Meningitis due to H influenzae occurs at an estimated rate of 0.08 cases per 100,000 in the United States, and it has been reported as the etiologic agent of bacterial meningitis in 20% to 50% of cases worldwide over the last several decades. In areas with routine vaccination, the majority of invasive H influenzae infections are caused by nontypeable strains and remain an important cause of meningitis, particularly for persons with predisposing conditions such as otitis or sinusitis, diabetes, immune deficiency, or head trauma with CSF leakage. Prior to widespread use of the H influenzae type b (Hib) conjugate vaccines, Hib caused more than 80% of invasive H influenzae infections, predominantly in children under the age of 5, with a mortality rate of 3% to 6% and a further 20% to 30% developing permanent sequelae ranging from mild hearing loss to intellectual disability. Encapsulated strains are further divided into 6 serotypes (a through f). Strains of H influenzae are divided into 2 groups based on the presence or absence of a polysaccharide capsule. Haemophilus influenzae is a gram-negative coccobacillus that is isolated exclusively from humans. Infections in adults are less common and generally opportunistic in nature, following exposure of sterile organs to contents of the gastrointestinal tract following trauma or surgical procedures the mortality rate for adults is reported to be 28% to 36%. These infections are of particular concern for preterm babies and neonates, and they are responsible for nearly 45% and 30% of meningitis cases in these age groups with a mortality rate of 13% and 25%, respectively. The K1 antigen is a capsule that protects the bacteria from the immune system. E coli associated with meningitis contain virulence factors that contribute to their pathogenesis by allowing them to spread through the blood, hijack normal host cell functions, infiltrate endothelial cells, and gain access to the tissues of the central nervous system (CNS). While most E coli are harmless enteric organisms residing in the intestines of humans and animals, some cause gastrointestinal illness and extraintestinal infections (eg, urinary tract infections, bacteremia, and meningitis). Escherichia coli K1 strains account for nearly 80% of E coli isolated from cerebrospinal fluid (CSF).
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