Biofilm formation on endovascular aneurysm repair (EVAR) grafts– a proof of concept in vitro model

Objectives
An endovascular aneurysm repair (EVAR) graft is a catheter-implanted vascular prosthesis and is the preferred treatment for patients with aortic aneurysm. If an EVAR graft becomes the focus of infection, the treatment possibilities are limited as it is technically difficult to remove the graft to obtain source control. This study examines whether Pseudomonas aeruginosa and Staphylococcus aureus form biofilm on EVAR prostheses.

Methods
EVAR graft sections were exposed to bacteria at 102 or 108 colony forming units (CFU)/ml in lysogeny broth and Krebs-Ringer at 37°C, bacterial biofilm formation was evaluated by scanning electron microscopy (SEM) and counting CFU on the graft sections following antibiotic exposure at x10 minimal inhibitory concentration (MIC). Bacteria were tested for tolerance to benzyl penicillin, tobramycin and ciprofloxacin.

Results
Bacterial exposure for 15 minutes established biofilms on all prosthesis fragments (6/6 replicates). After 4 hours, bacteria were firmly attached to the EVAR prostheses and resisted washing. After 18 to 24 hours the median CFU/g of EVAR graft reached 5.2 x 108 (1.15 x 108 – 1.1 x 109) for S. aureus and 9.1 x 107 (3.5 x 107 – 6.25 x 108) for P. aeruginosa. SEM showed bacterial attachment to the graft pieces. There was a time-dependent development of tolerance with approximately 20 (tobramycin), 560 (benzyl penicillin), and 600 (ciprofloxacin) times more S. aureus surviving antibiotic exposure in 24 compared to 0 hours old biofilm. Five (tobramycin) and 170 times (ciprofloxacin) more P. aeruginosa survived antibiotic exposure in 24 compared to 0 hours old biofilms.

Conclusions
Our results show that bacteria can rapidly adhere to and subsequently form antibiotic tolerant biofilms on EVAR graft material in concentrations equivalent to levels seen in transient bacteremia in vivo. Potentially, the system can be used for identifying optimal treatment combinations for infected EVAR prosthesis.