MICR5832 L4: Skin, Soft Tissue & Wounds Infection 8/4/25

What layer of the skin is this?

-Layers of squamous epithelial cells

-Keratinized outer layer called stratum corneum

Epidermis

What is this?

-Tough, dry barrier to invading pathogens

-Keratinized outer layer of epidermis

Stratum corneum

What layer of the skin is this?

-Connective tissue, blood vessels, nerves

-Contains sweat gland ducts, hair follicles and oil gland ducts producing sebum

Dermis

What layer of skin lies between the Dermis and Fascia (Muscle)?

Subcutaneous Tissue

What layer of the skin is this?

-Fibrous sheets of connective tissue

-Barrier to infection

Fascia of muscle

What are some normal flora that competitively inhibit pathogenic bacteria?

-Corynebacterium and other diphtheroids

-Coagulase-negative Staphylococcus

-Propionibacterium

What is this?

-Metabolises sebum to toxic fatty acids

-Inhibits growth of non-skin bacteria

-Associated with acnes, opportunistic pathogen

Propionibacterium acnes

True or False: Corynebacterium diphtheriae is a normal flora

False

What do these skin secretions found in sebum/sweat help to do?

1) Fatty Acids

2) NaCl

3) Lysozyme

4) Defensins

1) Low pH

2) Inhibitory

3) Disrupt cell wall

4) Disrupt cell membrane

1) Low pH

2) Inhibitory to bacteria

3) Disrupts cell wall

4) Disrupts membranes

What is this?

-Group of inflammatory skin disorders caused by bacteria that produce pus

-Examples: Impetigo, Cellulitis, Folliculitis, Erysipelas, Furuncle, Carbuncle

Pyoderma

What is this?

-Emerging coagulase-negative pathogen causing pyodermas

-Severe invasive disease, septicaemia, wound infections, and abscesses

Staphylococcus lugdunensis

What Hair Follicle Infection is this?

-Minor infection of hair follicle

-Blockage of hair follicle with sebum

-Minor trauma, friction from rubbing

Agents:

-Staphylococcus aureus, Enterobacteriaceae, Pseudomonas aeruginosa

Folliculitis

Folliculitis is caused by what bacteria?

-Staphylococcus aureus

-Enterobacteriaceae

-Pseudomonas aeruginosa

Where is Pseudomonas aeruginosa commonly found?

-Swimming pools, hot tubs

-Rarely causes Folliculitis

What Epidermis/Dermis Infection is this?

-Dermis/Subcutaneous Tissue infection (superficial parts)

-Red, swollen, painful, indurated patches with raised margin

-Local lymphadenitis, often fever

Agents:

-GAS, BCG Strep, Staph Aureus

Erysipelas

Erysipelas is caused by what bacteria?

-Group A Streptococcus

-B, C, G Streptococci

-Staph Aureus

True or False: Fever associated with Erysipelas is rare and indicates life-threatening bacteremia

True

What groups of people tend to get Erysipelas?

Infants/Elderly

What Epidermis/Dermis Infection is this?

-"School sores" that are highly infectious, especially to children

-Red, bullous or non-bullous lesions

-Painless, do not scar

-May have local lymphadenitis

Agents:

-Staph aureus, Strep pyogenes, GAS

Impetigo

Impetigo is caused by what bacteria?

-Staph aureus (Bullous)

-Strep pyogenes (Non-bullous)

True or False: Impetigo is most common in cold, dry climates and spreads via body fluids and aerosols

False, it is common in hot humid climates and spreads via direct contact with skin or clothing

What Epidermis/Dermis Infection is this?

-Strep pyogenes antibodies cross-react with glomerular basement membrane after skin infection

-Autoimmune inflammatory disease of kidneys

-Leads to kidney failure, dialysis and renal transplant

Glomerulonephritis

Glomerulonephritis is caused by what bacteria?

Streptococcus pyogenes

What is another example of Type III immune reaction where streptococcal M proteins cause antibodies to crossreact with the heart tissue?

Rheumatic fever

What Epidermis/Dermis Infection is this?

-Skin peels off in sheets due to secretion of exfoliative toxin

-Skin, wound or more generalised infection

-Usually affects infants & neonates and immunosuppressed patients

Agents:

-Staph aureus

Scalded Skin Syndrome (SSS)

Scalded Skin Syndrome (SSS) is caused by what bacteria?

Staphylococcus aureus

True or False: Epiderminal/dermal SSS can progress to subcutaneous infection

True

What Epidermis/Dermis Infection is this?

