Bacterial Meningitis: Etiology, Pathogenesis, and Health Promotion

Introduction

Meningitis is an infection of the meninges — the delicate membranes that enclose the spinal cord and brain. It is a relatively rare infection that can affect both adults and children. The disease manifests in several forms, including viral, bacterial, and fungal varieties. Bacterial meningitis is the most dangerous and infectious type and is considered a life-threatening condition that spreads through physical contact. Viral meningitis, by contrast, is not as severe; most patients who develop it recover on their own without requiring treatment (Wang et al., 2014). Fungal meningitis is the rarest form and typically occurs in people whose immune systems have been compromised.

Etiology of Meningitis

Bacterial meningitis is caused by bacteria that enter the body and migrate to the brain and spinal cord, or that directly invade the meninges. Points of entry include skull fractures, ear or sinus infections, or surgery. Bacteria responsible for bacterial meningitis include Streptococcus pneumoniae (the most common cause), Neisseria meningitidis, Haemophilus influenzae, and Listeria monocytogenes — the last being particularly dangerous to people with weakened immune systems. In the United States and many other countries, S. pneumoniae is the most frequent cause. The common serotypes associated with bacterial meningitis include 4, 6B, 9V, 14, 18C, 19, and 23. Pneumococcal strain 19A is among the most notable serotypes, capable of attacking both children and adults (Kaplan et al., 2014; Wang et al., 2014).

Viral meningitis is caused by enteroviruses, which are most prevalent in late summer and early fall. It is generally mild and resolves without medication. Chronic meningitis, on the other hand, is caused by slow-growing organisms such as Mycobacterium tuberculosis and certain fungi. Fungal meningitis is uncommon; Cryptococcal meningitis is the most recognized form and predominantly affects persons with immune deficiencies such as AIDS. Other causes of meningitis are non-infectious and include drug allergies, chemical reactions, inflammatory diseases, and some types of cancer.

Pathogenesis of Bacterial Meningitis

The entry point for pathogens that cause meningitis is typically the nasopharynx or ears, from which they access the bloodstream. They pass through endothelial cells to enter the subarachnoid space and then penetrate the porous capillaries of the choroid plexus; they may also be transported by granulocytes. The cerebrospinal fluid (CSF) is an ideal medium for bacterial growth because it provides sufficient nutrients while housing relatively few phagocytic cells and maintaining low antibody concentrations (McGill et al., 2016). During the early phase, bacteria multiply uninhibited and can be identified through cultures, smears, or enzyme-linked immunosorbent assay (ELISA) detection of antigens prior to the onset of significant inflammation.

Bacteria have the potential to damage blood capillaries and brain tissue directly. The toxins they generate cause neuronal apoptosis. Endotoxins produced by bacteria trigger clotting, which in turn can lead to disseminated intravascular coagulation — a state that compounds injury through the subsequent inflammatory response. If neutrophils increase markedly in number, they can damage brain nerves, vessels, and tissue. Cerebral infarcts may result from vasculitis and clotting. This pattern of brain damage during a meningitis attack is partly attributable to direct bacterial action and partly to the immune system's response to that action.

The brain is equipped to manage inflammation, but disproportionate or dysregulated defense responses can themselves be injurious. Increased intracranial pressure caused by cerebral edema is the most severe complication of bacterial meningitis. The edema may be vasogenic — arising from heightened vascular permeability — interstitial, cytotoxic, or secondary to cerebral hypoxia, elevated complement-fixing antibody levels, or a combination of these factors (Liechti et al., 2015). Elevated intracranial pressure then leads to decreased cerebral perfusion, neuronal necrosis, and hypoxia or ischemia.

References

Kaplan, S. L., Edward, M. S., Nordli Jr, D. R., & Torchia, M. M. (2014). Bacterial meningitis in children older than one month: Clinical features and diagnosis. Waltham (MA): UpToDate.

Liechti, F. D., Grandgirard, D., & Leib, S. L. (2015). Bacterial meningitis: Insights into pathogenesis and evaluation of new treatment options: A perspective from experimental studies. Future Microbiology, 10(7), 1195–1213.

McGill, F., Heyderman, R. S., Panagiotou, S., Tunkel, A. R., & Solomon, T. (2016). Acute bacterial meningitis in adults. The Lancet, 388(10063), 3036–3047.

Wang, A. Y., Machicado, J. D., Khoury, N. T., Wootton, S. H., Salazar, L., & Hasbun, R. (2014). Community-acquired meningitis in older adults: Clinical features, etiology, and prognostic factors. Journal of the American Geriatrics Society, 62(11), 2064–2070.