Surgical Infections


Microbiology

  1. Bacteria
    • prokaryotes (lack a nucleus and chromosomes)
    • classified morphologically as rods, cocci, or spirals
    • also classified according to the Gram-staining characteristics of their cell walls
    • external to the cell membrane, bacteria have a cell wall that is much thicker in gram-positive than in gram-negative bacteria
    • many antibiotics work by inhibiting cell wall synthesis
    • gram-negative bacteria, in addition to their cell wall, have an outer membrane containing lipopolysaccharide (endotoxin)
    • in some bacteria, surrounding the cell wall is a thick gel-like structure called a capsule
    • capsules appear to protect the bacteria from phagocytosis, thereby acting as a virulence factor

  2. Fungi
    • eukaryotic cells (contain a nucleus with a nuclear membrane)
    • contain a cell wall that is external to the plasma membrane
    • also have a capsule that inhibits phagocytosis
    • some fungi are primary pathogens (Histoplasma, Blastomyces, Coccidiodes)
    • other fungi cause opportunistic infections (Candida, Aspergillus, Cryptococcus)
    • Candida albicans is the most frequent clinical isolate

  3. Protozoa
    • eukaryotes
    • human pathogens include Pneumocystis carinii and Trichomonas vaginalis

  4. Viruses
    • most primitive form of infectious pathogen
    • obligate intracellular parasites
    • must invade cells and use their energy-generating machinery in order to replicate
    • Hepatitis B, C and HIV infections may be transmitted from patients to health care workers

Determinants of Infection

  1. Microbial Pathogenicity
    • ability of a microbe to cause infection is a balance between host defenses and microbial pathogenicity
    • some bacteria possess specific characteristics that make them virulent

    1. Virulence Factors
      1. Exotoxins
        • some bacteria can secrete highly potent toxins
        • Clostridium tetani produces a potent neurotoxin
        • Clostridium botulinum produces a neuromuscular toxin
        • S. aureus is responsible for toxic shock syndrome
        • Group A Streptococci secrete potent hemolysins

      2. Endotoxins
        • lipopolysaccharide-protein complexes that are normal constituents of the cell wall of gram-negative bacteria
        • Lipid A appears to be the toxic portion of LPS
        • activates both the intrinsic and extrinsic coagulation cascades
        • activates the alternative pathway of the complement cascade
        • activates macrophages and stimulates the production of numerous cytokines (IL-1, IL-6, TNF)

      3. Additional Virulence Factors
        • many bacteria and fungi have thick capsules, making them resistant to phagocytosis
        • S. epidermidis secretes a ‘slime’ that facilitates its binding to foreign surfaces and makes it less vulnerable to phagocytic cells
        • some bacteria produce various enzymes that thwart the bactericidal mechanisms of phagocytic cells
        • collagenase produced by streptococci and staphylococci facilitates dissection of infection along fascial planes

      4. Antibiotic Resistance
        • each group of antibiotics has a different mechanism of action that exploits a metabolic or structural vulnerability of the sensitive organism
        • many antibiotics interfere with cell wall synthesis; others inhibit protein synthesis by targeting bacterial ribosomes
        • antibiotic resistance may occur through several different mechanisms: plasmids (most common) and spontaneous mutations (rare)

  2. Host Defenses
    1. Local Host Defenses
      1. Skin
        • principle defenses of the skin are dryness and desquamation
        • bacteria need moisture to grow and normal skin has a low moisture content
        • keratin constantly sloughs, and the flora is sloughed as well
        • sebaceous glands also secrete a lipid inhibitory to most pathogens
        • colonization is sparse on normal skin except in intertriginous areas where moisture accumulates

      2. Respiratory Tract
        • most inhaled particles, including microbes, never reach the alveoli
        • large particles are filtered out in the nares
        • mucous layers in the trachea and bronchi contain a number of defenses (IgA, lysozyme, complement)
        • cilia move the mucous layer towards the mouth, where it is expelled by coughing
        • small particles that make it to the lower airway are phagocytized by pulmonary alveolar macrophages

