Possible horizontal transmission of Salmonella via reusable egg trays in Thailand

Salmonella enterica serovar Braenderup shows clade-specific source associations and a high proportion of molecular epidemiological clustering

Salmonella enterica serovar Braenderup (S. enterica ser. Braenderup) is an important clinical serovar in the United States. This serovar was reported by the CDC in 2017 as the fifth most common Salmonella enterica serovar associated with outbreaks in the United States, which have been linked to both fresh produce and food animal products. The goals of this study were to compare the relatedness of human clinical isolates from southeastern USA (Tennessee (n = 106), Kentucky (n = 48), Virginia (n = 252), South Carolina (n = 109), Georgia (n = 159), Alabama (n = 8), Arkansas (n = 26), and Louisiana (n = 91)) and global clinical (n = 5,153) and nonclinical (n = 1,053) isolates obtained from the NCBI. Additionally, we also examined the population structure of S. enterica ser. Braenderup strains (n = 3,131) on EnteroBase and found that all the strains of this serovar are associated with a single cgMLST eBurst group (ceBG 185), confirming that this serovar is monophyletic. We divided the S. enterica ser. Braenderup population into two clades (Clade I and Clade II) and one clade group (Clade Group III). The composition of distinct environmental isolates in the clades differed: Clade I was significantly associated with produce (90.7%; P < 0.0001) and water, soil, and sediment (76.9%; P < 0.0001), and Clade II was significantly associated with poultry environments (62.8%; P < 0.0001). The clade-specific gene associations (e.g., Clade I-associated competence proteins and cytochrome_c_asm protein and Clade II-associated heme-exporter protein and dimethyl sulfoxide [DMSO] reductase-encoding genes) provide potential insights into possible mechanisms driving environmental adaptation and host-pathogen interaction. Phylogenetic analyses identified 218 molecular epidemiological clusters in the current study, which represented a greater proportion of potentially outbreak-related isolates than previously estimated.

IMPORTANCE

This study provides insights into the genomic diversity of S. enterica ser. Braenderup by revealing distinct clade-specific source attribution patterns and showing that a greater proportion of isolates were associated with epidemiological clusters based on the genomic relatedness than previously estimated. Specifically, we analyzed the diversity of human clinical isolates from southeastern USA and compared them with the global clinical and nonclinical isolates. Our analysis showed different clades of S. enterica ser. Braenderup linked to different environments, providing insights on the potential source of human sporadic infection and outbreaks. These findings can enhance public health surveillance and response strategies targeting S. enterica serovar Braenderup by expanding our understanding of potential transmission pathways and the genomic diversity of clinical and environmental isolates.

Occurrence and characterization of Salmonella isolates from commercial eggs in Phayao Province, Thailand

Background and Aim

Salmonella contamination in eggs poses a significant public health risk, particularly in alternative egg production systems where contamination and antimicrobial resistance remain underexplored. This study aimed to determine the occurrence of Salmonella contamination in three different egg production systems in Phayao, Thailand, and analyze serovar diversity, antimicrobial resistance, virulence genes, and genetic profiles.

Materials and Methods

A total of 750 eggs were sampled from cage, free-range, and organic egg production systems, purchased from supermarkets in Phayao Province. Eggshells and contents were separately analyzed using conventional microbiological methods to isolate Salmonella. Phenotypic identification, serotyping, and antimicrobial susceptibility testing were performed. Genotypic characterization, including virulence and antimicrobial resistance gene detection, was conducted using polymerase chain reaction. Multilocus sequence typing (MLST) was employed to determine genetic diversity.

Results

Salmonella contamination was detected in three eggshell samples (0.4%), with one positive sample from each production system. The identified serovars were Salmonella Mbandaka (cage eggs), Salmonella Corvallis (free-range eggs), and Salmonella Cerro (organic eggs). Antimicrobial resistance was observed in only one isolate, S. Mbandaka, which exhibited resistance to sulfamethoxazole/trimethoprim and carried the sul1 and sul2 genes. All Salmonella isolates harbored virulence genes (invA, sopB, and stn). MLST analysis identified three distinct sequence types (ST413, ST1541, and ST1593) corresponding to the detected serovars.

Conclusion

This study demonstrates a low occurrence of Salmonella contamination in eggshells across different production systems, with no contamination detected in egg contents. The presence of distinct serovars and genetic types suggests varying contamination sources. Although antimicrobial resistance was minimal, the presence of virulence genes in all isolates highlights the potential risk of infection. Continuous monitoring and improved biosecurity measures in egg production and distribution are recommended to enhance food safety and public health.

