skin and soft tissue infections I - Kozel Flashcards
How are infections of the skin classified?
- Pyoderma
- Subcutaneous tissue infection
- Secondary infections complicating pre-existing skin lesions
- Cutaneous involvement in systemic infection
- Scarlet fever syndromes
- Wound infections
What is pyoderma?
Any skin infection that is pyogenic or pus-forming. Primary pyodermas can be purulent or non-purulent.
What is the definition of pyogenic?
Pus forming or pus producing.
What is the definition of purulent?
Consisting of, containing or discharging pus.
Name some primary pyodermas.
- Impetigo - bullous and non-bullous
- Ecthyma
- Cutaneous abcess
- Folliculitis, carbuncles and furuncles
- Chancriform lesions
What is impetigo?
It is a pyoderma that causes a superficial infection of the skin which presents with vesicles and later forms a honey-colored crust.
Describe non-bullous impetigo.
- begins as erythematous papules that evolve into vesicles and pustules that later rupture
- the ruptured vesicles form a honey-colored crust on an erythematous base
- infection is superficial so it typically heals without scarring
What bacteria are responsible for impetigo?
- Staph aureus is the most common cause
- Strep pyogenes causes about 20-30% of cases
- mixed infections are possible
How is bullous impetigo different from non- bullous?
Bullous impetigo is caused by a strain of S. aureus that produces an exfoliating toxin. This toxin cleaves the dermal-epidermal junction and causes a blister.
What is Ecthyma?
This infection is an ulcerative pyoderma of the skin.
Describe some characteristics of Ecthyma.
- a deeper form of impetigo
- follows insect bites or minor trauma
- heals with scarring since it affects deeper tissues
- caused by S. aureus and/or S. pyogenes
What is a cutaneous abscess?
A primary pyoderma that is purulent. It is characterized by collections o cpus within the dermis and deeper tissues. Typically caused by S. aureus but can be polymicrobial.
What is folliculitis?
A pyodermic infection within a hair follicle.
What is a Furuncle or boil?
A pyodermic infection that is characterized by an inflammatory nodule extending into subcutaneous tissue. It follows folliculitis.
What is a carbuncle?
A pyodermic infection that is characterized by a coalescing involving multiple follicles or furuncles.
What microbe causes folliculitis, carbuncles and furuncles?
S. aureus.
What is a chancriform lesion? Give examples.
A chancriform lesion is an ulcerative, pyodermic, purulent lesion. Examples are cutaneous anthrax, some venereal infections and some other infections also, such as those caused by Francisella tularensis, Mycobacterium ulcerans, and Mycobacterium marinum.
Describe cutaneous anthrax.
- caused by direct inoculation with Bacillus anthracis
- begins as a painless pruritic papule that enlarges and vesiculates
- the malignant pustule becomes necrotic and is covered by an eschar
- can cause periorbital edema if on the face and local edema at site of inoculation - due to edema factor
What are some microbes that cause chancriform lesions in venereal infections?
- Treponema pallidum
2. Haemophilus ducreyi - not really found in US, mainly in Africa where it was associated with HIV infection
What are Erysipelas and cellulitis?
These are diffuse, superficial spreading skin infections. These types of infections are not associated with pus.
Would you drain a lesion caused by Erysipelas or cellulitis?
No. These infections are not associated with pus. They require antimicrobial therapy to heal whereas a purulent lesion often heals after draining.
Describe some characteristics of Cellulitis.
- infection involving upper dermis and subcutaneous fat
- follows previous trauma, often minor or underlying skin lesion such as a furuncle or ecthyma
- lesions are painful, erythematous and often present as very red, hot and swollen
- Caused mainly by Group A strep but sometimes S. aureus
- very rarely can involve other bacteria - especially if history includes trauma, water contact or animal insect or human bites
Describe Erysipelas.
- superficial cellulitis with prominent lymphatic involvement
- painful, sharp demarcation from adjacent normal skin
- almost entirely caused by S. pyogenes
Name some different types of subcutaneous tissue infections.
- necrotizing fasciitis
- synergistic necrotizing cellulitis
- clostridial anaerobic cellulitis
- misc. infections secondary to trauma
Describe necrotizing fasciitis.
- aggressive subcutaneous infection that tracks along the superficial fascia - all tissues between skin and underlying muscle
- most often an extension from a skin lesion
- causes systemic toxicity
- can involve S. pyogenes, S. aureus, Vibrio vulnificus, Aeromonas hydrophila and can be polymicrobial
Name some infections that are examples of systemic bacterial infections that produce rashes or skin lesions.
