Introduction 1 2 3 4 5 4 6 11 12 15 2 2 The aim of this paper is to provide useful insights into the use of animal models to study bacterial translocation during acute pancreatitis, in the light of current knowledge of pathophysiology. Animal Species and Housing Conditions Before the late 1970s, larger laboratory animals such as dogs were predominantly used to study acute pancreatitis. But since the introduction of models of acute pancreatitis in small laboratory animals, mice or rats are generally favored for financial and ethical or practical reasons. Because of physiological and anatomical differences between species, choice of laboratory animal has important implications on the study results and extrapolation to the human situation. 16 18 19 20 21 22 23 Experiments using small animals (e.g., mouse or rat) usually incorporate a larger number of animals compared to experiments with large laboratory animals (e.g., cat or dog). The use of a larger number of small laboratory animals improves statistical power of an experiment. On the other hand, the use of larger animals could resemble human pathophysiology better, but a smaller number of animals means lower statistical power and increased potential false negative or false positive results. Models of Acute Pancreatitis 1 2 Table 1 Characteristics of Several Animal Models of Acute Pancreatitis Model Animal Species Pancreatic Necrosis Pancreatic Infection Mortality Invasiveness 24 25 Rat No Considerable High Laparotomy 30 32 Mouse Yes Little High Minimal 34 37 Rat/opossum No/Yes Little Low Laparotomy 44 Mouse/rat Yes/No Little Low Minimal 48 Rat/dog/pig Yes Considerable Moderate to high Laparotomy 52 Rat Yes Considerable Moderate Laparotomy Table 2 Aspects of Bacterial Translocation and Potential Confounding Factors of Animal Models Aspect Confounding Factor Model Intestinal motility and flora Animal species Potentially all models Housing conditions (SPF) Potentially all models Diet CDE diet Analgesics Invasive models Laparotomy Invasive models Bile flow Duct ligation Cerulein Cerulein models Intestinal manipulation Invasive models Mucosal barrier function Stress Potentially all models  Diet CDE diet Anesthetics Invasive models Pancreatic proteases Duct ligation Intestinal manipulation/puncture Duct perfusion Immune system Stress Potentially all models Diet CDE diet Disease course/severity Species-dependent Obstructive jaundice Duct ligation, duodenal loop Intestinal manipulation Invasive models Duodenal Loop 24 25 26 27 29 Ethionine-supplemented Choline Deficiency 30 31 32 32 31 33 Biliopancreatic Duct Ligation 34 37 28 38 39 40 42 Cerulein Infusion 43 44 45 46 6 47 Biliopancreatic Duct Perfusion 48 48 49 50 48 49 51 52 2 53 Advantages of this model are the quick procedure of acute pancreatitis induction and the reproducibility of results. Other than duodenal puncturing and intestinal handling during surgery, both potentially affecting mucosal barrier function, no direct effects on intestinal flora or immune function are expected in this model. Biliopancreatic Duct Injection and Cerulein Hyperstimulation 52 2 52 Disease Course 54 2 55 56 52 57 2 4 58 Severity 1 32 52 59 60 33 Culturing, Controls, and Route of Bacterial Translocation 61 62 63 Proper sterile surgical techniques are very important when investigating bacterial translocation. If abdominal surgery is involved, control cultures of the peritoneal cavity to trace surgical contamination are of special importance. If peritoneal cultures are found to be positive, extra caution should be taken with interpretation of bacteriological analysis of abdominal organs. In case of surgical contamination or transperitoneal bacterial translocation, the peritoneal covering of the organ samples might be the cause of positive organ cultures, not the bacterial colonization in the organ itself (false positive culture). 15 6 14 15 64 66 Discussion 6 Escherichia coli Enterococcus 67 69 70 71 72 73 74 75 74 76 Although animal models were proven indispensable in acute pancreatitis research, model-related problems are most likely the reason for important questions on pathophysiology and treatment strategies to remain unanswered. Current topics of debate include the route and origin of bacterial translocation and optimal prophylaxis and treatment strategies. 77 78 79 80 6 61 81 52 80 23 When experimentally evaluating therapy, treatment often starts before induction of acute pancreatitis. Obviously, this is an important reason why results cannot directly be translated to the clinical situation. On the other hand, these experimental studies provide proof of principle concerning the tested therapy. If prophylactically successful, the tested treatment strategy might be beneficial when started after the onset of acute pancreatitis and should therefore be further investigated. On the other hand, the faster course of acute pancreatitis in rodent models provides only a very short treatment window between the onset of the disease and early or late phase complications. This may lead to false negative effects of the therapy tested. In conclusion, animal models of acute pancreatitis are indispensable tools, but model-related drawbacks often interfere with one or more pathophysiological aspects of bacterial translocation, complicating interpretation of results. When the ideal model of acute pancreatitis is not at hand, it is of major value that numerous alternatives are available. But with each experimental hypothesis, special care should be taken to select the most suitable model. Despite all the experimental work done, the route by which pancreatic infection occurs and gives rise to septic complications and mortality has not yet fully been elucidated. Optimal prophylactic and treatment strategies are also still widely debated. In the future, animal models will undoubtedly provide increasing understanding of these subjects, but model-related drawbacks should always be kept in mind when designing a study or when interpreting results.