Tilapia (Oreochromis niloticus) as a Putative Reservoir Host for Survival and Transmission of Vibrio cholerae O1 Biotype El Tor in the Aquatic Environment

Prevalence and antimicrobial susceptibility pattern of Vibrio cholerae isolates from cholera outbreak sites in Ethiopia

BACKGROUND

Cholera is an acute infectious disease caused by ingestion of contaminated food or water with Vibrio cholerae. Cholera remains a global threat to public health and an indicator of inequity and lack of social development. The aim of this study was to assess the prevalence and antimicrobial susceptibility pattern of V. cholerae from cholera outbreak sites in Ethiopia.

METHODS

Across-sectional study was conducted from May 2022 to October 2023 across different regions in Ethiopia: Oromia National Regional State, Amhara National Regional State and Addis Ababa City Administration. A total of 415 fecal samples were collected from the three regions. Two milliliter fecal samples were collected from each study participants. The collected samples were cultured on Blood Agar, MacConkey Agar and Thiosulfate Citrate Bile Salt Sucrose Agar. A series of biochemical tests Oxidase test, String test, Motility, Indole, Citrate, Gas production, H2S production, Urease test were used to identify V. cholerae species. Both polyvalent and monovalent antisera were used for agglutination tests to identify and differentiate V. cholerae serogroup and serotypes. In addition, Kirby-Bauer Disk diffusion antibiotic susceptibility test method was done. Data were registered in epi-enfo version 7 and analyzed by Statistical Package for Social Science version 25. Descriptive statistics were used to determine the prevalence of Vibrio cholerae. Logistic regression model was fitted and p-value < 0.05 was considered as statically significant.

RESULTS

The prevalence of V. cholerae in the fecal samples was 30.1%. Majority of the isolates were from Oromia National Regional State 43.2% (n = 54) followed by Amhara National Regional State 31.2% (n = 39) and Addis Ababa City Administration 25.6% (n = 32). Most of the V. cholerae isolates were O1 serogroups 90.4% (n = 113) and Ogawa serotypes 86.4% (n = 108). Majority of the isolates were susceptible to ciprofloxacin 100% (n = 125), tetracycline 72% (n = 90) and gentamycin 68% (n = 85). More than half of the isolates were resistant to trimethoprim-sulfamethoxazole 62.4% (n = 78) and ampicillin 56.8% (n = 71). In this study, participants unable to read and write were about four times more at risk for V. cholerae infection (AOR: 3.8, 95% CI: 1.07-13.33). In addition, consumption of river water were about three times more at risk for V. cholerae infection (AOR: 2.8, 95% CI: 1.08-7.08).

CONCLUSION

our study revealed a high prevalence of V. cholerae from fecal samples. The predominant serogroups and serotypes were O1 and Ogawa, respectively. Fortunately, the isolates showed susceptible to most tested antibiotics. Drinking water from river were the identified associated risk factor for V. cholerae infection. Protecting the community from drinking of river water and provision of safe and treated water could reduce cholera outbreaks in the study areas.

Vibrio cholerae O1 Inhabit Intestines and Spleens of Fish in Aquaculture Ponds

Vibrio cholerae is the causative agent of cholera, an acute diarrheal disease that spreads locally and globally in epidemics and pandemics. Although it was discovered that fish harbor V. cholerae strains in their intestines, most investigations revealed non-toxic V. cholerae serogroups in fish. Due to the rarity of toxigenic V. cholerae serogroups, it is difficult to cultivate these strains from environmental samples. Hence, here we aimed to uncover evidence of the occurrence of toxigenic V. cholerae in the intestines and spleens of various fish species. By using molecular detection tools, we show that V. cholerae O1 and strains positive for the cholera toxin inhabit both healthy and diseased fish intestines and spleens, suggesting that fish may serve as intermediate vectors of toxigenic V. cholerae. No significant differences were found between the abundance of toxigenic V. cholerae (either O1 or cholera toxin positive strains) in the healthy and the diseased fish intestines or spleens. In conclusion, a variety of fish species may serve as potential vectors and reservoirs of toxigenic V. cholerae as they form a link between the other reservoirs of V. cholerae (chironomids, copepods, and waterbirds). Similarly, they may aid in the spread of this bacterium between water bodies.

Advances in cholera research: from molecular biology to public health initiatives

The aquatic bacterium Vibrio cholerae is the etiological agent of the diarrheal disease cholera, which has plagued the world for centuries. This pathogen has been the subject of studies in a vast array of fields, from molecular biology to animal models for virulence activity to epidemiological disease transmission modeling. V. cholerae genetics and the activity of virulence genes determine the pathogenic potential of different strains, as well as provide a model for genomic evolution in the natural environment. While animal models for V. cholerae infection have been used for decades, recent advances in this area provide a well-rounded picture of nearly all aspects of V. cholerae interaction with both mammalian and non-mammalian hosts, encompassing colonization dynamics, pathogenesis, immunological responses, and transmission to naïve populations. Microbiome studies have become increasingly common as access and affordability of sequencing has improved, and these studies have revealed key factors in V. cholerae communication and competition with members of the gut microbiota. Despite a wealth of knowledge surrounding V. cholerae, the pathogen remains endemic in numerous countries and causes sporadic outbreaks elsewhere. Public health initiatives aim to prevent cholera outbreaks and provide prompt, effective relief in cases where prevention is not feasible. In this review, we describe recent advancements in cholera research in these areas to provide a more complete illustration of V. cholerae evolution as a microbe and significant global health threat, as well as how researchers are working to improve understanding and minimize impact of this pathogen on vulnerable populations.

