Infectious Diseases of the Skin, Eyes, Respiratory, and Nervous Systems

Infectious Diseases: Skin, Respiratory, Nervous System

Overview

This module covers the microbial diseases affecting the skin, eyes, respiratory tract, and nervous system. It discusses the physical barriers to infection, common pathogens, disease mechanisms, and clinical features relevant to microbiology students.

Microbial Diseases of the Skin and Eyes

Physical Barriers Against Infectious Agents

The skin and mucous membranes serve as the body's first line of defense against microbial invasion.

• Skin: Composed of a tight layer of cells with constant shedding, which removes attached microbes.

• Keratin: A tough protein in skin cells that provides structural strength and resistance to microbial penetration.

• Sebum: An oily substance secreted by sebaceous glands, containing fatty acids and antimicrobial peptides that inhibit pathogen growth.

• Mucous Membranes: Line body cavities open to the exterior; also have tightly packed cells and constant shedding.

• Normal Microbiota: Compete with pathogens for nutrients and space, preventing colonization.

Additional info: The skin's acidic pH and salty environment further inhibit microbial growth.

Skin Rashes and Lesions

Skin manifestations are common in infectious diseases and can be classified based on their appearance and contents.

• Rash: General term for skin eruption due to disease.

• Vesicle: Small, fluid-filled lesion.

• Bulla: Larger fluid-filled lesion.

• Macule: Flat, reddened lesion.

• Pustule: Raised lesion containing pus.

Type	Description
Vesicle	Small, fluid-filled
Bulla	Large, fluid-filled
Macule	Flat, reddened
Pustule	Raised, pus-filled

Bacterial Skin Infections Caused by Staphylococci

Staphylococcus aureus is a major cause of skin infections, often found in the nasal passages and on the skin.

• Characteristics: Gram-positive cocci in clusters; part of normal flora in 20% of population.

• Virulence Factors:

  • Produces damaging toxins (e.g., exfoliative toxin, TSST-1) and enzymes (coagulase, hyaluronidase, staphylokinase).

  • Resists host defenses by killing phagocytic cells.

  • Some strains are antibiotic-resistant (e.g., MRSA).

• Clinical Manifestations:

  • Folliculitis: Infection of hair follicles.

  • Sty: Infection of eyelash follicle.

  • Furuncle: Abscess; localized pus surrounded by inflamed tissue.

  • Carbuncle: Damage and inflammation of deep tissue.

  • Impetigo: Crusting sores, often spread by autoinoculation.

  • Scalded Skin Syndrome: Exfoliation due to toxin.

  • Toxic Shock Syndrome (TSS): Fever, shock, organ failure due to TSST-1 toxin.

Staphylococcus epidermidis is less virulent but can cause healthcare-associated infections, especially via biofilm formation on catheters.

• Coagulase-negative; major component of normal skin microbiota.

• Forms biofilms, especially on medical devices.

Bacterial Skin Infections Caused by Group A Streptococci (GAS)

Streptococcus pyogenes (GAS) is a Gram-positive cocci in chains, part of normal flora of mouth and respiratory tract.

• Produces enzymes and toxins (e.g., streptolysins, hyaluronidase) that damage tissues and evade immune response.

• Can cause impetigo, erysipelas, necrotizing fasciitis, and other skin infections.

• Classified into groups (A-T) based on antigenic cell wall carbohydrates.

Other Bacterial Skin Infections

• Pseudomonas aeruginosa: Common in burn victims and swimmers; resistant to many antibiotics; forms biofilms.

• Acne: Infections of hair follicles and sebaceous glands; affected by hormones, not diet; treated with topical agents.

Viral Skin Infections

• Varicella-zoster virus (VZV): Causes chickenpox and shingles; can become latent in nerve ganglia.

• Herpes simplex virus (HSV): HSV-1 causes oral lesions; HSV-2 causes genital herpes; both can remain latent in nerve cells.

• Measles (Rubeola): Characterized by Koplik's spots; can cause severe complications.

• Rubella: Can cause congenital rubella syndrome in fetuses.

• Human papillomavirus (HPV): Causes warts; not a rash.

Fungal Skin and Nail Infections

• Dermatophytes: Cause ringworm (tinea capitis, tinea corporis, tinea pedis).

• Candida (yeast): Opportunistic overgrowth can cause thrush and vaginitis, especially in immunocompromised patients.

Eye Infections Caused by Bacteria

• Neisseria gonorrhoeae: Causes ophthalmia neonatorum in newborns; can lead to blindness if untreated.

• Chlamydia trachomatis: Causes inclusion conjunctivitis and trachoma; leading cause of blindness worldwide.

Microbial Diseases of the Nervous System

Structure and Function of the Nervous System

The nervous system is divided into the central nervous system (CNS: brain and spinal cord) and peripheral nervous system (PNS: nerves branching from CNS). Protective layers include dura mater, arachnoid mater, and pia mater. The blood-brain barrier restricts pathogen entry.

Inflammatory Diseases

• Meningitis: Inflammation of the meninges.

• Encephalitis: Inflammation of the brain.

• Menigoencephalitis: Inflammation of both meninges and brain.

Bacterial Meningitis

Bacterial meningitis is a serious infection with symptoms of fever, headache, stiff neck, nausea, and vomiting. Can progress to convulsions and coma.

