ed. St. Louis, MO: Mosby, 2005:273–78.

HomeMicrobiology Case FileListeria monocytogenes Case File