The efficacy of the antibacterial peptide, pyrrhocoricin, is finely regulated by its amino acid residues and active domains

Alternatives to overcoming bacterial resistances: State-of-the-art

Worldwide, bacterial resistance to chemical antibiotics has reached such a high level that endangers public health. Presently, the adoption of alternative strategies that promote the elimination of resistant microbial strains from the environment is of utmost importance. This review discusses and analyses several (potential) alternative strategies to current chemical antibiotics. Bacteriophage (or phage) therapy, although not new, makes use of strictly lytic phage particles as an alternative, or a complement, in the antimicrobial treatment of bacterial infections. It is being rediscovered as a safe method, because these biological entities devoid of any metabolic machinery do not possess any affinity whatsoever to eukaryotic cells. Lysin therapy is also recognized as an innovative antimicrobial therapeutic option, since the topical administration of preparations containing purified recombinant lysins with amounts in the order of nanograms, in infections caused by Gram-positive bacteria, demonstrated a high therapeutic potential by causing immediate lysis of the target bacterial cells. Additionally, this therapy exhibits the potential to act synergistically when combined with certain chemical antibiotics already available on the market. Another potential alternative antimicrobial therapy is based on the use of antimicrobial peptides (AMPs), amphiphilic polypeptides that cause disruption of the bacterial membrane and can be used in the treatment of bacterial, fungal and viral infections, in the prevention of biofilm formation, and as antitumoral agents. Interestingly, bacteriocins are a common strategy of bacterial defense against other bacterial agents, eliminating the potential opponents of the former and increasing the number of available nutrients in the environment for their own growth. They can be applied in the food industry as biopreservatives and as probiotics, and also in fighting multi-resistant bacterial strains. The use of antibacterial antibodies promises to be extremely safe and effective. Additionally, vaccination emerges as one of the most promising preventive strategies. All these will be tackled in detail in this review paper.

Insect antimicrobial peptides show potentiating functional interactions against Gram-negative bacteria

Antimicrobial peptides (AMPs) and proteins are important components of innate immunity against pathogens in insects. The production of AMPs is costly owing to resource-based trade-offs, and strategies maximizing the efficacy of AMPs at low concentrations are therefore likely to be advantageous. Here, we show the potentiating functional interaction of co-occurring insect AMPs (the bumblebee linear peptides hymenoptaecin and abaecin) resulting in more potent antimicrobial effects at low concentrations. Abaecin displayed no detectable activity against Escherichia coli when tested alone at concentrations of up to 200 μM, whereas hymenoptaecin affected bacterial cell growth and viability but only at concentrations greater than 2 μM. In combination, as little as 1.25 μM abaecin enhanced the bactericidal effects of hymenoptaecin. To understand these potentiating functional interactions, we investigated their mechanisms of action using atomic force microscopy and fluorescence resonance energy transfer-based quenching assays. Abaecin was found to reduce the minimal inhibitory concentration of hymenoptaecin and to interact with the bacterial chaperone DnaK (an evolutionarily conserved central organizer of the bacterial chaperone network) when the membrane was compromised by hymenoptaecin. These naturally occurring potentiating interactions suggest that combinations of AMPs could be used therapeutically against Gram-negative bacterial pathogens that have acquired resistance to common antibiotics.

Mechanism of Escherichia coli resistance to Pyrrhocoricin

Due to their lack of toxicity to mammalian cells and good serum stability, proline-rich antimicrobial peptides (PR-AMPs) have been proposed as promising candidates for the treatment of infections caused by antimicrobial-resistant bacterial pathogens. It has been hypothesized that these peptides act on multiple targets within bacterial cells, and therefore the likelihood of the emergence of resistance was considered to be low. Here, we show that spontaneous Escherichia coli mutants resistant to pyrrhocoricin arise at a frequency of approximately 6 × 10(-7). Multiple independently derived mutants all contained a deletion in a nonessential gene that encodes the putative peptide uptake permease SbmA. Sensitivity could be restored to the mutants by complementation with an intact copy of the sbmA gene. These findings question the viability of the development of insect PR-AMPs as antimicrobials.

Small molecule DnaK modulators targeting the beta-domain

The molecular chaperone DnaK is essential for the survival of bacterial pathogens in the hostile environment of the host. Hence, it is in principle a promising target for drug design but for which no current inhibitors are available apart from certain antimicrobial peptides. To this end, we have screened libraries of small molecules for their ability to interact with the substrate-binding domain of DnaK. The most promising hit from the screen was synthesized and along with its analogs subjected to further assays to determine their binding affinity and ability to interfere with bacterial growth. This work resulted in the identification of a number of compounds that bind with submicromolar affinity and capable of inhibiting Yersinia pseudotuberculosis growth more effectively than the previously characterized peptides.

Development of novel antibacterial peptides that kill resistant isolates

The rapid emergence of bacterial strains that are resistant to current antibiotics requires the development of novel types of antimicrobial compounds. Proline-rich cationic antibacterial peptides such as pyrrhocoricin kill responsive bacteria by binding to the 70 kDa heat shock protein DnaK and inhibiting protein folding. We designed and synthesized multiply protected dimeric analogs of pyrrhocoricin and optimized the in vitro antibacterial efficacy assays for peptide antibiotics. Pyrrhocoricin and the designed dimers killed beta-lactam, tetracycline- or aminoglycoside-resistant strains of Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis in the submicromolar or low micromolar concentration range. One of the peptides also killed Pseudomonas aeruginosa. The designed dimers showed improved stability in mammalian sera compared to the native analog. In a murine H. influenzae lung infection model, a single dose of a dimeric pyrrhocoricin analog reduced the bacteria in the bronchoalveolar lavage when delivered intranasally. The solid-phase synthesis was optimized for large-scale laboratory preparations.

Identification of crucial residues for the antibacterial activity of the proline-rich peptide, pyrrhocoricin

Members of the proline-rich antibacterial peptide family, pyrrhocoricin, apidaecin and drosocin appear to kill responsive bacterial species by binding to the multihelical lid region of the bacterial DnaK protein. Pyrrhocoricin, the most potent among these peptides, is nontoxic to healthy mice, and can protect these animals from bacterial challenge. A structure-antibacterial activity study of pyrrhocoricin against Escherichia coli and Agrobacterium tumefaciens identified the N-terminal half, residues 2-10, the region responsible for inhibition of the ATPase activity, as the fragment that contains the active segment. While fluorescein-labeled versions of the native peptides entered E. coli cells, deletion of the C-terminal half of pyrrhocoricin significantly reduced the peptide's ability to enter bacterial or mammalian cells. These findings highlighted pyrrhocoricin's suitability for combating intracellular pathogens and raised the possibility that the proline-rich antibacterial peptides can deliver drug leads into mammalian cells. By observing strong relationships between the binding to a synthetic fragment of the target protein and antibacterial activities of pyrrhocoricin analogs modified at strategic positions, we further verified that DnaK was the bacterial target macromolecule. Inaddition, the antimicrobial activity spectrum of native pyrrhocoricin against 11 bacterial and fungal strains and the binding of labeled pyrrhocoricin to synthetic DnaK D-E helix fragments of the appropriate species could be correlated. Mutational analysis on a synthetic E. coli DnaK fragment identified a possible binding surface for pyrrhocoricin.