Microbiology, Dstl Porton Down, Salisbury, Wiltshire SP4 0JQ, UK As antibiotic resistance increases worldwide, there is an increasing pressure to develop novel classes of antimicrobial compounds to fight infectious disease. Peptide therapeutics represent a novel class of therapeutic agents. Some, such as cationic antimicrobial peptides and peptidoglycan recognition proteins, have been identified from studies of innate immune effector mechanisms, while others are completely novel compounds generated in biological systems. Currently, only selected cationic antimicrobial peptides have been licensed, and only for topical applications. However, research using new approaches to identify novel antimicrobial peptide therapeutics, and new approaches to delivery and improving stability, will result in an increased range of peptide therapeutics available in the clinic for broader applications.

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Rhizobium leguminosarum bv. viciae 3841 contains six putative quaternary ammonium transporters (Qat), of the ABC family. Qat6 was strongly induced by hyperosmosis although the solute transported was not identified. All six systems were induced by the quaternary amines choline and glycine betaine. It was confirmed by microarray analysis of the genome that pRL100079-83 (qat6) is the most strongly upregulated transport system under osmotic stress, although other transporters and 104 genes are more than threefold upregulated. A range of quaternary ammonium compounds were tested but all failed to improve growth of strain 3841 under hyperosmotic stress. One Qat system (gbcXWV) was induced 20-fold by glycine betaine and choline and a Tn5::gbcW mutant was severely impaired for both transport and growth on these compounds, demonstrating that it is the principal system for their use as carbon and nitrogen sources. It transports glycine betaine and choline with a high affinity (apparent K(m), 168 and 294 nM, respectively).

Proline-rich antibacterial peptides protect experimental animals from bacterial challenge even if they are unable to kill the microorganisms in vitro. Their major in vivo modes of action are inhibition of bacterial protein folding and immunostimulation. Here we investigated whether the proline-rich antibacterial peptide dimer A3-APO was able to inhibit Bacillus cereus enterotoxin production in vitro and restrict the proliferation of lethal toxin-induced Bacillus anthracis replication in mouse macrophages. After 24 h incubation, peptide A3-APO and its single chain metabolite reduced the amount of properly folded B. cereus diarrhoeal enterotoxin production in a concentration-dependent manner leading to only 10-25% of the original amount of toxin detectable by a conformation-sensitive immunoassay. Likewise, after 4 h incubation, A3-APO restricted the proliferation of B. anthracis in infected macrophages by 40-45% compared to untreated cells both intracellularly and in the extracellular cell culture milieu. Although the peptide had a minimal inhibitory concentration of >512 mg/L against B. anthracis in vitro, in systemic mouse challenge models it improved survival by 20- 37%, exhibiting statistically significant cumulative efficacy when administered at 3x5 mg/kg intraperitoneally or intramuscularly. We hypothesize that the activity in isolated murine macrophages and in vivo is due to deactivation of bacterial toxins. Bacterial protein folding inhibition in synergy with other types of antimicrobial modes offers a remarkable novel strategy in combating resistant or life-threatening infections.

We have previously examined the mechanism of antimicrobial peptides on the outer membrane of vaccinia virus. We show here that the formulation of peptides LL37 and magainin-2B amide in polysorbate 20 (Tween 20) results in greater reductions in virus titer than formulation without detergent, and the effect is replicated by substitution of polysorbate 20 with high-ionic-strength buffer. In contrast, formulation with polysorbate 20 or high-ionic-strength buffer has the opposite effect on bactericidal activity of both peptides, resulting in lesser reductions in titer for both Gram-positive and Gram-negative bacteria. Circular dichroism spectroscopy shows that the differential action of polysorbate 20 and salt on the virucidal and bactericidal activities correlates with the α-helical content of peptide secondary structure in solution, suggesting that the virucidal and bactericidal activities are mediated through distinct mechanisms. The correlation of a defined structural feature with differential activity against a host-derived viral membrane and the membranes of both Gram-positive and Gram-negative bacteria suggests that the overall helical content in solution under physiological conditions is an important feature for consideration in the design and development of candidate peptide-based antimicrobial compounds.