Antibacterial properties of lidocaine.

The inhibitory effect of lidocaine and lidocaine plus methylparaben as a preservative was studied with 25 strains of bacteria. The tested strains were mixed with each topical anesthetic preparation, and quantitative cultures were performed on aspirates at 0, 30, 60, and 120 minutes. Results were compared to counts obtained in a lactated Ringer's solution, which were sampled at the same intervals. Lidocaine (1 percent solution) reduced quantitative counts after 120 minutes of contact by over one log for only six of the 25 strains tested. Lidocaine plus methylparaben reduced quantitative counts to a somewhat greater extent; nevertheless, all strains except Bacteroides melaninogenicus were recovered in relatively high concentrations at each sampling interval. Numerous previous studies have shown that topical anesthetic agents are toxic to bacteria. The results of this study show that this toxic effect is not sufficiently severe to prevent the recovery of most pathogens of the lower respiratory tract in bronchoscopic aspirates.