Abstract:
The rising threat of antibiotic resistance, mainly driven by the indiscriminate use of antibiotics, has compromised the effectiveness of many treatments and placed an increasing burden on global health systems. This study investigates how different Gram-negative bacterial species, namely Escherichia coli, Pseudomonas aeruginosa, and Vibrio cholerae, respond to individual antibiotics and antibiotic combinations. My thesis aims to better understand the species-specific mechanisms underlying drug action and resistance, with the goal of informing more targeted therapeutic strategies in the future. The first project focused on combinations of β-lactams and aminoglycosides across various species. The results demonstrated that the effectiveness of this combination is highly species-specific and driven by the binding specificity of β-lactams. These findings suggest that while antibiotic combinations hold clinical promise, their application must be guided by a deeper understanding of species-specific interactions and cannot yet be generalised across bacterial taxa. The second project explored the emergence of antibiotic-resistant populations following short-term treatments, especially in E. coli. Short-term treatments were chosen to avoid biases inherent in long-term evolution studies, which often miss low-fitness resistant mutants. The study found that aminoglycosides uniquely enabled the rapid development of resistance, but only in E. coli. Survivor isolates showed reduced growth rates, indicating the presence of low-fitness mutants. The results highlight the importance of using alternative experimental designs to capture the full spectrum of resistance mechanisms. The third project examined how non-antibiotic-related elements, such as the phage defence system CBASS, modulate antibiotic susceptibility. The CBASS system was found to sensitise bacteria to antifolate antibiotics, transforming these typically bacteriostatic agents into bactericidal ones. However, this effect depended on the presence of CBASS components, in this case, the CD-NTase, underlining the complexity and species-specific nature of this interaction. Interestingly, our data suggest a co-occurrence of CBASS systems with antifolate resistance genes, pointing to a trade-off between phage defence and antibiotic resistance markers. Together, these projects reinforce that antibiotic efficacy and resistance are highly dependent on bacterial species and genetic context, way beyond the antibiotic target. A more personalised, species-level approach to antibiotic therapy, rather than generalising by Gram stain or taxonomy, may be necessary to improve clinical outcomes and combat resistance more effectively.
Antibiotic