Evolution of colistin resistance in Acinetobacter baumannii and disrupting the colistin resistance mechanisms

Abstract

The emergence of antibiotic resistance is a global threat, rendering our reservoir of antibiotics ineffective against many bacterial pathogens. In 2019, 4.95 million people died with an antibiotic-resistant associated infection. One major contributor to this crisis is Acinetobacter baumannii, a Gram-negative multi-drug resistant (MDR) pathogen listed by the World Health Organisations (WHO) as a priority for novel therapeutic interventions. This thesis explores innovative approaches against MDR A. baumannii, focusing on the therapeutic properties of phytochemicals and plant extracts, more specifically kaempferol and tormentil. Kaempferol, a phytochemical derived from capers and strawberries, in combination with colistin, reduces the growth of A. baumannii and inhibits biofilm formation when used on its own. Additionally, kaempferol disrupts iron homeostasis, resulting in increased reactive oxygen species under colistin stress, leading to bacterial death. Similarly, tormentil, a plant used in traditional Irish folklore medicine for treating burn wounds, and its constituents exhibit significant antimicrobial and antibiofilm activity against A. baumannii. Our mechanistic studies reveal that these extracts also impact bacterial iron homeostasis. These findings demonstrate the potential of iron-chelating compounds as colistin potentiators or standalone antimicrobials against MDR A. baumannii. Additionally, we investigate the fitness and virulence costs associated with colistin resistance. Our laboratory evolved colistin-resistant mutants (CRMs) show varied growth rates in the presence of colistin and slow growth rates in the absence. The CRMs also show an increased biofilm formation and reduced virulence, illustrating the trade-offs of evolved colistin resistance. In vivo analysis of known and novel mutations in PmrB revealed structural changes that may enhance kinase activity and mediate colistin resistance. These findings uncover metabolic vulnerabilities in A. baumannii, suggesting new strategies to enhance colistin efficacy through phytochemicals and plant extracts and further our knowledge in understanding the trade-offs of evolved resistance in A. baumannii. Together, these insights can contribute to the design and development of more effective treatments against MDR A. baumannii.