An ovine septic shock model of live bacterial infusion
Authors: Obonyo Nchafatso G. , Raman Sainath , Suen Jacky Y. , Peters Kate M. , Phan Minh-Duy , Passmore Margaret R. , Bouquet Mahe , Wilson Emily S. , Hyslop Kieran , Palmieri Chiara , White Nicole , Sato Kei , Farah Samia M. , Gandini Lucia , Liu Keibun , Fior Gabriele , Heinsar Silver , Ijuin Shinichi , Kyun Ro Sun , Abbate Gabriella , Ainola Carmen , Sato Noriko , Lundon Brooke , Portatadino Sofia , Rachakonda Reema H. , Schneider Bailey , Harley Amanda , See Hoe Louise E. , Schembri Mark A. , Li Bassi Gianluigi , Fraser John F. 
Published: October 2024

Abstract
Background
Escherichia coli is the most common cause of human bloodstream infections and bacterial sepsis/septic shock. However, translation of preclinical septic shock resuscitative therapies remains limited mainly due to low-fidelity of available models in mimicking clinical illness. To overcome the translational barrier, we sought to replicate sepsis complexity by creating an acutely critically-ill preclinical bacterial septic shock model undergoing active 48-h intensive care management.

Aim
To develop a clinically relevant large-animal (ovine) live-bacterial infusion model for septic shock.

Methods
Septic shock was induced by intravenous infusion of the live antibiotic resistant extra-intestinal pathogenic E. coli sequence type 131 strain EC958 in eight anesthetised and mechanically ventilated sheep. A bacterial dose range of 2 × 105–2 × 109 cfu/mL was used for the dose optimisation phase (n = 4) and upon dose confirmation the model was developed (n = 5). Post-shock the animals underwent an early-vasopressor and volume-restriction resuscitation strategy with active haemodynamic management and monitoring over 48 h. Serial blood samples were collected for testing of pro-inflammatory (IL-6, IL-8, VEGFA) and anti-inflammatory (IL-10) cytokines and hyaluronan assay to assess endothelial integrity. Tissue samples were collected for histopathology and transmission electron microscopy.

Results
The 2 × 107 cfu/mL bacterial dose led to a reproducible distributive shock within a pre-determined 12-h period. Five sheep were used to demonstrate consistency of the model. Bacterial infusion led to development of septic shock in all animals. The baseline mean arterial blood pressure reduced from a median of 91 mmHg (71, 102) to 50 mmHg (48, 57) (p = 0.004) and lactate levels increased from a median of 0.5 mM (0.3, 0.8) to 2.1 mM (2.0, 2.3) (p = 0.02) post-shock. The baseline median hyaluronan levels increased significantly from 25 ng/mL (18, 86) to 168 ng/mL (86, 569), p = 0.05 but not the median vasopressor dependency index which increased within 1 h of resuscitation from zero to 0.39 mmHg−1 (0.06, 5.13), p = 0.065, and. Over the 48 h, there was a significant decrease in the systemic vascular resistance index (F = 7.46, p = 0.01) and increase in the pro-inflammatory cytokines [IL-6 (F = 8.90, p = 0.02), IL-8 (F = 5.28, p = 0.03), and VEGFA (F = 6.47, p = 0.02)].

Conclusions
This critically ill large-animal model was consistent in reproducing septic shock and will be applied in investigating advanced resuscitation and therapeutic interventions.