Identification of a Salmonella Abortusequi isolate based on morphological and serological characteristics and assessment of its antimicrobial resistance profile

Abstract

Background and Aim. Equine salmonellosis-associated abortion remains an important infectious disease of horses, with significant implications for reproductive health and horse breeding. Salmonella enterica subsp. enterica serovar Abortusequi is recognized as one of the etiological agents associated with abortion in mares and may persist within affected horse populations. This study is aimed to characterize and identify an S. Abortusequi isolate recovered from pathological material obtained from an aborted equine fetus in the Aiyrtau District of the North Kazakhstan Region through comprehensive assessment of its morphological, cultural, biochemical, and serological characteristics, and to determine its antimicrobial susceptibility profile.Materials and Methods. A pure bacterial culture was obtained through sequential subculturing followed by the selection of colonies exhibiting characteristic morphological features. Identification of the isolate was performed using an integrated approach based on morphological, cultural, biochemical, and antigenic characteristics. Cellular morphology and staining properties were examined by light microscopy, while cultural characteristics were evaluated based on growth patterns and colony morphology on selective and differential diagnostic media. Biochemical activity was assessed using standard diagnostic tests. The antigenic profile of the isolate was determined by slide agglutination with polyvalent and monoreceptor diagnostic antisera specific for Salmonella O- and H-antigens. Species-level identification was additionally confirmed by matrix-assisted laser desorption/ionizationtime-of-flight mass spectrometry (MALDI-TOF MS). Antimicrobial susceptibility was assessed against 12 antimicrobial agents using the disk diffusion method, with results evaluated based on the diameters of bacterial growth inhibition zones.Results. The recovered isolate exhibited morphological, cultural, and biochemical characteristics consistent with the genus Salmonella, including Gram-negative, motile, rod-shaped cells, facultative anaerobic growth, characteristic colony morphology on selective and differential media, and hydrogen sulfide (H2S) production. Biochemical testing demonstrated glucose fermentation with acid and gasproduction, absence of lactose and sucrose fermentation, negative indole and urease reactions, and positive citrate utilization. Serological analysis revealed specific agglutination with O:4 (O-IV) and O:12 (O-XII) diagnostic antisera, assigning the isolate to Salmonella serogroup B; the antigenic formula was determined as 4,12:-: n,x, consistent with Salmonella enterica subsp. enterica serovar Abortusequi. Identification was additionally supported by MALDI-TOF MS analysis. Antimicrobial susceptibility testing showed inhibition zone diameters ranging from 26 to 41 mm, with the largest zones observed for imipenem (41 mm), ofloxacin (40 mm), and ceftriaxone (38 mm), followed by amoxicillin/clavulanic acid and norfloxacin (35 mm each), cefoxitin (34 mm), co-trimoxazole and tobramycin (32mm each), ampicillin (31 mm), gentamicin (30 mm), azithromycin (27 mm), and tetracycline (26 mm).Conclusion. Comprehensive assessment of the morphological, cultural, biochemical, serological, and mass spectrometric characteristics enabled identification of the isolate recovered from pathological material of an aborted equine fetus as S. Abortusequi. Disk diffusion testing demonstrated growth inhibition zone diameters ranging from 26 to 41 mm across the panel of 12 antimicrobial agents tested. These findings provide a phenotypic characterization of the S. Abortusequi isolate and describeits antimicrobial susceptibility profile, contributing to the laboratory characterization of this pathogen associated with equine abortion.