A multi-city outbreak of Salmonella Enteritidis infections linked to bakery products, Republic of Korea

Abstract

Objectives

In May 2025, clusters of salmonellosis were identified in 7 cities in the Republic of Korea, all associated with consumption of identical bakery products. This investigation aimed to characterize the outbreak, identify potential contributing factors, and inform strategies for preventing similar multi-facility foodborne outbreaks.

Methods

A case series study was conducted among individuals who consumed Manufacturer H’s Product I and Product II on May 15–16, 2025 at 7 facilities (n=1,235). Clinical specimens from symptomatic individuals, retained food samples, and environmental samples were collected and tested. Food-exposure histories were assessed, and active case finding was implemented across all supplied facilities. Traceback investigations were conducted at the manufacturer, distributor, and egg farms. Human and food isolates underwent pulsed-field gel electrophoresis (PFGE) and whole-genome sequencing (WGS).

Results

A total of 323 cases met the outbreak case definition (attack rate, 26.2%), of which 48 were laboratory-confirmed. Salmonella Enteritidis was isolated from both clinical specimens and retained bakery products. PFGE patterns were indistinguishable between human and food isolates, and WGS demonstrated high genetic relatedness. These findings confirmed a common-source outbreak linked to the implicated bakery products.

Conclusion

This outbreak underscores the value of integrating epidemiological investigation, active case finding, and molecular typing to identify common food vehicles in outbreaks involving widely distributed manufactured foods. Coordinated collaboration between public health and food safety authorities is essential for the effective detection, response, and prevention of multi-facility foodborne outbreaks.

Graphical abstract

Introduction

Salmonellosis caused by non-typhoidal Salmonella (NTS) is an acute gastroenteritis of global public health concern. Salmonella is a Gram-negative, rod-shaped bacterium belonging to the Enterobacteriaceae family [1]. Based on O, H, and Vi antigens, Salmonella is classified into 6 subspecies and over 2,500 serotypes, approximately 10% of which are associated with human infection [1,2]. Salmonella spp. primarily reside in domestic and wild animals, including poultry, swine, and cattle. Most human infections are foodborne and originate from contaminated animal products; secondary transmission occurs through contaminated water, fresh produce, or direct contact. Typical symptoms of salmonellosis include watery diarrhea, abdominal pain, fever, headache, nausea, and vomiting. The incubation period is typically 6–72 hours but can occasionally extend up to 16 days [3–7].

Globally, NTS is a leading cause of diarrheal disease. The Centers for Disease Control and Prevention estimates that NTS causes approximately 1.35 million infections annually in the United States, most of which are attributable to foodborne transmission [4]. In the Republic of Korea, Salmonella is designated as a notifiable infectious disease and is monitored through a nationwide sentinel surveillance system managed by the Korea Disease Control and Prevention Agency (KDCA) [3]. Salmonella is among the most frequently reported bacterial agents of water- and foodborne outbreaks, with approximately 3,000 cases of salmonellosis reported annually [8,9]. Incidence generally increases during the hot, humid summer months [1,4], and common vehicles include mixed dishes (e.g., gimbap) and egg products [10]. When a cluster of salmonellosis is suspected, local governments initiate a comprehensive epidemiological investigation. This mandated process involves patient interviews, environmental assessments, and the collection of human, retained food, and environmental specimens for laboratory testing [3]. This framework enables a rapid public health response to prevent further disease spread.

Among Salmonella serotypes, Salmonella Enteritidis is the most frequently isolated serotype worldwide and in the Republic of Korea, largely due to its strong association with egg-laying poultry [11–13]. Although localized outbreaks linked to single food establishments are common, large-scale, multi-city outbreaks involving complex manufactured foods, such as bakery products, pose unique challenges for public health authorities [14–19]. In such outbreaks, prompt and accurate identification of the contaminated source, along with precise characterization of transmission pathways using integrated epidemiological and laboratory data, is essential for effective control and prevention [20].

