# Assessing the Effectiveness of Antibiotic Irrigation to Reduce Bacterial Load at the Spinal Surgical Site: An In-Vitro Study

## TL;DR

This study tests if antibiotic irrigation during spine surgery can reduce bacteria, finding that Gentamicin works against Pseudomonas aeruginosa but not other bacteria.

## Contribution

Demonstrates that Gentamicin is effective against Pseudomonas aeruginosa in surgical irrigation, but not for other tested bacteria-antibiotic pairs.

## Key findings

- Gentamicin inhibited Pseudomonas aeruginosa growth after both 5-minute and 8-hour exposures.
- Vancomycin and Cefazolin did not significantly inhibit bacterial growth in other tested combinations.
- Short-term and long-term antibiotic exposure had no effect on most bacteria-antibiotic pairs.

## Abstract

Introduction

Intra-operative surgical site irrigation with antibiotics is believed to reduce the risk of infection in spine surgeries involving instrumentation. However, despite its frequent use, there is limited supporting evidence for this practice. This prospective in-vitro study aims to evaluate the effect of short-term antibiotic exposure on the growth of common pathogens associated with wound infections. Furthermore, this study aims to determine the optimal duration of antibiotic exposure to eradicate common surgical site infection organisms.

Methods

A suspension of one of three micro-organisms: (1) Staphylococcus aureus, (2) Staphylococcus epidermidis, or (3) Pseudomonas aeruginosa was added to 2-ml vials of an enriched medium, containing one of three antibiotics: (a) Vancomycin, (b) Gentamicin, or (c) Cefazolin. The final inoculum of each micro-organism was 103 CFU/ml, representing a contaminated surgical wound in spine surgery. Antibiotics were washed out from the suspension by a centrifugation technique after (i) 5 minutes, or (ii) 8 hours. The recovery of growth of the micro-organisms was monitored by laser light scattering technology.

Results

P. aeruginosa inoculated in vials with Gentamicin showed no 24-hour bacterial growth after 5-minute and 8-hour exposure to the antibiotic. Vials of all other bacteria-antibiotic combinations showed bacterial growth curves similar to the control vials after both 5-minute and 8-hour exposures to antibiotics, with no signs of bacterial growth inhibition.

Conclusion

The study demonstrated that Gentamicin effectively inhibited P. aeruginosa growth after both short-term (5-minute) and long-term (8-hour) exposures. However, no significant bacterial growth inhibition was observed with other bacteria-antibiotic combinations, regardless of the exposure time. These findings suggest that while Gentamicin may be effective against P. aeruginosa in the context of surgical site irrigation, the use of Vancomycin and Cefazolin does not appear to provide the same level of effectiveness for the other tested pathogens. Further studies are needed to evaluate alternative antibiotic strategies for broader infection control in spine surgeries.