Bactofencin A Displays a Delayed Killing Effect on a Clinical Strain of Which Is Greatly Accelerated in the Presence of Nisin.

Bacteriocins can be considered a novel source of natural alternatives to antibiotics or chemical food additives with the potential to fight against clinical and food pathogens. A number have already been commercialised as food preservatives, but they also have the potential to treat drug-resistant clinical pathogens and can play a role in immune modulation. To achieve their full potential, an understanding of their mode of action is required. Bactofencin A and nisin A were purified to homogeneity by reversed-phase HPLC and their effect on the mastitis pathogen DPC5246 was assessed by cell viability assays and flow cytometry. We report that bactofencin A displays a delayed inhibitory effect against the mastitis pathogen, DPC5246, suggesting an unusual mode of action. This characteristic was clearly visible on BHI plate media, where formation of inhibition zones against the staphylococcal strain took 23 h compared to 6 h for the well-characterised nisin. This delayed killing and injury was also demonstrated using flow cytometry, where damage was evident 4 h after bacteriocin addition. Treatment with 2 μM bactofencin A resulted in approximately 20-fold higher numbers of injured and 50-fold higher numbers of dead cells when compared to untreated cells. Combining bactofencin A with the lantibiotic nisin A resulted in faster killing at lower bacteriocin concentrations. When combined in an equal ratio, the combination exhibited a 4-fold increase in inhibition compared to nisin A alone. These results demonstrate that the combination may be very effective in therapeutic applications against pathogenic staphylococci.