Surgical site infections (SSIs) are infections that occur in the part of the body where a surgical procedure took place. These complications represent a significant challenge in healthcare, accounting for an estimated 2 to 5% of patients undergoing inpatient surgical procedures, making them one of the most common complications of surgery. An SSI can range from a superficial skin infection to a deep infection involving organs or implanted materials, often requiring extended hospital stays, re-operation, and increased healthcare costs. Understanding the sources of infectious agents and the factors that compromise a patient’s natural defenses is the first step in comprehending what causes these infections.
Primary Sources of Pathogens
The fundamental cause of a surgical site infection is the introduction of microorganisms into the incision, and these pathogens originate from two main categories: the patient’s own body or the external environment. Most cases, estimated to be between 70% and 95%, are caused by the patient’s native bacterial flora, known as the endogenous source. When the surgeon makes an incision, the skin’s natural barrier is breached, allowing organisms that colonize the skin, such as Staphylococcus aureus or coagulase-negative staphylococci, to enter the deeper tissues. The type of surgery dictates the specific endogenous threat; for example, gastrointestinal procedures expose the surgical site to enteric gram-negative organisms like Escherichia coli and Enterococcus spp. Pathogens can also originate from exogenous sources outside the patient, including the operating room environment, surgical instruments, or the surgical team.
The air in the operating room is a known source of contamination, with airborne particles carrying microorganisms that can settle onto the sterile field or the open wound. Modern ventilation systems use high-efficiency particulate air (HEPA) filters to minimize this risk. Surgical staff can also shed skin flakes and hair, which carry bacteria, emphasizing the strict requirements for surgical attire and sterile technique. While rare, a failure in the sterilization process of reusable surgical instruments can also introduce pathogens.
Patient Health and Susceptibility Factors
A patient’s underlying health status is a major determinant of their susceptibility to developing an SSI, as several systemic conditions can compromise the body’s immune response and wound healing capacity. Uncontrolled diabetes is a significant risk factor because high blood glucose levels impair the function of white blood cells, such as neutrophils and macrophages, which are responsible for fighting infection. High glucose also disrupts wound healing by hindering collagen synthesis and new blood vessel formation, leaving the surgical site vulnerable.
Obesity increases SSI risk through poor circulation in adipose tissue and a state of chronic, low-grade inflammation. Adipose tissue, particularly visceral fat, can secrete pro-inflammatory chemicals that impair insulin action and contribute to a generally compromised immune state. The fatty tissue itself has a poor blood supply, which limits the delivery of oxygen, nutrients, and infection-fighting immune cells and antibiotics to the wound site.
Extremes of age also affect immune function. In infants, the immune system is immature with limited ability to kill pathogens and poor white blood cell movement. Older adults experience a decline in immune function, known as immunosenescence, characterized by reduced T-cell diversity and function. Furthermore, patients who are already colonized with drug-resistant organisms, such as methicillin-resistant Staphylococcus aureus (MRSA), are at an increased risk of developing an SSI caused by that organism.
Intraoperative Conditions that Increase Risk
The events that occur during the surgery itself can create an environment within the wound that is highly favorable for bacterial proliferation. The duration of the operation is a well-established factor, with longer procedures increasing the time the surgical site is exposed to potential contamination from the air and the surgical team. Longer surgery also increases the amount of tissue trauma and the likelihood of compromised tissue viability.
Poor tissue handling, which includes excessive trauma or leaving collections of blood (hematoma) or fluid in the wound, creates conditions that promote infection. Traumatized tissue with poor blood flow (ischemia) lacks oxygen, nutrients, and immune cells, while hematomas serve as an excellent, nutrient-rich culture medium for bacteria. Maintaining a normal body temperature, or normothermia, is also an important factor, as intraoperative hypothermia (a core temperature below 36.0°C) impairs the patient’s immune system by causing blood vessels in the skin to constrict. This vasoconstriction leads to tissue hypoxia, which lowers the oxygen concentration at the wound site, reducing the body’s ability to resist infection.
The introduction of foreign materials, such as surgical implants (e.g., joint replacements) or even certain types of suture material, significantly lowers the bacterial load required to cause an infection. Bacteria can adhere to the surface of these materials, forming a protective structure called a biofilm. Once a biofilm is established, the bacteria become shielded from the body’s immune defenses and are less susceptible to antibiotic treatment, making the infection much more difficult to eradicate.
Post-Surgical Care and Management Gaps
The continuum of SSI risk extends well past the operating room and into the patient’s recovery period, where errors in wound management or unmanaged systemic issues can allow infection to take hold. Improper handling of the wound dressing, particularly during changes, introduces a risk of contamination from the hands of the caregiver or the immediate environment. Meticulous hand hygiene and adherence to established protocols are necessary to prevent external pathogens from entering the healing incision.
Poorly managed post-operative systemic complications also contribute to infection risk, especially conditions that lead to inadequate blood flow to the wound site. Post-operative hypotension, or critically low blood pressure, can compromise local tissue perfusion, reducing the supply of oxygen and immune cells to the incision.
Failure to follow discharge instructions can also complicate the recovery process and increase the risk of infection. Patient non-adherence to prescriptions, such as a course of antibiotics, can lead to an incomplete eradication of lingering pathogens. Similarly, ignoring restrictions on activity or wound care instructions can place undue stress on the incision, potentially causing the wound to separate and allowing bacteria a clear path into the deeper tissues.