On May 21, 2025, two initial clusters of diarrheal illness were identified at 2 facilities located in separate cities in Chungcheongbuk-do. In response, local governments, a regional office of the Ministry of Food and Drug Safety (MFDS), and the provincial office of education initiated an epidemiological investigation. On May 22, MFDS identified a shared exposure to bakery products from the same manufacturer and proactively requested a temporary suspension of distribution and sales to prevent fcenter staff in the 7 affected cities conducted spread. On June 2, cultures of patient specimens and retained food samples from both initial facilities yielded Salmonella of the same serotype. The KDCA subsequently classified the events as a common-source outbreak and identified 2 additional facilities that had consumed the same products, bringing the total to 4. Active case finding across all recipient facilities later identified 3 additional symptomatic clusters, resulting in a total of 7 affected facilities. This event represents a significant multi-city foodborne outbreak linked to a complex manufactured food item in the Republic of Korea.

Therefore, the KDCA conducted an extensive epidemiological investigation and laboratory analysis. The objective of this study was to characterize the multi-city outbreak of S. Enteritidis infections linked to bakery products, identify the specific food source and transmission route through the integration of epidemiological and laboratory findings, and provide critical insights for the prevention and control of future complex foodborne outbreaks in the Republic of Korea.

Materials and Methods

Study Population

The study population comprised symptomatic individuals who developed illness after consuming the suspected bakery products, namely Product I (use-by October 12, 2025) and II (use-by September 21, 2025), produced by Manufacturer H. Among 250 facilities supplied with these bakery items, 7 facilities reported symptomatic individuals (n=1,811) who experienced symptoms such as diarrhea, vomiting, or abdominal pain, or who received a clinical diagnosis of Salmonella infection. Public health center staff in the 7 affected cities conducted food-consumption (exposure) surveys. Among 1,596 respondents, duplicate responses, individuals who were absent on the serving date or did not consume the bakery items, and individuals whose symptom onset preceded product consumption were excluded. The final analytic cohort comprised 1,235 persons.

To identify additional clusters, the presence of symptomatic persons in the remaining 246 facilities was verified, excluding the 4 Facilities (A–D) already reported in the KDCA outbreak reporting system. In parallel, among 282 waterborne and foodborne outbreak reports submitted between January and May 2025, eight events with pulsed-field gel electrophoresis (PFGE) patterns matching those of the present outbreak were reviewed to assess whether the implicated bakery items had been consumed.

To evaluate potential contamination points along the production–distribution chain, inspections were conducted at 2 manufacturing sites, including the bakery and the liquid egg–processing facility, as well as at the distributor and supplying egg farms (Figure 1).

Figure 1.

Subjects of multi-city outbreak of Salmonella Enteritidis. ^a)Seven facilities including three identified through active surveillance.

Case Definition

A case was defined as an individual from a facility that served either of the 2 bakery products on May 15–16, 2025, who developed illness within the maximum incubation period for Salmonella infection after consumption and met one or more of the following criteria: (1) ≥3 episodes of diarrhea within 24 hours; (2) ≥2 episodes of diarrhea within 24 hours accompanied by fever or abdominal pain; (3) ≥1 episode of vomiting.

In addition, any individual in whom Salmonella was detected by rectal swab or stool culture was included as a case regardless of symptoms.

Data Collection

Local public health centers administered questionnaires adapted from national guidelines. For each facility, food histories were obtained for the 3-day period preceding the first symptomatic case or the documented serving date of the implicated products. The scope of data collection varied by facility: in Facilities E–G, questionnaires focused specifically on the implicated bakery items, whereas in Facilities D and G, surveys were limited to symptomatic persons only.

Study Design

Multiple lines of evidence supported the identification of a common-source outbreak, including shared consumption of identical bakery products among affected individuals, appropriate temporality between product consumption and illness onset, and laboratory confirmation of an identical Salmonella serotype in both patient specimens and retained food samples. Based on these converging findings, a descriptive case series was conducted to summarize clinical features, incubation periods, and epidemic curves among reported cases.

