Detection and isolation of Shiga Toxin-producing Escherichia coli (STEC) strains in caecal samples from pigs at slaughter in Italy

A Comprehensive Review on Shiga Toxin Subtypes and Their Niche-Related Distribution Characteristics in Shiga-Toxin-Producing E. coli and Other Bacterial Hosts

Shiga toxin (Stx), the main virulence factor of Shiga-toxin-producing E. coli (STEC), was first discovered in Shigella dysenteriae strains. While several other bacterial species have since been reported to produce Stx, STEC poses the most significant risk to human health due to its widespread prevalence across various animal hosts that have close contact with human populations. Based on its biochemical and molecular characteristics, Shiga toxin can be grouped into two types, Stx1 and Stx2, among which a variety of variants and subtypes have been identified in various bacteria and host species. Interestingly, the different Stx subtypes appear to vary in their host distribution characteristics and in the severity of diseases that they are associated with. As such, this review provides a comprehensive overview on the bacterial species that have been recorded to possess stx genes to date, with a specific focus on the various Stx subtype variants discovered in STEC, their prevalence in certain host species, and their disease-related characteristics. This review provides a better understanding of the Stx subtypes and highlights the need for rapid and accurate approaches to toxin subtyping for the proper evaluation of the health risks associated with Shiga-toxin-related bacterial food contamination and human infections.

Prevalence and Virulent Gene Profiles of Sorbitol Non-Fermenting Shiga Toxin-Producing Escherichia coli Isolated from Goats in Southern Thailand

Shiga toxin-producing Escherichia coli (STEC) is the pathogenic E. coli causing disease in humans via the consumption or handling of animal food products. The high prevalence of these organisms in ruminants has been widely reported. Among STECs, O157 is one of the most lethal serotypes causing serious disease in humans. The present study investigated the prevalence of sorbitol non-fermenting STECs in goats reared in the lower region of southern Thailand and described the virulent factors carried by those isolates. Sorbitol non-fermenting (SNF)-STECs were found in 57 out of 646 goats (8.82%; 95% CI 6.75% to 11.28%). Molecular identification revealed that 0.77% of SNF-STEC isolates were the O157 serotype. Shiga toxin genes (stx1 and stx2) and other virulent genes (i.e., eaeA, ehxA, and saa) were detected by molecular techniques. The presence of stx1 (75.44%) was significantly higher than that of stx2 (22.81%), whereas 1.75% of the total isolates carried both stx1 and stx2. Most of the isolates carried ehxA for 75.44%, followed by saa (42.11%) and eaeA (12.28%). In addition, 21.05% of STEC isolates did not carry any eaeA, ehxA, or saa. The first investigation on SNF-STECs in goat was conducted in the lower region of southern Thailand. The present study revealed that goats could be one of the potential carriers of SNF-STECs in the observing area.

A review of Shiga-toxin producing Escherichia coli (STEC) contamination in the raw pork production chain

Epidemiological evidence of Shiga toxin-producing Escherichia coli (STEC) infections associated with the consumption of contaminated pork highlight the need for increased awareness of STEC as an emerging pathogen in the pork supply chain. The objective of this review is to contribute to our understanding of raw pork products as potential carriers of STEC into the food supply. We summarize and critically analyze primary literature reporting the prevalence of STEC in the raw pork production chain. The reported prevalence rate of stx-positive E. coli isolates in live swine, slaughtered swine, and retail pork samples around the world ranged from 4.4 % (22/500) to 68.3 % (82/120), 22 % (309/1395) to 86.3 % (69/80), and 0.10 % (1/1167) to 80 % (32/40), respectively, depending upon the sample categories, detection methods, and the hygiene condition of the slaughterhouses and retail markets. In retail pork, serogroup O26 was prevalent in the U.S., Europe, and Africa. Serogroup O121 was only reported in the U.S. Furthermore, serogroup O91 was reported in the U.S., Asia, and South American retail pork samples. The most common virulence gene combination in retail pork around the globe were as follows: the U.S.: serogroup O157 + stx, non-O157 + stx, unknown serogroups+stx + eae; Europe: unknown serogroups+(stx + eae, stx₂ + eae, or stx₁ + stx₂ + eae); Asia: O157 + stx₁ + stx₂ + ehxA, Unknown+stx₁ + eaeA + ehxA, or only eae; Africa: O157 + stx₂ + eae + ehxA. STEC strains derived from retail pork in the U.S. fall under low to moderate risk categories capable of causing human disease, thus indicating the need for adequate cooking and prevention of cross contamination to minimize infection risk in humans.

