Commensal bacteria do translocate across the intestinal barrier in surgical patients

Background The “gut origin of sepsis” hypothesis proposes that enteric bacteria may cause sepsis at distant extra-intestinal sites. Whilst there is much circumstantial evidence to support this hypothesis, there is no conclusive proof in humans. The nature of translocating bacteria remains unclear. The aim of this study was to establish the origin of Escherichia coli (E. coli) cultured from mesenteric lymph nodes (MLN) and determine if they belonged to any recognized pathotypes known to cause infections in humans. Methods MLN and faecal samples were obtained from 98 patients undergoing colonic resection. E. coli were isolated from 9/98 MLN samples. DNA fingerprints of MLN isolates were compared with faecal isolates from the same patient. MLN isolates were tested for adherence and invasion using HEp-2 epithelial cells, and screened for DNA markers indicative of different pathotypes of E. coli. MLN isolates were also tested for internalisation into Caco-2 cells. Results All the nine E. coli cultured from MLNs were found to have identical DNA fingerprints to at least one and often several E. coli isolates cultured from faecal samples of the same patient. 8/9 (89%) MLN isolates were weakly adherent and 2/9 (22.2%) were invasive. 8/9 (89%) tested negative for DNA markers. All the nine MLN strains were internalised by Caco-2 cells. Conclusion This study confirms the gut origin of translocating bacteria. Most translocating E. coli do not belong to any recognised pathotype and are therefore normal commensal microflora. Our results suggest that bacterial translocation is more dependent upon the gut epithelium rather than the virulence properties of resident enteric bacteria.