High pressure increases bactericidal activity and spectrum of lactoferrin, lactoferricin and nisin

We have studied the inactivation of a panel of eight test bacteria (two Escherichia coli strains, Salmonella enteritidis, Salmonella typhimurium, Shigella sonnei, Shigella flexneri, Pseudomonas fluorescens and Staphylococcus aureus) by high pressure in the presence of bovine lactoferrin (500 microg/ml), pepsin hydrolysate of lactoferrin (500 microg/ml), lactoferricin (20 microg/ml) and nisin (100 IU/ml). None of these compounds, at the indicated dosage, were bactericidal when applied at atmospheric pressure, except nisin, which caused a low level of inactivation of the bacteria. Under high pressure, lactoferrin, lactoferrin hydrolysate and lactoferricin displayed bactericidal activity against some of the test bacteria, however, the former had a narrower bactericidal spectrum than the two latter compounds. The bactericidal efficiency and spectrum of nisin were also enhanced under high pressure. The sensitisation of the test bacteria to these antimicrobials under pressure was transient, since no bactericidal activity was observed when bacteria were pressure treated before exposure to the compounds. We propose a mechanism of pressure-promoted uptake of these antimicrobial proteins and peptides in gram-negative bacteria to explain this sensitisation.