Probiotics in Prevention of Surgical Site Infections

Background

The human gastrointestinal tract is a complex microenvironment interwoven between host cells and the indigenous microflora or “microbiome,” which includes not only bacteria but also fungi, viruses, and archaea. These two systems, the host and the microbiome, create a “super-organism” that allows macro-nutrient and micro-nutrient extraction for the host while serving as a barrier to toxins and other detrimental bacterial end-products [3]. There is no doubt that without the human microbiome, humans could not survive.

The microbiome has been implicated in changes to the physiology of the entire host, not just in the tissues directly in contact with the microbiome. We now know that the microbiome influences the nutritional status, immune function, and oncogenesis as well as psychological and overall well-being of the host. Not only does the microbiome influence the host, the host influences the microbiome. In passage through the gastrointestinal tract, bacteria adapt to changing physical factors such as pH, temperature, oxygenation, redox potential, and the presence of commensal or pathogenic bacteria [4].

Under normal conditions, the intestinal microbiome offers protection or resistance to pathogenic bacteria. The human microbiome is modified rapidly by a variety of factors that include diet; psychological or physiologic stress; changes in circadian rhythms; medications, especially antibiotics; eating habits; and lifecycle stages. Surgery has dramatic effects on the microbiome. Currently, most studies of these changes pertain to visceral surgery, but any surgery-related stressor may alter the entire host microbiome. Such alterations may begin in the pre-operative setting with bowel preparations and antibiotics and continue through surgery into the post-operative setting as a result of the physiologic stress of the surgery, including its duration, alterations in tissue perfusion and pH, and anesthesia among many factors. In mouse models, even brief exposure of the bowel to the external environment increases the number of Enterococcus and Escherichia coli by 200–500 times after a colectomy. These changes, as well as dramatic local changes in anastomotic-tissue–associated bacteria, persist as long as six days post-operatively, despite otherwise-normal surrounding luminal bacteria [5]. In addition, during an intestinal anastomosis, the exposure of the bowel lumen to air changes the partial pressure of oxygen, allowing certain species of bacteria to proliferate. This hypothesis was corroborated by data from patients undergoing ileostomy takedown in whom the lumen was exposed to the environment and resected at the time of surgery [6].

The role of the microbiome in infection is complex. Typically, the intestinal epithelium is separated from the gastrointestinal lumen by a single-cell barrier layer that serves as the first line of defense against invasive pathogens [7]. Along this barrier is a thick mucus layer that is inhabited by bacteria. This layer helps determine which dietary substances are absorbed into the body and offers extra protection from luminal pathogens. Breakdown of this mucus results in proliferation of pathogenic bacteria, which can become invasive by attaching to the mucosal cells, resulting in a localized transfer into the cell or another entry point into the host, potentially causing disruption of gut homeostasis, systemic inflammation, and distant infections. Beyond the mucosal barrier, the microbiome affects the immune system directly. It secretes key metabolites that cause cellular signaling effects via epithelial-cell receptors resulting in changes in immune function. These tonic inputs help regulate the immune response, including inflammation and wound healing [8].

Surgical site infections occur at a rate of 2%–5% for all operations, but rates differ by specialty. For example, colorectal surgery carries a higher rate, estimated to be 15%–30% [9]. These SSIs are associated with morbidity and increased length of stay and with re-admissions. The re-admission rate for SSI after colorectal surgery is 11%–23%. According to a study following Veterans Affairs patients for one month post-operatively, the increase in the cost to the hospital per SSI was exceeded $11,000 [10]. The mortality rate approaches 3% for patients with an SSI after colorectal surgery.

The microbiome and its involvement with post-operative complications such as anastomotic leaks, ileus, and nosocomial infections including pneumonia and SSIs, has been evaluated by several studies. Despite dramatic improvement in the rates of death and infection, intestinal anastomotic leaks persist at an overall rate of 5%–10% [11]. Most recommendations to limit anastomotic leaks center on the prevention of ischemia, with adequate perfusion via a tension-free anastomosis. Recent data have shown that the microbiome is an essential component of a successful anastomosis. Bacteria at the anastomotic site can stimulate epithelial cells to grow, and collagen-producing bacteria can supplement epithelial-cell collagen to create structural integrity. Animal studies show that gnotobiotic mice have lower turnover of epithelial cells with higher rates of anastomotic leaks than their wild-type counterparts. Nosocomial infections—especially pneumonia—occur more frequently without a healthy microbiome. The etiology of this defense is the maintenance of the intestinal barrier and the prevention of bacterial translocation. The benefits of the microbiome against SSIs are less clear.

