# Soil-Dwelling Bacteria Display Tolerance Against Typical Antibiotic Serum Levels in an Ex Vivo Model of Traumatized Tissue

## TL;DR

Soil bacteria in traumatic wounds are resistant to typical antibiotic levels, suggesting a need for stronger local treatments.

## Contribution

Demonstrates antibiotic tolerance of soil microbes in traumatized tissue using an ex vivo model.

## Key findings

- Serum-level antibiotics failed to significantly reduce microbial load in soil-contaminated tissue.
- Only 10× higher antibiotic concentrations achieved significant bioburden reduction.
- Fungal species were the most commonly cultured organisms in soil-contaminated samples.

## Abstract

Military wounds often contain large amounts of necrotic, avascular tissue that provide an ideal home for environmental and endogenous flora to dwell. Debridement mitigates infection but cannot be performed until the casualty is transported to a surgically equipped site, sometimes days after initial injury. Meanwhile, opportunistic pathogens spread throughout the wound site. Systemic antibiotic therapies fail to eradicate these soil microorganisms potentially because they dwell in an antibiotic-tolerant phenotype. Laboratory tests that incorporate environment soil may promote the development of point-of-care therapies effective against biofilm-like organisms. We hypothesized that, in an ex vivo model of traumatized tissue, blood-serum antibiotic concentrations would fail to eradicate soil-dwelling organisms.

Sterile, excised tissue was collected from concurrent sheep studies. Tissue samples were coated with topsoil that was confirmed to contain naturally occurring viable organisms, then blasted with an air cannon to simulate trauma and distribute soil throughout. The tissue test environment was then moved into a 6 well-plate for treatment with saline suspensions of systemic levels of antimicrobials, or a 10× concentration of the same. Following a 24-hour incubation, tissues and remaining treatment was diluted and quantified on selective and non-selective agar plates. Colony forming units (CFU)/g of tissue were calculated between agar type and treatment group than compared using a Student’s t-test.

Non-treated, soil-contaminated tissues had an average of 8.43 ± 0.36 log10 CFU/g tissue. Fungal species were the most common organism cultured among all groups, though this may change between soil samples. Serum-level antimicrobials failed to reduce bioburden to a significant degree. Only 10× systemic concentrations achieved statically significant (P < .001) bioburden reduction and lowered organisms to below 5 log10 CFU/g tissue.

Outcomes suggested that antimicrobial levels that would typically be administered prophylactically fail to kill soil microbes dwelling in compromised tissue. The findings support the premise for local, high-dose antimicrobial therapies capable of managing biofilm-life bacterial contamination. These results translate towards improved triage methods: to prolong the time from injury to debridement, anti-infective technologies must have the potential to kill organisms that otherwise tolerate low levels of antimicrobials. Systemic prophylaxis alone may be insufficient.