Active Case Finding

A line list of 250 recipient facilities supplied by Distributor I with the implicated bakery products was obtained. Excluding the 4 Facilities (A–D) already reported to the KDCA outbreak reporting system, active case finding was conducted in the remaining 246 facilities to identify additional outbreaks. Case-finding activities were coordinated through the Ministry of Education and local governments.

In parallel, 282 waterborne and foodborne outbreak reports submitted to the KDCA outbreak reporting system between January 1 and May 31, 2025, were reviewed, and the PulseNet Korea database was queried. Eight outbreaks with PFGE patterns indistinguishable from those of the initial 4 clusters were identified, and consumption of the implicated bakery products at those facilities was assessed.

Laboratory Testing

Testing of Salmonella-positive stool specimens, including rectal swabs, retained food samples, and environmental specimens (e.g., food-preparation utensils), was conducted by the Research Institutes of Public Health and Environment. For specimens tested at healthcare facilities, polymerase chain reaction was used to identify organisms at the Salmonella genus level (i.e., Salmonella spp.).

For Facilities E, F, and G, which were identified through active case finding, the maximum incubation period had elapsed by the time of investigation, and no actively symptomatic cases remained. Therefore, additional clinical specimens could not be collected from affected individuals. However, stool specimens were collected from food-handling staff at Facility E for testing.

PFGE and whole-genome sequencing (WGS) analyses of human and retained food specimens that tested positive for Salmonella were performed by the Division of Bacterial Diseases at the KDCA.

Traceback Investigation

To elucidate potential contamination routes, on-site field investigations were conducted across the full production–distribution continuum, including the bakery manufacturer (Manufacturer H), the primary distributor (Distributor I), the liquid egg–processing facility, and the source egg farms. The investigation was guided by 3 predefined hypotheses addressing distinct stages of exposure risk: contamination introduced via raw materials, workforce-associated transmission during handling, and cross-contamination occurring within the production process.

Manufacturer H

At Manufacturer H, hygiene practices and production protocols were evaluated in a systematic and structured manner. Raw material safety was assessed through a comprehensive review of ingredient lists, along with microbiological testing of available ingredient lots and process water. Workforce hygiene was evaluated by examining employee health records, gastrointestinal symptom logs, mandatory hygiene training documentation, and food-handler certification status. To assess the production environment, the facility layout was mapped in detail to verify segregation between hygiene zones, and 11 environmental swabs were collected from food-contact surfaces, production equipment, and packaging areas. In addition, microbiological testing was performed on 4 finished bakery products produced at the manufacturer. Both environmental and finished-product samples obtained from Manufacturer H were tested for Salmonella.

Downstream and upstream facilities

The investigation extended downstream to Distributor I to evaluate cold-chain management and recall implementation. Upstream investigations were carried out by the MFDS at the liquid egg–processing facility and its supplying farms. Microbiological tracing was conducted by collecting 3 liquid egg samples from the processor and 84 raw egg samples from the source farms to identify the origin of the implicated pathogen.

Statistical Analysis

Local public health center staff administered surveys and compiled responses using a standardized Microsoft Excel data-entry template to ensure consistency across facilities. The assembled dataset was analyzed using Excel ver. 2021 (Microsoft) and RStudio ver. 4.5.1 (Posit, PBC) to generate descriptive statistics, summarize outbreak characteristics, and calculate incubation periods.

Ethical Considerations

This study was approved by the Institutional Review Board (IRB) of the KDCA (IRB No. KDCA-2025-10-08-PE-01). Because the research utilized secondary data collected by local governments as part of routine public health response activities, informed consent from individual participants was not required.

