It has been reported that some of the local anaesthetic agents possess antimicrobial activity against clinically-significant bacteria. Although bupivacaine exhibits a bacteriostatic effect at concentrations above 0.25% there are concerns that it might interact with some of the other antibiotics administered to patients. Whilst these interactions may be potentially benign, the risk is that they are antagonistic and that local bupivacaine might predispose the patient to a higher risk of infection.

Bupivacaine is commonly administered as a local anaesthetic following knee arthroplasy; the purpose of this study was to assess its potential interactions with gentamicin eluting from the cement used to fix the device.

A strain of Saphylococcus aureus (29213) with established susceptible Minimal Inhibition Concentration (MIC) and Minimal Bactericidal Concentration (MBC) for gentamicin was used. This organism was inoculated into four types of broth; Mueller-Hinton broth (MH), MH with different concentrations of gentamicin, MH with 0.25% and 0.125% bupivacaine and MH with various combinations of gentamicin and bupivacaine. The broths were incubated at 37C and at 0.5, 1, 2, 3, 6 and 24 hours post inoculation the number of bacteria remaining were counted. From these data kill-curves were generated describing the absolute and individual rates of killing seen with bupivacaine and gentamicin alone and when in combination.

Bupivacaine showed a bacteriostatic effect only at concentrations of 0.25% and higher. All concentrations of gentamicin above or equal to the expected MBC showed bactericidal effect. However, in combination with both strengths of Bupivacaine (0.25 and 0.125%) the bacteriocidal effect of gentamicin was seen at a lower concentration and the rate of killing of bacteria was enhanced.

Bupivacaine has bacteriostatic effect at concentrations above 0.25% in line with published data. In these experiments we have shown that the use of bupivacaine together with gentamicin does not reduce the bactericidal property of the antibiotic and that the bactericidal effect of gentamicin appears to be enhanced by bupivacaine. This would suggest that the local use of bupivacaine is unlikely to increase the risk of infection in patients undergoing knee arthroplasty and may actually be beneficial.