Microbial Diseases of the Skin and Nervous System: Study Notes

Diseases of the Skin and Nervous System

Introduction

The skin and nervous system are critical barriers and targets for microbial infection. This guide summarizes the major microbial diseases affecting these systems, including bacterial, viral, fungal, protozoan, and prion-related conditions, with emphasis on pathogenesis, diagnosis, and prevention.

Skin: Structure and Function

Functions of the Skin

• Prevents excessive water loss

• Regulates temperature

• Involved in sensory phenomena

• Barrier against microbial invaders

• Composed of two main layers:

  • Dermis

  • Epidermis

Skin Microbiota

Normal Microbiota

• Harmless microbes present on the skin; cannot be completely removed by cleaning.

• Compete with pathogens and secrete chemicals that kill pathogens.

• Includes Malassezia (yeast), Staphylococcus, Micrococcus, and diphtheroids (bacteria).

• Protects from skin infections but can cause disease if penetrating the epidermis or if immune system is suppressed.

Bacterial Diseases of the Skin

Staphylococcal Infections

• Staphylococcus is the most common skin pathogen.

• Facultative anaerobic, Gram-positive bacteria; tolerant of salt and desiccation.

• Two main species: Staphylococcus epidermidis and Staphylococcus aureus.

Virulence Factors Comparison

Virulence Factor	S. aureus	S. epidermidis
Enzymes	Coagulase, Hyaluronidase, Staphylokinase	None
Toxins	Exfoliative, Toxic shock, Enterotoxins	None
Other	Protein A, Capsule	Capsule

Staphylococcus aureus

• Can clot blood; diagnosed by this ability.

• Treated with penicillin alternatives; MRSA strains are resistant to many antibiotics.

• Hospital-acquired infections are a major concern due to antibiotic resistance.

Acne

• Commonly caused by Propionibacterium acnes / Cutibacterium acnes.

• Gram-positive, rod-shaped diphtheroids; part of normal skin microbiota.

• Begins in adolescence but can occur later in life.

Stages of Acne Development

• Normal skin: Sebum protects and is discharged onto skin surface.

• Whitehead: Pore swells from blocked sebum and bacteria.

• Blackhead: Dead and dying bacteria block pore.

• Furuncle: Severe inflammation from tissue destruction.

Rocky Mountain Spotted Fever

• Transmitted via bite of infected tick (Rickettsia rickettsii).

• Non-itchy spotted rash on trunk and appendages.

• Pathogen avoids digestion in host cell, invades immune system.

• Treated with antimicrobials; prevented by tick repellents and avoiding tick-infested areas.

Viral Diseases of the Skin

Poxviruses

• Include smallpox, orf (sheep), cowpox, monkeypox.

• Smallpox: First human disease eradicated by vaccination.

• Disease progresses through a series of stages.

Chickenpox and Shingles

• Chickenpox: Lesions on back and trunk, spread across body.

• Shingles: Lesions localized to skin along infected nerve.

• Caused by Varicella-zoster virus (VZV).

• Virus becomes latent in nerve ganglia; reactivation causes shingles.

Measles (Rubeola)

• Characterized by Koplik's spots; rarely causes brain swelling.

• Caused by measles virus; adhesion and fusion proteins help avoid immune recognition.

• Immune response to infected cells causes most symptoms.

• Eradicated in many regions by vaccination, but outbreaks still occur.

Impact of Immunization (US Data)

Disease	Annual Morbidity (20th Century)
Smallpox	29,005
Diphtheria	21,053
Measles	530,217
Mumps	162,344
Pertussis	200,752
Polio (paralytic)	13,316
Rubella	47,745
Congenital rubella syndrome	152
Tetanus	580
Haemophilus influenzae	20,000

Warts

• Benign epithelial growths caused by papillomaviruses.

• Transmitted via direct contact and fomites.

• Diagnosed by observation; various removal techniques exist.

• New warts can develop due to latent viruses.

Parasitic Infestations of the Skin

Leishmaniasis

• Caused by protozoan parasites of the genus Leishmania.

• Infected macrophages stimulate inflammatory responses.

• Endemic in tropics and subtropics.

• Diagnosed by microscopic identification; most cases heal without treatment, but antimicrobials needed for severe cases.

• Prevention: Reduce exposure to reservoir host.

Scabies

• Intense itching and rash at infection site.

• Caused by mite Sarcoptes scabiei.

• Transmitted via prolonged bodily contact; epidemics in crowded conditions.

• Treated with mite-killing lotions and cleaning contaminated items.

• Prevention: Good personal hygiene.

Nervous System: Structure and Function

Central and Peripheral Nervous System

• CNS: Brain and spinal cord.

• PNS: Nerves outside the CNS, transferring commands to muscles and glands.

• Brain parts: Cerebrum (voluntary muscles, perception, thinking), Cerebellum (involuntary movements), Brain stem (breathing, heart rate, blood pressure).

• Spinal cord: Extends from brain stem to lumbar region.

Peripheral Nervous System

• Cranial nerves: Extend from brain through cranial bones.