      3. Alimentary Tract
        • saliva possesses lysozyme and IgA
        • ingested organisms are killed in the highly acidic stomach
        • peristalsis in the small intestine makes it difficult for bacteria to adhere, although they do proliferate
        • drugs, operations, or disease states that diminish gastric acidity allow bacteria to proliferate in the stomach
        • prolonged ileus will also allow bacteria to overgrow in the small intestine

        1. Large Intestine
          • low-oxygen, low motility environment of the colon allows exponential growth of bacteria
          • anaerobes outnumber aerobes by 100:1
          • only a few of these species are ever isolated in intra-abdominal infections
          • normal colonic flora suppresses growth of pathogenic species
          • prolonged use of broad-spectrum antibiotics will greatly reduce the normal colonic flora

      4. Urinary Tract
        • protected by periodic flushing of urine
        • urine is somewhat bacteriostatic by virtue of its high urea content, high osmolarity, acidic pH
        • long urethra in males limits microbial access
        • short female urethra facilitates microbial access
        • urinary catheters introduce bacteria into the bladder

    2. Systemic Host Defences
      1. Neutrophils (PMNs)
        • once microbes have breached the skin or mucosal barrier, the most important host defense against infection is the phagocytic system (innate response)
        • PMNs respond long before the production of specific antibody and cell-mediated responses
        • phagocytes must recognize that an organism is ‘bad’
        • this is mediated by opsonins
        • opsonins bind to the microbial surface and then to specific receptors on the phagocytic cell membrane

      2. Macrophages
        • also act as phagocytic killers
        • found throughout the body’s tissues
        • secrete a vast array of cytokines
        • function as antigen-presenting cells for lymphocytes

      3. Complement System
        • activated by direct contact with microbes and via IgM > IgG binding to microbes
        • also activated when the coagulation cascade is activated
        • membrane attack complex destroys pathogens
        • some complement protein fragments are biologically active (C3a, C5a)

    3. Local Environmental Factors
      1. Bacterial Load
        • a critical threshold of bacteria is necessary to cause clinical infection
        • in general, 100,000 bacteria per gram of tissue must be present to cause a clinical infection
        • more virulent bacteria may require much fewer numbers to cause severe or life-threatening infections (clostridial gangrene or tetanus)
        • the presence of adjuvant factors may also lower the number of bacteria required to cause infection

      2. Adjuvant Factors
        • amplify bacterial virulence or repress host responsiveness
        • hemoglobin (clot or hematoma) may be the most potent adjuvant factor
        • foreign bodies (suture, prosthetic materials)
        • necrotic tissue

Common Surgical Pathogens

  1. Staphylococci
    1. S. aureus
      • possesses virulence factors which permit it to invade normal tissues
      • skin organism found in the anterior nares and areas of moist skin in up to 40% of healthy people
      • suture material decreases by 500 times the number of bacteria required to produce an infection
      • most common isolate from wound infections after clean procedures
      • most hospital strains are methicillin-resistant (MRSA)

      1. MRSA
        • vancomycin is the standard first-line therapy
        • linezolid (Zyvox) is an oral agent useful for outpatient treatment

    2. S. epidermidis
      • normal resident of the skin
      • causes infection in the presence of foreign bodies such as intravenous catheters, heart valves, prosthetic joints
      • some strains produce a glycocalyx (‘slime’), which allow bacteria to resist phagocytosis and adhere to prosthetic materials

  2. Streptococci
    • classified according to cell surface antigens and their ability to cause hemolysis on blood agar

    1. Group A Streptococci
      • comprised of a single species: Strep pyogenes
      • β-hemolytic
      • responsible for necrotizing soft tissue infections and erysipelas (streptococcal cellulitis and lymphangitis)
      • may secrete a number of exotoxins
      • hyaluronidase and streptokinase promote the spread of infection along tissue planes

    2. Group B Streptococci
      • harbored in the female genital tract
      • responsible for neonatal meningitis and bacteremia

    3. Strep. pneumoniae
      • common resident of the nasopharynx
      • possesses a capsule with antiphagocytic properties
      • responsible for overwhelming post splenectomy sepsis
      • antipneumococcal vaccine is 65% to 70% effective in preventing infection

