Skin and Soft Tissue Infections

• Infections occurring in the skin, soft tissues, bones, and joints
• Normal flora of skin are a barrier separating outer environment and inner tissues
• Skin's normal microflora has a protective function by producing bacteriocines

Normal Microflora of Skin:

• Dry locations: predominantly coagulase-negative Staphylococcus (S. epidermidis)
• Wet locations: diverse flora, coagulase-negative Staphylococcus, coagulase-positive Staphylococcus, Corynebacterium, Propionibacterium, Candida

Types of Skin Infections:

• Primary: morphologies and courses, initiated by single organisms, occurring in normal skin
• Secondary: occur as a complication of diseased skin
• Systemic: infections with manifestation on skin

Primary Bacterial Skin Infections:

• Folliculitis: infection and inflammation of a hair follicle; the causative agent is Staphylococcus aureus
• Furuncle (boil): a deep folliculitis lesion filled with pus and necrotic debris, painful and swollen; therapy consistis of pus release with drainage
• Rare complications of furuncles are spread to other locations, like bacteremia, osteomyelitis, or brain abscess
• Carbuncle: a cluster of furuncles
• Impetigo: common in children, less frequent in adults, typically localized on the face
• Transmission occurs through human contact
• Bullous impetigo causes fluid-filled blisters, which later turn into yellowish scabs
• Contagious impetigo presents as a red sore with pus before scabbing
• Diagnosis is by swab cultivation on blood agar; causative agents are Staphylococcus aureus and Streptococcus pyogenes

Borreliosis (Lyme Disease):

• Caused by Borrelia burgdorferi, a spirochete transmitted by ticks (Ixodes ricinus)
• Reservoir is other mammals
• Early stage: dermal phase with an expanding erythematous rash (erythema migrans) at the tick bite site, growing 2-3 cm per day
• Late stage: variable, affecting the heart, joints, CNS, and may be asymptomatic
• Diagnosis is done through serology

Infections of Skin and Subcutaneous Tissues: Cellulitis and Erysipelas:

• Diffuse inflammation of subcutaneous tissue
• Manifests as erythematous, edematous lesions in skin with febrilia
• Causative agents: Streptococcus pyogenes and Staphylococcus aureus
• Infections can start on intact or broken skin
• Erysipelas: more superficial with a sharper margin compared to cellulitis
• Cellulitis: may cause more severe forms with lymphadenitis or necrosis

Infections of Skin and Subcutaneous Tissues: Necrotizing Fasciitis:

• Progressive infection of the fascia with necrosis of surrounding soft tissues
• May develop from cellulitis or as a wound infection
• Symptoms: early hardening and swelling, pain; later: necrotic changes, bleeding, bullae, febrilia, sepsis, and septic shock
• Causative agents: Streptococcus pyogenes, Staphylococcus aureus, Clostridium sp., and polymicrobic infections
• Requires medical emergency intervention: empiric ATB therapy, de-escalation, surgical debridement, or amputation

De-escalation Therapy

• Antimicrobial de-escalation means stopping some parts of an empirical therapy and switching from a broad-spectrum to a more specific antimicrobial. This occurs when the causative agent is unknown, then changed when the bacteria is found after 48-72 hours to a specific narrow-spectrum antimicrobial.

Surgical Site Infection (SSI):

• Hospital-acquired infections
• Superficial incisional: infection restricted to the incision area of the skin
• Deep incisional: infection beneath the incision area, in muscle and surrounding tissues
• Organ/space: infection involving a body organ or space between organs
• Symptoms: swelling, pain, purulent discharge, wound dehiscence
• Common causative agents: Streptococcus pyogenes, Staphylococcus aureus, Pseudomonas aeruginosa, non-spore-forming anaerobes

Clostridial Myonecrosis:

• Gas gangrene from Clostridium (Gram-positive rods, spore-forming, anaerobic)
• Clostridia are intestinal flora in warm-blooded animals; spores are in the environment (soil, survival)
• Histolytic clostridia include C. perfringens, C. sordellii, and C.septicum, which cause myonecrosis
• Disease develops through infection after a deep wound, causing local hypoxia and contamination by environmental material (soil); spore implantation causes germination
• Causes Clostridia grow and spread in tissues
• Lysis of soft tissues leads to liquefaction, necrosis, and gas production, expanding between tissue layers, increasing bacteria spread, resulting in malodorous foamy discharge
• Toxin production leads to sepsis/septic shock and high lethality
• Deep injuries in car accidents, open fractures occur: rarely HAI/SSI (hospital acquired infections/surgical site infection)
• Frequent complications of war wounds are present
• Diagnosis/Therapy: medical emergency, clinical findings and surgical debridement with amputation, in many cases empiric ATB therapy, de-escalation by cultivation results

Wounds Infections - Anaerobic:

