Salmonella Enteritidis

Background: Salmonella enterica subsp. enterica serovar Enteritidis (antigenic formula 1,9,12:g,m:- ) is a serovar of the O:9 (D₁) serogroup. This serovar is mainly associated with table eggs and chickens. Eggs can become contaminated either through penetration of the eggshell by Salmonella Enteritidis from the colonized gut or contaminated feces during or after oviposition (horizontal transmission). Alternatively, contamination can occur via direct infection of the yolk, albumen, eggshell membranes, or eggshells before oviposition, which results from the infection of reproductive organs with this serovar (vertical transmission). Since the 1990s, infection rates have dropped as the egg industry implemented voluntary controls on egg flocks and consumers have reduced their consumption of raw eggs. However, large outbreaks have still occurred. According to the World Health Organization, it is the most common serovar reported from human salmonellosis cases in Europe, North and South America, Africa and Asia.

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

Genetic characteristics: This serovar is further classified according to the susceptibility to typing phages (phage typing (PT)), the most common PTs are PT8 and PT4. Molecular subtyping with Pulse field gel electrophoresis (PFGE) shows that approx. 40% of the isolates fall in the same PFGE type. Invasive strains of serovar Enteritidis emerged in sub-Sahara regions of Africa and have caused systemic salmonellosis in travelers returning from Africa. Serovar Nitra and Enteritidis have antigenic formulas that differ only in their O antigens. Although serovar Nitra is very rare, it is genetically indistinguishable from serovar Enteritidis. Chen et al. found that Salmonella Enteritidis is a polyphyletic serovar comprised of six phylogenetic groups and two stand-alone singletons that do not cluster with any other Salmonella Enteritidis isolates. Within Enteritidis-A-7 (chicken-associated; estimated emergence in 1633), they identified four clonal sub-clades. Cao et al. analyzed Enteritidis isolates collected worldwide from humans and concluded that in clades I and IV, most isolates were pan-susceptible, while 81.8% (9/11) of isolates in clade III and 22.4% (13/58) in clade II were multidrug-resistant (MDR). Notably, 77% (10/13) of the MDR isolates in clade II originated from China. The most common antimicrobial resistance genes (ARGs) found in the Chinese isolates were aph(3′)-IIa, bla_CTX-M-55, and bla_TEM-1B, whereas the most prevalent ARGs in the African isolates (clade III) were bla_TEM-1B, sul1, sul2, drfA7, aph(3")-Ib/strA, and aph(6)-Id/strB. Of the 14 plasmid types identified, IncX1 and IncFII(pHN7A8) were exclusive to the Chinese MDR isolates, while IncQ1 was strongly linked to the African MDR isolates. A Chinese study revealed that serovar Enteritidis triggers an inflammatory response through SPI-1 and SPI-2, which enhances intracellular proliferation and bacterial virulence. A protein–protein interaction prediction revealed a connection between virulence proteins and T3SS proteins, with PagN interacting with T3SS-related proteins to collectively mediate the invasion of the Enteritidis 27A strain into the human body.

Animal reservoir: Chicken is the main reservoir. Serovar Enteritidis is also found in low percentage (<1%) in turkey, cattle, and dogs.

Geographical distribution: The distribution of serovar Enteritidis is global.

Human outbreaks: Numerous Enteritidis outbreaks have been reported and they are mainly associated with eggs or poultry. Below are some examples.

Year	Location	Associated source	Number of cases
2024	US: multistate	Backyard poultry	470¹
2018	US: multistate	Eggs	44
2016	Europe: multi-country²	Eggs	112³
2012	US: multistate	Ground beef	46
2012	US: multistate	Restaurant chain A	68
2011	US: multistate	Turkish pine nuts	43
2011	US: multistate	Alfalfa sprouts and spicy sprouts	25
2011	England	Eggs	63
2011	Singapore	Seafood (marinara pasta seafood)	270
2010	US: multistate	Shell - eggs	1,939
2010	Austria	Eggs	66

¹ Multiple serovars, including Salmonella Altona, Cerro, Enteritidis, Indiana, Infantis, Johannesburg, Mbandaka, and Typhimurium, were linked to this outbreak. The case number represents the total number of cases associated with the outbreak and does not specifically indicate the number of people infected by Salmonella Enteritidis.

²Belgium, Denmark, Luxembourg, the Netherlands, Norway, Sweden and the UK, and potentially Croatia

³112 confirmed, 148 probable

Border rejections: There are numerous border rejections linked to Salmonella Enteritidis. The majority of them are associated with chicken, eggs, and egg products, while a few can also be related to other commodities. Below are examples.

Year	Exporting country	Importing country	Associated source	Product category
2025	Poland	Italy	Frozen duck¹	Poultry meat and poultry meat products
2024	Brazil	Netherlands	Fresh chicken meat	Poultry meat and poultry meat products
2019	Poland	Czech Republic	Chilled Beef	Meat and meat products (other than poultry)

¹ Salmonella Enteritidis and Newport were both found.

Recalls: There are numerous recalls linked to Salmonella Enteritidis. The majority of them are associated with chicken, eggs, and egg products, while a few can also be related to other commodities. Below are examples.

Year	Location	Recalled food	Type
2024	Romania	Frozen chicken legs	Poultry meat and poultry meat products
2022	Estonia	Chicken thigh boneless, skinless from Poland	Poultry meat and poultry meat products
2021	Spain	Frozen pasteurized wedge clams (Donax trunculus) from Turkey	Bivalve molluscs and products thereof
2021	Slovakia	Cereal-based foods for infants and young children	Cereals and bakery products
2020	Poland	Eggs	Eggs and egg products

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Salmonella Enteritidis