Maintaining a healthy microbiome in the peri-operative period may offer a unique solution for multi–drug-resistant (MDR) organisms. A clear example of this is the treatment of Clostridium difficile with fecal transplantation. Antibiotics promote opportunistic C. difficile proliferation, increasing the number of spores released locally into spaces such as hospital beds [12]. The infectious risks of these locally released antibiotic-associated organisms can be long-lasting; in a retrospective cohort study of 100,615 pairs of patients, antibiotic receipt by previous hospital bed occupants was significantly related to a higher risk of C. difficile infection in subsequent occupants, even if those occupants did not receive antibiotics themselves [13]. Such side effects of antibiotic use mandate alternative treatment methods. The ability of bacteria to compete with one another allows commensal bacteria to inhibit pathogenic bacteria and their spores competitively, providing a novel antibiotic-independent treatment. Given the benefits of a healthy microbiome in preventing and fighting infection, the question arises whether the microbiome can be manipulated to aide in fighting infection.

Why Has the Microbiome Been Neglected?

Before we can attempt to answer the question of whether the microbiome can be manipulated to decrease infections, we must first understand why this novel strategy has not already been explored more extensively. The microbiome has become a research topic only recently for two major reasons. The first is the lack of understanding of how commensal bacteria can prevent infection. The basic principle of germ theory has been that a single pathogenic bacterium proliferates and causes an infection, so the way to combat this infection is to eradicate this pathogen with antibiotics or by allowing the immune system to act. In reality, an infection is much more complex, arising as a result of dysbiosis between the pathogenic bacteria, commensal bacteria, and the host. Although this idea is contrary to germ theory, the administration of pro-biotic bacteria promotes cytokine activation [14], releases factors that help mediate inflammatory responses [15], and even inhibits pathogenic bacterial strains competitively [16]. Conversely, use of antibiotics simply resets the dysbiotic relation by eliminating multiple strains of bacteria but may also contribute to other infectious processes, as in the case of C. difficile. The idea of giving bacteria to fight bacteria is not intuitive running as it does contrary to our traditional teaching of “germ” theory.

Another reason the microbiome has been neglected is its tremendous complexity. There are more than 10¹⁴ bacteria in the typical gastrointestinal tract, with more than 1,000 species and unknown numbers of viruses, fungi, and archaea. There are more bacterial cells in the human colon than there are tissue cells. We are just beginning to understand how these indigenous bacteria contribute to an individual's nutritional status and its complicated relations with the immune system. Previously, bacteria could be studied only using cultures, limiting our ability to identify and quantify them. Modern techniques with sequencing technologies have revolutionized and advanced our study of the microbiome. These modern techniques allow rapid, reliable, and reproducible analysis, perhaps someday offering the opportunity to abandon the one-size-fits-all antibiotic method and to adopt personalized pro-biotic regimens for patients [4]. These modern techniques now allow us to analyze the microbiome and possibly to manipulate it [17].

Current Data on Prevention of Infections from a Microbiome Perspective

The mechanism of post-operative surgical infections from the microbiome perspective is complicated. Surgery causes physiologic stress that can change the concentrations of bacteria and diversity within the microbiome. These changes often alter the ability to ferment soluble fibers and even to produce nutrients, including short-chain fatty acids, which are thought to possess systemic anti-inflammatory effects and to promote mucosal healing [18]. Concentrations of short-chain fatty acids decrease rapidly after surgery, and lack of these luminally delivered nutrients results in disruption of the mucosal barrier and increased membrane permeability. This disruption allows toxic bacterial metabolites and even bacteria to cross the mucosal barrier. This exposes these products to extra-intestinal sites such as the mesenteric lymph node complex and the blood stream [19]. Although the phenomenon of bacterial translocation probably is a regular, benign process in the healthy population, the increase in translocation into the mesenteric lymphatics and blood stream in the immunosuppressed person (i.e., surgical patients) promotes bacteremia and systemic infection [20]. A meta-analysis found that surgery caused significant changes in the microbiome of patients pre-operatively and post-operatively, with decreased numbers of bacteria and more prevalent pathologens.