  3. Enterococcus
    • part of the normal flora of the GI tract and vagina
    • often found in polymicrobial infections of the peritoneum and pelvis
    • may also cause UTIs and endocarditis
    • now resistant to many antibiotics, including vancomycin (VRE)
    • VRE may be treated with linezolid, daptomycin, or tigecycline

  4. Neisseria
    • gram-negative cocci
    • N. gonorrhoeae is harbored in the urethra, cervix, pharynx, conjunctiva
    • transmission is usually by sexual contact
    • contiguous spread from the endocervix can result in PID
    • spread of gonococci into the upper abdomen can result in perihepatitis (Fitzhugh-Curtis syndrome)

  5. Gram-Negative Bacilli
    1. Enterobacteriaceae
      • facultative aerobes
      • commensal in the GI tract and vagina
      • contain endotoxin in their cell walls
      • E. coli is the most common GNR in the GI tract and is the most common organism isolated from biliary, urinary, and intra-abdominal infections
      • Klebsiella is an encapsulated organism commonly associated with hospital-acquired pneumonias
      • multidrug resistant organisms are becoming more common

    2. Pseudomonads
      • obligate aerobes
      • able to survive in water almost without other nutrients
      • normal host defenses are highly effective against colonization
      • colonization occurs in debilitated patients whose own flora has been depleted by antibiotics
      • pseudomonads flourish in ICUs when they contaminate wet equipment (respirators) and are spread by personnel from patient to patient
      • Pseudomonas aeruginosa is the leading cause of death from nosocomial infections in the ICU

  6. Anaerobic Bacteria
    • require a low-oxygen environment for growth
    • found predominantly in the mouth, GI tract, and vagina
    • in most infections, aerobic bacteria are also present
    • anaerobes outnumber aerobes by 10 to 1 in the mouth and by 100 to 1 in the colon
    • predisposing conditions for anaerobic infection include poor vascular supply, tissue necrosis, foreign bodies

    1. Bacteroides
      • low virulence organisms
      • have an endotoxin, but it is of poor biologic activity
      • their slow growth in culture (up to one week) may delay their diagnosis and make empiric treatment necessary

    2. Clostridia
      • gram-positive spore-forming rods
      • most virulent anaerobic species
      • all are soil organisms and are found in the gut as normal flora

      1. C. tetani
        • tetanus occurs in nonimmunized or partially immunized individuals who have contaminated wounds
        • produces an exotoxin that causes muscle spasms
        • treatment involves penicillin to eradicate the source of the toxin and tetanus immune globulin to neutralize circulating and unbound toxin

      2. C. perfringens
        • causes necrotizing soft tissue infections
        • secretes many exotoxins

      3. C. difficile
        • produces an exotoxin that causes pseudomembranous colitis
        • occurs in patients who have received broad-spectrum antibiotics
        • oral vancomycin or metronidazole are effective therapies

  7. Fungi
    • fungal surgical infections are caused primarily by the opportunistic fungi

    1. Aspergillus
      • colonizes the upper airway in healthy individuals
      • in the compromised host, may infect the ear, sinuses, orbit, lung, and brain

    2. Mucormycosis
      • caused by fungi of the mucor and rhizopus genera
      • occurs in the context of uncontrolled diabetes, immunosuppression, thermal burns
      • begins in the nose and may spread contiguously to the orbits, palate, eye, and brain
      • treatment consists of aggressive surgical debridement and amphotericin B

    3. Candida
      • normal inhabitant of the mouth, upper airways, GI tract
      • overgrowth occurs when competing bacteria are eliminated by antibiotic therapy
      • risk factors for invasion include malignancy, malnutrition requiring hyperalimentation, diabetes, systemic steroids, immunosuppression

  8. Viruses
    1. Hepatitis
      1. Hepatitis B
        • DNA virus
        • infection occurs through needle sticks, exposure to blood, sexual contact
        • needle sticks cause a 30% conversion rate in unvaccinated individuals
        • ~5% of those infected become chronic carriers
        • with blood bank screening, hepatitis B now rarely accounts for post transfusion hepatitis
        • hepatitis B vaccine is available and is 95% protective
        • 250,000 cases/year in the 1980s; 3350 cases in 2010
        • hepatitis B immune globulin is available for post exposure prophylaxis in individuals not vaccinated against hepatitis B