Genomic Characteristics of Stx2e-Producing Escherichia coli Strains Derived from Humans, Animals, and Meats

Shiga toxin (Stx) can be classified into two types, Stx1 and Stx2, and different subtypes. Stx2e is a subtype commonly causing porcine edema disease and rarely reported in humans. The purpose of this study was to analyze the prevalence and genetic characteristics of Stx2e-producing Escherichia coli (Stx2e-STEC) strains from humans compared to strains from animals and meats in China. Stx2e-STEC strains were screened from our STEC collection, and whole-genome sequencing was performed to characterize their genetic features. Our study showed a wide distribution of Stx2e-STEC among diverse hosts and a higher proportion of Stx2e-STEC among human STEC strains in China. Three human Stx2e-STEC isolates belonged to O100:H30, Onovel26:H30, and O8:H9 serotypes and varied in genetic features. Human Stx2e-STECs phylogenetically clustered with animal- and food-derived strains. Stx2e-STEC strains from animals and meat showed multidrug resistance, while human strains were only resistant to azithromycin and tetracycline. Of note, a high proportion (55.9%) of Stx2e-STEC strains, including one human strain, carried the heat-stable and heat-labile enterotoxin-encoding genes st and lt, exhibiting a STEC/enterotoxigenic E. coli (ETEC) hybrid pathotype. Given that no distinct genetic feature was found in Stx2e-STEC strains from different sources, animal- and food-derived strains may pose the risk of causing human disease.

Diversity of Non-O157 Shiga Toxin-Producing Escherichia coli Isolated from Cattle from Central and Southern Chile

Simple Summary

Cattle are the main reservoir of Shiga toxin-producing E. coli (STEC), foodborne pathogens that cause severe disease and outbreaks. However, not all STEC cause human illnesses or have the same virulence potential. Characterizing strains isolated worldwide allows insights into how strains spread and which isolates have a more significant risk potential. This study described STEC isolation rates from cattle in Chile and characterized 30 isolates. We obtained 93 STEC isolates from 56/446 (12.6%) fecal cattle samples. Then, 30 non-O157 STEC isolates were selected for complete characterization; we found isolates of 16 different sequence types and 17 serotypes. One isolate was resistant to tetracycline and carried resistance genes against the drug. Surveyed virulence genes (n = 31) were present from 13% to 100% of isolates, and one isolate carried 26/31 virulence genes. Most isolates (90%; 27/30) carried the stx2 gene, which is frequently linked to strains causing severe disease. A phylogenetic reconstruction demonstrated that isolates clustered based on serotypes, independent of their geographical origin (Central or Southern Chile). These results indicate that cattle in Chile carry a wide diversity of STEC potentially pathogenic for humans based on the presence of virulence genes.

Abstract

Cattle are the main reservoir of Shiga toxin-producing Escherichia coli (STEC), one of the world’s most important foodborne pathogens. The pathogen causes severe human diseases and outbreaks. This study aimed to identify and characterize non-O157 STEC isolated from cattle feces from central and southern Chile. We analyzed 446 cattle fecal samples and isolated non-O157 STEC from 12.6% (56/446); a total of 93 different isolates were recovered. Most isolates displayed β-glucuronidase activity (96.8%; 90/93) and fermented sorbitol (86.0%; 80/93), whereas only 39.8% (37/93) were resistant to tellurite. A subgroup of 30 representative non-O157 STEC isolates was selected for whole-genome sequencing and bioinformatics analysis. In silico analysis showed that they grouped into 16 different sequence types and 17 serotypes; the serotypes most frequently identified were O116:H21 and O168:H8 (13% each). A single isolate of serotype O26:H11 was recovered. One isolate was resistant to tetracycline and carried resistance genes tet(A) and tet(R); no other isolate displayed antimicrobial resistance or carried antimicrobial resistance genes. The intimin gene (eae) was identified in 13.3% (4/30) of the genomes and 90% (27/30) carried the stx2 gene. A phylogenetic reconstruction demonstrated that the isolates clustered based on serotypes, independent of geographical origin. These results indicate that cattle in Chile carry a wide diversity of STEC potentially pathogenic for humans based on the presence of critical virulence genes.

