Bactericidal activity of preservative-free bupivacaine on microorganisms in the human skin flora

The rate at which the bactericidal activity of preservative-free bupivacaine develops at body temperature and at room temperature is not known. We studied the bactericidal activity of preservative-free bupivacaine on two strains of methicillin-resistant Staphylococcus aureus (MRSA), two strains of methicillin-susceptible S. aureus (MSSA), and each of Staphylococcus epidermidis and Escherichia coli.

The pathogen was exposed to 0.5% bupivacaine for 1, 3, 6, 12, and 24 h at 37 degrees C and room temperature. In addition, each strain of MRSA, MSSA, and S. epidermidis was exposed to distilled water, and 0.125%, 0.25%, 0.5%, and 0.75% bupivacaine at 37 degrees C. The inocula from the suspensions were cultured for 48 h at 37 degrees C.

The 1- through 24-h exposures of 4 strains of S. aureus to 0.5% bupivacaine at room temperature reduced the colony count by 21.7%, 34.7%, 51.1%, 65.6%, and 81.1%, respectively, and the exposure at 37 degrees C reduced the count by 34.1%, 50.8%, 66.3%, 94.5%, and 96.0%, respectively. The differences were significant at all exposure times (P < 0.001, respectively). No organisms grew in the strain of E. coli after 24-h exposure and in the strain of S. epidermidis after 12- and 24-h exposures at 37 degrees C. The percent change from controls 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 and 37 degrees C (P < 0.0001, respectively). Higher concentrations of bupivacaine were associated with lower colony count.

Our results show that preservative-free bupivacaine possesses a temperature- and concentration-dependent bactericidal activity, and S. aureus is more resistant to the bactericidal activity of bupivacaine than are S. epidermidis and E. coli.