Charge counter charge

Bacteria may encounter a very wide variety of conditions in nature. These include changes in salinity, temperature, pH, and the presence of antibacterial molecules that have evolved either to kill them or impede their growth. In response, some bacteria have evolved modifications of their membrane lipids that improve their survival. Among these are amino acyl derivatives of phosphatidylglycerol (PG) and cardiolipin (diphosphatidylglycerol). A lysine derivative of PG (Fig. 1) was first found in Staphylococcus aureus. Other amino acids linked to PG including ornithine and alanine have been detected in bacteria. The amino acid group is transferred from aminoacyl-tRNA to PG. Soon after the discovery of lys-PG it was observed that the ratios of PG and lys-PG changed as the pH of the culture medium was varied. In S. aureus grown at pH 7, PG and cardiolipin were the predominant lipids; however at pH 5 there was a marked increase in lys-PG and a decrease in PG. Using artificial bilayers formed from lipids purified from bacteria, Hopfer et al. found that membranes made from positively-charged lys-PG were selective for the negatively-charged chloride ion, whereas negativelycharged membranes made from PG and cardiolipin were highly cation selective. Thus an increase in lys-PG at the expense of the negatively charged lipids could potentially serve to protect the cell from increased proton concentrations at lower pH. The gene mprF (multi peptide resistance factor), which encodes aminoacyl-PG synthase, was discovered by screening a transposon insertion library of Staphylococcus xylosus C2a for sensitivity to the lantibiotic gallidermin, a polypeptide antibiotic. This gene conferred resistance of S. aureus to cationic proteins and polypeptides of the defensin and protegrin families, ancient and effective components of innate immunity. Aminoacyl-PG synthases are encoded in the genomes of a wide variety of grampositive bacteria (mainly firmicutes and actinobacteria) and in gram-negative proteobacteria. Members of the MprF family may be specific for alanine or lysine or in some cases, both. The genome of Listeria monocytogenes, a gram-positive, facultatively intracellular pathogen that infects a wide variety of animal species including humans, contains an mprF ortholog and the cells are able to synthesize both lys-PG and lys-cardiolipin. This foodborne pathogen has been responsible for several large outbreaks that mainly affect the immunocompromised, the very young, the elderly and pregnant women, resulting in approximately 20% mortality. Thedieck et al. found that an mprF mutant of L. monocytogenes had increased sensitivity to gallidermin, but only at a relatively low concentration, 0.5 μg/ ml, in liquid medium. At higher concentrations the wild-type and mutant were equally affected. The resistance conferred by MprF to the α-defensins HNP-1 and -2, were moderate and also concentration-dependent. These authors also reported that infection with an mprF mutant produced fewer bacteria after 1–2 h of infection than the wild type in Caco-2 epithelial cells, HeLa cells and the P388D1 macrophage cell line. In a mouse model of infection, growth of the mutant in the spleen was attenuated and even more so in the liver. Thus it appears that MprF serves as a virulence factor for this pathogen and that resistance to cationic antimicrobial peptides (CAMPs) contributes significantly to its intracellular survival and its pathogenicity. Lys-PG and lys-cardiolipin are not confined to pathogenic species of Listeria, they are also found in L. innocua, L. seeligeri, and L. welshimeri, which may encounter antimicrobial peptides in their environments. The findings of Dare et al. draw a clear distinction between Bacillus subtilis, a saprophytic non-pathogen and L. monocytogenes. It should be noted that B. subtilis has phosphatidylethanolamine as a major lipid, which is not present in L. monocytogenes. Thus the content of lys-PG was lower in B. subtilis when both were grown at 37 °C. Interestingly, the absence of MprF negatively affected the growth of B. subtilis in response to a much wider range of antibacterial substances than was the case for L. monocytogenes. These included antibiotics, membrane disrupters and a cationic surfactant. Indeed, the effects of chloroxylenol, a disrupter of membrane potential, were different for the two species; the wild-type strain of B. subtilis was more resistant than the ΔmprF mutant, but the contrary was true for L. monocytogenes. The presence of MprF (aa-PG synthase) in L. monocytogenes did confer resistance to two aminoglycosides, tobramycin and