Characterization of the hemolytic activity of Riemerella anatipestifer

In vivo pharmacokinetic and pharmacodynamic study and cutoff of florfenicol against Riemerella anatipestifer in ducks

Riemerella anatipestifer causes serious infections, characterized by septicemia and serositis, in ducks and geese. R. anatipestifer is mainly controlled through antimicrobial chemotherapy. This study investigated the pharmacokinetic/pharmacodynamic (PK/PD) integration of florfenicol (FF) against R. anatipestifer by establishing a systemic infection model in ducks. For PK studies, FF was administrated intramuscularly (i.m.) at single doses of 2.5, 10, 20, and 40 mg/kg body weight. The concentrations of FF in blood, lung, and liver were determined. FF was rapidly eliminated in R. anatipestifer-infected ducks with T1/2kel values of 1.67, 2.2, and 1.62 h in the plasma, lung, and liver, respectively. For PD analysis, the infected ducks were administered FF via the i.m. route at doses of 5-80 mg/kg body weight, using 2 dosing regimens involving the administration of FF either once or twice over 24 h. The bacteria were counted 24 h after drug administration. Bactericidal effects in tissues (including those of the heart, liver, spleen, lung, kidney, and brain) were achieved at doses of ≥20 mg/kg following 2 i.m. injections of FF within 24 h. The data obtained were fitted to a sigmoidal Emax model. The results demonstrated that AUC24h/minimum inhibitory concentration (MIC) (R2 = 0.930) and Cmax/MIC (R2 = 0.930) were the optimal PK/PD parameters for describing the antibacterial activity of FF. The magnitudes of AUC24h/MIC and Cmax/MIC required to produce a drop of 3 Log10CFU/mL in the bacterial count were 58.56 h and 15.10, respectively. The MIC distribution of 164 R. anatipestifer strains for FF ranged from 0.25 to 16 μg/mL. Both the values of COWT derived from the ECOFFinder program and the COPD based on a 10,000-subject Monte Carlo simulation of FF against R. anatipestifer were 1 μg/mL, confirming that infections caused by strains with MIC ≤ 1 μg/mL could be effectively treated. Our study results may prove useful in optimizing FF regimens to treat R. anatipestifer infections.

Immunogenicity of live phoP gene deletion strain of Riemerella anatipestifer serotype 1

Duck infectious serositis is an acute and infectious disease caused by Riemerella anatipestifer (R. anatipestifer) that leads to perihepatitis, pericarditis, meningitis, and airbag inflammation in ducks, which causes serious economic losses to the global duck industry. The phoP/phoR is a novel 2-component signal transduction system first reported in gram-negative bacteria, of which phoP acts as a global regulator and virulence factor. In this study, the phoP gene from the R. anatipestifer YM strain was knocked out using homologous recombination technology and replaced with the spectinomycin resistance gene (Spec). The virulence of the R. anatipestifer YMΔphoP strain was reduced by approximately 47,000 times compared to that of the wild-type R. anatipestifer YM strain. Ducks were immunized with live R. anatipestifer YMΔphoP strain by subcutaneous inoculation at a dose of 10⁶ to 10⁷ CFU (0.2 mL per duck) and challenged with the wild-type R. anatipestifer YM strain 14 days later. The protection rate in the immunized group was 100%. The growth characteristics of ducks in the immunized and negative control groups were normal, and the research demonstrated R. anatipestifer YMΔphoP strain have suitable immunogenicity and protective effects. Thus, the study findings suggest that the novel R. anatipestifer YMΔphoP strain may provide a candidate for the development of a gene deletion activated vaccine against duck infectious serositis.

Identification of genetic determinants of hemolytic activity of Riemerella anatipestifer using random transposon mutagenesis

Riemerella anatipestifer causes epizootic infectious disease in poultry resulting in serious economic losses especially to the duck industry. In our previous study, R. anatipestifer was found to lyse duck erythrocytes in vitro. In the present study, a random Tn4351 mutagenesis library of hemolytic R. anatipestifer strain SX containing 4000 mutants was constructed to investigate the genetic basis of hemolytic activity. Thirty mutants with reduced hemolytic activity and one with increased hemolytic activity were screened and insertions in 24 genes were identified. Of these genes, four were predicted to encode outer membrane proteins, one encoded a cytoplasmic membrane protein, 11 encoded cytoplasmic proteins, and eight encoded proteins with unknown locations. Based on current annotations of the R. anatipestifer genomes, of the 24 genes, 7 (29.17%) were involved in iron utilization. The hemolytic activities of the complemented strains M2 (pRES-Riean_0790) and M18 (pRES-Riean_0653) were restored, indicating that both Riean_0653 and Riean_0790 are involved in the hemolytic activity of strain SX. However, the recombinant proteins rRiean_0317, rRiean_0790, rRiean_0653, rRiean_1027, rRiean_1143, and rRiean_1561 had no hemolytic activity, suggesting that none were hemolysins.