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Background: Salmonella enterica subsp. enterica serovar Stanley (antigenic formula: 1,4,[5],12,[27]:d:1,2) is a serovar of the O:4 (B) serogroup. S. Serovar Stanley is commonly associated with human clinical infections in South East Southeast Asia, which appears to have spread to Europe. Initial cases in Denmark were associated with patients that traveled to Thailand; In Asia, cases have been found in children under the age of 5. Multidrug resistant strains that encode extended spectrum beta-lactamases (enzymes that confer resistance to third generation cephalosporin) have been reported in clinical cases in Asia and EuropeA study found that serovar Stanley was the second most prevalent serovar, responsible for 11% of human salmonellosis cases between 2002 and 2007. Its prevalence increased from 1.9% (64 cases) in 1993 to 10.1% (141 cases) in 2003 in Thailand. In Denmark, this serovar ranked fifth among 3,657 confirmed salmonellosis cases reported to the Statens Serum Institut (SSI) in 2008, with 55% (24 cases) linked to travel, mainly to Thailand. In France, serovar Stanley was the 36th most common strain out of 10,378 human isolates recorded by the Institut Pasteur in 2008. Among 13 French cases with travel history, 38% (5 patients) had visited Southeast Asia shortly before falling ill. Both Salmonella Typhimurium and Stanley were the two most common serogroup B Salmonella serovars affecting humans between 2004 and 2012 in Taiwan.

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

Genetic characteristics:

Serovar Stanley has been found to be polyphyletic with two lineages identified and two stand-alone singletons that do not cluster with any other Salmonella Stanley isolates. Hendriksen et al. confirmed that multiple plasmid lineages containing bla_CMY-2 gene, which encodes extended-spectrum cephalosporin resistance, were present in serovar Stanley isolates from central Thailand. Serovar Stanley isolates in a Chinese study showed resistance to six antimicrobials—ampicillin, ciprofloxacin, cefotaxime, azithromycin, trimethoprim/sulfamethoxazole, and chloramphenicol—and harbored 14 resistance genes across five classes: β-lactams (bla_TEM-1B, bla_OXA-1, bla_DHA-1), quinolones (qnrB4, aac(6')-Ib-cr), macrolides (mph(A), mph(E), msr(E)), folate pathway inhibitors (sul1, sul2, dfrA14), and chloramphenicol (floR, catA2, catB3). A T57S mutation in parC’s quinolone resistance-determining region was detected, along with three plasmid replicons (IncHI2/IncHI2A, IncR, IncN) and three MGEs. Among 42 virulence genes, 25 were secretion/transport-related, 10 encoded fimbrial adherence factors, and only the inv gene in SPI-1 was linked to invasiveness, with no typhoid toxin genes identified.

A Taiwanese study revealed that genetically diverse Salmonella Typhimurium and clonally disseminated Salmonella Stanley elicited distinct immune responses, with serovar Stanley showing higher phagocytosis rates, reduced intracellular survival, elevated reactive oxygen species (ROS) production, and increased expression of the pro-inflammatory cytokine IL-1β when exposed to dead bacterial cells. These differences in host immune interaction may contribute to the lower prevalence of Salmonella Stanley compared to Salmonella Typhimurium. Rao and Chauhan studied pathogenesis of serovar Stanley infection in experimental chicks and found after oral exposure, serovar Stanley could be found in different organs at varying time periods: present in the duodenum between 6 hours and 5 days post-infection, in the caecum from 12 hours to 9 days, and detectable in the liver, spleen and bloodstream between 24 hours and 7 days. The development of infection resistance was only observed in animals older than 10 days, with no significant protection seen in those 6 days old or younger.

Animal reservoir: Swine is the reservoir of serovar Stanley. According to the metadata from NCBI PD, this serovar was also isolated from other animals such as fish (Labeo rohita), snake (Naja siamensis), and frog.

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³ This recall was caused by an outbreak described above. Serovar Stanley, Newport, Kottbus, and Lexington were found in tested products.

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