Results

Epidemiologic Investigation

Between May 15 and May 30, 2025, cases were identified in 7 facilities located in 7 cities across 6 provinces. This represented 2.8% (7/250) of all facilities that received bakery products from the same manufacturer on May 15–16, 2025. Product I was served at all 7 facilities on May 15–16, whereas Product II was served only at Facility B on May 15. Accordingly, Facility B was the only facility at which both products were consumed.

Attack rate and demographic and clinical characteristics

A total of 323 individuals with gastrointestinal illness who met the outbreak case definition were identified. Among respondents who submitted stool specimens, 48 tested positive for Salmonella spp., yielding a confirmed-case attack rate (AR) of 3.9% (48/1,235) (Table 1). When 4 additional Salmonella-positive individuals identified among nonrespondents were included, the total number of positive individuals increased to 52.

Table 1.

Attack rate of cases and confirmed cases by facilities

Facility	No. of exposed	Case	AR of cases (%)	Confirmed casea)	AR of confirmed cases (%)
Total	1,235	323	26.2	48	3.9
A	502	173	34.5	18	3.6
B	178	32	18.0	7	3.9
C	52	18	34.6	4	7.7
D	77b)	31	40.3	13	16.9
E	208	45	21.6	3	1.4
F	216	22	10.2	2	0.9
G	2b)	2	100.0	1	50.0

AR, attack rate.

^a)Confirmed case defined as polymerase chain reaction or culture-confirmed Salmonella from a rectal swab or stool; 4 positives among nonrespondents were excluded.

^b)At Facilities D and G, the exposure denominators reflect data collected from groups with symptomatic cases only.

Age-specific ARs varied across age groups: 25.5% (97/380) among children aged <10 years, 30.6% (225/736) among adolescents aged 10–19 years, and 3.1% (1/32) among adults in their 40s, while no cases occurred in other age groups.

Among the 323 cases, 69.7% were aged 10–19 years, and 50.8% were male. Nearly all affected individuals (99.7%) were students. Illness onset began on May 15, 2025, at Facility E and on May 16, 2025, at the remaining facilities. Diarrhea was the most frequently reported symptom (96.9%), followed by abdominal pain and fever. The incubation period ranged from 6.0 to 347.6 hours, with a mean of 94.3 hours, a median of 77.8 hours, and an interquartile range of 78.5 hours (Table 2).

Table 2.

Demographic and clinical characteristics of cases in the outbreak of Salmonella Enteritidis

Characteristic	Case
Sex
Male	164 (50.8)
Female	159 (49.2)
Age (y)
≤9	97 (30.0)
10–19	225 (69.7)
≥20	1 (0.3)
Position
Students	322 (99.7)
School staff	0 (0)
Food handler	1 (0.3)
Symptomsa)
Diarrhea	313 (96.9)
Vomiting	65 (20.1)
Fever	178 (55.1)
Abdominal pain	258 (79.9)
Chills	137 (42.4)
Nausea	145 (44.9)
Headache	120 (37.2)
Othersb)	2 (0.6)
Incubation period	94.3 (6–347.6)

Data are presented as n (%) or mean (minimum–maximum).

^a)Data include multiple responses per respondent.

^b)Myalgia and body ache.

Epidemic curve

Products I and II were served on May 15–16, 2025, across the 7 facilities. Excluding 2 individuals with unknown symptom-onset times, facility-specific epidemic curves were constructed using data from 321 cases. The first symptomatic case occurred at facility E on May 15, 2025; in the remaining 6 facilities, symptom onset first occurred on May 16, 2025. Additional cases were reported from May 17 onward, with the final case occurring at Facility E on May 30, 2025 (Figure 2).

Figure 2.

Epidemic curve of the multi-city outbreak of Salmonella Enteritidis

Common exposure assessment

All 7 facilities received Product I, which was produced on the same day by Manufacturer H and distributed through Distributor I. No other common exposures were identified. Facility B also received Product II, which was produced by the same manufacturer (Table 3).

Table 3.