Pathogenicity of Shiga Toxin Type 2e Escherichia coli in Pig Colibacillosis

Shiga toxin type 2e (Stx2e) Escherichia coli is the causative factor of diarrhea and edema in swine. The aims of this study were to determine the prevalence of Stx2e-producing E. coli isolates and to characterize isolates from clinical cases of pig colibacillosis and healthy swine. During the 11 years of the study (2006–2017), a total of 233 Stx2e-producing isolates were detected−230 out of 2,060 (11.16%) E. coli isolated from diseased pigs and 3 out of 171 (1.75%) from healthy swine. Stx2e-producing isolates were indeed more present in clinical colibacillosis cases than in healthy pigs (p = 0.0002). The predominant serogroup was O139 (79.82%) and the most common fimbrial factor present in these isolates was F18 (177 isolates), followed by F6 (5 isolates). The enterotoxins LTI, STa, and STb were detected in 10.43, 41.73, and 48.26% of the isolates, respectively. The predominant virotypes F18-Stx2e and -STa-STb-Stx2e were similarly present in weaners (33.33 and 35.52%) and finishers (38.30 and 25.53%). Among isolates from diseased pigs, O139 and F18 were the more frequently identified serogroup and virulence factor, respectively. Of the tested 230 Stx2e-producing isolates isolated from diseased pigs, 29 (12.60%) harbored genes encoding ESBL, particularly TEM (79.30%), CTX-M1 (17.20%), and CMY-2 (3.40%). Antimicrobial resistance to tetracycline was the most common characteristic (98.25%), followed by ampicillin (93.91%), cephalotin (90.43%) and trimethoprim/sulfamethoxazole (82.17%). Our results showed that Stx2e-producing E. coli were more frequently associated with clinical forms of colibacillosis, with minimal probability to isolate these isolates from healthy pigs.

Identification and molecular characteristics of verotoxin-producing Escherichia coli (VTEC) from bovine and pig carcasses isolated in Poland during 2014-2018

The presence of verotoxin-producing Escherichia coli (VTEC) on bovine (n = 330) and pig (n = 120) carcasses in Poland was investigated using the ISO/TS 13136 standard. A total of 115 (34.8%) and 37 (30.8%) cattle and pig samples were positive in real-time PCR, respectively. Isolation of the bacteria revealed that from bovine carcasses 37 (32.2%) VTEC were obtained whereas only 5 (13.5%) pig carcasses were positive for the stx gene. The VTEC were characterized using whole genome sequencing (WGS) and bovine isolates were classified into 25 serotypes with the most prevalent O113:H21 (5 strains) whereas pig strains belonged to 5 different serotypes which were not identified among cattle strains. The majority of bovine VTEC (35; 94.6% isolates) were positive for the stx2 gene, either alone or together with the stx1 gene. All strains isolated from pig carcasses resulted positive for the stx2 gene only. Only two isolates of bovine origin contained the eaeA intimin gene, together with the ehxA and lpfA markers. VTEC were highly molecularly diverse as shown by classification into 29 different MLST STs. The obtained results suggest that further studies related to cattle and pig carcasses are needed to assess the role of these sources for human VTEC infections.

Structural and Functional Characterization of Stx2k, a New Subtype of Shiga Toxin 2

Shiga toxin (Stx) is the major virulence factor of Shiga toxin-producing Escherichia coli (STEC). Stx evolves rapidly and, as such, new subtypes continue to emerge that challenge the efficacy of existing disease management and surveillance strategies. A new subtype, Stx2k, was recently identified in E. coli isolated from a wide range of sources including diarrheal patients, animals, and raw meats, and was poorly detected by existing immunoassays. In this study, the structure of Stx2kE167Q was determined at 2.29 Å resolution and the conservation of structure with Stx2a was revealed. A novel polyclonal antibody capable of neutralizing Stx2k and an immunoassay, with a 10-fold increase in sensitivity compared to assays using extant antibodies, were developed. Stx2k is less toxic than Stx2a in Vero cell assays but is similar to Stx2a in receptor-binding preference, thermostability, and acid tolerance. Although Stx2k does not appear to be as potent as Stx2a to Vero cells, the wide distribution and blended virulence profiles of the Stx2k-producing strains suggest that horizontal gene transfer through Stx2k-converting phages could result in the emergence of new and highly virulent pathogens. This study provides useful information and tools for early detection and control of Stx2k-producing E. coli, which could reduce public risk of infection by less-known STECs.