Co-infections of Klebsiella pneumoniae and Elizabethkingia miricola in black-spotted frogs (Pelophylax nigromaculatus)

A Duplex PCR Assay for Rapid Detection of Klebsiella pneumoniae and Chryseobacterium in Large Yellow Croaker Fish

Both Klebsiella pneumoniae and Chryseobacterium cause an increasing number of diseases in fish, resulting in great economic losses in aquaculture. In addition, the disease infected with Klebsiella pneumoniae or Chryseobacterium exhibited the similar clinical symptoms in aquatic animals. However, there is no effective means for the simultaneous detection of co-infection and discrimination them for these two pathogens. Here, we developed a duplex polymerase chain reaction (PCR) method based on the outer membrane protein A (ompA) gene of Klebsiella pneumoniae and Chryseobacterium. The specificity and validity of the designed primers were confirmed experimentally using simplex PCR. The expected amplicons for Klebsiella pneumoniae and Chryseobacterium had a size of 663 and 1404 bp, respectively. The optimal condition for duplex PCR were determined to encompass a primer concentration of 0.5 μM and annealing temperature of 57°C. This method was analytical specific with no amplification being observed from the genomic DNA of Escherichia coli, Vibrio harveyi, Pseudomonas plecoglossicida, Aeromonas hydrophila and Acinetobacter johnsonii. The limit of detection was estimated to be 20 fg of genomic DNA for Chryseobacterium and 200 fg for Klebsiella pneumoniae, or 100 colony-forming units (CFU) of bacterial cells in both cases. The duplex PCR was capable of simultaneously amplifying target fragments from genomic DNA extracted from the bacteria and fish liver. For practical validation of the method, 20 diseased fish were collected from farms, among which 4 samples were PCR-positive for Klebsiella pneumoniae and Chryseobacterium. The duplex PCR method developed here is time-saving, specific, convenient, and may prove to be an invaluable tool for molecular detection and epidemiological investigation of Klebsiella pneumoniae and Chryseobacterium in the field of aquaculture.

Horizontal plasmid transfer promotes antibiotic resistance in selected bacteria in Chinese frog farms

The emergence and dissemination of antibiotic resistance genes (ARGs) in the ecosystem are global public health concerns. One Health emphasizes the interconnectivity between different habitats and seeks to optimize animal, human, and environmental health. However, information on the dissemination of antibiotic resistance genes (ARGs) within complex microbiomes in natural habitats is scarce. We investigated the prevalence of antibiotic resistant bacteria (ARB) and the spread of ARGs in intensive bullfrog (Rana catesbeiana) farms in the Shantou area of China. Antibiotic susceptibilities of 361 strains, combined with microbiome analyses, revealed Escherichia coli, Edwardsiella tarda, Citrobacter and Klebsiella sp. as prevalent multidrug resistant bacteria on these farms. Whole genome sequencing of 95 ARB identified 250 large plasmids that harbored a wide range of ARGs. Plasmid sequences and sediment metagenomes revealed an abundance of tetA, sul1, and aph(3″)-Ib ARGs. Notably, antibiotic resistance (against 15 antibiotics) highly correlated with plasmid-borne rather than chromosome-borne ARGs. Based on sequence similarities, most plasmids (62%) fell into 32 distinct groups, indicating a potential for horizontal plasmid transfer (HPT) within the frog farm microbiome. HPT was confirmed in inter- and intra-species conjugation experiments. Furthermore, identical mobile ARGs, flanked by mobile genetic elements (MGEs), were found in different locations on the same plasmid, or on different plasmids residing in the same or different hosts. Our results suggest a synergy between MGEs and HPT to facilitate ARGs dissemination in frog farms. Mining public databases retrieved similar plasmids from different bacterial species found in other environmental niches globally. Our findings underscore the importance of HPT in mediating the spread of ARGs in frog farms and other microbiomes of the ecosystem.

Outbreaks of Elizabethkingia miricola Caused Fatal Meningitis-Like Disease in Cultured Bullfrogs

Elizabethkingia miricola is an emerging nosocomial pathogen responsible for meningitis, sepsis, urinary tract infection, pneumonia, and joint infection in humans. These pathogens were also reported to be causal agents for meningitis-like disease in cultured frogs, which displayed high infectivity, mortality, and significant loss. In July 2023, 10 outbreaks of infectious meningitis-like disease in bullfrogs occurred in Tangshan area. To determine the causal agent, 70 diseased frogs from 10 farms were collected for etiological identification. Gram-negative bacilli were isolated from the brain and liver of sick bullfrogs and identified as members of E. miricola by biochemical characterization and 16S rRNA sequencing analysis. A total of 42 strains of E. miricola were isolated and further determined as the etiological agent by reproducing neurological symptoms and deaths in an artificial infection test. A representative isolate, HBTS-1, was picked up for the pathogenicity test, and the data showed that this stain was highly pathogenic to bullfrogs with an LD50 of 3.7 × 105 CFU. Notably, the isolate also showed high pathogenicity to 5-day-old suckling mice, with an LD50 of 3.1 × 106 CFU, indicating its potential threat to mammals. Moreover, all the 42 E. miricola isolates showed resistance to multiple antibotics without an apparent inhibition zone observed in the test, making the choice of antimicrobial therapy challenging. These novel findings prioritized E. miricola as an important zoonotic agent, which may provide a reference for human medicine.

