Fatal Attraction Evaded

Antimicrobial polypeptides are widely distributed effec-tors of host defense in animals and plants. They include enzymes that digest vital microbial structures (e.g., lysozyme, neutrophil elastase, phospholipase A2), substances that bind and sequester iron or other essential nutrients (e.g., lacto-ferrin), and polypeptides that insert into and disrupt microbial membranes (e.g., bactericidal permeability-inducing protein, defensins, cathelicidins; references 1 and 2). Despite their diverse sizes, structures, and mechanisms of action , nearly all antimicrobial proteins and peptides have a net cationic (positive) charge. It is thought that the electro-static attraction of cationic polypeptides increases the depo-sition of the polypeptides onto the negatively charged microbial surfaces and thereby promotes their effectiveness. In support of this model, the activity of most but not all anti-microbial polypeptides is competitively inhibited by increasing the ionic strength of the medium (3–5) whose sol-ute anions and cations shield the opposing charges of the cationic polypeptides and anionic microbial surfaces and diminish their mutual attraction. Microbes have evolved a repertoire of countermeasures that limit the effectiveness of antimicrobial polypeptides. Earlier clues that bacterial resistance to cationic polypep-tides could be due to the loss of negative surface charges came from exposing Pseudomonas fluorescens to phosphate limitation. Under these conditions, anionic phospholipids of bacterial membranes were largely replaced by positively charged ornithine-modified lipids (6) and the bacteria became completely resistant to the cationic peptide antibiotic polymyxin. In other studies, acquired resistance of enteric bacteria to the cationic antibiotic polymyxin and the anti-microbial peptide cecropin was associated with increased content of aminoarabinose and decreased anionic charge of lipopolysaccharide (7, 8). The strategy of transposon-mediated gene disruption in pathogenic bacteria has proven particularly fruitful in identifying bacterial genes essential for bacterial survival despite host defense. The inserted transposon allows rapid se-quencing and identification of the disrupted gene in bacterial mutants that lost resistance to host defense mechanisms. The method identified the PhoP-PhoQ two-component regulator of Salmonella typhimurium as an environmental sensor and master switch that promotes survival in mac-rophages and activates resistance to antimicrobial peptides (9–11). Among the many processes regulated by PhoP-PhoQ are several that covalently modify the lipid A moiety of lipopolysaccharide and diminish its negative charge by adding aminoarabinose, palmitate, and 2-hydroxymyristate (12). In Staphyloccus aureus , disruption of the dlt operon increased bacterial susceptibility to a number of cationic polypeptides, including human defensins human neutrophil peptide 1–3, animal-derived protegrins, tachyplesins, and magainin II, and even to the bacteria-derived gallidermin and …