| The growth of the foodborne pathogen Listeria monocytogenes can be controlled by nisin, an antimicrobial peptide. A spontaneous mutant of L. monocytogenes shows both resistance to nisin and increased acid sensitivity compared to the wild type. Although the mutant is more sensitive to both hydrochloric and lactic acids compared to the wild-type, the net influx of protons occurs more slowly when HCl is used compared to lactic acid for both strains.; A survey of the salts of lactic acid show that the synergy that exists between Zn and Al lactates and nisin to inhibit the wild-type strain does not occur for the nisin resistant strain. The mechanism of the synergy suggests that the metal ions interact with the cell membrane to facilitate the action of nisin in the wild-type strain.; The changes in the cell membrane that were previously thought to explain nisin resistance are not related to acid sensitivity. Changes in the lipid composition do not result in altered proton permeability of the mutant, and neither strain experiences membrane damage resulting from acid treatment. However, upon hydrochloric or lactic acid addition, intracellular ATP levels drop significantly in the mutant (P < 0.01) but not the wild type. Characterization of the F0F1 ATPase, which hydrolyzes ATP to pump protons from the cell cytoplasm, shows that the enzyme is more active in the mutant than the wild type, having nearly double the catalytic efficiency. These data support a model where, upon acid addition, the overactive ATPase depletes the mutant's supply of ATP, resulting in cell death.; A proteomic approach was taken to further examine differences. Because two-dimensional electrophoresis of membrane proteins shows that the mutant is lacking 5 proteins that the wild-type strain expresses, and underexpresses 5 and overexpresses 3 proteins compared to the wild type, a key regulator may be altered in the mutant. These protein changes may be responsible for the altered phenotypes of the mutant. Pulsed field gel electrophoresis resolved different banding patterns between the chromosomal DNA of the strains. This change in the genetic code could result in the myriad of protein changes observed. |
