shown in [Fig. 2](#f2){ref-type="fig"}, at 24 hours, morphine alone induced bacterial translocation into the peritoneal cavity, MLN, liver, and spleen, confirming that morphine itself can compromise the gut epithelial barrier, as previously reported[@b8]. In the placebo-treated CLP animals, the bacterial load in the peritoneal lavage fluid, MLN, liver, spleen, and blood reached its highest levels at 24 hours, and subsequently decreased at 72 hours. At 168 hours after the CLP, almost all bacteria that disseminated into the peritoneal organs were cleared in the placebo-treated animals. In the morphine-treated CLP mice, a significant increase in the amount of bacteria was observed in the peritoneal organs and blood compared with the placebo-treated mice at both 24 and 72 hours, indicating that morphine promoted bacterial dissemination and inhibited bacterial clearance. Bacterial translocation in liver homogenates was further evaluated using bacterial 16 s ribosomal DNA (rDNA). Consistent with data obtained using bacterial cultures and colony forming units, the bacterial load as measured by 16 s rDNA was also significantly higher in the morphine treated-CLP animals than all other groups ([Supplementary Fig. 1](#S1){ref-type="supplementary-material"}). Because none of the animals survived beyond 96 hours in the morphine-treated CLP group, data regarding the bacterial load were not available for the last time point.

Morphine treatment promotes Gram-positive bacterial dissemination and modulates the gut microbiome
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We next serotyped the bacterial species that disseminated into the MLN, liver, and spleen following CLP. In placebo-treated CLP animals, the most common bacterial species detected in the MLN, spleen, and liver were non-hemolytic Escherichia coli ([Fig. 3a](#f3){ref-type="fig"}), which are common Gram-negative commensal bacteria resident in the gut lumen; very few Enterococcus species were detect