First study on virulence genes, antimicrobial resistance, and integrons in Escherichia coli isolated from cage, free-range, and organic commercial eggs in Phayao Province, Thailand

Background and Aim:

Antimicrobial resistance (AMR) is a global problem that affects human and animal health, and eggs can act as a vehicle for pathogenic and non-pathogenic resistant bacteria in the food chain. Escherichia coli is an indicator of food contamination with fecal materials as well as the occurrence and levels of AMR. This study aimed to investigate the presence of AMR, integrons, and virulence genes in E. coli isolated from eggshell samples of three egg production systems, from supermarkets in Thailand.

Materials and Methods:

A total of 750 hen’s egg samples were purchased from supermarkets in Phayao Province: Cage eggs (250), free-range eggs (250), and organic eggs (250). Each sample was soaked in buffered peptone water (BPW), and the BPW samples were incubated at 37°C for 18–24 h. All samples were tested for E. coli by the standard conventional culture method. Then, all identified E. coli were tested for antimicrobial susceptibility to 15 antimicrobial agents by the agar disk diffusion method. All E. coli strains were subsequently found to have virulence genes and Classes 1 and 2 integrons by polymerase chain reaction.

Results:

Among the eggshell samples, 91 samples were identified as having E. coli (cage eggs, 24 strains; free-range eggs, 27 strains; and organic eggs, 40 strains). Then, among the E. coli strains, 47 (51.6%) were positive for at least one virulence gene. The proportion of AMR in the eggshell samples was 91.2% (83/91), and streptomycin (STR), ampicillin (AMP), and tetracycline (TET) had a high degree of resistance. Among the E. coli strains, 27 (29.7%) strains were positive for class 1 or 2 integrons, and integron-positive strains were commonly found in STR-, AMP-, and TET-resistant strains. Multidrug resistance (MDR) was detected in 57.1% (52/91) of the E. coli strains, with STR-AMP-TET (5.5%) as the most frequent pattern. The proportion of MDR in cage eggs was 75.0% (18/24), which was higher than in both free-range and organic eggs. On the other hand, 53.2% (25/47) of E. coli carrying virulence genes had MDR, distributed across the production systems as follows: Cage eggs, 76.9% (10/13); free-range eggs, 63.6% (7/11); and organic eggs, 34.8% (8/23).

Conclusion:

Escherichia coli was detected in eggshell samples from all three egg production systems. The high level of virulence genes, AMR, and integrons indicated the possibility of dissemination of AMR among pathogenic and commensal E. coli through eggshells. These findings could be a major concern to farmers, food handlers, and consumers, especially regarding raw egg consumption.

Prevalence and Genetic Analysis of Salmonella enterica from a Cross-Sectional Survey of the New Zealand Egg Production Environment

Epidemiological evidence suggests that Salmonella on New Zealand eggs is not an important pathway for human salmonellosis. However, robust nationally representative data for Salmonella contamination of eggs is not available to support this. To better understand the exposure of New Zealand commercial eggs to Salmonella, a cross-sectional survey collected data on prevalence and serotypes of Salmonella in the feed, laying sheds (feces, dust, and boot or manure belt swabs), and packhouses (egg contact surfaces) of New Zealand commercial egg layer farms. Salmonella was not detected on 16 of 28 surveyed farms, and 4 farms had only one positive sample. Of the 43 (13.3%) of 323 Salmonella-positive samples, dust samples had the highest prevalence (19 of 67, 28.4%), followed by boot or manure belt swabs (11 of 67, 16.4%), feces (7 of 67, 10.4%), packhouse egg contact surfaces (5 of 87, 5.7%), and feed (1 of 33, 3.0%). A significantly higher prevalence was from caged (33 of 75, 44.0%; P < 0.001) compared with cage-free (4 of 126, 3.2%) systems, yet multiple practices differ between laying systems, which could influence prevalence. Salmonella-positive packhouse samples were only identified on the three farms with the highest laying shed prevalence, and isolates were genetically related (as determined by single nucleotide polymorphism analyses) suggesting cross-contamination between the laying shed and packhouse surfaces. Serotypes isolated included Salmonella Infantis, Salmonella Thompson, Salmonella Typhimurium, Salmonella Anatum, and Salmonella Mbandaka. Importantly, Salmonella Enteritidis, which causes egg-associated outbreaks internationally, was not isolated. Genomic comparisons of isolates supported the presence of a common contamination source in the shed and farm environments rather than multiple sporadic contamination events. This survey establishes a benchmark of Salmonella prevalence and types in the New Zealand egg production environment and provides a reference point for assessing the impact of changes to practices on Salmonella prevalence.

