The Evolving Microbiology and Antimicrobial Resistance in Peritonitis of Biliary Origin: An Evidence-Based Update of the Tokyo Guidelines (TG18) for Clinicians

<b>Background</b>: Biliary peritonitis is a severe intra-abdominal emergency with high mortality. Effective management requires source control and appropriate antimicrobial therapy. <b>Methods</b>: This review synthesizes recent literature (2016-2025), as well as established guidelines recommendations on the evolving microbiology and antimicrobial resistance patterns in biliary tract infections, as data on biliary peritonitis is scarce and relatively heterogeneous. <b>Results</b>: The microbiological landscape is stratified by patient history. Community-acquired infections are typically caused by <i>Escherichia coli</i>, <i>Klebsiella pneumoniae</i>, and <i>Enterococcus</i> spp. In contrast, healthcare-associated infections show a shift, with highly resistant pathogens such as <i>Pseudomonas aeruginosa</i>, and a tendency towards polymicrobial infections. The rise of multidrug-resistant (MDR) organisms, including extended-spectrum β-lactamase (ESBL)-producing Enterobacterales, Carbapenem-Resistant Enterobacterales (CRE), and Vancomycin-Resistant Enterococci (VRE), is a critical challenge limiting therapeutic options. Resistance patterns vary geographically, necessitating the use of local data. <b>Conclusions</b>: This review argues for a paradigm shift from severity-based guidelines to a dual-axis model incorporating resistance risk factors (prior healthcare exposure, previous biliary interventions, a history of MDR infections). We propose a risk-stratified approach to empiric antibiotic selection, emphasizing microbiological diagnostics for therapy de-escalation. Future research should focus on prospective studies, novel antibiotics, and rapid diagnostics.