Types of bakery products and consumption date by facilities

Facility	Types of bakery products consumed	Manufacturer	Date of consumption
A	Product Ⅰ	Manufacturer H	2025-05-15
B	Product Ⅰ, Ⅱ	Manufacturer H	2025-05-15
C	Product Ⅰ	Manufacturer H	2025-05-16
D	Product Ⅰ	Manufacturer H	2025-05-15
E	Product Ⅰ	Manufacturer H	2025-05-15
F	Product Ⅰ	Manufacturer H	2025-05-15
G	Product Ⅰ	Manufacturer H	2025-05-15

All items in Product I and Product II were from the same lot number, respectively.

Active Case Finding

From January to May 2025, 282 reported outbreaks of waterborne and foodborne diseases were reviewed by local public health centers; none involved exposure to the 2 implicated bakery products. Among 246 additional facilities supplied with these products, 3 facilities (E, F, and G) reported gastrointestinal illness following consumption of products with the same use-by dates as those in the initial clusters. In addition, analysis of the PulseNet Korea database identified 8 other clusters sharing the same Salmonella genotype as those in Facilities A–D; exposure assessments in these clusters found no evidence of consumption of the implicated bakery products.

Laboratory Testing

Across the 7 facilities, 151 individuals were tested, of whom 52 (34.4%) were positive for Salmonella spp., including 41 cases of S. Enteritidis. Additionally, 2 individuals tested positive for Staphylococcus aureus (Table 4). Of the 52 Salmonella-positive individuals, 48 were classified as cases (including 1 food handler); detections identified at healthcare facilities are included.

Table 4.

Laboratory results from human specimens

Pathogen	No. of positive results	Remarks
Salmonella-positive	52	48 Were classified as cases; positives identified at healthcare facilities are included
Salmonella Enteritidis	41
Salmonella spp.	11
Staphylococcus aureus	2	Types B and C

Among the 7 facilities, retained bakery product samples from Facilities A and B tested positive for S. Enteritidis. Testing at the remaining facilities was not possible because the retention period had elapsed. All 214 environmental samples collected from Facilities A–E tested negative for Salmonella spp. (Table 5).

Table 5.

Environmental sampling results by facilities

Category	Facility	Specimen type	n	Result
Bakery-retained products	A	Product Ⅰ	1	Salmonella Enteritidis
	B	Product Ⅰ	1	S. Enteritidis
		Product Ⅱ	1	S. Enteritidis
Environmental sampling	A, B, C, D, Ea)	Others: retained food, cooking utensils, cooking water, drinking water	214	ND

ND, not detected.

^a)Environmental sampling was not performed at Facilities F and G because the field investigation occurred long after outbreak recognition.

The KDCA and the MFDS conducted PFGE and WGS analyses on S. Enteritidis isolates from human specimens (39 isolates across Facilities A–D) and retained food samples (3 isolates from Facilities A and B). PFGE patterns were identical across all isolates (SEGX01.049-A26.016). Core-genome multilocus sequence typing (cgMLST) placed all isolates within the same cluster (HC5-2301; Global IIa clade) (Figure 3). Whole-genome single-nucleotide polymorphism (wgSNP) analysis showed ≤7 SNP differences between clinical isolates (n=4) and retained food isolates (n=1), indicating very high genetic relatedness. All isolates also shared the same RE_2010 phage type, providing additional confirmation of the outbreak’s clonal origin.

Figure 3.

Pulsed-field gel electrophoresis (PFGE) profiles of outbreak-related Salmonella Enteritidis isolates associated with bakery product consumption

Traceback Investigation

Manufacturer H

Manufacturer H was a large-scale bakery company producing 88 items for corporate clients. Although the manufacturer maintained Hazard Analysis and Critical Control Points certification, last renewed in February 2025, a targeted audit conducted shortly after the outbreak identified specific nonconformities in process control and environmental maintenance.