• Haemophilus influenzae: Normal throat flora; major virulence factor is capsule; affects young children; prevented by Hib vaccine.

• Neisseria meningitidis: Gram-negative diplococcus with capsule; causes meningococcal meningitis; common in dorms and barracks; vaccine available for some types.

• Streptococcus pneumoniae: Encapsulated diplococcus; causes pneumococcal meningitis; common in children and elderly; conjugated vaccine available.

Additional info: Viral meningitis is more common and generally milder than bacterial forms.

Diagnosis and Treatment of Meningitis

• Diagnosis by lumbar puncture (spinal tap) to obtain cerebrospinal fluid (CSF).

• CSF samples must be processed quickly; bacteria do not survive storage or low temperature.

• Empiric antibiotic therapy often started before diagnosis is confirmed.

• Third-generation cephalosporins commonly used.

Other Bacterial Infections of the Nervous System

• Tetanus (Clostridium tetani): Deep wound infection; produces tetanospasmin toxin causing muscle spasms (lockjaw); prevented by DTaP vaccine.

• Botulism (Clostridium botulinum): Ingestion of botulinum exotoxin; causes flaccid paralysis; associated with improperly canned foods and honey.

• Leprosy (Mycobacterium leprae): Infects peripheral nerves and skin; causes loss of sensation and disfigurement.

Viral Infections of the Nervous System

• Poliomyelitis (Poliovirus): Fecal-oral transmission; can cause paralysis; vaccines include inactivated (injectable) and attenuated (oral).

• Rabies (Rabies virus): Transmitted by animal bites; causes fatal encephalitis; post-exposure prophylaxis includes immune globulin and vaccine.

• Arboviral encephalitis: Mosquito-borne viruses (e.g., EEE, SLE, West Nile virus); maintained in bird and mosquito populations.

Fungal and Prion Diseases of the Nervous System

• Cryptococcal meningitis: Associated with pigeon and chicken droppings; affects immunocompromised individuals.

• Creutzfeldt-Jakob Disease (CJD): Prion disease causing neurodegeneration.

Microbial Diseases of the Respiratory System

Physical Barriers and Defenses

• Nasal hairs: Filter large particles.

• Ciliary escalator: Moves particles toward the throat via ciliary action.

• Alveolar macrophages: Destroy microorganisms in the lungs.

• Respiratory mucus: Protects mucosal surfaces.

• Normal microbiota: Suppress pathogens by competing for resources.

Upper Respiratory Tract Infections

• Pharyngitis (Strep throat): Caused by group A streptococci (GAS); symptoms include sore throat, tonsillitis, and enlarged lymph nodes.

• Diphtheria (Corynebacterium diphtheriae): Produces tough grayish membrane in throat; toxin damages heart and kidneys; prevented by DTP (toxoid) vaccine.

• Common cold (Rhinoviruses): Over 200 varieties; symptoms include runny nose and fever; treatment is symptomatic.

• Otitis media: Middle ear infection; common in children due to smaller auditory tubes; caused by multiple bacteria (e.g., S. pneumoniae).

Lower Respiratory Tract Infections

• Bronchitis, Bronchiolitis, Pneumonia: Involve pulmonary alveoli; caused by bacteria and viruses.

• Pertussis (Whooping cough): Caused by Bordetella pertussis; destroys ciliated cells; prevented by DTaP vaccine.

• Tuberculosis (Mycobacterium tuberculosis): Acid-fast rod; resistant to drying; forms tubercles in lungs; diagnosed by tuberculin skin test, chest X-ray, acid-fast staining, and PCR.

Tuberculosis Treatment: Requires 6 months of drug therapy; common drugs include isoniazid, rifampin, ethambutol, pyrazinamide. MDR and XDR strains are resistant to multiple drugs.

Bacterial Pneumonia

• Streptococcus pneumoniae: Causes pneumococcal pneumonia; prevented by vaccine; resists phagocytosis.

• Haemophilus influenzae: Similar symptoms; affects young and old.

• Mycoplasma pneumoniae: Common in young adults; atypical pneumonia.

• Legionella pneumophila: Grows in water systems; causes Legionnaires' disease.

Viral Pneumonia

• Often a complication of influenza, measles, or chickenpox.

• SARS-CoV, MERS-CoV, SARS-CoV-2: Coronaviruses causing severe respiratory syndromes; COVID-19 pandemic caused by SARS-CoV-2.

• Respiratory Syncytial Virus (RSV): Most common viral respiratory disease in infants; causes cell fusion (syncytium); diagnosed by serological tests; treated with palivizumab.

• Influenza (Flu): RNA virus with envelope and spikes (HA and NA); undergoes antigenic drift and shift; treated with Tamiflu and Relenza.

Virus	Key Features
Influenza	Antigenic drift/shift, HA/NA spikes, pandemics
RSV	Syncytium formation, severe in infants/elderly
SARS-CoV-2	COVID-19 pandemic, severe pneumonia

Fungal Diseases of the Lower Respiratory System

• Histoplasmosis: Caused by Histoplasma capsulatum; associated with bat droppings; endemic in certain regions.

• Pneumocystis pneumonia: Caused by Pneumocystis jirovecii; indicator of AIDS; affects immunocompromised individuals.

• Other fungi: Grow in compost piles and can cause respiratory infections.

Additional info: Fungal respiratory diseases are more common in immunocompromised hosts.