Bacterial Meningitis: Etiology, Modes of Transmission, Epidemiology and Risk Factors

Abstract

Resulting in a grave and potentially life-threatening infection of the meninges — the protective membranes encapsulating and surrounding the spinal cord and brain — which causes them to swell, bacterial meningitis is a serious condition. Its ramifications exist in the form of severe complications such as seizures, neurological damage to the brain and even death if prompt treatment is not administered. This article aims to provide a comprehensive overview of the disease, beginning with a description of its etiology and methods of transmission. Moreover, it seeks to list down the risk factors and associated epidemiology of the condition as well.

Introduction 

Resulting in a grave and potentially life-threatening infection of the meninges — the protective membranes encapsulating and surrounding the spinal cord and brain — which causes them to swell, bacterial meningitis is a serious condition. Meningitis in essence is an inflammation of the meninges, and bacterial meningitis is just one type. Its ramifications exist in the form of severe complications such as seizures, neurological damage to the brain, learning differences, speech issues, vision or hearing loss, problems in balance, coordination and memory, concentration issues and even death if prompt treatment is not administered.[1][2] However, with quick and effective treatment, the majority of the patients tend to recover and not experience the  aforementioned problems permanently[1].

The causative organism, as the name suggests, are bacteria. Many different types of these pathogenic microorganisms can initially cause an upper respiratory infection and then proceed to travel via the bloodstream to the brain. Alternatively, some bacteria invade the meninges directly[3]. The meninges themselves are three layers of tissue covering the brain and the spinal cord. The subarachnoid space is situated between the innermost and middle layer,  and contains the cerebrospinal fluid. The fluids are responsible for cushioning the brain and spinal cord and are able to achieve this by filling the spaces within the brain and flowing through the meninges. 

When bacteria invade the meninges and subarachnoid space, an immune response is triggered which results in inflammation. This can lead to blood clots, cerebral edema, intracranial pressure, and subdural empyema (when pus collects under the outer layer of the meninges). Early symptoms include a fever, stiffness felt in the neck, confusion and decreased levels of alertness, sensitivity to light and headaches. In newborns and infants specifically, the symptomatic manifestations of the disease include vomiting, increased irritability and excessive crying, lethargy, feeding problems and high or low body temperatures. If these features continue to worsen, a diagnosis should be sought after[4].

Etiology and Methods of Transmission

The infection can be either acquired in a hospital (nosocomial) or community acquired, and the specific bacteria responsible varies by several factors, age being a preponderant one among them. A list a number of these etiological culprits responsible for bacterial meningitis would look as follows:

1. Neisseria meningitidis: This bacterial agent is particularly common in young adults and individuals going through adolescence and is a major cause of meningococcal meningitis, a condition that can subsequently lead to a blood infection.

2. Group B Streptococcus and Streptococcus pneumoniae:  The former is common in infants under the age of 2 months while the later is recurrent in all age groups besides teenagers ranging from 11 to 17 years of age. It can result in patients succumbing to pneumonia, sinusitis and ear infections[5][6]. 

3. Haemophilus influenzae: It is able to infiltrate the immune systems of non-vaccinated individuals to cause meningitis, but is at best occasionally encountered now given the fact that most are vaccinated[5]. Although it affects every age group, it is prevalent in children under the age of 5 years.

4. Listeria monocytogenes -  most of the cases involving it include neonates[6].

5. Staphylococcus aureus and Staphylococcus albus: These bacteria are responsible for nosocomial infections[5]. 

Moreover, meningitis is not contagious but it can spread among people. In the case of infants, the bacteria can be transmitted from mother to infant, especially during childbirth; pathogens responsible for early onset disease are acquired from the birth canal after a rupture of membranes. Less frequently, it can be transmitted to the neonate from contaminated hospital equipment or materials[7]. According to the World Health Organisation, it can spread through the air. The respiratory droplets of an infected person are contaminated by bacteria and if they’re inhaled in by a non-infected individual, it can lead to an infection. Furthermore, prolonged contact with the diseased increases the likelihood of transmission. Sharing items like utensils which are in contact with throat secretions (saliva and mucus) infected by the etiological agent also facilitate the spread of bacteria[8]. 

Epidemiology and Risk Factors

The epidemiology of causative pathogens have changed. As vaccines have been continually developed, utilised and administered to the general public, the incidence of acute bacterial meningitis has lessened and it is not as common in pediatric patients as before. Vaccinations have increased the median age of those infected. Of the 72,000 hospitalisations in the USA related to meningitis, a minority of the cases were bacterial infections; they accounted for around 21.8% of the total cases documented. The mortality rate for these patients stood at 8% and it saw a substantial increase for patients above the age of 45 years old. Analysts have reported that the annual incidence of meningitis in the country decreased from 2.00 cases per 100,000 of the population between 1998-1999 to 1.38/100,000 of the population between 2006-2007. However, the median age increased from 30.3 years to 41.9 years between the aforementioned ranges. With 350 cases of disease reported in 2017, the value has decreased to 0.11/100,000[5].

There are many risk factors that increase the likelihood of acquiring bacterial meningitis. Those include:

1. Age: Specific bacteria are more prevalent in certain age groups. The details have been stated above under the etiology of the disease 

2. Weakened Immune System: Certain medical conditions like HIV infection, functional and anatomic asplenia and complement component deficiencies weaken the immune system and increase the risk of an individual succumbing to meningococcal disease and bacterial meningitis

3. Receiving Specific Medication: Patients receiving complement inhibitors like eculizumab and ravulizumab are at a heightened risk of the disease. This medication is prescribed to those with atypical hemolytic uremic syndrome, paroxysmal nocturnal hemoglobinuria, neuromyelitis optica spectrum disorder and generalised myasthenia gravis[9].

Conclusion

Bacterial meningitis is a serious infection which results in the inflammation of the meninges. It can be a severe condition that adversely impacts the neurological system, motor skills and bodily functions of the patient. Further research is being conducted by scientists globally to understand the disease further. If any symptoms are experienced, medical treatment should be immediately sought after as prompt administration of medications and care can be life-saving.

Resources

1. https://www.hopkinsmedicine.org/health/conditions-and-diseases/bacterial-meningitis

2. https://my.clevelandclinic.org/health/diseases/11039-bacterial-meningitis

3. https://www.ninds.nih.gov/health-information/disorders/meningitis

4. Acute Bacterial Meningitis - By Robyn S. Klein, MD, PhD, University of Western Ontari - https://www.msdmanuals.com/home/brain-spinal-cord-and-nerve-disorders/meningitis/acute-bacterial-meningitis 

5. Runde TJ, Anjum F, Hafner JW. Bacterial Meningitis. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK470351/ 

6. Global etiology of bacterial meningitis: A systematic review and meta-analysis - Anouk M. Oordt-Speets, Renee Bolijn, Rosa C. van Hoorn, Amit Bhavsar, Moe H. Kyaw - https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0198772 

7. Remington and Klein's Infectious Diseases of the Fetus and Newborn Infant By Christopher B. Wilson, Victor Nizet, Yvonne Maldonado, Jack S. Remington, Jerome O. Klein

8. https://www.who.int/health-topics/meningitis#tab=tab_1

9. https://www.cdc.gov/meningococcal/risk-factors/index.html#:~:text=Receiving%20medicines,for%20four%20rare%20medical%20conditions: