A combination of silver nanoparticles (AgNPs) and an antibiotic can synergistically inhibit bacterial growth, especially against the drug-resistant bacteria Salmonella typhimurium. However, the mechanism for the synergistic activity is not known. This study chooses four classes of antibiotics, β-lactam (ampicillin and penicillin), quinolone (enoxacin), aminoglycoside (kanamycin and neomycin), and polykeptide (tetracycline) to explore their synergistic mechanism when combined with AgNPs against the multidrug-resistant bacterium Salmonella typhimurium DT 104. Enoxacin, kanamycin, neomycin, and tetracycline show synergistic growth inhibition against the Salmonella bacteria when combined with AgNPs, while ampicillin and penicillin do not. UV-vis and Raman spectroscopy studies reveal that all these four synergistic antibiotics can form complexes with AgNPs, while ampicillin and penicillin do not. The presence of tetracycline enhances the binding of Ag to Salmonella by 21% and Ag+ release by 26% in comparison to that without tetracycline, while the presence of penicillin does not enhance the binding of Ag or Ag+ release. This means that AgNPs first form a complex with tetracycline. The tetracycline-AgNPs complex interacts more strongly with the Salmonella cells and causes more Ag+ release, thus creating a temporal high concentration of Ag+ near the bacteria cell wall that leads to growth inhibition of the bacteria. These findings agree with the recent findings that Ag+ release from AgNPs is the agent causing toxicity.