The spectrum of activity and mode of action of a novel antibacterial agent, 135C, was investigated using a range of microbiological and genomic approaches.

Aims: The spectrum of activity and mode of action of a novel antibacterial agent, 135C, was investigated using a range of microbiological and genomic approaches.Methods and Results: Compound 135C was active against Gram-positive bacteria with MICs for S. aureus ranging from 0.12 – 0.5 µg ml-1. It was largely inactive against Gram-negative bacteria. The compound was bacteriostatic in time kills studies, and did not elicit bacterial cell leakage or cell lysis, suggesting that these sites are unlikely to be major targets. Chequerboard assays showed little synergy or antagonism when 135C was combined with a range of other antibacterial agents. Multi-step serial passage of four S. aureus isolates with increasing concentrations of 135C showed that resistance developed relatively quickly and was stable after drug-free passages. Growth curves showed minor differences in the fitness of 135C-resistant isolates compared to the parent wildtype and isolates resistant to 135C did not show cross-resistance to other antibiotics. Genomic comparison of mutant and wildtype strains showed changes in genes encoding cell wall teichoic acids.Conclusions: 135C shows promising activity against Gram-positive bacteria but is currently limited by rapid resistance development. It is likely that a mode of action of 135C is related to the synthesis and function of wall teichoic acids.Significant and Impact of the study: Whilst 135C remains an interesting bacteriostatic compound, further studies are required verify its interaction with wall teichoic acids as its mode of action and whether the issue of resistance development can be overcome.