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The increase in emerging drug resistant Gram-negative bacterial infections is a global concern. In addition, there is growing recognition that compromising the microbiota through the use of broad-spectrum antibiotics can impact long term patient outcomes. Therefore, there is the need to develop new bactericidal strategies to combat Gram-negative infections that would address these specific issues. In this study, we report and characterize one such approach, an antibody-drug conjugate (ADC) that combines (i) targeting the surface of a specific pathogenic organism through a monoclonal antibody with (ii) the high killing activity of an antimicrobial peptide. We focused on a major pathogenic Gram-negative bacterium associated with antibacterial resistance: Pseudomonas aeruginosa. To target this organism, we designed an ADC by fusing an antimicrobial peptide to the C-terminal end of the VH and/or VL-chain of a monoclonal antibody, VSX, that targets the core of P. aeruginosa lipopolysaccharide. This ADC demonstrates appropriately minimal levels of toxicity against mammalian cells, rapidly kills P. aeruginosa strains, and protects mice from P. aeruginosa lung infection when administered therapeutically. Furthermore, we found that the ADC was synergistic with several classes of antibiotics. This approach described in this study might result in a broadly useful strategy for targeting specific pathogenic microorganisms without further augmenting antibiotic resistance. Author summary: The increasing incidence of emerging drug resistant bacterial infections is a worldwide public health issue. Infections caused by antibiotic-resistant Gram-negative pathogens are particularly concerning. In addition, there is now growing recognition that disruption of the microbiota through the use of broad-spectrum antibiotics can have detrimental effects on long-term patient outcomes. Therefore, there is a need to develop new bactericidal strategies to combat Gram-negative infections while preserving the microbiota and avoiding the enhancement of antibiotic resistance. Here, we report on and characterize one such approach by using a specific monoclonal antibody associated with the potent killing activity of antimicrobial peptides in the form of an antibody-drug conjugate (ADC). The selected pathogenic bacterium was Pseudomonas aeruginosa, which presents numerous markers of both innate and acquired antibiotic resistance. The ADC lacked significant cytotoxicity against mammalian cells and was shown to be effective both in vitro and in vivo against P. aeruginosa. [ABSTRACT FROM AUTHOR]