Study of Skin Infections

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Questions and Answers

What is the primary role of catalase produced by Staphylococcus aureus?

To lyse white blood cells

To degrade collagen

To neutralize hydrogen peroxide (correct)

To enhance phagocytosis

Which enzyme allows staphylococci to spread through subcutaneous tissue?

Collagenase (correct)

Exfoliative toxin

Catalase

β-lactamase

What do the exotoxins produced by staphylococci primarily target?

Collagen tissues

Human red blood cells (correct)

Mucopolysaccharides

Pathogen recognition receptors

What syndrome is associated with exfoliative toxins in infants?

Scalded skin syndrome Signup and view all the answers

What role does the stratum corneum play in skin defense?

It provides a waterproof barrier and sheds dead skin cells. Signup and view all the answers

What is the function of hyaluronidase produced by Staphylococcus aureus?

To dissolve hyaluronic acid Signup and view all the answers

Which factor does NOT help organisms enter the skin?

Sweat glands producing excessive sweat. Signup and view all the answers

What unique component is involved in the action of Panton–Valentine leukocidin?

Bacteriophage encoded components Signup and view all the answers

How do β exotoxins exert their toxic effect?

By degrading sphingomyelin Signup and view all the answers

What is a primary component of sebum that contributes to skin defense?

Sapienic acid Signup and view all the answers

Which of the following enzymes produced by staphylococci is responsible for fibrinolysis?

Staphylokinase Signup and view all the answers

Which type of bacteria primarily makes up the normal microbiota of the skin?

Gram-positive cocci Signup and view all the answers

What is the result of vigorous washing of the skin?

Reduction in microbiota numbers but not elimination. Signup and view all the answers

Which organism is associated with the condition known as dandruff?

Malassezia furfur Signup and view all the answers

What contributes to body odor in moist areas of the skin?

Microbial metabolism of sweat gland secretions. Signup and view all the answers

What type of growth environment do skin microorganisms prefer?

Moist areas with high salt concentration. Signup and view all the answers

Which clinical manifestation is NOT associated with S. aureus infections?

Chickenpox Signup and view all the answers

What is the primary purpose of the catalase test in laboratory diagnosis?

To assess oxygen production from hydrogen peroxide Signup and view all the answers

What result would indicate a positive coagulase test?

Clot formation within 1-4 hours Signup and view all the answers

Which medium is used to observe pigmented colonies of S. aureus after 48 hours?

Salt Mannitol agar Signup and view all the answers

What type of gram-staining result is typical for staphylococci?

Gram-positive cocci in clusters Signup and view all the answers

Which sugar does S. aureus NOT ferment?

Lactose Signup and view all the answers

What does a positive Citrate test indicate?

Production of alkaline pH from citrate utilization Signup and view all the answers

Which characteristic distinguishes blood agar in culturing S. aureus?

Beta hemolysis due to hemolysin enzyme Signup and view all the answers

What is one of the main features of Staphylococcus aureus morphology?

Spherical cells forming irregular clusters Signup and view all the answers

Which media is selective for Staphylococcus aureus due to its mannitol fermenting ability?

Mannitol Salt Agar (MSA) Signup and view all the answers

What kind of hemolysis does Staphylococcus aureus exhibit on blood agar?

Beta hemolysis Signup and view all the answers

Which enzyme produced by S.aureus is associated with clotting oxalated plasma?

Coagulase Signup and view all the answers

Which characteristic enables Staphylococcus aureus to resist opsonization?

Lysozyme resistant cell wall Signup and view all the answers

What is a key response stimulated by S.aureus infection in the skin?

Attraction of macrophages and neutrophils Signup and view all the answers

Which of the following correctly describes a cultural characteristic of Staphylococcus aureus?

Forming gray to deep golden yellow colonies Signup and view all the answers

What type of toxin secretion is most commonly associated with Staphylococcus aureus to combat phagocytic cells?

Exotoxins Signup and view all the answers

What color change indicates that an organism can utilize citrate in Simmons citrate agar?

Green to blue Signup and view all the answers

What is the purpose of the Durham tube in carbohydrate fermentation tests?

To capture gas produced during fermentation Signup and view all the answers

Which of the following conditions will cause phenol red in broth media to turn yellow?

Production of acid from carbohydrate fermentation Signup and view all the answers

What type of bacteria is typically differentiated using Phenol Red Broth?

Gram-negative enteric bacteria Signup and view all the answers

What is the main biochemical purpose of the urease test?