-Localized collection of pus

-Can penetrate lower in hair follicles

Agent:

-Staphylococcus aureus

Abscesses

What Epidermis/Dermis Infection is this?

-Boil or abscess that starts in hair follicle

-Becomes painful and full of pus

Agent:

-Staphylococcus aureus

Furuncle

What Epidermis/Dermis Infection is this?

-Tissue infection surrounding a boil progresses into dermis/subcutaneous tissue

-Often a cluster of boils

-Fever, malaise

Agent:

-Staphylococcus aureus

Carbuncle

Abscesses, Carbuncles and Furuncles are caused by what bacteria?

Staphylococcus aureus

What Epidermis/Dermis Infection is this?

-Common spreading infection of connective tissue/deeper layers of skin

-Red, flat, painful, local lymphadenitis

-Poorly defined margins, warmth of area

-Pus drainage, fever, malaise may occur

Agents:

-Strep pyogenes, Staph aureus, Aeromonas hydrophila, Vibrio, Hemophilus influenzae

Cellulitis

Cellulitis is caused by what bacteria?

-Streptococcus pyogenes

-Staphylococcus aureus

-Aeromonas hydrophila

-Vibrio

-Hemophilus influenzae

Your patient has cellulitis. What bacteria caused the infection?

1) They got it from marine/freshwater

2) Patient is an unvaccinated child with facial cellulitis

1) Vibrio

2) Hemophilus influenzae

Are these abscess bacteria most likely aerobic or anaerobic?

-Perineal, inguinal & buttock abscesses

Anaerobic

Are these abscess bacteria most likely aerobic or anaerobic?

-Non-perineal

Facultative and obligately aerobic

What Subcutaneous Infection is this?

-Often in extremities of patients with poor circulation

-Diabetes mellitus, poor blood supply w/ venous insufficiency

-Acute cellulitis and lymphangitis associated with it

Agent:

-Staphylococcus aureus, but usually mixed (3-5 including anaerobes)

Ulcers

Ulcers are caused by what bacteria?

-Staphylococcus aureus

-Typically mixed with 3-4 other microbes including anaerobes

Why is an ulcer more likely to be polymicrobial if it infects the subcutaneous tissue?

-Not salty or dry

-More microbes grow there

What Subcutaneous Infection is this?

-Ulcer that progresses to infect underlying bone

Osteomyelitis

What diseases are ulcers associated with?

-Acute cellulitis

-Lymphangitis

What diseases cause a risk factor for ulcers?

-Diabetes mellitus due to poor circulation/venous insufficiency

-Poor blood supply, can't get immune cells to key sites

What Subcutaneous Infection is this?

-Decubitous ulcers found in sacral area of bedridden patients

-Poor circulation -> Ulcer -> Infection w/ bowel flora

Agents:

-Bacteroides fragilis, Enterobacteriaceae, Staph aureus, Ps. aeruginosa

Pressure sores

Pressure sores are caused by what bacteria?

1) Anaerobes (Hint: Gram Negative pleiomorphic rods)

2) Facultative Anaerobes

3) Nosocomial pathogens

-Bacteroides fragilis

-Enterobacteriaceae

-Staphylococcus aureus, Pseudomonas aeruginosa

What happens if the following epidermal/dermal infections progress to subcutaneous infections?

1) Erysipelas

2) Folliculitis

3) Cellulitis

1) Erysipelas → Subcutaneous cellulitis -> Streptococcal necrotising fasciitis

2) Folliculitis -> Cellulitis or Furuncle

3) Cellulitis often extends to Subcutaneous Tissue -> Anaerobic Cellulitis associated with subcutaneous gas production

What Subcutaneous Infection is this?

-Often localized in extremities, can be post-operative infection

-Common in patients w/ diabetes and poor blood circulation

-Bacteremia not usually present

Agents (Usually mixed anaerobes + aerobes):

-Staph. aureus, Strep. pyogenes, non-haemolytic Streptococci, Enterobacteriaceae

-Peptostreptococcus, Bacteroides fragilis, Clostridium (anaerobes)

Anaerobic Cellulitis

Anaerobic Cellulitis is caused by what bacteria?

Hint: Typically mixed aerobes and anaerobes

-Staph. aureus, Strep. pyogenes, non-haemolytic Streptococci, Enterobacteriaceae

-Peptostreptococcus, Bacteroides fragilis, Clostridium (anaerobes)

What Fascia/Muscle Infection is this?

