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Background: Salmonella enterica subsp. enterica serovar Orion (antigenic formula 3,{10}{15}{15,34}:y:1,5) is a serovar of the O:3,10 (E₁) serogroup. This serovar was first identified in 1945 from a rectal swab sample taken from a man in US. McAuley et al. found that serovar Orion was the most prevalent Salmonella serovar and was potentially endemic to seven dairy farms in Australia.

NCBI PD link for serovar Orion: https://www.ncbi.nlm.nih.gov/pathogens/isolates/#(taxgroup_name:%22Salmonella%20enterica%22)%20AND%20computed_types:(%22serotype=Orion%22)

Genetic characteristics: Serovar Orion, specifically its variant 15⁺,34⁺, was classified as belonging to the low-invasive groups of Salmonella serovars. Agga et al. isolated a serovar Orion strain from mink feces, which carried copper and silver resistance gene clusters (e.g., golS, pcoA, and silA) and multidrug resistance genes (i.e., mdsA and mdsB). Similarly,Hasegawa et al. also found that all 324 serovar Orion genomes analyzed contained the efflux pump genes mdsA/mdsB and gold tolerance genes golS/golT, while other resistance genes ( aminoglycosides, β-lactams, quinolones [via chromosomal mutations], colistin, lincosamides, macrolides, phenicols, sulfonamides, trimethoprim, tetracyclines, and disinfectants) were present at lower frequencies (0.3%–5.9%), with 17 genomes (5.2%) from Canada, the UK, US, and Tanzania showing multidrug-resistant potential. Metal tolerance genes for arsenic, copper, mercury, silver, and tellurium appeared in 0.3%–35.5% of isolates, and the plasmid Col(MGD2) was most prevalent (15.4%). Virulence profiling identified genes for adherence, macrophage induction, magnesium uptake, regulation, serum resistance, stress adaptation, type III secretion systems, and six SPIs (1–5, 9, 12–14, C63PI). Typing revealed 89.2% of isolates belonged to ST639.

Ragione et al. found that serovar Orion var. 15⁺ demonstrated comparable adherence and invasion capabilities to Salmonella Enteritidis in HEp-2 cell cultures. However, in one-day-old and seven-day-old chick models, serovar Orion var. 15⁺exhibited significantly lower virulence. While Salmonella Enteritidis colonized 100% of liver and spleen samples within 24 hours at a given dose, serovar Orion var. 15⁺ only colonized 25% of these organs. In another trial, var. 15⁺ failed to colonize any livers or spleens by 24 hours, whereas Salmonella Enteritidis successfully colonized 50% of organs at the same dose. Among the three tested serovar Orion var. 15⁺ isolates (F3720, B1, and B7), strain F3720 showed significantly greater invasiveness, colonizing more internal organs than the others. Interestingly, F3720 was also shed in lower quantities compared to B1 and B7, suggesting a potential inverse relationship between deep tissue colonization and gut persistence.

Animal reservoir: Unknown, but according to the metadata from NCBI PD, serovar Orion was isolated from pigs, cattle, chicken and other animals.

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² This recall was caused by a multi-country outbreak described above. Salmonella Orion and group E serovars were found.

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