To differentiate enteric bacilli based on urea splitting Signup and view all the answers

Which of the following carbohydrates is not commonly tested using Phenol Red Broth?

Fructose Signup and view all the answers

If an organism cannot utilize the carbohydrate in Phenol Red Broth but can use peptone, what will occur?

The medium will turn fuchsia Signup and view all the answers

During the carbohydrate fermentation procedure, at what temperature should the tubes be incubated?

37 °C Signup and view all the answers

Study Notes

Introduction to Skin Infections

Skin is the body's largest organ, playing a crucial role in protection and homeostasis.
Normal skin temperature is less than 37°C.
Composed of multiple layers: epidermis (outer waterproof barrier), dermis (contains connective tissues, hair follicles, sebaceous and sweat glands), and hypodermis (fat and connective tissue).

Skin as a Barrier

The outermost layer, stratum corneum, sheds dead skin cells, preventing long-term microbial settlement.
Sweat contains lysozyme, which targets bacterial cell walls.
Sebaceous glands secrete sebum, which provides lubrication and contains sapienic acid that combats some gram-positive bacteria.
Entry points for pathogens include skin pores, wounds, and bites.

Normal Microbiota of Skin

Skin hosts microorganisms resistant to drying and high salt levels, mainly gram-positive cocci like staphylococci and micrococci.
Vigorous washing reduces but does not eliminate these microbial populations, which quickly restore.
Moist areas such as armpits harbor more microbes, contributing to body odor.
Cutibacterium acnes, an anaerobic diphtheroid, thrives in hair follicles, associated with acne.

Dandruff and Skin Conditions

Malassezia furfur is a yeast that thrives on oily skin and contributes to dandruff, treated with antifungal agents like ketoconazole.

Staphylococcus aureus

Morphology: Spherical, about 1 μm in diameter, arranged in clusters, gram-positive, nonmotile, and encapsulated.
Growth conditions: Prefers 37°C but yields deep golden yellow colonies at lower temperatures; commonly demonstrates beta hemolysis on blood agar.
Uses selective media like Mannitol Salt Agar for isolation, indicating mannitol fermentation with yellow colonies.

Pathogenesis of S. aureus

Triggers inflammatory responses, attracting macrophages and neutrophils, while producing toxins to evade immune detection.
Resistant to opsonization and can survive within phagosomes.
Key virulent factors include coagulase, catalase, hyaluronidase, and exotoxins.

Virulence Factors: Enzymes and Toxins

Coagulase: Forms fibrin around bacteria, aiding in evasion from phagocytosis.
Catalase: Dismantles hydrogen peroxide produced by phagocytes, enhancing survival.
Hyaluronidase and collagenase facilitate the spread of S. aureus through tissues.
Exotoxins include various toxins (α, β, γ, δ) that damage cell membranes and lyse white blood cells.
Panton-Valentine Leukocidin specifically targets and kills white blood cells.

Clinical Manifestations of S. aureus Infections

Includes abscesses, painful pimples, boils, impetigo, cellulitis, scalded skin syndrome, and folliculitis.

Laboratory Diagnosis

Specimen Collection: Swabs from abscesses or pus, transported in Tryptic Soy Broth.
Smears: Stain shows gram-positive cocci in clusters.
Culturing: Grows on blood agar exhibiting beta hemolysis; Mannitol Salt Agar indicates S. aureus by yellow colonies.
Catalase Test: Produces bubbles in the presence of hydrogen peroxide, confirming staphylococci presence.
Coagulase Test: Clotting in plasma indicates S. aureus presence, with results visible within 1–4 hours.
Biochemical Tests: Can ferment multiple sugars like mannitol, maltose, and sucrose. Methyl Red and Voges-Proskauer tests yield positive results, indicating fermentation pathways.

Citrate and Carbohydrate Fermentation Tests

Citrate Test: Assesses ability to utilize citrate as carbon source, indicated by a color change from green to blue.
Carbohydrate Fermentations: Uses phenol red broth with various sugars; acid production turns the medium yellow, while gas production is captured in Durham tubes.

Urease Test

Identifies the ability to hydrolyze urea into ammonia through urease production, differentiating enteric bacilli.

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Description

This quiz explores the fascinating functions and layers of the skin, the body's largest organ. It covers topics such as temperature regulation, homeostasis, and the skin's role as a barrier against microbial invasion. Test your knowledge on skin infections and the importance of skin health.