Salmonella Oslo

Background: Salmonella enterica subsp. enterica serovar Oslo (antigenic formula 6,7,14:a:e,n,x) is a serovar of the serogroup O:7 (C₁). In US, serovar Oslo was considered rare with about 25 cases reported each year. Devi and Murray found that serovar Oslo was one of the most common Salmonella serotypes isolated from school children between 1981 to 1983 in South Kanara District, India.

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

Genetic characteristics: Serovar Oslo has been found to be polyphyletic with two lineages identified. A MDR strain of serovar Oslo isolated from urine and stool specimens was reported by McCarthy et al. Kumar et al. found that three out of eight isolates from tropical seafood in India were resistant to carbenicillin, while the remaining five were pan-susceptible. A US study identified one serovar Oslo isolate resistant to trimethoprim/sulfamethoxazole, sulfisoxazole, ampicillin, tetracycline, and chloramphenicol, which possessed a class 1 integron with a gene cassette array of dfrA1.

Jazeela et al. characterized an auxotrophic mutant of serovar Oslo and revealed that the mutant strain demonstrated greater invasion potential compared to the wild-type, although its replication rate was slower in both cell lines. Like the wild-type, the mutant retained cytotoxic activity in in vitro assays. Additionally, infection with the mutant strain led to increased expression of proinflammatory cytokines (TNF-α, IL-1β) and reduced levels of anti-inflammatory cytokines (TGF-β, IL-6, IL-10) in the infected cell lines. Their further analysis found downregulation of most virulence genes from Salmonella pathogenicity islands 1 and 2 (SPI-1 and SPI-2) in the mutant under in vitro conditions, except for invA, which remained active in HeLa cells. Their findings suggested that the auxotrophic mutant exhibits favorable characteristics for potential antitumor applications, particularly due to its ability to upregulate tumor-inhibiting cytokine genes in in vitro studies.

Animal reservoir: Unknown, serovar Oslo was previously isolated from seafood and monkey.

Geographical distribution: According to the metadata from NCBI PD (accessed May 13, 2025), serovar Oslo has primarily been reported in US and UK, with 441 and 269 isolates, respectively, out of 840 serovar Oslo isolates available on NCBI PD.

Human outbreaks:

Year	Location	Associated source	Number of cases
2023	US: LA	Boiled crawfish	28
2016	US: multistate	Cucumbers	14
1974	US: NY	Endoscope in a hospital	5

Border rejections:

Year	Exporting country	Importing country	Associated source	Product category
2020	Thailand	Germany	Frozen chicken breast¹	Poultry meat and poultry meat products

¹ Consignment was released later.

Recalls:

Year	Location	Recalled food	Type
2022	Germany, Luxembourg, Northern Ireland	Organic tahini from Germany, made with raw material from Uganda¹	Nuts, nut products and seeds

¹ Salmonella Oslo and Korovi were found.

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Salmonella Oslo