Establishment of an in vivo rat model for chronic musculoskeletal implant infection

Eivind Witsø,Kåre Bergh,Kirsti Løseth,Linh Hoang

doi:10.1186/s13018-020-1546-6

Abstract

BackgroundThe aim of the study was to establish an experimental chronic musculoskeletal infection model in vivo characterized by (a) a small bacterial inoculum, (b) no general or local signs of infection, (c) several parallels (implants) in each animal and finally (d) a model that is technically easy to perform.MethodsBone xenografts with steel plates were implanted intramuscularly in rats. To the xenografts, different inocula of Staphylococcus aureus and two strains of Staphylococcus epidermidis were added. The animals were observed for different time periods before the removal of the xenografts. The xenografts and steel plates were subjected to quantitative bacterial culture after sonication. Additional steel plates were subjected to scanning electron microscopy (SEM) for visualization of biofilm formation.ResultsInoculation of bone grafts with S. aureus did produce a pyogenic infection in all animals. A chronic infection was established in rats where the bone grafts were inoculated with S. epidermidis. A bacterial inoculum of 100 colony-forming units (CFU) of S. epidermidis was adequate as a minimum infective dose. During a period of up until 42 days, the animals infected with S. epidermidis had no general or local signs of infection. According to the results of the quantitative bacterial culture of sonicate fluid and SEM, a biofilm was developed on all implants.ConclusionIn the present in vivo model, a very small bacterial inoculum succeeded in establishing a chronic musculoskeletal implant infection where a biofilm was formed on the implants. The experimental model is easy to perform and allows several implants in each animal. The model could be useful for the study of biofilm formation in vivo on different implants and different surfaces.

Highlights

The aim of the study was to establish an experimental chronic musculoskeletal infection model in vivo characterized by (a) a small bacterial inoculum, (b) no general or local signs of infection, (c) several parallels in each animal and (d) a model that is technically easy to perform
Animals infected with S. aureus (ATCC 25923) In brief, inoculating the bone grafts with S. aureus resulted in an acute postoperative infection, and none of the animals (n = 13) infected with S. aureus developed a chronic infection
We found that a bone xenograft with implants contaminated with a very low inoculum of S. epidermidis in this in vivo model resulted in a chronic implant infection

Summary

Introduction

The aim of the study was to establish an experimental chronic musculoskeletal infection model in vivo characterized by (a) a small bacterial inoculum, (b) no general or local signs of infection, (c) several parallels (implants) in each animal and (d) a model that is technically easy to perform. The implant has to be removed, and the final functional result is often much inferior to what the patient would else have expected. These infections are considered to represent biofilm infections [1,2,3]. The results of in vitro biofilm studies are not readily reproduced in vivo [5,6,7], and in vitro biofilm models are not models for the chronic in vivo infection [4]. Several experimental models have been used to study the infected orthopaedic implant per se and biofilm formation on the implant in particular [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]

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