Abstract Introduction Riemerella anatipestifer (RA) is a causative agent of high-mortality septicaemia anserum exsudativa , which inflicts large economic losses on poultry farming. The growing concern over antibiotic resistance and imposed limitations on these drugs’ use in animals prompt the search for antibiotic alternatives. This study characterises the phylogenetic relationship of Polish RA strains and assesses their susceptibility to antibiotics and trans -cinnamaldehyde, geraniol, carvacrol and eugenol as antibiotic alternatives. Material and Methods A total of 24 provided strains were confirmed by RA-specific PCR. A phylogenetic tree was generated with partial 16S ribosomal RNA sequences, also including Riemerella -type strains obtained from GenBank. Antibiotic susceptibility was determined by the Kirby–Bauer method and phytogenic susceptibility by the minimum inhibitory concentration (MIC) method. Results The majority of the strains clustered together, and only one, which clustered with the RA type strain, was an outlier. The widest inhibition-zone diameters were obtained for cephalexin, ceftazidime and cefuroxime, while more than 70% of the strains showed no visible zone of inhibition under the action of enrofloxacin, clindamycin, erythromycin or streptomycin. The most effective phytogenic was trans -cinnamaldehyde, with the minimum concentrations inhibitory and bactericidal to 90% of isolates being only 75 μg/mL. Conclusion The RA strains showed relatively low genetic diversity. Despite this, antimicrobial resistance varied significantly between them. Continuous monitoring of the resistance pattern is necessary. Interpretive criteria for RA susceptibility testing not having yet been standardised, existing research is inconclusive. Some phytogenics are promising alternatives to antibiotics; however, more pharmacokinetics and pharmacodynamics data are needed.
Tyc et al. (Mon,) studied this question.