Abundance and biofilm formation capability of Vibrio cholerae in aquatic environment with an emphasis on Hilsha fish (Tenualosa ilisha)

The potentially deadly and sporadic diarrhea-causing agent, Vibrio cholerae, is present in a great number in the freshwater aquatic environment and can be transmitted to humans by different aquatic organisms. In the perspective of Bangladesh, an anadromous fish species Hilsha (Tenualosa ilisha) can act as a transmission vehicle of V. cholerae from the aquatic to the household kitchen environment. The present study was carried out to investigate the presence of V. cholerae in the aquatic habitat of Bangladesh with a major emphasis on freshly caught Hilsha fish, along with river water and plankton samples from the fish capture site. The study also detected the biofilm formation capability of V. cholerae within Hilsha fish that might help the transmission and persistence of the pathogen in aquatic habitat. Twenty out of 65 freshly caught fish (30.8%) and 1 out of 15 water samples (6.67%) showed the presence of V. cholerae and none of the plankton samples were positive for V. cholerae. The isolated strains were identified as non-O1 and non-O139 serogroups of V. cholerae and contain some major toxin and virulence genes. A few strains showed cellular cytotoxicity on the HeLa cell line. All strains were able to form biofilm on the microtiter plate and the detection of three genes related to biofilm formation (vpsA, vpsL, and vpsR) were also assayed using qPCR. In this study, the in vitro biofilm formation ability of the isolated strains may indicate the long-term persistence of V. cholerae in different parts of Hilsha fish. The abundance of V. cholerae only in freshly caught Hilsha fish and the absence of the pathogen in the surrounding aquatic environment could stipulate the role of Hilsha fish as one of the major transmission routes of V. cholerae from the freshwater aquatic environment of Bangladesh to the household kitchen environment.

Adaptation to an amoeba host drives selection of virulence-associated traits in Vibrio cholerae

Predation by heterotrophic protists drives the emergence of adaptive traits in bacteria, and often these traits lead to altered interactions with hosts and persistence in the environment. Here we studied adaptation of the cholera pathogen, Vibrio cholerae during long-term co-incubation with the protist host, Acanthamoeba castellanii. We determined phenotypic and genotypic changes associated with long-term intra-amoebal host adaptation and how this impacts pathogen survival and fitness. We showed that adaptation to the amoeba host leads to temporal changes in multiple phenotypic traits in V. cholerae that facilitate increased survival and competitive fitness in amoeba. Genome sequencing and mutational analysis revealed that these altered lifestyles were linked to non-synonymous mutations in conserved regions of the flagellar transcriptional regulator, flrA. Additionally, the mutations resulted in enhanced colonisation in zebrafish, establishing a link between adaptation of V. cholerae to amoeba predation and enhanced environmental persistence. Our results show that pressure imposed by amoeba on V. cholerae selects for flrA mutations that serves as a key driver for adaptation. Importantly, this study provides evidence that adaptive traits that evolve in pathogens in response to environmental predatory pressure impact the colonisation of eukaryotic organisms by these pathogens.

Vibrio cholerae was found in cultured bullfrog

Bullfrog is one of the most important economic aquatic animals in China that is widely cultured in southern China and is a key breed recommended as an industry of poverty alleviation in China. During recent years, a fatal bacterial disease has often been found in cultured bullfrogs. The clinical manifestations of the diseased bullfrogs were severe intestinal inflammation and an anal prolapse. A bacterial pathogen was isolated from the diseased bullfrog intestines. The bacterium was identified as Vibrio cholerae using morphological, biochemical and 16S rRNA phylogenetic analysis. In this study, V. cholerae was isolated and identified in diseased bullfrogs for the first time, providing a basis for the diagnosis and control of the disease. Therefore, attention should be paid to the modes of transmission of V. cholerae from bullfrog and formulate reasonable safety measures.

