Tyrocidine-induced modulation of the DNA conformation in Bacillus brevis

Multitarget Approaches against Multiresistant Superbugs

Despite efforts to develop new antibiotics, antibacterial resistance still develops too fast for drug discovery to keep pace. Often, resistance against a new drug develops even before it reaches the market. This continued resistance crisis has demonstrated that resistance to antibiotics with single protein targets develops too rapidly to be sustainable. Most successful long-established antibiotics target more than one molecule or possess targets, which are encoded by multiple genes. This realization has motivated a change in antibiotic development toward drug candidates with multiple targets. Some mechanisms of action presuppose multiple targets or at least multiple effects, such as targeting the cytoplasmic membrane or the carrier molecule bactoprenol phosphate and are therefore particularly promising. Moreover, combination therapy approaches are being developed to break antibiotic resistance or to sensitize bacteria to antibiotic action. In this Review, we provide an overview of antibacterial multitarget approaches and the mechanisms behind them.

The Multifaceted Antibacterial Mechanisms of the Pioneering Peptide Antibiotics Tyrocidine and Gramicidin S

Cyclic β-sheet decapeptides, such as tyrocidines and gramicidin S, were among the first antibiotics in clinical application. Although they have been used for such a long time, there is virtually no resistance to them, which has led to a renewed interest in this peptide class. Both tyrocidines and gramicidin S are thought to disrupt the bacterial membrane. However, this knowledge is mainly derived from in vitro studies, and there is surprisingly little knowledge about how these long-established antibiotics kill bacteria. Our results shed new light on the antibacterial mechanism of β-sheet peptide antibiotics and explain why they are still so effective and why there is so little resistance to them.

Cyclic β-sheet decapeptides from the tyrocidine group and the homologous gramicidin S were the first commercially used antibiotics, yet it remains unclear exactly how they kill bacteria. We investigated their mode of action using a bacterial cytological profiling approach. Tyrocidines form defined ion-conducting pores, induce lipid phase separation, and strongly reduce membrane fluidity, resulting in delocalization of a broad range of peripheral and integral membrane proteins. Interestingly, they also cause DNA damage and interfere with DNA-binding proteins. Despite sharing 50% sequence identity with tyrocidines, gramicidin S causes only mild lipid demixing with minor effects on membrane fluidity and permeability. Gramicidin S delocalizes peripheral membrane proteins involved in cell division and cell envelope synthesis but does not affect integral membrane proteins or DNA. Our results shed a new light on the multifaceted antibacterial mechanisms of these antibiotics and explain why resistance to them is virtually nonexistent.

Inhibition of malaria parasite blood stages by tyrocidines, membrane-active cyclic peptide antibiotics from Bacillus brevis

Tyrothricin, a complex mixture of antibiotic peptides from Bacillus brevis, was reported in 1944 to have antimalarial activity rivalling that of quinine in chickens infected with Plasmodium gallinaceum. We have isolated the major components of tyrothricin, cyclic decapeptides collectively known as the tyrocidines, and tested them against the human malaria parasite Plasmodium falciparum using standard in vitro assays. Although the tyrocidines differ from each other by conservative amino acid substitutions in only three positions, their observed 50% parasite inhibitory concentrations (IC(50)) spanned three orders of magnitude (0.58 to 360 nM). Activity correlated strictly with increased apparent hydrophobicity and reduced total side-chain surface area and the presence of ornithine and phenylalanine in key positions. In contrast, mammalian cell toxicity and haemolytic activities of the respective peptides were considerably less variable (2.6 to 28 microM). Gramicidin S, a structurally analogous antimicrobial peptide, was less active (IC(50)=1.3 microM) and selective than the tyrocidines. It exerted its parasite inhibition by rapid and selective lysis of infected erythrocytes as judged by fluorescence and light microscopy. The tyrocidines, however, did not cause an overt lysis of infected erythrocytes, but an inhibition of parasite development and life-cycle progression.

Psoralen cross-linking as probe of torsional tension and topological domain size in vivo

DNA within a cell is organized with unrestrained torsional tension, and each molecule is divided into multiple individual topological domains. Psoralen photobinding can be used as an assay for supercoiling and topological domain size in living cells. Psoralen photobinds to DNA at a rate nearly linearly proportional to superhelical density. Comparison of the rate of photobinding to supercoiled and relaxed DNA in cells provides a measure of superhelical density. For this, in vivo superhelical tension is relaxed by the introduction of nicks by either ionizing radiation or photolysis of bromodeoxyuridine in the DNA. Since nicks are introduced in a random fashion, the distribution of nicks is described by a Poisson distribution. Thus, after nicking, the fraction of topological domains containing no nicks is described by the zero term of the Poisson distribution. From measurement of the number of nicks introduced in the DNA and the fraction of torsional tension remaining, an average topological domain size can be estimated. Using this logic, procedures were designed and described for measuring supercoiling and domain size at specific sites in eukaryotic genomes.

Loloatins A-D, cyclic decapeptide antibiotics produced in culture by a tropical marine bacterium

Loloatins A (1) to D (4), a family of new cyclic decapeptide antibiotics, have been isolated from laboratory cultures of a tropical marine bacterium recovered from the Great Barrier Reef in Papua New Guinea. The structures of loloatins A-D were elucidated via a combination of spectroscopic analyses and chemical degradation. Loloatins A-D exhibit in vitro antimicrobial activity against methicillin-resistant Staphyloccoccus aureus, vancomycin-resistant enterococci, and drug-resistant Streptococcus pneumoniae.