A 3-week-old baby boy is brought to the emergency department with fever, irritability, and poor feeding. Examination of the cerebrospinal fluid shows a leukocyte count of 600 cells/µL (80% neutrophils), protein of 160 mg/dL, and glucose of 20 mg/dL. Blood cultures grow gram-negative rods that form pink colonies on MacConkey agar. Which of the following is the most important bacterial virulence factor for the development of this patient's condition?

Question

A 3-week-old baby boy is brought to the emergency department with fever, irritability, and poor feeding.  Examination of the cerebrospinal fluid shows a leukocyte count of 600 cells/µL (80% neutrophils), protein of 160 mg/dL, and glucose of 20 mg/dL.  Blood cultures grow gram-negative rods that form pink colonies on MacConkey agar.  Which of the following is the most important bacterial virulence factor for the development of this patient's condition?

Quizplus · Accepted Answer

11ec160b_3875_acee_9249_5b5df4b7b9a8_MD0001_00 Group B Streptococcus is the most common cause of neonatal meningitis (0-3 months) in the United States, followed by Escherichia coli and Listeria monocytogenes. In older infants (>3 months) and adults, the most common pathogens are Streptococcus pneumoniae and Neisseria meningitidis. This patient has meningitis secondary to E coli infection. E coli is a motile, gram-negative, facultative anaerobic rod that is able to ferment both lactose and glucose. It grows well on blood, MacConkey, and eosin methylene blue agar plates. MacConkey agar is a selective and differential medium used to isolate gram-negative organisms from contaminants in clinical specimens. The bile salts and crystal violet present in MacConkey agar prevent the growth of gram-positive organisms. Organisms that ferment lactose (eg, E coli, Klebsiella, Enterobacter) cause a local drop in pH, resulting in colonies with a pink-red appearance. Non-lactose fermenting organisms remain colorless. E coli can invade the blood stream of infants from the nasopharynx or gastrointestinal tract and can then travel hematogenously to the meninges. The K1 capsular antigen is present in 20%-40% of intestinal E coli isolates and is considered the major virulence factor among E coli strains that cause neonatal meningitis. Bacterial capsules are important for most meningeal pathogens. They help facilitate survival in the blood by preventing recognition of bacterial antigens, complement deposition, and subsequent phagocytosis. The K1 capsule is immunogenic and anti-capsular antibodies are protective against repeat infection. (Choice B) Fimbriae (pili) are a virulence factor that allows bacteria to adhere to the target tissue, thereby establishing infection. Examples of organisms that use pili are Neisseria gonorrhoeae, uropathogenic E coli, and Vibrio cholerae. (Choice C) The lipid A unit in lipopolysaccharides is similar for all enteric bacteria; it causes activation of macrophages, leading to widespread release of IL-1 and TNF-α, which in turn cause the signs and symptoms of septic shock. (Choice D) The O antigen is the outermost component of the lipopolysaccharide molecule. It is a variable polysaccharide that can be used to classify gram-negative bacteria. (Choice E) Verotoxin (shiga-like toxin) is synthesized by enterohemorrhagic E coli. Exotoxin expression is not present in any of the pathogens commonly associated with neonatal meningitis. Educational objective: Escherichia coli is a frequent cause of neonatal meningitis, second only to group B streptococci. E coli strains that cause neonatal meningitis possess the K1 capsular antigen. The K1 capsule is a virulence factor that allows the bacteria to survive in the bloodstream and establish meningeal infection.

A 3-Week-Old Baby Boy Is Brought to the Emergency Department