Microbial Diseases of the Nervous System: Structure, Pathogenesis, and Clinical Features

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Microbial Diseases of the Nervous System

Structure and Protection of the Nervous System

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all nerves branching from the CNS. The CNS is protected by three layers of connective tissue called meninges (dura mater, arachnoid mater, and pia mater), and the subarachnoid space contains cerebrospinal fluid (CSF) that cushions the brain and spinal cord. The blood-brain barrier (BBB) is a highly selective semipermeable barrier formed by endothelial cells, which protects the brain from pathogens and toxins in the bloodstream.

Diagram of the human nervous system showing CNS and PNS Diagram of the meninges and cerebrospinal fluid around the brain and spinal cord

Common Signs and Symptoms of Nervous System Infections

Microbial infections of the nervous system often present with characteristic symptoms due to inflammation of neural tissues:

Meningitis: Inflammation of the meninges
Encephalitis: Inflammation of the brain
Meningoencephalitis: Inflammation of both the brain and meninges

Common symptoms include fever, headache, stiff neck, nausea, vomiting, photophobia (sensitivity to light), and altered mental status. Severe cases may progress to convulsions, coma, and death due to shock or inflammation.

Symptoms of meningitis: fever, headache, stiff neck, photophobia, nausea, confusion

Bacterial Infections of the Nervous System

Bacterial Meningitis

Bacterial meningitis is a medical emergency that progresses rapidly and can be fatal. The most common causative agents are:

Haemophilus influenzae type b (Hib): Gram-negative aerobic bacteria, normal throat microbiota, primarily affects children (6 months to 4 years), prevented by Hib vaccine.
Neisseria meningitidis: Gram-negative aerobic cocci with capsule, causes meningococcal meningitis, outbreaks common in crowded settings, six serotypes, high mortality without antibiotics.
Streptococcus pneumoniae: Gram-positive encapsulated diplococcus, causes pneumococcal meningitis, most common in children and elderly, prevented by conjugated vaccine.

Diagnosis is often confirmed by analyzing CSF obtained via lumbar puncture.

Table comparing major pathogens, transmission, and treatment of nervous system infections Photograph of inflamed meninges covering the brain Electron micrograph of Neisseria meningitidis interacting with cilia Diagram of lumbar puncture for CSF collection

Listeriosis

Listeria monocytogenes is a Gram-negative aerobic rod, usually foodborne. It can invade the bloodstream, reproduce in phagocytes, and cross the placenta, causing sepsis, meningitis in immunocompromised individuals, and stillbirths. Prevention includes proper food handling and cooking.

Infographic on listeriosis: symptoms, high-risk groups, prevention, and treatment

Tetanus

Clostridium tetani is a Gram-positive, endospore-forming, obligate anaerobe. It grows in deep wounds and releases tetanospasmin, a neurotoxin that blocks inhibitory neurotransmitter release, causing rigid paralysis and death from respiratory muscle spasms. Prevention is by vaccination with tetanus toxoid (DTaP), with boosters every 10 years.

Drawing of tetanic spasm (opisthotonos) in tetanus Infographic: 10 facts you must know about tetanus

Botulism

Clostridium botulinum is a Gram-positive, endospore-forming, obligate anaerobe that produces a potent exotoxin. The toxin blocks acetylcholine release at neuromuscular junctions, causing flaccid paralysis. Transmission is usually via ingestion of preformed toxin in improperly canned foods, but can also occur in infants (from honey or soil) and wounds. Treatment includes respiratory support and antitoxins.

Diagram showing normal neurotransmitter release and blockage by botulinum toxin Mouse bioassay for botulism diagnosis Infographic: symptoms of botulism Infographic: Clostridium botulinum in infants and honey Photo of home-canned foods, a risk for botulism

Leprosy (Hansen’s Disease)

Mycobacterium leprae is an acid-fast rod that grows best at 30°C, with a slow generation time. It infects peripheral nerves and skin, surviving in macrophages and invading the myelin sheath. Transmission requires prolonged contact or inhalation of secretions; armadillos are a zoonotic reservoir. Two forms exist: tuberculoid (neural, with loss of sensation) and lepromatous (progressive, with disfiguring nodules). Treatment is with antibiotics (dapsone or rifampin) for 6–24 months.

Infographic: leprosy (Hansen's disease) transmission, symptoms, treatment, and prevention Photos of tuberculoid and lepromatous leprosy Photo of leprosy patient with facial involvement

Summary Table: Major Nervous System Infections

Disease	Pathogen	Portal of Entry	Method of Transmission	Treatment	Prevention
Haemophilus influenzae Meningitis	H. influenzae	Respiratory tract	Endogenous infection, aerosols	Cephalosporin	Capsular Hib vaccine
Meningococcal Meningitis	Neisseria meningitidis	Respiratory tract	Aerosols	Cephalosporin	Capsular vaccines against serotypes A, B, C, Y, W
Pneumococcal Meningitis	Streptococcus pneumoniae	Respiratory tract	Aerosols	Cephalosporin	Polysaccharide vaccine
Listeriosis	Listeria monocytogenes	Mouth	Foodborne infection	Penicillin G	Pasteurizing and cooking food
Cryptococcosis	Cryptococcus neoformans, C. grubii, C. gattii	Respiratory tract	Inhaling soil contaminated with spores	Amphotericin B, flucytosine	None
Primary Amebic Meningoencephalitis	Naegleria fowleri	Nasal mucosa	Swimming	Amphotericin B	None
Granulomatous Amebic Encephalitis	Acanthamoeba spp., Balamuthia mandrillaris	Mucous membranes	Swimming	Amphotericin B, miltefosine	None