In Vitro Synergistic Effects of Antibiotic Combinations Against Multidrug-Resistant Streptococcus suis from Diseased Pigs

<b>Background/Objectives:</b> Multidrug-resistant (MDR) strains of <i>Streptococcus suis</i> are increasingly prevalent and present significant challenges in clinical management. Given that the development of new antibiotics is a resource-intensive process and time-consuming, there is an urgent need for alternative therapeutic strategies to address resistance in the short term. One promising approach is the use of combination therapy, which involves pairing potent antibiotics with agents that may be less effective on their own, to enhance therapeutic efficacy and potentially overcome resistance mechanisms. This study aimed to investigate the in vitro antibacterial activity of combining two classes of antibiotics with distinct mechanisms of action-cell wall synthesis inhibitors and protein synthesis inhibitors-against MDR <i>S. suis</i> strains isolated from diseased pigs. <b>Methods:</b> A total of 36 MDR <i>S. suis</i> strains were tested using a microbroth dilution checkerboard assay to determine the minimum inhibitory concentration (MIC) of four cell wall synthesis inhibitors -amoxicillin/clavulanic acid (AMC), ampicillin (AMP), penicillin G (PEN), and vancomycin (VAN)- in combination with four protein synthesis inhibitors -gentamicin (GEN), neomycin (NEO), tilmicosin (TMS), and tylosin (TYL). Time-kill curve assays were conducted to evaluate the in vitro bactericidal activity of synergistic antibiotic combinations (PEN-GEN and AMP-NEO) against Beta-lactam-resistant and Beta-lactam-susceptible MDR <i>S. suis</i> strains. <b>Results:</b> Checkerboard analysis revealed that penicillin-gentamicin combination exhibited the most effective synergistic activity against the MDR <i>S. suis</i> strains (10/19, 52.6%), with ∑FIC values of 0.25-1.06 and MIC reductions from resistant to susceptible levels. Time-kill assays further confirmed the synergistic bactericidal effect of the combination, demonstrating complete bacterial clearance within 6-9 h, markedly rapid bacterial killing compared to monotherapy. <b>Conclusions</b>: This study demonstrates that antibiotic combinations, particularly Beta-lactams combined with aminoglycosides, show synergistic activity against pig-isolated <i>S. suis</i> MDR strains. The PEN-GEN combination exhibited strong synergistic and bactericidal effects, supporting combination therapy as a potential strategy to address antimicrobial resistance. Further evaluation in diverse strain backgrounds and prudent antibiotic use are essential to confirm efficacy and limit the emergence of antibiotic resistance.