Background: Streptococcus  and Staphylococcus  are the major contagious organisms causing dairy cow mastitis. Our previous studies have demonstrated that solid lipid nanoparticles (SLNs) can effectively enhance the antimicrobial activity of tilmicosin against Staphylococcus . This study aimed to evaluate the antibacterial efficacy of tilmicosin-loaded SLN (Til-SLN) against Streptococcus agalactiae .
Methods: Til-SLN was prepared using a hot homogenization and ultrasonication method as described previously. Til-SLN was labeled with rhodamine B for nanoparticle tracking. In vitro antibacterial experiments were carried out by broth dilution technique. Pharmacokinetics of the drug and distribution of the nanoparticles in mammary gland were studied after subcutaneous injection in Kunming mice. The therapeutic study was conducted in a mouse mastitis model infected with S . agalactiae  . 
Results: The results showed that the diameter, polydispersity index, zeta potential, encapsulation efficiency, and loading capacity of the nanoparticles were not significantly affected by fluorescence labeling. Til-SLN showed a sustained and enhanced antibacterial activity in vitro. Til-SLN maintained a sustained drug concentration above 17 µg/g for at least 6 days in the mammary gland, as compared with only 3 days for the same amount of tilmicosin phosphate solution. The mean residence time and elimination half-life (T_1/2) of Til-SLN were much longer than those of tilmicosin phosphate solution. Most of the nanoparticles remained at the injection site and a few were transferred to the mammary glands, indicating that the drug was slowly released at the injection site and then distributed to the mammary glands. SLN significantly enhanced the therapeutic efficacy of tilmicosin as determined by lower colony forming unit counts.
Conclusion: These results demonstrate that SLN could effectively enhance the antibacterial activity of tilmicosin against Streptococcus .
Keywords: tilmicosin, solid lipid nanoparticles, Streptococcus agalactiae , mastitis