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

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

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Understand that the coagulase test is used to differentiate Staphylococcus species based on their ability to produce the enzyme coagulase, which clots plasma. A positive coagulase test indicates the presence of Staphylococcus aureus, a pathogenic species known for this trait.

Recognize that group A beta-hemolytic bacteria refer to a classification based on hemolysis patterns on blood agar and Lancefield grouping. Beta-hemolysis means complete lysis of red blood cells around colonies, producing a clear zone.

Recall that group A beta-hemolytic bacteria are primarily Streptococcus pyogenes, which is known for causing various infections such as pharyngitis, impetigo, and rheumatic fever.

Summarize that a positive coagulase test identifies Staphylococcus aureus, while group A beta-hemolytic bacteria are identified as Streptococcus pyogenes based on their hemolytic pattern and Lancefield grouping.

Note the clinical relevance: Staphylococcus aureus (coagulase positive) is often associated with skin infections and abscesses, whereas Streptococcus pyogenes (group A beta-hemolytic) is linked to throat infections and post-infectious complications.

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Key Concepts

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

Coagulase Test

The coagulase test detects the presence of the enzyme coagulase, which causes blood plasma to clot. It is primarily used to differentiate Staphylococcus aureus, which is coagulase-positive, from other Staphylococcus species that are coagulase-negative. This test helps identify pathogenic bacteria involved in infections.

Group A Beta-Hemolytic Streptococci

Group A beta-hemolytic streptococci (GAS) are bacteria classified by their ability to completely lyse red blood cells on blood agar, producing clear zones of hemolysis. The most common species is Streptococcus pyogenes, known for causing strep throat, skin infections, and other diseases. Lancefield grouping identifies these bacteria based on carbohydrate antigens.

Hemolysis Patterns on Blood Agar

Hemolysis refers to the breakdown of red blood cells by bacterial enzymes on blood agar plates. Beta-hemolysis indicates complete lysis, alpha-hemolysis partial lysis (greenish discoloration), and gamma-hemolysis no lysis. Recognizing these patterns aids in bacterial identification and understanding pathogenic potential.

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Related Practice

Textbook Question

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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Textbook Question

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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Textbook Question

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

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Textbook Question