Beyond direct effects, the microbiome exerts influence through neurologic actions along the brain–gut neurologic axis. These effects are mediated in part by oxytocin, a neuropeptide that enhances wound healing via activation of regulatory T-cells. Oral administration of Lactobacillus rheuteri, a common pro-biotic bacterium, results in up-regulation of oxytocin in mouse models, causing L. rheuteri-treated mice to exhibit faster wound healing than their control counterparts [21]. Conversely, loss of microbial diversity or abundance can slow wound healing. Animal studies have shown that germ-free mice have reduced liver regeneration compared with wild-type animals. The exact mechanisms of these downstream effects of the microbiome are still being elucidated.

Specifically for SSIs, there is a contribution from both the gut and the skin microbiomes. The skin has a microbiome that is large and diverse. Alterations can contribute to cellulitis, acne, psoriasis, and dermatitis. Staphylococcus epidermidis potentiates the antimicrobial effects of the immune system [22]. This keeps pathogenic bacteria from causing infections. In addition to directly causing infections, skin flora can affect the ability of a wound to heal. Biofilms created by bacteria can change how a wound is able to heal. Manipulation of the microbiome on the skin can allow a wound to heal more quickly [23].

Treatment Options for the Microbiome

Given the relations between the microbiome and surgical infections, manipulation of the microbiome with either pre-biotics or pro-biotics has been reported to result in fewer SSIs. Pre-biotics are non-digestible compounds obtained from foods such as soluble fiber or fiber additives. When pre-biotics are metabolized or fermented by micro-organisms to produce energy, the organisms synthesize short-chain fatty acids that impact the health of the host favorably. For this reason, promoting the intake of pre-biotics in the peri-operative setting may manipulate the microbiome positively. A number of animal studies on the ability of pre-biotics to prevent infections support the idea of efficacy through several mechanisms, including preserving a stable microenvironment and maintaining microbial biodiversity, thus promoting competitive inhibition of pathogens [24]. Further, soluble fibers or pre-biotics fermented by pro-biotic bacteria produce short-chain fatty acids that have anti-inflammatory effects [25]. However, the significant heterogeneity of the results of these studies make clinical recommendations difficult [26].

Pro-biotics might be used to increase the number of commensal bacteria, which could compete with pathogenic bacteria to maintain homeostasis, preventing infection. Pro-biotics are live bacteria that add to or replace lost beneficial bacteria normally present in the gastrointestinal tract. Pro-biotics come in many forms as food and dietary supplements, with little standardization. These products have a variety of bacteria at different concentrations. Typical pro-biotics consist of formulations of Lactobacillus or Bifidobacterium with a variety of species.

Prophylactic pro-biotics have been evaluated in several studies as a way to prevent infection, but the effects are unclear. One meta-analysis found that pro-biotics given pre-operatively can alter a patient's microbiome, and the changes persist post-operatively. These patients had a lower infection rate, but the study covered only pneumonia, not SSIs [17]. However, another meta-analysis assessed the efficacy of pro-biotics against infection-related complications in colorectal surgery and concluded that multistrain pro-biotic combinations were beneficial against SSI (odds ratio 0.48, 95% confidence interval 0.25–0.8; p = 0.02). There was no difference in total infections or SSI in the studies using fewer than three strains of organisms [27]. A subsequent meta-analysis that pooled 10 studies found that pre-operative pro-biotics decreased overall infections. The other major finding from this larger meta-analysis was that the diversity of the pro-biotics is important. As found in the Liu et al. meta-analysis, multistrain pro-biotic combinations decreased infection rates more than those with only one strain [28].

Summary and Recommendations

Clearly, dramatic changes in both bacterial diversity and absolute numbers in the gut microbiome after surgery can increase the risk of SSI. The significance of this contribution and the ability to manipulate it is unclear, but the pieces of this complex puzzle are beginning to come together. There is a clear need for randomized trials to synthesize firm conclusions and make clinical recommendations. Much of the current literature on pro-biotics is biased and overlooks several major issues, namely the lack of standardization of pro-biotic regimens both in research and in clinical medicine. The ability of pro-biotics to change the gut microbiome is obvious, but understanding the full effect on SSI will require extensive methodical investigation.