Fatty Acids Synergize with Bacteriophages Against Multidrug-Resistant Pathogens

In the face of rising antimicrobial resistance (AMR), phage therapy presents a promising alternative to antibiotics, but efficacy is limited by biofilms, narrow host range, and hosts rapidly developing phage resistance. Here we present a new concept for phage–based antimicrobials: using small–molecules to help phages infect and kill bacteria. Using checkerboard synergy assays, growth curves, and biofilm assays, we evaluate short– and medium–chain fatty acids (SCFAs/MCFAs) with diverse lytic phages against 18 strains of Stenotrophomonas maltophilia and extend our analysis to include more than 120 clinical isolates of Staphylococcus aureus , Enterococcus spp. , and Pseudomonas aeruginosa . pH–neutral MCFAs (C6–C10) are bactericidal, do not diminish phage viability, and synergized with phages to delay or prevent phage–resistant bacterial rebound over 48 h. MCFAs lowered both attached and free bacterial biomass and were synergistic with envelope–active antibiotics (polymyxin B, ceftazidime, meropenem), supporting a mechanism involving bacterial envelope disruption. Phage–MCFA cooperativity was observed across both Gram–negative and Gram–positive bacteria: across 18 S. maltophilia strains and four lytic phages, phage–MCFA combinations were predominantly additive–to– synergistic; in 91 strains of Enterococcus spp. , pH-neutral C8 produced widespread synergy with phage CAR, and enabled killing of strains recalcitrant to phage alone; in 18 strains of S. aureus , effects were stoichiometry–dependent, showing both synergy and antagonism depending on the ratio of phage–to– MCFA; in 14 strains of P. aeruginosa , interactions were largely additive. Together, this work elevates MCFAs as potential therapeutic accompaniments that stabilize and broaden phage efficacy, and highlights the general potential of small molecules as phage adjuvants.