Controlled Release of LL-37-Derived Synthetic Antimicrobial and Anti-Biofilm Peptides SAAP-145 and SAAP-276 Prevents Experimental Biomaterial-Associated Staphylococcus aureus Infection

Bacterial populations produce persister cells that neither grow nor die in the presence of microbicidal antibiotics. Persisters are largely responsible for high levels of biofilm tolerance to antimicrobials, but virtually nothing was known about their biology. Tolerance of Escherichia coli to ampicillin and ofloxacin was tested at different growth stages to gain insight into the nature of persisters. The number of persisters did not change in lag or early exponential phase, and increased dramatically in mid-exponential phase. Similar dynamics were observed with Pseudomonas aeruginosa (ofloxacin) and Staphylococcus aureus (ciprofloxacin and penicillin). This shows that production of persisters depends on growth stage. Maintaining a culture of E. coli at early exponential phase by reinoculation eliminated persisters. This suggests that persisters are not at a particular stage in the cell cycle, neither are they defective cells nor cells created in response to antibiotics. Our data indicate that persisters are specialized survivor cells.

Water-membrane soluble protein and peptide toxins are used in the defense and offense systems of all organisms, including plants and humans. A major group includes antimicrobial peptides, which serve as a nonspecific defense system that complements the highly specific cell-mediated immune response. The increasing resistance of bacteria to conventional antibiotics stimulated the isolation and characterization of many antimicrobial peptides for potential use as new target antibiotics. The finding of thousands of antimicrobial peptides with variable lengths and sequences, all of which are active at similar concentrations, suggests a general mechanism for killing bacteria rather than a specific mechanism that requires preferred active structures. Such a mechanism is in agreement with the "carpet model" that does not require any specific structure or sequence. It seems that when there is an appropriate balance between hydrophobicity and a net positive charge the peptides are active on bacteria. However, selective activity depends also on other parameters, such as the volume of the molecule, its structure, and its oligomeric state in solution and membranes. Further, although many studies support that bacterial membrane damage is a lethal event for bacteria, other studies point to a multihit mechanism in which the peptide binds to several targets in the cytoplasmic region of the bacteria.