Raw materials

Both implicated products contained unpasteurized liquid eggs (whole egg in Product I and egg white in Product II) supplied by the same processor. Although lot-specific raw materials from the implicated production date were unavailable for testing, except for salt, storage conditions were compliant with regulatory standards. Routine potable-water testing conducted from January to May 2025 confirmed the absence of bacteriological contamination.

Workforce

No symptomatic illness or recent sick leave was reported among the 107 staff members. All production personnel held valid health certificates, and routine hygiene compliance audits showed no significant violations on the implicated production dates.

Process-associated and environmental factors

While finished bakery products from other dates and environmental swabs were negative for Salmonella spp., the investigation identified a critical cross-contamination risk. Production areas were segregated by product type and internally organized into general, semi-clean, and clean zones with restricted inter-zone movement. In the semi-clean zone, dough shaping and oven baking were conducted in proximity, and shared transport racks were used across different hygiene zones. This layout suggested a plausible pathway for cross-contamination between raw egg–handling areas and heat-treated finished products.

Distributor I

On-site inspections of Distributor I conducted in June 2025 confirmed that cold-chain integrity was maintained during storage and transport. Transaction logs showed no irregularities, and the recall of Products I and II was successfully verified.

Liquid egg processor and source farms

Microbiological analysis of products from the liquid egg–processing facility met all regulatory standards. Among the 3 source farms (Farm J, K and L), Salmonella Thompson was isolated exclusively from eggshell surfaces at Farm J. Samples from Farm I and J, as well as from the internal contents of eggs from Farm J, yielded negative results.

Discussion

This outbreak resembled a previous nationwide salmonellosis outbreak in 2018 that was linked to chocolate cake. In that earlier event, investigations were conducted in 60 schools across 12 provinces, assessing 33,918 individuals; of these, 2,975 (8.8%) were classified as cases and 749 (2.2%) were laboratory confirmed [12]. In the present investigation, among the 1,235 exposed individuals, 323 (26.2%) met the case definition and 52 (4.2%) had laboratory-confirmed Salmonella infection. Accordingly, although fewer individuals were affected overall, the AR was approximately 2- to 3-efold higher.

The identification of this outbreak as a common-source event was fundamentally supported by laboratory-based genotyping. PFGE demonstrated indistinguishable patterns between clinical and retained food isolates, while WGS further placed all isolates within the same cluster. This laboratory evidence provided strong epidemiological support implicating the commonly consumed food item and facilitated the early identification of 3 additional epidemiologically linked clusters. Querying the PulseNet Korea database identified other Salmonella outbreaks with matching genotypes; however, the absence of epidemiological linkage to the present event allowed delimitation of the spatial and temporal scope of this outbreak. These findings reaffirm that genotyping is central to source attribution, assessment of geographic spread, supply-chain traceback, production stoppage, and product recalls. Internationally, PFGE and WGS, deployed through the PulseNet network, are established tools used at the national level to rapidly identify etiologic agents of foodborne outbreaks and to trigger early control measures, including interruption of distribution and initiation of recalls [14,15,21–24]. In the Republic of Korea, the application of PFGE and WGS is recommended as early as possible during investigations of waterborne and foodborne outbreaks, together with strengthened capacity for rapid analysis, interpretation, and interagency data sharing.

With the common source confirmed, we evaluated 3 hypotheses regarding the specific contamination route. For raw materials, only lots produced on different dates were available, and all tested negative. Detection of S. Thompson on eggshells from one supplier farm suggested possible upstream egg contamination; however, the serotype mismatch with the outbreak isolates (S. Enteritidis) limited causal inference. Personnel records identified no gastrointestinal illness or hygiene breaches on the production day. Regarding the production process, negative results from other products and environmental samples argued against persistent contamination. However, dough shaping involving unpasteurized liquid egg and oven baking occurred within the same semi-clean zone. In addition, shared transport racks and dual handling by some workers were consistent with transient cross-contamination. In summary, while a definitive route could not be determined due to discarded specimens, transient cross-contamination during production remained the most plausible explanation.

