Exploring the interactions of antibiotic combinations against multidrug-resistant Gram-negative bacteria

Sammanfattning: Antimicrobial resistance is emerging and available treatment options are lacking. Antibiotics is a cornerstone in modern medicine where medical procedures such as surgery, care of premature babies or treatment of cancer is dependent on efficient drugs. The need for novel treatment alternatives is increasing as bacteria continue to develop new resistance mechanisms.The main goal of this thesis was to screen for antimicrobial combinations efficient against Gram-negative bacteria. The complex membrane structure of Gram-negative bacteria is very protective against antimicrobial activity making many antibiotics ineffective. Polymyxin B was therefore used as a main component in the combinations evaluated due to its membrane disruptive mode of action. Previously neglected or disused antibiotics was used in combination with polymyxin B as a part of a Joint Programming Initiative on Antimicrobial Resistance (JPIAMR) called CO-ACTION. The prevailing theory of polymyxin B combinations is that the membrane disruptive effect will facilitate entry of a second antibiotic and reduce efflux activity. In contrast, the combination will not be as efficient in the presence of bacterial enzymatic activity as the resistance mechanism is not affected by changes in the membrane composition. To increase knowledge on underlying mechanisms behind the success of antimicrobial combinations an extensive genetic analysis was performed. Several promising polymyxin B combinations were found which could offer a treatment option in caring for severely ill patients for which few alternatives exist. Associations between genetic background and efficient bacterial killing was also established. The chance of synergistic effect by the combination was increased if the antibiotic used in combination with polymyxin B could normally not enter the bacterial cell or in presence of resistance mechanisms increasing efflux activity. This thesis highlights the fact that only phenotypical antimicrobial susceptibility testing would not be used in forecasting the success of antimicrobial combinations. Information on antimicrobial susceptibility in combination with knowledge on resistance mechanisms present and how it influences the antibiotics used in combination is equally important.  With this work increased knowledge on genetic background of resistance mechanisms and bacterial killing by polymyxin B combinations and was provided. Antimicrobial combinations offer an interesting feature when no other treatment alternatives are available. The lack of diagnostics in forecasting the success of combination therapy in a clinical microbiology lab is of concern. The knowledge obtained in this work contributes to the general knowledge on antimicrobial combinations and provides an example of how to evaluate their effect.