Isolation of Edwardsiella tarda from swine

Establishment and clinical application of a droplet digital PCR method for the detection of Edwardsiella tarda

Edwardsiella tarda (E. tarda) can infect humans and a variety of animals, including fish, amphibians, reptiles, birds, and mammals. However, a more highly sensitive, specific, and repeatable test for its detection is lacking. The objective of this study was to develop a highly sensitive, specific, and repeatable droplet digital polymerase chain reaction (ddPCR)-based method for the quantitative detection of E. tarda. The gyrB gene was selected as the target gene, and primers and probe were designed and synthesized. Using E. tarda genomic DNA as templates, the reaction method was optimized to establish a linear relationship with real-time PCR detection methods. The sensitivity, specificity, and repeatability of the method were analyzed, and clinical samples were tested. When the primer and probe concentrations were 900 and 300 nM, respectively, and the annealing temperature was 57°C, the efficiency of the ddPCR amplification reaction was highest and the boundary between positive and negative droplet distribution was clearest. The sensitivity was high, with detection limit being as low as 0.56 copies·μL-1; additionally, and a good linear relationship (R 2 = 0.9962) between ddPCR and real-time PCR detection, within the range of 1-25,000 copies·μL-1, was evident. The repeatability was good, with a detection coefficient of variation of 2.74%. There was no cross-reactivity with 15 other common pathogenic microorganisms in aquatic animals (Streptococcus agalactiae, Streptococcus iniae, Streptococcus suis type 2, Nocardia seriolae, Vibrio parahaemolyticus, Aeromonas sobria, red sea bream iridovirus, decapod iridescent virus 1, enterocytozoon hepatopenaei, carp edema virus, Koi herpesvirus, goldfish hematopoietic necrosis virus, tilapia lake virus, viral nervous necrosis virus, or grass carp reovirus) in positive samples. Among the 48 clinical samples, including Bahaba taipingensis and its live food fish, pond water samples, and routine monitoring samples (Koi), 21 were positive for E. tarda, consistent with the bacterial isolation and identification results. The E. tarda ddPCR detection method has high specificity, sensitivity, and repeatability, can more accurately quantify E. tarda, and provides a useful reference for research related to this bacterium.

Multiplex PCR assay for correct identification of the fish pathogenic species of Edwardsiella genus reveals the presence of E. anguillarum in South America in strains previously characterized as E. tarda

AIMS

Develop a species-specific multiplex PCR to correctly identify Edwardsiella species in routine diagnostic for fish bacterial diseases.

METHODS AND RESULTS

The genomes of 62 Edwardsiella spp. isolates available from the National Center for Biotechnology Information (NCBI) database were subjected to taxonomic and pan-genomic analyses to identify unique regions that could be exploited by species-specific PCR. The designed primers were tested against isolated Edwardsiella spp. strains, revealing errors in commercial biochemical tests for bacterial classification regarding Edwardsiella species.

CONCLUSION

Some of the genomes of Edwardsiella spp. in the NCBI platform were incorrectly classified, which can lead to errors in some research. A functional mPCR was developed to differentiate between phenotypically and genetically ambiguous Edwardsiella, with which, we detected the presence of Edwardsiella anguillarum affecting fish in Brazil.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that the misclassification of Edwardsiella spp in Brazil concealed the presence of E. anguillarum in South America. Also, this review of the taxonomic classification of the Edwardsiella genus is a contribution to the field to help researchers with their sequencing and identification of genomes, showing some misclassifications in online databases that must be corrected, as well as developing an easy assay to characterize Edwardsiella species in an end-point mPCR.

