Inhibition of bacterial growth by lignocaine in propofol emulsion

Contamination of propofol, in an emulsion formulation, has been associated with infective complications. Local anaesthetics,some of which are known to have antibacterial properties, are frequently added to the solution to reduce pain on injection. We examined the growth rates of E. coli, S. aureus, S. epidermidis and P. aeruginosa in propofol with and without lignocaine 0.1%-2% after incubation for 2, 5 and 24 hours at 37 degrees C. Growth of microorganisms in each solution was compared by counting the number of colony forming units (CFU). Propofol supported the growth of all microorganisms. An increase in the number of CFUs was observed in all drug combinations 2, 5 and 24 hours after inoculation except for S. aureus (P<0.05). No difference was found in CFU numbers between 2 and 5 hours for this microorganism. With E. coli, a significant decline in colony counts was observed in mixtures of 1% and 2% lignocaine (P<0.05). With the other microorganisms only 2% lignocaine showed a significant reduction in the number of CFUs (P<0.05). We conclude that lignocaine in recommended clinical doses (0.05%-0.1%) did not exhibit adequate antibacterial activity to prevent infective complications.