Streptococcus suis has garnered increasing attention due to its implication in severe infections in both swine and humans, as well as its development of multidrug resistance. The phenomenon of collateral sensitivity, whereby resistance to one antibiotic leads to increased sensitivity to another, provides new opportunities for mitigating the evolution of resistance. In this study, we evolved resistance in S. suis to 11 clinically used antibiotics and characterized the resulting collateral sensitivity profiles, revealing a complex network of interactions. Based on our findings, we identified dozens of such drug pairs and demonstrated collateral sensitivity to gamithromycin in ciprofloxacin-resistant S. suis both in vitro and in vivo. Gamithromycin effectively limits the evolution of resistance and reduces the mutant selection window for ciprofloxacin-resistant S. suis strains. Mechanistic studies indicated that the heightened sensitivity of ciprofloxacin-resistant S. suis to gamithromycin was associated with increased intracellular gamithromycin accumulation due to membrane potential alterations and reduced functions of proton motive force (PMF)-dependent efflux pumps. Furthermore, collateral sensitivity-based treatments significantly resensitized ciprofloxacin-resistant S. suis strains to gamithromycin, resulting in superior efficacy, lower pharmacodynamic targets, and higher treatment success rates in a murine thigh infection model. Our results indicate that gamithromycin sensitivity in S. suis is a collateral consequence of resistance to ciprofloxacin, providing valuable insight for the strategic design of collateral sensitivity-based antibiotic therapies for S. suis infections.

Methicillin-resistant Staphylococcus aureus (MRSA) is a current global public health problem due to its increasing resistance to the most recent antibiotic therapies. One critical approach is to develop ways to revitalize existing antibiotics. Here, we show that the phytogenic compound cinnamaldehyde (CIN) and β-lactam antibiotic combinations can functionally synergize and resensitize clinical MRSA isolates to β-lactam therapy and inhibit MRSA biofilm formation. Mechanistic studies indicated that the CIN potentiation effect on β-lactams was primarily the result of inhibition of the mecA expression by targeting the staphylococcal accessory regulator sarA. CIN alone or in combination with β-lactams decreased sarA gene expression and increased SarA protein phosphorylation that impaired SarA binding to the mecA promoter element and downregulated virulence genes such as those encoding biofilm, α-hemolysin, and adhesin. Perturbation of SarA–mecA binding thus interfered with PBP2a biosynthesis and this decreased MRSA resistance to β-lactams. Furthermore, CIN fully restored the anti-MRSA activities of β-lactam antibiotics in vivo in murine models of bacteremia and biofilm infections. Together, our results indicated that CIN acts as a β-lactam adjuvant and can be applied as an alternative therapy to combat multidrug-resistant MRSA infections.