In vitro antibacterial activities of a thiazolyl peptide antibiotic PM2409

Compound 2 was profiled against several Gram-positive bacteria, including anaerobes. It inhibited the growth of methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE) with MIC at 0.12 μg ml⁻¹ for both. It also inhibited Bacillus spp. with MIC 0.06–0.12 μg ml⁻¹ (Table 1). Compound 2 was also tested against 16 clinical strains of Clostridium difficile. Owing to concern over the possible deleterious effects of the blood component in the standard reference medium (supplemented brucella agar (SBA)), the compound was also tested in Wilkens–Chalgren agar (WCA) medium. It exhibited potent activity against the clinical C. difficile isolates with MIC range, MIC₅₀ and MIC₉₀ values of 0.25 μg ml⁻¹ (Table 2). As SBA contains blood and WCA does not, the data indicate that the presence of blood in the test medium has some effect on the activity of PM2409 against C. difficile as the MIC₉₀ in SBA showed a fourfold-increased value (1 μg ml⁻¹). It implies that possibly the compound binds to serum components in the blood. To rule out the contribution of the cytotoxicity (if any) of the compound 2 toward the observed in vitro antibacterial activities, cytotoxicity studies were carried out for the compound against different cancer and normal mammalian cell lines. The compound exhibited no cytotoxicity up to 100 μg ml⁻¹ in all the cell lines tested (Supplementary Data S2 and S3).

Table 1 Summary of the in vitro antibacterial activities of PM2409 (2) and Kocurin (1)

Table 2 Comparative in vitro activity of compound PM2409 (2), Kocurin (1), vancomycin and metronidazole against C. difficile

To determine the mechanism of action of PM2409, it was tested for its ability to inhibit the synthesis of major macromolecules that are essential for bacterial growth in a whole-cell assay using Bacillus subtilis strains. When B. subtilis were incubated with PM2409 at its MIC (0.12 μg ml⁻¹), protein synthesis was inhibited selectively (∼40% incorporation of the labeled amino-acid mixture during the macromolecular synthesis). There was no significant inhibition of RNA, DNA or peptidoglycan synthesis at this concentration (Figure 1c). Like other antibiotics of this class (for example, GE2270A⁷ and LFF571⁴), we envisage that the compound PM2409 curtails protein synthesis at translational level by binding to bacterial elongation factor thermo unstable (EF-Tu).

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