Rifampicin impregnated Dacron grafts: no development of rifampicin resistance in an animal model.

AIM

Rifampicin impregnated Dacron grafts have been shown to be effective at preventing vascular graft infection in different animal models. The development of resistance to rifampicin would be a major drawback to the widespread use of such a graft. We aimed to determine how readily this would occur by using a sheep animal model.

METHODS

Under general anaesthetic a 2cm long, 5mm diameter Dacron interposition graft inpregnated with 1.2 mg/ml rifampicin was placed in the left carotid artery. An extreme challenge of methicillin resistant Staphylococcus aureus (MRSA) using an inoculum of 10(9) colony forming units was placed directly onto the graft. The grafts were harvested at 3 weeks and cultures of the graft and tissues were taken. The presence or absence of any abscess formation, anastomotic disruption and graft thrombosis was noted. Any positive growths were identified and if found to be the same as the inoculum, the bacteria were used as the inoculum for another sheep. This was repeated once more. Thus we started with three sheep initially and used a total of nine sheep.

RESULTS

There were no deaths. All grafts were infected with the same MRSA strain, confirmed on phage typing. There were three abscess and one anastomotic disruption. Seven of the grafts were occluded. The minimal inhibitory concentration (MIC) of the infecting inoculum and the bacteria retrieved were determined using the agar dilutional method. The MIC for the three initial inocula was < 0.007 mg/l. All subsequent strains isolated had an MIC of < 0.015 mg/l. This was a difference of one dilution and not significant.

CONCLUSION

There was no development of rifampicin resistance using this animal model.