Microbiology of Skin Infections: Pathogens, Syndromes, and Clinical Management

Skin Infections: Overview and Learning Goals

This section provides an overview of the major pathogens responsible for skin infections, their clinical syndromes, routes of transmission, and strategies for diagnosis, treatment, and prevention. Understanding these aspects is crucial for identifying and managing infectious diseases of the skin.

Classification of Skin Pathogens

Helminths, Protozoa, Fungi, Bacteria, and Viruses

• Helminths: Parasitic worms such as Taenia (tapeworms), Schistosoma (blood flukes), and Enterobius (pinworm) can cause cutaneous and systemic infections.

• Protozoa: Includes Entamoeba, Giardia, Leishmania, and Plasmodium, which may affect the skin, GI tract, blood, or tissues.

• Fungi: Dermatophytes (e.g., Microsporum, Trichophyton), Candida, and Aspergillus cause superficial and systemic mycoses.

• Bacteria: Gram-positive cocci (e.g., Staphylococcus, Streptococcus), Gram-negative rods (e.g., E. coli, Pseudomonas), and spirochetes (e.g., Borrelia).

• Viruses: DNA and RNA viruses such as Herpes simplex, Varicella zoster, Measles, and Rubella are common causes of exanthems.

Skin Immune Defenses

Physical and Chemical Barriers

• Melanin: Provides antimicrobial properties.

• Perspiration: Acidic pH, high salt content, and lysozyme help wash away microbes.

• Sebum: Oily secretion with low pH; metabolized by skin bacteria to produce toxic fatty acids.

Primary and Secondary Skin Lesions

Types and Clinical Significance

• Primary lesions: Vesicles, bullae, macules, papules, pustules, cysts, and nodules are initial manifestations of infection or inflammation.

• Secondary lesions: Crusts, scales, purpura, petechiae, and ulcers develop from progression or complications of primary lesions.

Viral Exanthems and Enanthems

Definitions and Major Childhood Diseases

• Exanthem: Widespread rash, often abrupt, typically in children, associated with viral infections, toxins, or immune responses.

• Enanthem: Lesions on mucous membranes, often accompanying exanthems (e.g., Koplik spots in measles).

• Major exanthems: Measles (rubeola), Rubella (German measles), Roseola (HHV-6/7), Chickenpox (varicella-zoster), Erythema infectiosum (parvovirus B19).

Measles (Rubeola)

Etiology, Pathogenesis, and Clinical Features

• Agent: Measles virus, family Paramyxoviridae, ss(-) RNA, enveloped.

• Transmission: Respiratory droplets; highly contagious.

• Clinical features: High fever, malaise, conjunctivitis, cough, coryza, photophobia, Koplik spots (bluish-gray on buccal mucosa), maculopapular rash starting on face and spreading downward.

• Complications: Subacute sclerosing panencephalitis (SSPE), pneumonia, otitis media.

• Diagnosis: Clinical signs, viral antigen detection.

• Prevention: MMR(V) vaccine (live, attenuated).

Rubella (German Measles)

Etiology, Pathogenesis, and Clinical Features

• Agent: Rubella virus, family Togaviridae, ss(+) RNA, enveloped.

• Transmission: Respiratory droplets, vertical (placental) transmission.

• Clinical features: Mild fever, pale pink maculopapular rash, lymphadenopathy.

• Complications: Congenital rubella syndrome (CRS): microcephaly, cataracts, patent ductus arteriosus (PDA).

• Diagnosis: Rubella-specific IgM antibodies.

• Prevention: MMR(V) vaccine (not for pregnant women).

Parvovirus B19 (Erythema Infectiosum, Fifth Disease)

Etiology, Pathogenesis, and Clinical Features

• Agent: Parvovirus B19, family Parvoviridae, ssDNA, non-enveloped.

• Transmission: Respiratory route.

• Clinical features: Fever, coryza, headache, "slapped cheek" rash, possible lacy rash on limbs.

• Complications: Aplastic crisis in patients with hemolytic anemia.

• Diagnosis: Detection of viral nucleic acid, antibody, or antigen.

• Treatment: Supportive; immunoglobulin for immunocompromised.

• Prevention: No vaccine available.

Roseola Infantum (Exanthem Subitum, Sixth Disease)

Etiology, Pathogenesis, and Clinical Features

• Agent: Human herpesvirus 6 or 7 (HHV-6/7), family Herpesviridae.

• Transmission: Respiratory droplets.

• Clinical features: Sudden high fever (3-5 days), followed by a faint maculopapular rash as fever subsides.

• Complications: Febrile seizures, leukopenia.

• Prevention: No vaccine available.

Varicella-Zoster Virus (VZV): Chickenpox and Shingles

Etiology, Pathogenesis, and Clinical Features

• Agent: Varicella-zoster virus, family Herpesviridae, dsDNA, enveloped.

• Transmission: Respiratory droplets, direct contact.

• Chickenpox: Fever, vesicular rash, highly contagious; virus remains latent in neurons.