      2. Hepatitis C
        • RNA virus
        • 2% conversion rate from a needle stick
        • no vaccine available
        • most patients are asymptomatic
        • progression to chronic persistent and chronic active hepatitis occurs in 75% of patients and may progress to cirrhosis
        • first or second most common indication for liver transplantation
        • rarely transmitted through blood transfusions now because of sensitive blood bank testing
        • interferon alpha and ribavirin are used to successfully treat chronic hepatitis C

    2. Cytomegalovirus
      • member of the herpes virus family
      • often complicates organ transplantation
      • commonly transmitted during blood transfusions
      • transfusion of CMV-seropositive blood to an immunocompetent patient will result in only a mild infection, if any
      • diagnosis is made by demonstrating intranuclear inclusions (owl’s eyes) in tissue sections
      • immunocompromised patients may be treated with ganciclovir

    3. Human Immunodeficiency Virus (HIV)
      1. Microbiology
        • RNA retrovirus with an affinity for the CD4 receptor on T lymphocytes
        • after binding to the CD4 receptor, the virus is internalized and uncoated
        • reverse transcriptase synthesizes DNA complementary to the viral RNA
        • this DNA is then incorporated into the host cell’s genome, leading to a lifelong infection

      2. Pathophysiology
        • infection with HIV leads to T helper cell deficiency (CD4), resulting in profound depression in cell-mediated immunity
        • once the CD4 lymphocyte count drops below 200/μL, the chance of developing an opportunistic infection is high

      3. Serologic Events
        • 6 to 12 weeks after infection, antibody to HIV develops (may take up to 6 months)
        • serologic testing examines antibodies to HIV
        • since it may take up to 6 months to develop antibodies to HIV, during this time a patient may have circulating virus and yet test negative for HIV

      4. Surgery in HIV-infected Patients
        • patients do not require any special preoperative preparation
        • prophylactic antibiotics are given for the same indications as for non-HIV patients
        • these patients do not have difficulty with wound healing and do not have a higher rate of infectious complications

      5. HIV Post Exposure Management
        • risk of developing seropositivity after a needle stick is about 0.3%
        • blood from the source patient should be drawn and tested for HBV, HCV, and HIV
        • following the initial test, seronegative workers should be retested at 6 weeks, 12 weeks, and 6 months
        • if the source patient is HIV positive, post exposure prophylaxis is associated with a lower risk of HIV transmission
        • if the HIV status of the source patient is unknown, the use of post exposure prophylaxis should be decided on a case-by-case basis (high risk vs low risk)

Specific Infections

  1. Wound Infections (SSIs)
    1. Wound Classification
      1. Clean
        • elective, primarily closed, undrained
        • nontraumatic, uninfected
        • respiratory, alimentary, GU, oropharyngeal tracts not entered
        • infection rate of 1.3% - 2.9%
        • hernia, breast biopsy

      2. Clean Contaminated
        • alimentary, respiratory, GU tracts entered under controlled conditions
        • mechanical drainage
        • minor break in technique
        • elective GI tract surgery (not colon), elective cholecystectomy
        • infection rate of 2.4% - 7.7%

      3. Clean Contaminated – Colorectal
        • infection rate of 4% - 14% in adequately prepped patients

      4. Contaminated
        • open, fresh traumatic wound
        • gross spillage from the GI tract
        • biliary surgery in presence of infected bile
        • incision through inflamed, but nonpurulent tissue
        • penetrating abdominal trauma
        • infection rate of 6.4% - 15.2%

      5. Dirty wounds
        • perforated viscus
        • pus encountered
        • perforated diverticulitis, necrotizing STI
        • infection rate of 7.1% - 40%

    2. Prophylaxis
      • SSIs are associated with increased morbidity/mortality, patient dissatisfaction
      • also associated with substantial costs to the health care system
      • many payers are refusing to pay hospitals for SSIs
      • all hospitals that participate in Medicare must report SSIs within 30 days of the procedure or 90 days if prosthetic material is implanted
      • good glucose control may minimize SSIs
      • intraoperative hypoxia and hypothermia should be prevented