Risk Characterization of Antibiotic Resistance in Bacteria Isolated from Backyard, Organic, and Regular Commercial Eggs

This study was conducted to assess the risk due to antimicrobial-resistant strains of Salmonella spp., Listeria monocytogenes, and Escherichia coli isolated from the eggshell and the contents of eggs bought in markets in Valencia (Spain). Thirty-four samples from three different production styles were analyzed: standard ( n = 34), organic ( n = 16), and backyard ( n = 10) eggs. L. monocytogenes was not isolated in any style of production. Only one strain of Salmonella was isolated from standard production, which was resistant to ciprofloxacin and amoxicillin. E. coli strains were resistant in 22% of the isolates from organic production, 12.25% from standard production, and 11.23% from backyard production. In all cases, the highest resistance was observed for amoxicillin-clavulanate. None of the isolates from standard and backyard eggs were resistant to chloramphenicol, ciprofloxacin, gentamycin, and streptomycin, while only ceftriaxone was found to be effective against all E. coli isolates from organic eggs. β-Lactamase genes bla_TEM , bla_SHV, and bla_CMY-2 and the resistance genes for tetracycline tetA, tetB, and tetC were tested. The most commonly detected antimicrobial resistance genes among the E. coli isolates were tetA (49.30%), bla_TEM (47.89%), and tetB (36.62%). Overall, a maximum public health risk is associated with β-lactam antibiotics.

Prevalence and molecular characterization of Salmonella enterica isolates throughout an integrated broiler supply chain in China

A total of 1145 samples were collected from chicken breeder farms, hatcheries, broiler farms, a slaughterhouse and retail refrigerated chicken stores in an integrated broiler supply chain in Guangdong Province, China, in 2013. One-hundred and two Salmonella enterica strains were isolated and subjected to serotyping, antimicrobial susceptibility testing, virulence profile determination and molecular subtyping by pulsed field gel electrophoresis (PFGE). The contamination rates in samples from breeder farms, hatcheries, broiler farms, the slaughterhouse and retail stores were 1·46%, 4·31%, 7·00%, 62·86% and 54·67%, respectively. The isolated strains of S. enterica belonged to 10 serotypes; most of them were S. Weltevreden (46·08%, 47/102) and S. Agona (18·63%, 19/102). Isolates were frequently resistant to streptomycin (38·2%), tetracycline (36·3%), sulfisoxazole (35·3%) and gentamicin (34·3%); 31·4% of isolates were multidrug resistant. The isolates were screened for 10 virulence factors. The Salmonella pathogenicity island genes avrA, ssaQ, mgtC, siiD, and sopB and the fimbrial gene bcfC were present in 100% of the strains. PFGE genotyping of the 102 S. enterica isolates yielded 24 PFGE types at an 85% similarity threshold. The PFGE patterns show that the genotypes of S. enterica in the production chain are very diverse, but some strains have 100% similarity in different parts of the production chain, which indicates that some S. enterica persist throughout the broiler supply chain.

Application of Molecular Approaches for Understanding Foodborne Salmonella Establishment in Poultry Production

Salmonellosis in the United States is one of the most costly foodborne diseases. Given that Salmonella can originate from a wide variety of environments, reduction of this organism at all stages of poultry production is critical. Salmonella species can encounter various environmental stress conditions which can dramatically influence their survival and colonization. Current knowledge of Salmonella species metabolism and physiology in relation to colonization is traditionally based on studies conducted primarily with tissue culture and animal infection models. Consequently, while there is some information about environmental signals that control Salmonella growth and colonization, much still remains unknown. Genetic tools for comprehensive functional genomic analysis of Salmonella offer new opportunities for not only achieving a better understanding of Salmonella pathogens but also designing more effective intervention strategies. Now the function(s) of each single gene in the Salmonella genome can be directly assessed and previously unknown genetic factors that are required for Salmonella growth and survival in the poultry production cycle can be elucidated. In particular, delineating the host-pathogen relationships involving Salmonella is becoming very helpful for identifying optimal targeted gene mutagenesis strategies to generate improved vaccine strains. This represents an opportunity for development of novel vaccine approaches for limiting Salmonella establishment in early phases of poultry production. In this review, an overview of Salmonella issues in poultry, a general description of functional genomic technologies, and their specific application to poultry vaccine developments are discussed.