Surgical Site Infections: Skin Microflora

• Surgical site infections (SSI) and skin and soft tissue infections are the focus.

Human Normal Microflora (Microbiome)

• The microbial population that colonizes the human body is both numerous and diverse.
• Colonization can be permanent or transient, lasting hours or days.
• Organisms typically don't cause disease in their normal setting, but can if introduced to unprotected, sterile sites like blood or tissues.

Skin Microflora

• Gram-Positive Bacteria:
• Coagulase-negative staphylococci, such as S. epidermidis and S. hominis.
• Corynebacterium spp.
• Propionibacterium spp.
• Micrococcus spp.
• Staphylococcus aureus (carrier state).
• Streptococcus pyogenes (carrier state).
• Gram-Negative Bacteria:
• Acinetobacter spp.
• Escherichia coli
• Gram-negative bacteria do not permanently colonize the skin because the skin environment is too dry.
• They do not multiply on the skin surface.
• Easily removed.
• Fungi:
• Candida spp.
• Malassezia spp.

Risk Factors for Skin Infections

• Changes in skin microbiota
• Staphylococcus aureus and Streptococcus pyogenes carrier state
• Traumas like burns, fractures, bites, and stings
• Surgical wounds
• Diabetes
• AIDS

Folliculitis

• A minor pyogenic infection of the hair follicles.
• Presents as a raised and reddened base of the follicle.
• Small collection of pus beneath the epidermal surface
• Is generally caused by Staphylococcus aureus.
• Often associated with areas of friction and sweat gland activity, frequently on the neck, face, axillae, and buttocks.
• Can manifest as a stye at the base of the eyelid.
• Can also be caused by Pseudomonas aeruginosa.
• Candida albicans can occasionally cause it, particularly in immunocompromised hosts.
• Treatment involves drainage of lesions and antimicrobial treatments like topical aminoglycosides, tetracyclines, macrolides, lincosamides, and mupirocin.

Furuncle

• An extension of folliculitis.
• Is a large, painful, raised nodule containing underlying necrotic tissue.
• Can drain spontaneously or after a surgical incision.
• Is usually caused by S. aureus.
• May be solitary or multiple (furunculosis).
• Can be a recurrent issue.
• Can involve spread of infection to the dermis and subcutaneous tissues, leading to a more extensive, multiloculated abscess called a carbuncle.
• Treatment includes drainage of nodules, topical mupirocin for solitary furuncles, and systemic antibiotics like cloxacillin, clindamycin, cefadroxil, or a macrolide for furunculosis.

Carbuncle

• Occurs when furuncles merge and extend into deeper subcutaneous tissue.
• Patients may exhibit chills and fevers, indicating systemic spread of staphylococci via bacteremia to other tissues.

Acne Vulgaris

• Inflammation of hair follicles and associated sebaceous glands.
• Caused by Propionibacterium acnes, Corynebacterium spp., and Brevibacterium spp.
• Primarily caused by hormonal influences on sebum secretion during puberty.
• Typically resolves in early adulthood.
• Treatment: may involve drainage of lesions; severe cases may require macrolides, tetracycline, or clindamycin.

Infection of Keratinized Layers

• Only dermatophyte fungi can use keratin.
• Dermatophytoses are fungal infections of the skin, hair, and nails.

Impetigo (Pyoderma)

• Caused by Streptococcus pyogenes (group A streptococci).
• The initial lesion is a small vesicle that develops and ruptures, leading to a superficial spread characterized by skin erosion and a serous exudate that dries into a honey-colored crust.
• Exudate and crust are highly communicable due to numerous infecting streptococci.
• S. aureus may contaminate the lesions.
• Epidemic impetigo: common in childhood, linked to heat, humidity, poor hygiene, and overcrowding.
• Infection spreads via fomites like shared clothing and towels.
• Nephritogenic strains of S. pyogenes can cause it, potentially leading to acute glomerulonephritis.
• Treatment: Penicillins or cephalosporins.

Bullous Impetigo

• A distinct disease caused by S. aureus strains that produce exfoliative toxin.
• More common in small children, but can occur at any age.
• Characterized by large serum-filled bullae (blisters) within the skin layers.
• Culture positive, indicating high communicability.
• Epidemic spread can occur under conditions similar to streptococcal impetigo.
• Minor infections are treated topically with aminoglycosides, tetracyclines, macrolides, lincosamides, or mupirocin.
• Serious disease in infants requires systemic antimicrobial treatment.

Staphylococcal Scalded Skin Syndrome (SSSS)

• Also known as Ritter disease.
• Caused by S. aureus strains producing exfoliative toxin.
• Has abrupt onset of localized perioral erythema that spreads over the entire body within 2 days.
• Cutaneous blisters form, followed by desquamation of the epithelium.
• Blisters contain clear fluid without organisms, because the disease is caused by a bacterial toxin.
• Slight pressure displaces the skin (positive Nikolsky sign).
• The epithelium becomes intact again within 7-10 days as antibodies against the toxin appear.
• Primarily affects neonates and young children, with a mortality rate less than 5%.
• In adults with immunocompromised hosts, the mortality may be as high as 60%.

Erysipelas

• A serious, rapidly spreading infection of the deeper layers of the dermis.
• It is caused by Streptococcus pyogenes.
• Associated with edema of the skin, marked erythema, pain, systemic manifestations of fever, and lymphadenopathy.
• Lower extremities are commonly involved.
• As the infection is intradermal, streptococci cannot be isolated from the skin surfaces.
• Can progress to bacteremia or local necrosis of skin.
• Treatment involves penicillins or cephalosporins.