Pathogenicity of Streptococcus iniae causing mass mortalities of yellow catfish (Tachysurus fulvidraco) and its induced host immune response

The outbreak of mass mortality occurred in Tachysurus fulvidraco farm in Hubei province of China. The pathogenic strain of Streptococcus iniae (termed 2022SI08) was isolated and identified from diseased T. fulvidraco, based on morphological, physiological, and biochemical characteristics, as well as 16S rRNA gene sequence and phylogenetic analysis. Further, the whole genome of isolate S. iniae was sequenced and predicted to contain one single circular chromosome of 1,776,777 bp with a GC content of 37.14%. The genomic sequence analysis showed that 2022SI08 was positive for 204 virulent and 127 antibiotic resistant genes. The experimental challenge demonstrated the high pathogenicity of the retrieved isolate of S. iniae, with a median lethal dosage (LD50) 9.53 × 105 CFU/g. Histopathological examination indicated that the 2022SI08 strain could induce extensive tissue cell degeneration, necrosis, hemorrhage, and inflammation in the skin, gill, fin, spleen, liver, kidney, intestine, eye, and brain. Moreover, the innate immune enzyme activities in serum such as acid phosphatase and alkaline phosphatase were increased significantly at 24 and 48 h post infection (hpi) and then decreased at 168 hpi. The transcriptional profile of immune associated gene in T. fulvidraco following bacterial infection was detected at each point of time, and the results revealed clear transcriptional activation of those genes, which proving their reacting and regulatory role during the response of the host against S. iniae infection. The results revealed that S. iniae was an etiological agent in the mass mortalities of T. fulvidraco and this research will be conducive for increasing our understanding on pathogenesis and host defensive system in S. iniae invasion.

Identification and drug resistance of pathogen of ulcerative skin disease and its immune responses and protective efficacy after vaccination in a giant spiny frog, Quasipaa spinosa

OBJECTIVE

In order to explore the pathogen of the ulcerative skin disease in giant spiny frog (Quasipaa spinosa), and to provide theoretical basis for the prevention and control of the disease in practical production, this study was carried out to isolate and identify the pathogenic bacteria from the sick frogs suffering from rotting skin disease and to carry out the immunization test of the inactivated vaccine.

METHODS

Physiological and biochemical characterization, and molecular biology of the pathogenic bacteria were identified, and drug screening and immunization responses were also carried out.

RESULTS

The dominant bacterium QS01 was isolated from the lesions of diseased giant spiny frogs, which was confirmed to be the causative agent of the rotting skin disease of giant spiny frogs by artificial regression infection test. Based on the fact that the pathogen is a gram-negative short bacterium, its phenotypic characteristics and 16S rRNA and gyrB gene sequences were analyzed, and the bacterium was determined to be Citrobacter freundii. The results of the drug sensitivity test showed that the bacterium was sensitive to 11 antibiotics, including Enrofloxacin, Fleroxacin and Ciprofloxacin, including three non-polluting drugs such as Florfenicol, Roxithromycin and Thiamphenicol, as well as three Chinese herbal medicines such as Rheum officinale Baill, Coptis chinensis Franch and Scutellaria baicalensis Georgi. Most non-specific immune responses could go to recovery in 24h. The frogs were vaccinated with QS01 formaldehyde inactivated vaccine by injection, immersion and spraying, and the serum antibody potency of the three immunized groups with the average potency reached the peak at the 20th d after immunization, and the serum antibody potency of the injected immunized group was at the highest ratio of 1:64-128 (101.6), while the immersed group and the spraying group attained the ratio of 1:16-32 (20.2) and 1:16-32 (16) respectively, and lasted until the 30th d. The control group that was not immunized had the highest serum antibody potency of 1:16-32 (20.2) and 1:16-32 (16), and continued until the 30th d. The control group that was not immunized was not immunized. The serum antibody potency of the unimmunized control group was 1:2 to 2(2). The immunoprotection rates after takedown were 100 %, 85.71 % and 71.43 %, respectively.

CONCLUSION

C. freundii is the pathogen of the disease in this farm, and the vaccination by immersion and spraying can effectively prevent and control the rotting skin disease in frogs. These results revealed pathogenicity of C. freundii and its activation of host immune response, which will provide a scientific reference for the aquaculture and disease prevention in Q. spinosa culture.