-Infection of fascia overlying muscle, aka hospital gangrene

-Rapid spread, very serious

Agents:

-Strep pyogenes, Staph aureus, Bacteroides, Clostridium

Necrotizing Fasciitis

Necrotizing Fasciitis is caused by what bacteria?

1) Flesh Eating Bacteria

2) Anaerobes

-Streptococcus pyogenes, Staphylococcus aureus

-Bacteroides, Clostridium

What Fascia/Muscle Infection is this?

-An infrequent postoperative complication after abdominal or thoracic surgery

-Needs broader range of antibiotics due to mixed infection

-Take both aerobe and aerobe culture samples

Agents (Mixed):

-Staph aureus, microaerophilic Streptococci like S. anguinosis

Progressive bacterial synergistic gangrene

Progressive bacterial synergistic gangrene is caused by what bacteria?

-Staph aureus

-Streptococcus anguinosis (microaerophile)

What Fascia/Muscle Infection is this?

-Muscle inflammation, possibly gas gangrene

Agents:

-S. aureus, Clostridium perfringens

Myositis

Myositis is caused by what agents?

1) Acute bacterial myositis, can have skin involvement

2) Gas gangrene

-Staphylococcus aureus

-Clostridium perfringens

What are some common aerobic pathogens introduced into wound infections via surgery/trauma?

-Staph. aureus

-Strep. pyogenes

-Enterobacteriaceae

-Ps. aeruginosa

-Enterococcus

What are some common anaerobic pathogens introduced into wound infections via surgery/trauma?

-Bacteroides

-Clostridium

-Peptostreptococcus (Gram P cocci)

What Wound Infection is this?

-Subcutaneous infections with draining sinuses, cellulitis

Traumatic wound infection with Nocardia

Traumatic wounds are caused by what bacteria?

Hint:

-Brittle texture

-Aerobic and Acid Fast Gram P branching rods

-Slow growth on SAB or routine culture media

Nocardia

How would you identify the species of Nocardia in a traumatic wound?

-PCR amplification/sequencing of 16S rDNA

-EX: MicroSeq 500 system

Sporothrix schenckii is suspected. What medium would you use for this fungi?

Sabouraud Agar

True or False: Nocardia will not show up in a Kinyoun's stain, but it will stain Gram Positive

False, it is both Gram Positive and Acid-Fast

Actinomycetes or Nocardia?

-Gram Positive, long branching filaments similar to fungi

-Aerobe and Acid-Fast

-Pulmonary infections in immunocompromised, like TB

-Cutaneous infections after trauma in immunocompetent

-Can spread to CNS

Nocardia

Actinomycetes or Nocardia?

-Gram Positive, long branching filaments similar to fungi

-Anaerobe, does not show up in Kinyoun's stain

-Normal oral, reproductive, and GI flora

-Causes oral/facial abscesses that drain through sinus tracts

-Often associated with dental caries/extraction

-Yellow "sulfur granules"

-Cause PID with IUDs

Actinomyces

What do you use to treat Nocardia?

Sulfonamides (TMP-SMX)

What do you use to treat Actinomyces?

Penicillin

What Wound Infection is this?

-Gram Positive, Acid-Fast rods

-Identified via Biochemical tests, PCR/Hybridization assay via 16S rDNA like Hain Lifescience

Traumatic wounds w/ Non-TB Mycobacteria

Traumatic wounds are caused by what bacteria?

Hint:

-Gram Positive, acid fast rods

-Grow up to a week on routine culture media

M. abscessus, chelonei, fortuitum

What are some aerobes that cause bite infections from cats/dogs?

-Pasteurella

-Strep mitis

-Staph aureus

What are some anaerobes that cause bite infections from cats/dogs?

-Fusobacterium nucleatum

-Porphyromonas

-Bacteroides testus

What is this?

-Slow-growing Gram Negative capnophilic bacteria

-Rare cause of serious infections from dog saliva

-"Bite of death"

Capnocytophaga canimorsus

Why do bacteria associated with cat/dog bites cause rapidly-spreading cellulitis, abscess formation, and infection of underlying bones/joints?

-Capnophiles (high CO2 lovers)

-Makes it easier to spread

What skin lesion is this?

1) Bullae ulcerate and form black eschar on skin

2) Arising from seafood or seawater exposure

3) Bull's eye lesion

4) Initial rash, papule, ulceration

5) Symptoms depend on species

1) Ecythma gangrenosum

2) Bullae, ulcers, necrosis

3) Erythema migrans

4) Syphilitic chancre

5) Lepros, gummas

What bacteria causes this?