      1. Operating Room Environment
        • air filtration systems can reduce the number of dust particles to which microbes can adsorb
        • operating rooms should have a positive pressure relative to air in the corridor so that unfiltered air does not enter the OR
        • laminar flow rooms are used when prosthetic joints are implanted

      2. Patient Preparation
        • preop shower with an antiseptic soap can reduce the resident skin bacteria
        • however, the evidence is unclear whether this reduces wound infections
        • elective operations should be delayed until remote site infections have been cleared up
        • shaving should be done in the OR with clippers, not the night before
        • skin should be prepped with povidone-iodine or chlorhexidine
        • bowel prep prior to elective colon surgery

      3. Surgical Technique
        • meticulous hemostasis
        • remove all devitalized tissues

      4. Prophylactic Antibiotics
        • indicated in most clean-contaminated cases
        • also indicated in clean operations in which an infection could lead to a disastrous outcome, such as prosthetic joint, vascular graft, or cardiac valve procedures
        • choose an antibiotic effective against the pathogens most likely to be encountered and has low toxicity
        • should be given 30 to 60 minutes prior to the incision
        • give a second dose if the procedure lasts longer than 4 hours
        • should be discontinued within 24 hours of surgery
        • cefazolin is active against most gram-positive organisms (not MRSA) and some gram-negative organisms
        • second-generation cephalosporins have broader coverage against gram-negative organisms and also have GI anaerobic coverage, making them useful in colon surgery, appendectomy
        • for patients with a true penicillin allergy, vancomycin or clindamycin can be substituted

    3. Management
      • primarily consists of opening and packing the wound
      • antibiotics are reserved for when significant cellulitis or fever is present
      • wound VACs can be very helpful in managing large or complex wounds

  2. Central Line Infections
    1. Prevention
      • hand washing
      • full barrier precautions (cap, mask, sterile gloves, sterile gowns, and sterile drapes) lower line infection rates dramatically
      • skin preparation with chlorhexidine has lower infection rates when compared with povidone-iodine solutions
      • subclavian vein insertion site has the lowest infection rate; the femoral vein has the highest
      • antibiotic-impregnated catheters have lower infection rates in catheters left in place for more than 5 days
      • multilumen catheters have higher infection rates
      • dedicated catheter care teams results in fewer catheter-related infections
      • central lines should be removed when they are no longer clinically necessary

      1. Replacement of Central Lines
        • routine replacement of lines that have no signs of infection is not recommended, except in high-risk populations like burn or transplant patients
        • replacing a line over a guidewire should rarely be done because of higher infection rates

    2. Diagnosis
      • a high index of suspicion is necessary
      • purulence and inflammation around the insertion site is a specific but uncommon finding
      • fever is a common finding, but it is not very sensitive for line infections
      • definitive diagnosis requires peripheral blood cultures to grow the same organism as the catheter tip
      • if clinical suspicion is high, then the line should be removed and the tip sent for culture

    3. Microbiology of Central Line Infections
      • S. aureus and S. epidermidis originate from the skin and cause most catheter-related infections
      • most candida infections result from hematogenous dissemination

    4. Treatment
      1. Antibiotic Selection
        • vancomycin is the preferred empiric drug because most hospital staph species are methicillin resistant
        • daptomycin can be substituted if vancomycin is contraindicated
        • pseudomonas coverage can be added in high risk patients
        • fluconazole is the treatment choice for candida infections

      2. Catheter Removal
        • the great majority of vascular catheter infections will require removal of the catheter
        • catheter infections with S. epidermidis can occasionally be treated with antibiotics instilled into the catheter and long-term systemic antibiotics







References

  1. Schwartz, 10th ed. Pgs 135 -157
  2. O’Leary, 4th ed. Pgs 218 - 257
  3. Simmons and Steed, pgs 56 – 83
  4. Sabiston, 20th ed., Pgs 240 - 250
  5. UpToDate, Antimicrobial Prophylaxis for Prevention of Surgical Site Infection in Adults. Anderson, Deverick. March, 2018. Pgs 1 – 48.
  6. UpToDate, Overview of Control Measures for Prevention of Surgical Site Infection in Adults. Anderson, Deverick. October 2018, pgs 1 - 34
  7. Cameron, 11th ed., pgs 1259 - 1262