• Shingles (zoster): Reactivation of latent virus, painful vesicular rash along a dermatome, postherpetic neuralgia possible.

• Diagnosis: Clinical appearance of lesions.

• Treatment: Acyclovir for zoster; avoid aspirin in children (Reye’s syndrome).

• Prevention: Varivax (children), Zostavax (adults >60), both live attenuated vaccines.

Smallpox (Variola)

Etiology, Pathogenesis, and Clinical Features

• Agent: Variola virus, family Poxviridae, dsDNA, enveloped.

• Transmission: Aerosol, direct contact, fomites.

• Clinical features: Fever, malaise, centrifugal rash (face, arms, legs), lesions progress synchronously from macules to pustules to scabs.

• Diagnosis: Clinical presentation, epidemiological context.

• Prevention: Live vaccinia virus vaccine; eradicated globally in 1980.

Bacterial Skin Infections: Streptococcus pyogenes

Diseases, Pathogenesis, and Clinical Management

• Agent: Streptococcus pyogenes (Group A Strep), Gram-positive cocci in chains, β-hemolytic.

• Diseases: Pharyngitis (strep throat), scarlet fever, rheumatic fever, impetigo, cellulitis, necrotizing fasciitis.

• Virulence factors: M protein, hemolysin, hyaluronidase, streptokinase, erythrogenic toxin (scarlet fever).

• Diagnosis: Throat swab, blood agar, rapid strep test.

• Treatment: Penicillin, erythromycin.

• Prevention: No vaccine; proper hygiene and coughing technique.

Bacterial Skin Infections: Staphylococcus aureus

Diseases, Pathogenesis, and Clinical Management

• Agent: Staphylococcus aureus, Gram-positive cocci in clusters, catalase and coagulase positive.

• Diseases: Food poisoning, skin infections (boils, impetigo, scalded skin syndrome), toxic shock syndrome.

• Virulence factors: Coagulase, hyaluronidase, staphylokinase, lipases, β-lactamase, cytolytic toxins, exfoliative toxins, TSS toxin, enterotoxins.

• Diagnosis: Catalase and coagulase tests, mannitol fermentation, β-hemolysis on blood agar.

• Treatment: Trimethoprim, doxycycline, IV vancomycin for severe cases.

• Prevention: Hand hygiene, infection control in healthcare settings.

Bacterial Skin Infections: Clostridium perfringens

Diseases, Pathogenesis, and Clinical Management

• Agent: Clostridium perfringens, anaerobic Gram-positive, endospore-forming rods.

• Diseases: Gas gangrene, cellulitis, foodborne disease.

• Virulence factors: α-toxin (phospholipase C), other toxins and enzymes causing tissue necrosis.

• Diagnosis: Clinical observation, bacteriologic studies.

• Treatment: Debridement, penicillin, hyperbaric oxygen.

• Prevention: Hand hygiene, wound care.

Summary Table: Exanthems and Enanthems

Name	Exanthem & Enanthem	Pearls
Measles (Rubeola)	Maculopapular rash, Koplik spots	Rash after 4 days of flu-like symptoms
Rubella	Pale pink maculopapular rash, Forchheimer spots	Rash after 1-2 days of mild symptoms; CRS in pregnancy
Erythema Infectiosum (Parvovirus B19)	Bright red cheeks, lacy rash	Most contagious prior to rash; affects red cell precursors
Roseola (HHV-6/7)	Pink maculopapular rash after high fever	Rash appears suddenly when fever stops

Summary Table: Fifth Disease, Roseola, Hand, Foot, and Mouth Disease

Disease	Causative Agent	Epidemiology	Transmission & Prevention	Signs & Symptoms	Pathogenesis & Features	Diagnosis & Treatment
Fifth Disease	Parvovirus B19	Common in children 5-15 years	Respiratory droplets	Slapped cheek rash	Affects red cell precursors	Clinical diagnosis; supportive therapy
Roseola	HHV-6/7	Infants, young children	Respiratory droplets	High fever, sudden rash	Virus persists in salivary glands	Clinical diagnosis; supportive therapy
Hand, Foot, and Mouth Disease	Coxsackievirus A16	Children under 5	Respiratory, fecal-oral	Vesicular rash on hands, feet, mouth	Virus replicates in oral mucosa	Clinical diagnosis; supportive therapy

Clinical Case Studies

Application of Knowledge

• Case 1: Transmission of chickenpox without direct contact—airborne spread.

• Case 2: Bright red cheek rash in a child—likely parvovirus B19 (erythema infectiosum).

• Case 3 & 4: Cellulitis and abscess formation—diagnosis and management of staphylococcal and streptococcal skin infections, including antibiotic resistance considerations.

Key Concepts and Takeaways

• Skin infections are caused by a wide range of pathogens, each with distinct clinical features and management strategies.

• Recognition of characteristic rashes and lesions is essential for diagnosis.

• Prevention relies heavily on vaccination, hygiene, and infection control.

• Antibiotic resistance is a growing concern in bacterial skin infections.