1) Ecythma gangrenosum

2) Bullae, ulcers, necrosis

3) Erythema migrans

4) Syphilitic chancre

5) Lepros, gummas

1) Pseudomonas aeruginosa

2) Vibrio vulnificus

3) Borrelia burgdorferi

4) Treponema pallidum

5) Mycobacteria

Name some zoonoses where the type of rash is determined by the species

1) Rickettsiosis

2) Leptospirosis

3) Bartonellosis

4) Tularemia

How would you collect this specimen?

-Fluid/pus-filled lesions (Boils, Bullae, Carbuncles)

-Erysipelas

-Cellulitis

-Needle aspirate

-Swabs of ruptured lesions (less useful due to skin flora, need to go deep to where anaerobes are)

How would you collect this specimen?

-Subcutaneous infections

Biopsy of tissue at base of nodules/ulcers

How would you collect this specimen?

-Necrotizing Fasciitis

Tissue biopsy, pus aspirate, swab of infected tissue

How would you collect this specimen?

-Wounds

-Needle aspirate from deepest part (best)

-Swabs of open wounds (discouraged)

What transport media would you use for swabs, considering anaerobes lose viability after 48 hours?

-Amie's Medium if aerobic

-Stuarts Medium if anaerobic (up to 24hrs viable)

What anaerobic transport media would you use for liquids, aspirates and biopsies?

-Vacutainer

-Oxygen-elimination system

-Activated by depressing the plunger

-Includes a color change indicator

What transport media would you use for anaerobic liquid specimens?

-Port-A-Cul

-Vial contains anaerobic atmosphere

-Specimen injected through rubber stopper

-Resazurin redox indicator turns pink-lavender when oxidized

What transport media would you use for anaerobic tissue biopsies?

-GasPack Pouch

-Pouch generates anaerobic atmosphere

How would you process a specimen?

1) Gram stain on direct smear

2) Kinyoun stain for acid-fast bacilli (Nocardia/Mycobacterium)

3) Selective media to isolate polymicrobial infections

What are some good media for superficial lesions with aerobes/facultative anaerobes?

-Blood agar (BA)

-Colistin-Nalidixic Acid Agar (CNA)

-MacConkey Agar (MAC)

-Chocolate Agar (less common)

-Incubate at 37°C with 5% CO2 for 48h

What routine culture medium would you use to culture anaerobes from wounds and deeper lesions?

-CDC Anaerobe Blood Agar (ABA)

-KV Agar (Kanamycin + Vancomycin + ABA)

What anaerobic routine culture medium is this?

-Nonselective medium supports growth of most anaerobes

-TSA (Tryptic soy agar) base with Vitamin K1, Hemin and Sheep blood

-Prepared and packaged under O2-free conditions

-Incubate 37°C, anaerobic atmosphere, 48h

CDC Anaerobe Blood Agar (ABA)

What anaerobic routine culture medium is this?

-Selects against facultative Gram Negative anaerobes (Kms) and Gram Positive bacteria (Vans)

-Useful in primary isolation of Gram Neg obligate anaerobes

-Incubate 37°C, anaerobic atmosphere, 48h

KV Agar (Kanamycin + Vancomycin + ABA)

What would you do if you found out your routine culture was Mycobacteria or Nocardia due to finding Gram positive acid fast bacteria?

Nocardia:

-Incubate up to 7 days

-SAB Dextrose Agar culture

Mycobacterium:

-Incubate up to 7 days

Non culture-based tests:

-Serum antibody tests, PCR assays, DNA probes for M. tuberculosis

Scalded skin syndrome is caused by:

A. Ps aerogenosa

B. Group A streptococcі

C. Aeromonas hydrophilia

D. Staphylococcus aureus

D. Staphylococcus aureus

Nocardia is:

A. An obligate anaerobe

B. Weakly positive for the the Kinyoun stain for acid-fast bacilli

C. Common skin commensal

D. Usually found in water

B. Weakly positive for the the kinyoun stain for acid fast bacilli

Describe how would process a swab sample for identification of fastidious anaerobic bacteria

-Specimen should be collected using a sterile swab.

-Place into an anaerobic transport medium (Amie's, or Stuart's will last up to 24hr).

-Store and transport at room temperature, as low temperatures can increase oxygen diffusion.

-Aim to get the sample to the lab within 2 hours.