The difficulty in pinpointing the contamination source was likely linked to the unusually long incubation period observed in this outbreak. Although Salmonella infection generally manifests within 6 to 72 hours [1,3], substantially longer incubation periods have been documented in low-dose exposures [5,6,25,26]. In this outbreak, although the bacterial load could not be quantified, the observation that cases were identified in only 7 of 250 recipient facilities suggests that contamination was not uniform and likely occurred at low concentrations. This interpretation is consistent with previous outbreak investigations in which lightly contaminated food vehicles were associated with prolonged incubation periods [6,19,20,25], supporting the inference that low-dose exposure contributed to the extended incubation observed here.

These biological characteristics, namely low-dose contamination and extended incubation, pose significant challenges for early outbreak recognition. Compared with the 2018 chocolate-cake outbreak, which enabled early recognition due to its immediate reporting [11], the present outbreak experienced delays in the implementation of control measures because of limited early case identification. Similar patterns have been reported internationally. Outbreaks in Japan, Singapore, and the United States have highlighted how cross-contamination and complex supply chains can obscure early detection [8,15,18]. Multi-country outbreaks in Europe further indicate that fragmented surveillance systems can delay the identification of a common exposure [27–29].

To overcome these inherent challenges, strengthening coordinated and proactive collaboration between public health and food safety authorities is essential for improving responses to future multi-institutional foodborne outbreaks [8,14,15,18,27–29]. In this outbreak, rapid interagency communication was facilitated through structured information sharing between the KDCA and the MFDS. Specifically, the list of recipient facilities provided by the MFDS enabled the KDCA to expand active case finding across multiple jurisdictions in a timely manner. Upon notification, the MFDS promptly conducted an on-site sanitary inspection and recommended a voluntary temporary suspension of sales. As the potential for nationwide spread emerged, investigative leadership was escalated to the national level, enabling a coordinated response involving local governments and the Ministry of Education.

Building on this experience, future response measures should include strengthened real-time, preemptive sharing of outbreak signals and systematic integration of clinical and food genomic data. Such coordinated sharing would provide essential foundational data for rapid source attribution and enhance preparedness for complex, multi-facility foodborne outbreaks [8,14,15,18,27–29].

Finally, several limitations of this investigation must be acknowledged. First, the staggered recognition of clusters resulted in some cases resolving before clinical specimens could be collected, and some facilities were unable to secure retained food samples. Second, the retention period for food samples had elapsed, and lot-matched raw materials were no longer available, limiting environmental and ingredient sampling and constraining definitive elucidation of the contamination route. Third, recall bias may have affected reports of food exposure, particularly among young children, for whom histories relied on proxy reports from caregivers.

Conclusion

This multi-city outbreak, linked to the consumption of bakery products across multiple facilities nationwide, highlights the necessity of coordinated, multi-agency responses in managing large-scale foodborne outbreaks. Collaborative efforts among relevant government agencies enabled timely detection of the outbreak and prevention of further spread. Moreover, the application of molecular analyses, including PFGE and WGS, facilitated rapid source identification and detection of additional cases. Collectively, these findings underscore the pivotal role of both interagency collaboration and laboratory-based evidence in strengthening foodborne outbreak investigations and response systems.

HIGHLIGHTS

• This study investigated a multi-city outbreak of Salmonella Enteritidis associated with commercially manufactured bakery products in the Republic of Korea.

• An integrated epidemiological investigation combined with active case finding and molecular typing was conducted to identify the common food vehicle and transmission pathway.

• Genomic analyses using pulsed-field gel electrophoresis and whole-genome sequencing provided strong evidence linking clinical cases to contaminated bakery products.

• The findings demonstrate the value of coordinated public health and food safety responses supported by laboratory-based surveillance in managing complex foodborne outbreaks.