Bactericidal activity of clinically used local anesthetics on Staphylococcus aureus

The rate of onset of antimicrobial activity of local anesthetics is unknown. Similarly, whether the activity is bactericidal or bacteriostatic is also unknown. The antimicrobial activity of local anesthetics with preservatives has not been studied. This study investigated the rate and potency of the antimicrobial activity of 0.125%, 0.25%, and 0.5% bupivacaine, 2.0% mepivacaine and 2.0% lidocaine with preservatives, and 2.0% lidocaine without preservatives on two strains of methicillin-resistant Staphylococcus aureus.

The pathogen was exposed to each local anesthetic for 1, 3, 6, 12, and 24 hours at room temperature. The inocula from these suspensions were diluted to 1:1,000 with physiological saline to inactivate the antimicrobial activity of the local anesthetics and then were cultured for 24 hours at 37 degrees C on agar plates.

Lower colony counts were observed with a 3-hour or longer exposure to 0.5% bupivacaine in both strains of S. aureus (P < .05). The 3-hour exposure reduced the count by approximately 60%, the 6-hour exposure by 70%, and the 24-hour exposure by more than 99%. The bactericidal activity was lowest with 0.125% bupivacaine and 2.0% mepivacaine.

Antimicrobial activity was observed shortly after exposure of S. aureus to local anesthetics and appeared to be bactericidal rather than bacteriostatic. However, the observed bactericidal activity, although it developed rapidly, may be insufficient to account for the low incidence of epidural infection related to epidural cannulation.