Inhibition of antimicrobial resistance by RNase 3/1 chimeras in biofilm cultures of Acinetobacter baumannii

The worldwide spread of bacterial resistance against conventional antibiotics motivates the development of novel antimicrobial drugs. Bacterial biofilms are highly present in the hospital setting and are responsible for several types of infections that contribute to the acquisition and diffusion of resistance determinants. Human RNases of the ribonuclease A superfamily contribute to the maintenance of body fluid sterility, showing interesting immuno-modulatory and antimicrobial properties. Among them, RNase 1 and 3 are the members exhibiting the highest catalytic and bactericidal activities, respectively. Based on structure-functional studies, three RNase 3/1 chimeric constructs combining the key properties of RNase 1 and 3 (RNase 3/1-v1, -v2, and -v3) were designed and demonstrated to be promising antimicrobic compounds against Gram-negative bacteria. In this study, we evaluate the activity of RNase 3/1 chimeras against biofilm preparations of Acinetobacter baumannii, the pathogen at the top of the World Health Organization’s priority list for research and development of new antibiotics. Biofilms were treated with RNase 3/1-v1 and -v3 in combination with colistin, an antibiotic clinically used to treat multi-drug resistant Gram-negative infections. Results showed that RNase 3/1 addition reduces by half the colistin effective antibacterial dose, highlighting its potential as an adjuvant to colistin. We also tested RNase 3/1-v3 ability to inhibit the generation of antimicrobial resistance by performing a resistance evolution assay in which biofilms were cyclically exposed to increasing concentrations of colistin during nine consecutive days. According to minimum inhibitory concentration (MIC) determinations, after the seventh cycle of selection, bacterial samples supplemented with RNase 3/1-v3 displayed average colistin MIC values more than 3-fold lower than those of the ones treated with colistin alone. These results highlight the potential of RNase-based compounds as antibiotic adjuvants for the treatment of bacterial biofilms, suggesting that they may be effective in reducing the emergence of resistance.