Retention of virulence in a viable but nonculturable Edwardsiella tarda isolate

Edwardsiella tarda is pathogen of fish and other animals. The aim of this study was to investigate the viable but nonculturable (VBNC) state and virulence retention of this bacterium. Edwardsiella tarda CW7 was cultured in sterilized aged seawater at 4 degrees C. Total cell counts remained constant throughout the 28-day period by acridine orange direct counting, while plate counts declined to undetectable levels (<0.1 CFU/ml) within 28 days by plate counting. The direct viable counts, on the other hand, declined to ca. 10(9) CFU/ml active cells and remained fairly constant at this level by direct viable counting. These results indicated that a large population of cells existed in a viable but nonculturable state. VBNC E. tarda CW7 could resuscitate in experimental chick embryos and in the presence of nutrition with a temperature upshift. The resuscitative times were 6 days and 8 days, respectively. The morphological changes of VBNC, normal, and resuscitative E. tarda CW7 cells were studied with a scanning electron microscope. The results showed that when the cells entered into the VBNC state, they gradually changed in shape from short rods to coccoid and decreased in size, but the resuscitative cells did not show any obvious differences from the normal cells. The VBNC and the resuscitative E. tarda CW7 cells were intraperitoneally inoculated into turbot separately, and the fish inoculated with the resuscitative cells died within 7 days, which suggested that VBNC E. tarda CW7 might retain pathogenicity.

Penetration and replication of Edwardsiella spp. in HEp-2 cells

The ability of 22 Edwardsiella strains to penetrate and replicate in cultured epithelial cells was initially evaluated by light microscopy methods and by the recovery of gentamicin-resistant (Gmr) bacteria from the Triton X-100 cell lysates of HEp-2-infected monolayers. Giemsa-stained HEp-2 cells revealed the presence of numerous internalized bacteria 3 h postinfection, often appearing as parallel rows of replicated bacteria within the cytosol and sometimes obliterating the cytoplasm because of the large numbers of bacilli present. Invasive bacteria were also sometimes found within cytoplasmic vacuoles in infected cells; thin-section electron micrographs of HEp-2-infected cells supported these conclusions. Results of light microscopy studies and cell lysate assays indicated that most Edwardsiella tarda (92%) and some Edwardsiella hoshinae strains were invasion positive on one or more occasions, while Edwardsiella ictaluri isolates were uniformly negative. HEp-2 invasion by E. tarda was a microfilament-dependent (cytochalasin B- and D-sensitive) process, with maximum numbers of Gmr CFU recorded between 3 and 6 h postinfection. The small percentage (0.01 to 1.0%) of the challenge inoculum recoverable as Gmr progeny 3 to 6 h postinfection was attributed to a strong cell-associated (not filterable) hemolysin that was produced by a majority (85%) of the E. tarda strains but not by E. ictaluri and only minimally by E. hoshinae. This cytolysin/hemolysin was responsible for the toxic effects observed in HEp-2 cells during the infection-replication process of edwardsiellae and appears to play a role in the release of internalized and replicated bacteria from infected cells. The results suggest an invasion strategy with some similarities to and differences from those of other recognized enteroinvasive pathogens.

Edwardsiella tarda isolated in Israel between 1961 and 1980

Edwardsiella tarda was isolated from patients, water tortoises (Clemmys caspica), and samples of water from Lake Kinnereth, the river Jordan, well water, and sewage water. Of the 53 isolates, 35 belonged to completely identified serotypes, among them 7 new ones. Fourteen cultures had O antigens, and one had an H antigen, different from those previously described. Three serotypes isolated from patients were also found in other sources: water tortoises, lake water, or both.

Extraintestinal human infection caused by Edwardsiella tarda

Edwardsiella tarda is an uncommon enteric bacterium which has been found generally in animal hosts and occasionally in human feces. Three cases of extraintestinal infection caused by E. tarda which are described herein include a typhoid-like illness, peritonitis with sepsis, and cellulitis from a wound acquired while fishing. The microbiology of E. tarda and the previous reports of infection due to this organism are reviewed.

Edwardsiella tarda in freshwater catfish and their environment

Edwardsiella tarda was isolated from 47, 88, and 79% of skin, visceral, and dressed-fish samples, respectively. This species was also isolated from 30% of imported dressed fish, 75% of catfish pond water samples, 64% of catfish pond mud samples, and 100% of frogs, turtles, and crayfish from catfish ponds. The incidence of Edwardsiella increased during the summer months, as water temperatures increased. Of several isolation media evaluated, the most effective was selective enrichment in double-strength Salmonella-Shigella broth and subsequent plating on single-strength Samonella-Shigella agar. The significance of the incidence of Edwardsiella in catfish, catfish disease, and public health could not be substantiated.