Modeling sepsis, with a special focus on large animal models of porcine peritonitis and bacteremia

Abstract

Infectious diseases, which often result in deadly sepsis or septic shock, represent a major global health problem. For understanding the pathophysiology of sepsis and developing new treatment strategies, reliable and clinically relevant animal models of the disease are necessary. In this review, two large animal (porcine) models of sepsis induced by either peritonitis or bacteremia are introduced and their strong and weak points are discussed in the context of clinical relevance and other animal models of sepsis, with a special focus on cardiovascular and immune systems, experimental design, and monitoring. Especially for testing new therapeutic strategies, the large animal (porcine) models represent a more clinically relevant alternative to small animal models, and the findings obtained in small animal (transgenic) models should be verified in these clinically relevant large animal models before translation to the clinical level.

Keywords: SOFA score; bacteremia; cardiovascular system; immune system; large animal models; peritonitis; pig; sepsis.

FIGURE 1

SOFA score. According to the Sepsis-3 definitions (Singer et al., 2016), sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. For clinical operationalization, organ dysfunction can be represented by an increase in the Sequential [Sepsis-related] Organ Failure Assessment (SOFA) score of two points or more. The SOFA score is based on grading six vital systems: respiration (PaO₂/FiO₂), coagulation (number of thrombocytes), the liver (bilirubin plasma levels), the cardiovascular system (mean arterial pressure and vasopressor support), the central nervous system (Glasgow Coma Scale), and the renal system (creatinine plasma levels, urine output), each of them scoring 0–4 points. In porcine experiments conducted under general anesthesia, the Glasgow coma scale-based neurologic component is excluded.

FIGURE 2

Chronic instrumentation and monitoring in porcine sepsis experiments. The experimental setting may include: central venous catheter for drug and fluid infusion (inserted through the left jugular vein); balloon-tipped thermodilution pulmonary artery catheter (placed via the right jugular vein); femoral arterial catheter for blood pressure recording and blood sampling; fiberoptic catheter for thermal-dye double-indicator dilution measurements; ultrasound transit time flow probes (e.g., around the portal vein, the common hepatic artery, and the left renal artery); catheters in the portal, renal, and hepatic veins; laser Doppler flowmetry for monitoring ileal mucosal and renal cortex microcirculation; cystostomy catheter for urine collection (percutaneous insertion under ultrasound guidance).

FIGURE 3

Flowchart of porcine sepsis experiments. After induction of general anesthesia and mechanical ventilation, the experiment starts with a surgical preparatory phase, during which access to various vital organ systems is secured and chronic instrumentation devices (see Figure 2) are installed. A postsurgical stabilization period of 6 h is allowed before baseline measurements are obtained (time point: 0 h). Afterwards, sepsis is induced either by bacteremia (continuous central venous infusion of the bacteria of interest, e.g., live P. aeruginosa) or fecal peritonitis (inoculating autologous feces suspended in saline into the abdominal cavity through the drainage tubes). Sepsis progression is followed for 24 h, during which, in a usual setting, irreversible septic shock develops. Throughout the experiment, general anesthesia and mechanical ventilation are maintained with proper volume resuscitation to maintain cardiac filling pressures and administration of vasopressor support, if needed, to maintain the mean blood pressure above 65 mmHg. Continuous hemodynamic monitoring is accompanied by blood and urine sampling for biochemical analyses at the time points of interest (usually 6, 12, 18, and 24 h from the induction of sepsis). At the end of the in vivo experiment, the animals are euthanized by anesthetic overdose and excision of the heart. The total duration of the in vivo porcine sepsis experiment is 36 h and it is followed by in vitro analysis of tissues, cells, and subcellular organelles.

FIGURE 4

Selected parameters of sepsis progression in a porcine model of bacteremia- or peritonitis-induced sepsis. The bacteremia was induced by continuous central venous infusion of live P. aeruginosa (strain O1 isolated from a patient with suppurative otitis, 1 x 109 colony-forming units/mL) (Stengl et al., 2010a); the peritonitis was induced by inoculating autologous feces (0.5–2 g·kg-1, suspended in 200 mL saline) into the abdominal cavity (Jarkovska et al., 2016). Peritonitis, n = 13; bacteremia, n = 14; based on our previous publications (Stengl et al., 2008; 2010a; 2010b). TP1–4: time points 0, 12, 18, and 24 h after sepsis induction. Data are presented as the mean ± SD; *p < 0.05, vs. TP1; #p < 0.05, bacteremia vs. peritonitis. (A) Similar progression of sepsis in pigs with bacteremia and peritonitis documented by rising SOFA scores. (B) Similar hemodynamic alterations in both peritonitis- and bacteremia-induced sepsis documented by reduced systemic vascular resistance (SVR). (C) Plasma levels of lactate as example of differential response in peritonitis- and bacteremia-induced sepsis.