• Spinal nerves: Extend from spinal cord through vertebral gaps.

• Types: Sensory (to CNS), Motor (from CNS), Mixed (both directions).

Microbial Diseases of the Nervous System

Portals of Infection

• CNS is axenic (free of other organisms).

• Pathogens access CNS via breaks in bones, medical procedures, travel in peripheral neurons, or infect meninges.

Bacterial Diseases

• Infect cells of nervous system: Meningitis, Leprosy

• Bacteria growing elsewhere release toxins: Botulism, Tetanus

Bacterial Meningitis

• Sudden high fever, severe meningeal inflammation.

• Encephalitis can cause behavioral changes, coma, death.

• Five species cause 90% of cases: Neisseria meningitidis, Streptococcus pneumoniae, Haemophilus influenzae, Listeria monocytogenes, Streptococcus agalactiae.

Hansen's Disease (Leprosy)

• Signs: Tuberculoid (nonprogressive), Lepromatous (progressive tissue destruction).

• Caused by Mycobacterium leprae; Gram-positive bacillus with mycolic acid.

• Grows best in cooler body regions; infects peripheral nervous system.

• Diagnosis: Acid-fast bacilli in samples; treated with multiple antimicrobials; BCG vaccine offers some protection.

• Strong immune response kills bacteria; weak response allows tissue destruction.

• Armadillos are natural carriers; animal-to-human transmission is called zoonosis.

Botulism

• Caused by ingested toxin from Clostridium botulinum.

• Produces seven neurotoxins; forms: foodborne, wound, infant (most common).

• Treatment: Washing intestinal tract, antitoxin, antibiotics.

Tetanus

• Signs: Lockjaw, muscle spasms, contractions.

• Caused by Clostridium tetani; produces neurotoxin tetanospasmin.

• Acquired through breaks in skin/mucous membrane.

• Mortality rate ~50% if untreated; diagnosis by muscle contraction.

• Treatment: Passive immunotherapy, antimicrobials, active immunization; vaccine available.

Viral Diseases

Viral Meningitis

• Symptoms milder than bacterial meningitis.

• 90% of cases caused by Enterovirus.

• Diagnosed by signs/symptoms and absence of bacteria in CSF; no specific treatment.

Poliomyelitis (Polio)

• Signs: Asymptomatic (90%), minor (nonspecific), nonparalytic (muscle spasms), paralytic (paralysis).

• Caused by poliovirus; transmitted by contaminated water.

• Mostly eliminated worldwide due to vaccination.

Polio Vaccine Comparison

Vaccine	Advantages	Disadvantages
Inactivated polio vaccine (IPV)	Effective, no risk of vaccine-derived polio	Requires injection, less effective in developing countries
Oral polio vaccine (OPV)	Easy to administer, induces gut immunity	Rare risk of vaccine-derived polio

Rabies

• Almost always fatal before vaccine.

• Transmitted via bite/scratch from infected animal; virus replicates in muscle, then moves to neurons.

• Diagnosis: Unique neurological symptoms, Negri bodies in brain (postmortem).

• Treatment: Human rabies immunoglobulin, vaccine injections, cleansing infection site.

• Prevention: Control rabies in domestic animals.

Arboviruses

• Arthropod-borne viruses (mosquitoes, ticks).

• Can cause arboviral encephalitis; usually mild, coldlike symptoms.

• Confirmed by arbovirus-specific antibodies in CSF.

• Treatment: Supportive; prevention: limit mosquito contact, use repellents, eliminate stagnant water.

• Vaccines for horses: EEE, WEE, VEE, WNV.

Fungal Diseases

Fungal Effects on the Nervous System

• Mushroom toxins can cause hallucinations and neurological problems.

• Spread from lungs to CNS via blood may produce fungal meningitis.

Cryptococcal Meningitis

• Infections follow inhalation of spores/dried yeast cells.

• Occurs in terminal AIDS patients and transplant recipients.

• Diagnosis: Detection of fungal antigen in CSF.

• Treatment: Intravenous antifungal drugs.

Protozoan Diseases

African Sleeping Sickness and Meningoencephalitis

• Caused by trypanosomes; change cell surface proteins to evade immune system.

• Difficult to treat due to lack of healthcare in endemic regions.

• Diagnosed by microscopic observation in blood, lymph, or spinal fluid.

Life Cycle of Trypanosoma brucei

• Trypanosoma matures in tsetse fly salivary gland.

• Injected into human during feeding.

• Carried via bloodstream to other sites; multiplies in body fluids.

Primary Amebic Meningoencephalopathy

• Caused by Acanthamoeba and Naegleria.

• Enters through abrasions or inhalation of contaminated water.

• Diagnosis: Detection in CSF; prevention: avoid contaminated water.

Prion Diseases

• Prions cause spongiform encephalopathies (holes in the brain).

• Progressive, fatal neurodegenerative diseases.

Additional info: These notes expand on the original slides with definitions, examples, and tables for comparison, providing a comprehensive overview suitable for microbiology exam preparation.