Tyrothricin

The tyrocidines and analogues are cationic cyclodecapeptides [cyclo (D-Phe1-L-Pro2-L-(Phe3/Trp3)-D-(Phe4/Trp4)-L-Asn5-L-Gln6-L-(Tyr7/Phe7/Trp7)-L-Val8-L-(Orn9/Lys9)-L-Leu10], produced together with the neutral linear pentadecapeptide gramicidins, in the antibiotic tyrothricin complex by Brevibacillus parabrevis. Despite discovery 80 years ago, it was still uncertain whether these peptides are secreted or sequestered intracellularly. We resolved this by utilising high resolution electrospray mass spectrometry to confirm the predominantly intracellular sequestration of the peptides in the tyrothricin complex. A "peptidomics" approach allowed us to map the intracellular production of 16 cyclodecapeptides and 6 gramicidins over 16 days of culturing. Gramicidin production remained relatively constant, with Val-gramicidin A the predominant analogue produced throughout the 16 day fermentation period. The tyrothricin cyclodecapeptides have four variable positions and there was a culturing time related shift from the Phe-rich A analogues, containing a L-Phe3-D-Phe4aromatic dipeptide unit, to the Trp-rich C analogues with L-Trp3-D-Trp4. For the other variable aromatic residue position, Tyr7was preferentially incorporated above Trp7, with a minor incorporation of Phe7over the whole culturing period. For the variable basic amino acid residue, there was time-sensitive shift from Orn9to Lys9incorporation. Modulation of the cyclodecapeptide profile over time does not correlate with the reported non-ribosomal peptide synthetase affinity, specifically for Trp in the variable aromatic residue positions, indicating additional supply-demand control in the cyclodecapeptides production by B. parabrevis. These novel observations are not only of importance for production and purification of selected peptide analogues from the tyrothricin complex, but also for insight into microbial control of non-ribosomal peptide production that extends beyond the peptide synthetase machinery.

Background/aims:Antibiotic-induced drug resistance requires new approaches in topical acne treatment. Tyrothricin is known to produce no resistance. In this study, it was tested for the first time in topical acne treatment. The efficacy and tolerability of topical tyrothricin 0.1% was evaluated.Methods:A randomized, active comparator-controlled, exploratory, observer-blind clinical study was conducted in 24 patients with acne papulopustulosa. Randomization on a split-face was either tyrothricin versus clindamycin + benzoyl peroxide (BPO) (n = 12) or tyrothricin versus BPO 5% (n = 12). The main outcome was change in inflammatory and noninflammatory lesion counts.Results:The mean differences in inflammatory lesion counts from baseline were -12.3 (95% CI: -20.5 to -4.1) in clindamycin + BPO, -10.2 (95% CI: -15.3 to -5.0) in BPO 5%, and -7.7 (95% CI: -11.7 to -3.7) in tyrothricin. Tyrothricin reduced noninflammatory lesions (mean difference: -6.5 (95% CI: -11.6 to -1.4) and caused less product-related adverse events (n = 31) compared to BPO (n = 37) and clindamycin + BPO (n = 20).Conclusion:The results indicate that tyrothricin might be a candidate for treating acne and it seems to be more tolerable than both comparator treatments.

The antimicrobial agent tyrothricin is a representative of the group of antimicrobial peptides (AMP). It is produced by Bacillus brevis and consists of tyrocidines and gramicidins. The compound mixture shows activity against bacteria, fungi and some viruses. A very interesting feature of AMPs is the fact, that even in vitro it is almost impossible to induce resistances. Therefore, this class of molecules is discussed as one group that could serve as next generation antibiotics and overcome the increasing problem of bacterial resistances. In daily practice, the application of tyrothricin containing formulations is relatively limited: It is used in sore throat medications and in agents for the healing of infected superficial and small-area wounds. However, due to the broad spectrum antimicrobial activity and the low risk of resistance development it is worth to consider further fields of application.