Bactericidal activity of 0.5% bupivacaine with preservatives on microorganisms in the human skin flora

The bactericidal activity of 0.5% bupivacaine with preservatives at body temperature and at room temperature is not known. We studied the bactericidal activity of 0.5% bupivacaine with 0.08% methyl para-oxybenzoate and 0.02% propyl para-aminobenzoate as preservatives and of the preservatives alone at 37 degrees C and at room temperature on two strains of methicillin-resistant Staphylococcus aureus, two strains of methicillin-susceptible S. aureus, and one strain each of Staphylococcus epidermidis and Escherichia coli.

The pathogen was exposed to 0.5% bupivacaine with preservatives or to the preservatives alone for 1, 3, 6, 12, and 24 hours at 37 degrees C and at room temperature. The inocula from these suspensions were cultured for 48 hours at 37 degrees C after the antimicrobial activity of bupivacaine was inactivated by 1:1,000 dilution with physiological saline.

The 1- through 12-hour exposures of four strains of S. aureus to 0.5% bupivacaine with preservatives at room temperature reduced the mean colony count by 24.2%, 49.2%, 71.3%, and 89.6%, respectively, and the exposure at 37 degrees C reduced the count by 74.1%, 95.2%, 99.9%, and 99.8%, respectively. The differences for 1- through 12-hour exposures were significant (P < .001). The percentage kill in the strains of E. coli and S. epidermidis was significantly higher than that in the strains of S. aureus at all exposure times at room temperature (E. coli, P < .001; S. epidermidis, P < .0001) and at 1- and 3-hour exposures at 37 degrees C (E. coli, P < .001; S. epidermidis, P < .0001). The bactericidal activity of the preservatives was markedly lower that that of 0.5% bupivacaine with preservatives (P < .0001).

The bactericidal activity of 0.5% bupivacaine with preservatives is stronger at body temperature than at room temperature; the bactericidal activity may be due, to a large extent, to bupivacaine rather than to the preservatives; and S. aureus is more resistant to the bactericidal activity of bupivacaine than are S. epidermidis and E. coli.