| Streptococcus suis is a zoonotic agent causing severe infection in both pigs and human beings, such as meningitis,septicemia, endocarditis and pneumonia. Specially, Streptococcus suis serotypeⅡis the most prevalent, virulent and hurtful serotype. β-lactams are the gold standard for therapy in treating Streptococcus suis infection, however, the emergence of antimicrobial resistance make it important to concent the prudent drug use in the treatment of infection in the livestock.Cefquinome, a fouth-generation cephalosporin developed solely for veterinary use, is a good choice for the treatment of Streptococcus suisinfection. However, there is no data about the PK/PD profiles of antimicrobial against Streptococcus suisⅡ.Therefore, the aim of this study was to characterize the PK/PDprofiles of Streptococcus suisⅡusing in vitro experiments and in vivo infection models. It was proposed that these models be used to provide a rational basis for designing dosage regimen, which will provide maximal efficacy and minimal opportunity for the emergence of resistant microorganisms. The contents of this thesis consisted of six parts:1. Antibacterial activity ofcefquinome against Streptococcus suisⅡ The minimum inhibitory concentrations(MICs) of cefquinome against Streptococcus suisⅡat various inoculum sizes(106CFU/m L and108CFU/m L) were determined by the agar dilution method according to the guidelines of the clinical and laboratory Standards Institute(CLSI). The MICs of ATCC 43765 and three clinical strains were 0.03, 0.03, 0.12 and 0.24 μg/m L at 106CFU/m L,and were 0.06, 0.06, 0.5and 0.5μg/m L at 108CFU/m L. There results were not judged to be significant(P =0.25). We next evaluated the time-dependent killing action of cefquinome against ATCC 43765. In the 106CFU/m L group, the killing action showed great bactericidal activity with significant non-concentration-dependence. However, in the 108CFU/m L group, the cefquinome bactericidal activity was markedly attenuated. Meantime, we also evaluated the MPC of ATCC 43765.The MPC was 0.12μg/m L, with MSWranged0.03 to 0.12μg/m L, which means the chance to produce antibiotical resistance is low.2. Pharmacokinetics of cefquinome in infected mouse model Pharmacokinetic studies were performed with Streptococcus suisⅡstrain ATCC 43765 infected neutropenic mice given single doses of cefquinome at 2.5, 10, 40, 160, 640 mg/kg s.c. Blood was sampled by retro-orbital puncture using 3 groups of 3 mice per dose at 0.083, 0.167, 0.25, 0.5, 0.75, 1, 2, 3, 4, 5, 6 h after dosing. A one-compartment model was used to depict the cefquinome concentration versus time profile. The t1/2 ranged from 0.32 to 0.38 h. Tmax rangd from 0.25 to 0.28 h. The kinetics of escalating doses were linear for both Cmax and AUC and ranged from 1.96 to 606.71 mg/liter and 1.58 to 551.40 mg h/liter, respectively. This significant linearity correlation allowed us to extrapolate the PK/PD parameters for other doses which not specifically studied.3. Selective medium and in vivo post-antibiotic effects. Streptococcus suisⅡgrows well in the 5% defibrinated sheep blood, however, it was easily polluted by other bacterials during the experiments. Therefore, we developed a selective and differential media for the identification and quantitation of Streptococcus suis Ⅱ. The selective medium was 5% defibrinated sheep blood with addition of nalidixin acid(60 μg/m L)and polymyxin B sulfate(30 μg/m L). Infection mice were injected s.c. with 0.2 m L single doses of cefquinome at 2.5 and 40 mg/kg at 2 h post-infection. Untreated control growth was determined at seven sampling times(two mice per time point) over 24 h with sampling at 0, 2, 4, 6, 8, 12 and 24 h. The treated groups(two mice per time point) were sampled nine times over 24 h at 1, 2, 3, 4, 6, 8, 12, and 24 h. The PAE values were2.45 and 8.55 h for 2.5 and 40 mg/kg, respectively.4. Study on the in vivo PK/PD model of cefquinome in infected mouse model In vivo activity was determined for mice with systemic infection of standard S. suisⅡATCC 43765 using two inoculum sizes. The therapy started at 2 h post-infection and 28 dosing regimens were used to determine the effects of dosing level and interval on cefquinome efficacy. The results showed that for both inoculum sizes, %?T>MIC held the best correlation at R2= 91% and R2= 63% for For 106CFU/m L and108CFU/m L, respectively. Furthermore, when the experiment was expanded to three more cilincal strains, The %?T>MIC were not statistically different in the degree of cefquinome exposure required to achieve a net bacteriostatic effect between SI and HI(P=0.19). There was, however, roughly a 2-fold increase in the 1-log killing for the HI group(P=0.01).IE index for the stasis effect was low(1.59±0.67) and for the 1-log kill increased to 4.47±1.21.HI showed a statistically significant effect on the magnitude of dosage required for 1-log kill efficacy and were almost 4-fold higher than those for the SI(P=0.006).Using Monte Carlo Simulation to evaluate recommended cefquinome dosing regimens in pigs. The results showed that 2mg/kg/24 h i.m was suitable when MIC ≤ 0.12μg/m L, and 2mg/kg/12 h i.m was better for MIC=0.24μg/m L.5. Septicemia model Septicemia model was built to further study the activity of cefquinome against Streptococcus suisⅡ. Clinical GD110, carried with virulence efand mrp, was chose as the infected strain. 10 mg/kg achieved 100 % survival, and relevant %?T>MIC was 22.5%~42.5%, ED50 was 2.87mg/kg.In conclusion, Theseinvestigations provide a solid foundation for the usage of Cefquinome against Streptococcus suisⅡinfections. |
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