- bacteremia - caused by S. aureus, group A strep, N. meningitidis
- Leptospirosis or Weil’s disease - caused by Leptospira interrogans
- Rat-bite fever - caused by Streptobacillus monofiliformis
- Annular erythema seen in Lyme’s disease - caused by Borrelia burgdorferi
- Rocky mountain spotted fever - caused by Rickettsia rickettsii
What are some toxin-induced skin reactions?
- Scarlett fever - follows pharyngitis by Group A strep, caused by streptococcal pyogenic exotoxin or SpeA
- Scalded skin syndrome - follows local infection by S. aureus, caused by Staphylococcal exfoliating toxin
- Toxic shock syndrome - follows infection by S. aureus, caused by Staphylococcal TSST-1 super- antigen
Name some infections that can follow animal contact.
- cutaneous anthrax - vetrinarians at risk, caused by Bacillus anthraces
- Cat-scratch fever - caused by Bartonella henselae, follows cat scratch or bite
- Pasteurella multicoda causes infection after dog or cat bites
- Bubonic plague - caused by Yersinia pests
- Tularemia - caused by Francisella tularensis
- Erysipeloid - caused by Erysipelothrix rhusiopathiae, follows exposure to infected fish, marine animals, swine or poultry
Name some microbes that cause wound infections.
- Serratia marcesens - infects surgical wounds, found in hospitals and moist environments
- Aeromonas spp.- infects cuts and abrasions, found in fresh water
- Pseudomonas aeruginosa - infects burn wounds and is found in the environment
- Vibrio parahemolyticus - infects cuts and abrasions, found in salt water and brackish water
- Vibrio vulnificus - infects cuts and abrasions, found in salt water
- Acinetobacter baumanii - infects battlefield injuries, found in hospitals
- Eikenella corrodens - infects human bite wounds and fistfight wounds, found in the human mouth
What is Pyomyositis?
An infection characterized by presence o cpus within individual muscle groups. 90% of cases caused by S. aureus and most cases are in the tropics.
What is Myonecrosis/gas gangrene?
- an infection characterized by necrotic damage to muscle tissue
- occurs after muscle injury and contamination with soil or other material containing spores
- involves extreme pain, crepitus and yellowish-bronze discoloration
- most commonly caused by Clostridium perfringens and other Clostridial species
Name some general features of streptococci.
- gram-positive cocci that form chains or pairs
- most are facultative anaerobes
- typically require complex media
- are catalase-negative
How are streptococci classified?
- hemolytic patterns on blood agar
- antigens - Lancefield grouping
- biochemical/physiological properties
What are the two general groups of streptococci?
- B-hemolytic - classified by Lancefield grouping
2. a-hemolytic and y-hemolytic streptococci - classified by biochemical testing
Antigens of Streptococci are usually what kinds of structures?
Polysaccarhides.
What is one test that can differentiate between Staph and Strep?
The catalase test. Staph are catalase positive and Strep are catalase negative.
What is alpha hemolysis?
When some bacteria are put on a blood agar, they cause reduction of hemoglobin of the RBC’s. This will show up in the growing colony as a greening of the surrounding agar.
Which Strep species are alpha hemolytic?
- S. salivaris
- S. mitis
- both of these are normal flora of mucous membranes
What is Beta hemolysis?
When some bacteria are grown on blood agar they cause a complete hemolysis of the RBC’s under and around them. This will look like a lightening or clearing on the agar.
What toxins are associated by Beta hemolysis?
- Streptolysin O
2. Streptolysin S
What is gamma hemolysis?
This is the absence of hemolysis when some bacteria are grown on blood agar. Some miscellaneous normal flora, opportunistic bacteria and anaerobic bacteria are classified as gamma hemolytic.
What is Lancefield grouping?
A classification system based on type of carbohydrate antigen present in the cell walls of primarily Beta hemolytic streptococci (some are alpha or gamma hemolytic).
What are the Lancefield groups?
- Group A - S. pyogenes (most are human pathogens)
- Groups B,C,F,G,H,L and L - normal mucosal flora that are occasional pathogens
- Groups E, M and N - found in lower animals
- Group D - Enterococcus faecalis
How are streptococci in the same Lancefield group differentiated?
Via their biochemical properties.
What Lancefield group are strep pyogenes in?
Group A. Group A strep is another name for S. pyogenes. The carbohydrate antigen on it’s surface is called the C carbohydrate.
Describe the C carbohydrate.
This is the name for the carbohydrate antigen found in the matrix of the cell wall of S. pyogenes. It is a polymer of rhamnose and N-acetyl glucosamine.