A Comprehensive Review of the Underlying Mechanisms of Resistance and Therapeutic Options for DTR-PA in Bloodstream Infection and Pneumonia: A Clinical Vignette-Based Approach

Pseudomonas aeruginosa is a ubiquitous opportunistic pathogen and a leading cause of severe healthcare-associated infections, particularly ventilator-associated pneumonia (VAP) and bloodstream infections (BSIs) in critically ill patients. Its remarkable capacity for both intrinsic and acquired resistance leads to complex phenotypic profiles, culminating in difficult-to-treat resistant P. aeruginosa (DTR-PA). Managing DTR-PA presents a daunting clinical challenge characterised by a narrow therapeutic armamentarium, treatment delays, and high attributable mortality. To bridge the gap between complex molecular microbiology and bedside decision-making, this narrative review employs a pragmatic, clinical vignette-based approach. Through six representative fictional scenarios of pneumonia and BSI, we systematically dissect the underlying mechanisms of resistance and their direct therapeutic implications. The vignettes explore combinations of intrinsic adaptations—such as Pseudomonas-derived cephalosporinase (PDC/AmpC) hyperexpression, OprD porin loss, and efflux pump upregulation—as well as the acquisition of serine- (e.g., GES) and metallo-β-lactamases (e.g., VIM, NDM, IMP). Grounded in the updated 2026 Infectious Diseases Society of America (IDSA) guidelines, we evaluate the optimal deployment of newer β-lactam/β-lactamase inhibitor combinations. We highlight the specific preference for ceftolozane–tazobactam in pneumonia, the roles of ceftazidime–avibactam and imipenem–relebactam, and the critical use of cefiderocol for metallo-β-lactamase producers and highly resistant phenotypes. Furthermore, the review addresses pressing clinical controversies: the superiority of targeted monotherapy over historical, toxic combination regimens; the risks of unconditionally applying “shorter-is-better” duration paradigms to DTR-PA; and the alarming frequency of treatment-emergent cross-resistance among novel agents. We also clarify common clinical misconceptions, such as the limited utility of meropenem–vaborbactam and aztreonam–avibactam against specific pseudomonal mechanisms. Ultimately, effective DTR-PA management precludes class-based empirical assumptions. By providing a phenotype-driven bedside aide-mémoire, this review reinforces that rapid recognition, direct agent-specific antimicrobial susceptibility testing (AST), pharmacokinetic/pharmacodynamic (PK/PD)-optimised dosing, and prompt source control remain the absolute cornerstones of survival for patients afflicted by these formidable infections.