Ch. 21 - Microbial Diseases of the Skin and Eyes

Tortora - Microbiology: An Introduction 14th Edition

Tortora14th EditionMicrobiology: An IntroductionISBN: 9780138200398Not the one you use?Change textbook

Tortora 14th Edition

Ch. 21 - Microbial Diseases of the Skin and Eyes

Problem 2

Chapter 21, Problem 2

A 6-year old girl was taken to the physician for evaluation of a slowly growing bump on the back of her head. The bump was a raised, scaling lesion 4 cm in diameter. A fungal culture of material from the lesion was positive for a fungus with numerous conidia.
Besides the scalp, this disease can occur on all of the following except
a. Feet.
b. Nails.
c. The groin.
d. Subcutaneous tissue.
e. The disease can occur on all of these areas.

Verified step by step guidance

Step 1: Identify the disease based on the clinical presentation and fungal culture results. A slowly growing, raised, scaling lesion on the scalp with numerous conidia in culture suggests a dermatophyte infection, commonly known as tinea capitis when on the scalp.

Step 2: Understand that dermatophytes infect keratinized tissues such as skin, hair, and nails. They typically cause superficial infections and do not invade deeper tissues like subcutaneous tissue.

Step 3: Review the common sites of dermatophyte infections: tinea pedis (feet), tinea unguium (nails), tinea cruris (groin), and tinea capitis (scalp). These are all superficial infections involving keratinized layers.

Step 4: Recognize that subcutaneous tissue infections are not caused by dermatophytes but rather by other fungi or pathogens that invade deeper layers, so this site is an exception for this disease.

Step 5: Conclude that the disease can occur on feet, nails, and groin but not in subcutaneous tissue, which helps answer the question about the exception.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Dermatophytosis and Its Clinical Presentation

Dermatophytosis is a fungal infection caused by dermatophytes that affects keratinized tissues like skin, hair, and nails. It commonly presents as scaling, raised lesions, often with hair loss on the scalp (tinea capitis). Understanding its typical clinical features helps identify the infection and differentiate it from other skin conditions.

Sites of Dermatophyte Infection

Dermatophytes infect areas rich in keratin such as the scalp, feet (tinea pedis), nails (onychomycosis), and groin (tinea cruris). These fungi do not invade deeper tissues like subcutaneous tissue because they require keratin for growth. Recognizing the typical infection sites is essential for diagnosis and treatment.

Fungal Culture and Identification of Conidia

Fungal culture is used to identify dermatophytes by growing the fungus from clinical samples. The presence of numerous conidia (asexual spores) is characteristic of dermatophytes and helps confirm the diagnosis. This laboratory method is crucial for distinguishing fungal infections from other causes of skin lesions.

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Discuss the usual mode of entry of bacteria into the skin. Compare bacterial skin infections with infections caused by fungi and viruses with respect to mode of entry.

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A 12-year old boy had a fever, rash, headache, sore throat, and cough. He also had a macular rash on his trunk, face, and arms. A throat culture was negative for Streptococcus pyogenes.

The boy most likely had

a. Streptococcal sore throat.

b. Measles.

c. Rubella.

d. Smallpox.

e. Hand-foot-mouth disease.

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Textbook Question

A 6-year old girl was taken to the physician for evaluation of a slowly growing bump on the back of her head. The bump was a raised, scaling lesion 4 cm in diameter. A fungal culture of material from the lesion was positive for a fungus with numerous conidia.

The girl’s disease was

a. Rubella.

b. Candidiasis.

c. Dermatomycosis.

d. A cold sore.

e. None of the above.

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On the following figure, show the sites of the following infections: impetigo, folliculitis, acne, warts, shingles, sporotrichosis, pediculosis.

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What bacteria are identified by a positive coagulase test? What bacteria are characterized as group A beta-hemolytic?

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