A 14-day-old baby is admitted to the hospital with a fever, hyperactivity, and a stiff neck. At the time of giving birth to the baby, the mother complained of flulike symptoms. Blood and CSF were collected for culture. No organisms were seen in the Gram stain of the CSF; however, the culture of blood and CSF became positive after 48 hours of incubation. Colonies appeared weakly beta-hemolytic, and the Gram stain was interpreted as gram-positive cocci or coccobacilli arranged in single cells and pairs. Which organism is most likely responsible for this baby’s infection?
- Escherichia coli
- Listeria monocytogenes
- Neisseria meningitidis
- Group B Streptococcus agalactiae
- Streptococcus pneumonia
Although L. monocytogenes is most likely the bacterium that caused the meningitis, it is important to note first that all five of the bacteria listed in this question can also cause the disease. E. coli and group B S. agalactiae are primarily restricted to babies less than 1 month of age, whereas L. monocytogenes, N. meningitidis, and S. pneumoniae can cause meningitis in all age groups. (N. meningitidis is most common in young adults, whereas the other two bacteria cause disease most commonly in the young and the very old.) The baby’s meningitis is most likely caused by L. monocytogenes because the organism is a slow-growing, weakly beta-hemolytic, gram-positive coccobacillus. In contrast with other organisms, L. monocytogenesgrows slowly in CSF and typically is not observed in a Gram stain of CSF because relatively few organisms are present. When observed by Gram stain, L. monocytogenes may be mistaken forS. pneumoniae. Organisms isolated in culture are readily differentiated by colonial morphology (S. pneumoniae is alpha-hemolytic) and simple phenotypic tests. E. coli and N. meningitidisare gram-negative and should not be mistaken for L. monocytogenes. S. agalactiae is a gram-positive cocci but grows well on blood agar and appears as long chains rather than single cells or pairs.
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