Quorum-sensing signaling by chironomid egg masses' microbiota, affects haemagglutinin/protease (HAP) production by Vibrio cholerae

Vibrio cholerae, the causative agent of cholera, is commonly isolated, along with other bacterial species, from chironomid insects (Diptera: Chironomidae). Nevertheless, its prevalence in the chironomid egg masses' microbiota is less than 0.5%. V. cholerae secretes haemagglutinin/protease (HAP) that degrades the gelatinous matrix of chironomid egg masses and prevents hatching. Quorum sensing (QS) activates HAP production in response to accumulation of bacterial autoinducers (AIs). Our aim was to define the impact of chironomid microbiota on HAP production by V. cholerae. To study QS signaling, we used V. cholerae bioluminescence reporter strains (QS-proficient O1 El-Tor wild-type and QS-deficient mutants) and different bacterial species that we isolated from chironomid egg masses. These egg mass isolates, as well as a synthetic AI-2, caused an enhancement in lux expression by a V. cholerae QS-deficient mutant. The addition of the egg mass bacterial isolate supernatant to the QS-deficient mutant also enhanced HAP production and egg mass degradation activities. Moreover, the V. cholerae wild-type strain was able to proliferate using egg masses as their sole carbon source, while the QS-deficient was not. The results demonstrate that members of the chironomid bacterial consortium produce external chemical cues that, like AI-2, induce expression of the hapA gene in V. cholerae. Understanding the interactions between V. cholerae and the insects' microbiota may help uncover the interactions between this pathogen and the human gut microbiota.

Molecular Characteristics and Zoonotic Potential of Salmonella Weltevreden From Cultured Shrimp and Tilapia in Vietnam and China

Salmonella Weltevreden is increasingly reported from aquatic environments, seafood, and patients in several Southeast Asian countries. Using genome-wide analysis, we characterized S. Weltevreden isolated from cultured shrimp and tilapia from Vietnam and China to study their genetic characteristics and relatedness to clinical isolates of S. Weltevreden ST-365. The phylogenetic analysis revealed up to 312 single-nucleotide polymorphism (SNP) difference between tilapia isolates, whereas isolates from shrimp were genetically more closely related. Epidemiologically unrelated isolates from Vietnam were closely related to isolates from China, e.g., 20 SNPs differences between strains 28V and 75C. In comparison with strains from other parts of the world, our environmental isolates predominantly clustered within the continental South Asia lineage, constituted mostly of strains from human stool with as low as seven SNPs difference, e.g., 30V versus Cont_ERR495254. All sequenced isolates were MLST type ST-365 and contained the major virulence-related genes encoded by the Salmonella Pathogenicity Islands 1–5. Ten of the isolates harbored the IncFII(S) plasmid similar to the virulence genes-mediated plasmid pSPCV of S. Paratyphi C, and one isolate had the IncQ1 plasmid on the same contig with strA/B, sul2, and tetA resistance genes similar to the IncQ1 type, pNUC of S. Typhimurium. A pangenomic analysis yielded 7891 genes including a core genome of 4892 genes, with a closely related accessory genome content between clinical and environmental isolates (Benjamini p > 0.05). In a search for differences that could explain the higher prevalence of S. Weltevreden in aquatic samples, genomes were compared with those of other Salmonella enterica serovars. S. Weltevreden revealed specific regions harboring glpX (Fructose-1;6-bisphosphatase; class II), rfbC (dTDP-4-dehydrorhamnose 3;5-epimerase), and cmtB (PTS Mannitol-specific cryptic phosphotransferase enzyme IIA component) involved in carbohydrate biosynthesis pathways. Our study builds grounds for future experiments to determine genes or pathways that are essential when S. Weltevreden are in aquatic environments and microbial interactions providing survival advantages to S. Weltevreden in such environments.

Genomic insights into Vibrio cholerae O1 responsible for cholera epidemics in Tanzania between 1993 and 2017

BACKGROUND

Tanzania is one of seven countries with the highest disease burden caused by cholera in Africa. We studied the evolution of Vibrio cholerae O1 isolated in Tanzania during the past three decades.

METHODOLOGY/PRINCIPAL FINDINGS

Genome-wide analysis was performed to characterize V. cholerae O1 responsible for the Tanzanian 2015-2017 outbreak along with strains causing outbreaks in the country for the past three decades. The genomes were further analyzed in a global context of 590 strains of the seventh cholera pandemic (7PET), as well as environmental isolates from Lake Victoria. All Tanzanian cholera outbreaks were caused by the 7PET lineage. The T5 sub-lineage (ctxB3) dominated outbreaks until 1997, followed by the T10 atypical El Tor (ctxB1) up to 2015, which were replaced by the T13 atypical El Tor of the current third wave (ctxB7) causing most cholera outbreaks until 2017 with T13 being phylogenetically related to strains from East African countries, Yemen and Lake Victoria. The strains were less drug resistant with approximate 10-kb deletions found in the SXT element, which encodes resistance to sulfamethoxazole and trimethoprim. Nucleotide deletions were observed in the CTX prophage of some strains, which warrants further virulence studies. Outbreak strains share 90% of core genes with V. cholerae O1 from Lake Victoria with as low as three SNPs difference and a significantly similar accessory genome, composed of genomic islands namely the CTX prophage, Vibrio Pathogenicity Islands; toxin co-regulated pilus biosynthesis proteins and the SXT-ICE element.

CONCLUSION/SIGNIFICANCE

Characterization of V. cholerae O1 from Tanzania reveals genetic diversity of the 7PET lineage composed of T5, T10 and T13 sub-lineages with introductions of new sequence types from neighboring countries. The presence of these sub-lineages in environmental isolates suggests that the African Great Lakes may serve as aquatic reservoirs for survival of V. cholerae O1 favoring continuous human exposure.