• Non-spore forming
• Endogenous source of infection
• Causes many general and species -Species in infected site includes more than one bacteria
• Has mild to high pathogenity
• High incidence
• Spore forming
• Exogenous source of infection
• Causes of anaerobic infections (bacteria) include Clostridium sp.
• Typically one bacteria species in infected site
• Very high pathogenity
• Low infection incidence

Infection of Joints - Infectious (Septic) Arthritis:

• Ways of infection include hematogenous spread, inoculation, contiguous spread
• Knee joint most frequently affected
• Damages connective tissues
• Symptoms: severe pain, impaired joint movement, swelling, warmth, redness, and febrilia
• Most frequent causative agents – Staphylococcus aureus and Streptococcus pyogenes

Gonococcal Arthritis:

• STD form of gonorrhea
• Frequently infects more joints, migrates, causes inflammation of synovium/dermatitis

Other Causes of Infectious Arthritis:

• Borreliosis in the second stage (Borrelia burgdorferi)
• Syphilis in the third stage (Treponema pallidum)
• Pseudomonas aeruginosa
• Salmonella enterica
• Mycobacterium tuberculosis
• viruses
• Cultivation of the synovial fluid is diagnostic
• Borreliosis, syphilis, viruses diagnosed by serology
• Arthritis can also be non-infectious etiology

Osteomyelitis:

• Infection of bone may occur through hematogenous spread from a distant focus, spread from surrounding tissues, or result of trauma
• Pathogenesis: infection/inflammation increases pressure, causing local ischemia, bone necrosis, chronic infection, sequestration of necrotic tissue, bone deformity, and purulent drainage
• Symptoms include febrilia, pain, and redness

Osteomyelitis: Causative Agents:

• Staphylococcus aureus is the most common
• Streptococcus agalactiae in newborns
• Streptococcus pyogenes, Haemophilus influenzae in infants
• Mycobacterium tuberculosis in the secondary stage of tuberculosis
• Diagnosis: cultivation of bone biopsy or debridement
• Therapy: surgical debridement and long-term ATB therapy

Systemic Infections with Manifestation in Skin:

• Scarlet fever
• Pharyngotonsillitis caused by strain of Streptococcus pyogenes with gene for erythrogenic toxin, experienced in childhood
• Disease has antibodies against erythrogenic toxin
• Meningococcal meningitis is purulent meningitis caused by Neisseria meningitidis, causing petechial rash
• Viral exanthematous infections (topic of viral infections)

Bacterial Causative Agents: Staphylococcus sp.:

• Microscopy: Gram-positive coccus, forms clusters
• Cultivation: pigment production produces beige, yellow, or orange colonies, blood agar shows β-hemolysis or non-hemolytic activity
• Tests:
• Catalase positive (genus Staphylococcus)
• Plasmacoagulase test (groups: coagulase-positive Staphylococcus and coagulase-negative Staphylococcus)
• Precise species identification done through biochemical tests or mass spectrometry

Bacterial Causative Agents: Streptococcus pyogenes:

• Microscopy: Gram-positive coccus, forms chains
• Cultivation: blood agar shows β-hemolysis
• Tests:
• Catalase negative (genus Streptococcus)
• PYR-test positive
• Further identification through biochemical tests, mass spectrometry, and Lancefield group identification

Lancefield Groups of Streptococci:

• Rebecca Lancefield developed groups in 1933 from antigens in the cell wall
• Tested by specific antibodies using latex agglutination
• Group A (S. pyogenes): mucus membranes; causes tonsillitis, cellulitis, septic arthritis, osteomyelitis, necrotizing fasciitis
• Group C (S. dysgalactiae, S. equi, S. equisimilis, S. zooepidemicus): mucus membranes and animals, causes tonsillitis, cellulitis, septic arthritis, osteomyelitis
• Group G (S. dysgalactiae): mucus membranes (possibly bowel), causes tonsillitis, cellulitis, septic arthritis, osteomyelitis, bowel cancer
• Group B (S. agalactiae): bowel, female genital tract, causes neonatal sepsis, septic arthritis, infective endocarditis, infection in diabetes, association with cancer
• Group D (E. faecalis, E. faecium): bowel, causes intra-abdominal infection and urinary catheter infections
• Group F (S. anginosus, S. intermedius, S. constellatus): bowel, causes empyema and bowel perforation

Bacterial Causative Agents: Pseudomonas aeruginosa:

• Common for nosocomial infections, SSI and RTI
• Microscopy: Gram-negative rods
• Cultivation: blood agar exhibits B-hemolysis (variable intensity), Endo lactose negative, XLD saccharides fermentation negative
• Soluble pigment is yellow or green, visible on colorless media
• Non-fermenting rod group

Bacterial Causative Agents: Clostridium sp. (Histolytic):

• Includes C. perfringens, C. histolyticum, C. sordellii
• Microscopic features include Gram-positive, large rods, and spore-forming
• Cultivation is anaerobic, using specific media for anaerobes