Wounds Infections

• Wounds subject to infection can be surgical, traumatic (deep cuts, compound fractures, frostbite necrosis, thermal burns) or physiologic (endometrial surface after separation of the placenta, and the umbilical stump).
• Sources of wound infection:
• The patient’s normal flora, material from infected individuals or carriers, pathogens from the environment.

Factors Contributing to Wound Infections Include

• Contaminating dose and virulence of microorganisms, condition of the wound, and general health of the patient.
• The condition of the wound, areas of necrosis, vascular strangulation from excessively tight sutures, excessive edema, poor blood supply and oxygenation.
• The general health, nutritional status, and ability of patients to mount an inflammatory response.
• Prevention of wound infections: the first 3 hours after contamination is the critical period to avoid contamination.
• Restrict prophylactic chemotherapy to operative and immediate perioperation

Classification of Wounds

• Clean: Surgical wounds made under aseptic conditions that do not traverse infected tissues or extend into sites with normal flora.
• Clean Contaminated: Operative wounds that extend into sites with normal flora (except the colon) without known contamination.
• Contaminated: Fresh surgical and traumatic wounds with a major risk of contamination; incisions entering nonpurulent infected tissues.
• Dirty and Infected: Old, infected traumatic wounds, wounds substantially contaminated with foreign material, and wounds contaminated with spillage from perforated viscera.

Etiologic Agents

• Traumatic wounds may involve Clostridium spp., Enterobacteriaceae, or Pseudomonas aeruginosa.
• Surgical (Clean) wounds: Staphylococcus aureus, Enterobacteriaceae, Streptococcus pyogenes.
• Surgical (Dirty) wounds: Staphylococcus aureus, Enterobacteriaceae, Anaerobes (Bacteroides fragilis, Fusobacterium spp., Peptostreptococcus spp.).
• Burns: Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus pyogenes, Enterobacteriaceae, Acinetobacter spp., Candida albicans, Anaerobes (often mixed etiology).
• Animal Bites: Pasteurella multocida.
• Human Bites: Anaerobes.
• Postpartum uterine infections: Can range from self-limiting infections to severe infections leading to bacteremia, mostly caused by gram-positive and gram-negative anaerobes.
• Streptococcus agalactiae (GBS – group B streptococci): all pregnant women should be screened for colonization (vaginal and rectal) with GBS at 35 to 37 weeks of gestation; perinatal chemoprophylaxis is recommended for pregnant women with positive vaginal or rectal culture for GBS at 35 to 37 weeks of gestation.
• Postpartum Uterine Infections: Normal delivery without retained fragment rarely results in infection. A prolonged delivery after membrane rupture with retained placental fragments, results in a increased risk of infection

Gas Gangrene (Clostridial Myositis)

• Develops within hours of traumatic injury, leading to death.
• Compound fractures, gunshot wounds, and injuries allow entry of clostridial spores.
• Associated with muscle trauma and necrosis needed for anaerobic multiplication.
• Clostridium perfringens is the most common cause; its α-toxin causes spreading tissue damage and muscle death.
• Other aerobic and anaerobic bacteria may have a major etiologic role also.
• Prevention requires surgically debriding necrotic tissue and administering high-dose penicillin.

Streptococcus Pyogenes Infections

• Necrotizing fasciitis (streptococcal gangrene): A deep skin infection with muscle and fat layer destruction.

Cellulitis

• Not a skin infection but can develop with expansion from skin or wound infection
• It is an acute inflammation of subcutaneous connective tissue with swelling, pain, fever, and lymphadenopathy
• Caused by various bacterial pathogens, S. aureus and S. pyogenes are the most common
• H. influenzae type b: Common in infants and children.
• Enteric Gram-negative rods, clostridia, and other: May cause cellulitis particularly in immunocompromised and diabetics.
• Treatment: Systemic, penicillins.

Laboratory Diagnosis

• Involves microscopic examination and culture.
• Staphylococci culture grow rapidly on nonselective media, aerobically or anaerobically.
• They form large, smooth colonies within 24 hours.
• S. aureus colonies turn yellow.
• Most isolates of S. aureus and some CNS strains produce: hemolysis on sheep blood agar, strep is similar and is catalase negative whereas staph is positive
• Gram-positive cocci are identified using a catalase test.
• Staphylococcus gives a positive result
• Streptococcus spp and Enterococcus spp give a negative result.
• Simple biochemical tests are used to distinguish between different S. aureus species that is catalase positive.
• Coagulase tests are positive
• Mannitol fermentation is positive.

Surgical Site Infections (SSI)

• Ranked as the 2nd or 3rd most common nosocomial infection (18-20%).
• The most frequent nosocomial infection among surgery patients.
• 2/3 are incisional SSI, 1/3 are organ SSI.
• Definitions include colonization (bacteria present in wound without signs), contamination (transient exposure of a wound to bacteria/prophylaxis is the best strategy), and infection (systemic and local signs of inflammation with bacterial counts ≥ 105 cfu/mL).
• Surgical wound infection defined as SSI.
• Primary SSIs occur at the primary site of infection, while secondary occur as a complication elsewhere.
• Early infections present within 30 days of procedure, intermediate infections occur between 1-3 months, and late infections present after 3 months of surgery.
• Antibiotics must be given before first incision, delivery of antibiotics gets impaired once the incision is made

Wound Culture

• Involves swab, biopsy, and aspiration.
• Sterile saline solution should be used to clean wound prior to swabbing, antiseptic should not be used prior to sampling.