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Background:  Salmonella enterica subsp. enterica serovar serovar Thompson (antigenic formula 6,7,14:k:1,5) is part of Group O:7 (C1). Members of this group can be lysogenized by phage 14 (O:6,7 ––> O:6,7,14, former group C4). Salmonella enterica subsp. enterica serovar Cardiff has been combined with Salmonella enterica subsp. enterica serovar Thompson. The first isolation of serovar Thompson was reported by Scott (1926) during a foodborne illness outbreak in Yorkshire, suspected to be linked to the consumption of rabbit and beef pie. Surveillance data indicate that serovar Thompson has remained one of the top ten Salmonella serovars causing diarrhea in both China and US over the last ten years. Zhang et al. suggested that serovar Thompson can be transmitted between poultry farms and humans. Serovar Thompson has been found to form biofilms on eggshells, leading to potential contamination risks.

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

Genetic characteristics: Serovar Thompson has been found to be polyphyletic with four lineages identified and one stand-alone singleton that does not cluster with any other Salmonella Thompson isolates. Zhang et al. revealed that all serovar Thompson strain in their study carried aac(6′)-Iaa and the polymyxin resistance gene mcr-9, though susceptibility to polymyxin B confirmed that mcr-9 did not produce a resistant phenotype. Antimicrobial sensitivity testing against 18 antibiotics showed that serovar Thompson isolates were resistant to streptomycin (100%), ampicillin (35.7%), and doxycycline (14.3%). Additionally, all serovar Thompson isolates harbored well-characterized virulence genes, including those from Salmonella pathogenicity islands (SPI1-5), fimbriae (fimA), flagellar assembly (bcfC, flhD, fliA, fliC, fljB, flgK, lpfC), and other virulence factors (iroN, pagC, cigR). Pathogenicity was further confirmed through yolk sac inoculation in 1-day-old chicks, suggesting that clinical serovar Thompson strains are highly pathogenic to newborn chicks.

Another Chinese study analyzed 141 serovar Thompson ST26 isolates from diverse sources and found that 29 strains—25 from diarrhea patients and four from animal-derived foods—exhibited co-resistance to ciprofloxacin, cefotaxime, and azithromycin (CIP^RCTX^RAZI^R), key frontline antimicrobials for treating human Salmonella infections. The predominant mobile genetic element in these CIP^RCTX^RAZI^R isolates was the IncC plasmid, which carried four resistance genes (qnrS1, qepA4, bla_CMY-2, and mph(A)) conferring resistance to these priority antibiotics. Closely related IncC-harboring isolates (0–23 SNPs) suggested clonal dissemination across multiple Chinese provinces. Notably, the presence of CIP^RCTX^RAZI^R strains in aquatic food products implied the potential role of aquaculture in driving the emergence and spread of high-risk antimicrobial resistance. Sheng et al. stated that, in China, serovar Thompson is the dominant Salmonella serovar carrying the mcr-9 gene. They found that chicken juice substantially increases the proportion of mcr-9-positive serovar Thompson transconjugants. The dissemination of mcr-9 is primarily linked to the IncHI2-IncHI2A plasmid. 

Animal reservoir: Unknown, chicken may be the potential reservoir for serovar Thompson. According to the metadata from NCBI PD, serovar Thompson was isolated from other animals such as cattle, horses, and turtles. 

Geographical distribution: Serovar Thompson has been reported worldwide. 

Human outbreaks: 

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³ This recall was caused by a multistate outbreak described above. Five Salmonella serovars, Salmonella Thompson, Kiambu, Agona, Gaminara, and Senftenberg, were involved. Grande Produce, Agroson’s LLC, and Freshtex Produce, LLC issued recalls of their papayas. 

Relevant links:

1. https://pubmed.ncbi.nlm.nih.gov/20474899/
2. https://onehealthadv.biomedcentral.com/articles/10.1186/s44280-023-00001-3
3. https://stacks.cdc.gov/view/cdc/58450/cdc_58450_DS1.pdf
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC9323645/
5. https://www.sciencedirect.com/science/article/pii/S0956713523004085
6. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2025.1547190/full
7. https://journals.asm.org/doi/10.1128/msystems.01018-24
8. https://www.sciencedirect.com/science/article/pii/S0963996925001000
9. https://kingcounty.gov/en/dept/dph/health-safety/disease-illness/foodborne-illness-outbreaks/2024-feb-1-ihop
10. https://www.cdc.gov/salmonella/outbreaks/onions-10-23/investigation.html
11. https://www.cdc.gov/mmwr/volumes/72/wr/mm7219a2.htm
12. https://www.cdph.ca.gov/Programs/CEH/DFDCS/CDPH%20Document%20Library/FDB/FoodSafetyProgram/EnvInvReports/ElPolloLoco_Chicken_Salmonella_Outbreak%20Report_Final_060820_redacted%20for%20posting.pdf
13. https://archive.cdc.gov/#/details?url=https://www.cdc.gov/salmonella/kratom-02-18/index.html
14. https://e-epih.org/journal/view.php?doi=10.4178/epih.e2019002
15. https://archive.cdc.gov/#/details?url=https://www.cdc.gov/salmonella/kiambu-07-17/index.html
16. https://outbreakdatabase.com/outbreaks/2006-salmonella-thompson-at-a-pumpkin-festival
17. https://www.fda.gov/safety/recalls-market-withdrawals-safety-alerts/gills-onions-voluntarily-recalls-select-lots-fresh-diced-onion-products-because-possible-health-risk
18. https://www.fda.gov/food/outbreaks-foodborne-illness/outbreak-investigation-salmonella-thompson-seafood-october-2021
19. https://archive.cdc.gov/www_cdc_gov/salmonella/kiambu-07-17/advice.html