Abstract
Studies have reported the occurrence of Vibrio cholerae in fish but little is known about the interaction between fish and toxigenic V. cholerae as opposed to phytoplankton, which are well-established aquatic reservoirs for V. cholerae. The present study determined the role of tilapia (Oreochromis niloticus) as a reservoir host for survival and transmission of V. cholerae in aquatic environments. Three experiments were performed with one repetition each, where O. niloticus (∼2 g) kept in beakers were inoculated with four V. cholerae strains (5 × 107 cfu/mL). Firstly, infected tilapia were kept in stagnant water and fed live brine shrimp (Artemia salina) larvae daily. Secondly, infected tilapia were kept without feeding and water was changed every 24 h. Thirdly, infected tilapia were fed and water was renewed daily. Infected tilapia and non-infected controls were sacrificed on days 1, 2, 3, 7, and 14 post-inoculation and V. cholerae were enumerated in intestinal content and water. Another experiment assessed the transmission of V. cholerae from infected to non-infected tilapia. The study revealed that El Tor biotype V. cholerae O1 and V. cholerae non-O1 colonized tilapia intestines and persisted at stable concentrations during the second week of the experiment whereas the Classical biotype was undetectable after 1 week. In stagnant water with feeding, V. cholerae counts dropped to 105 cfu/ml in water and from 107 to 104 cfu/intestine in fish after 14 days. When water was renewed, counts in water decreased from 107 to 103 cfu/ml and intestinal counts went from 106 to 102 cfu/intestine regardless of feeding. All strains were transmitted from infected to naïve fish after 24 h of cohabitation. Tilapia like other fish may play an essential role in the survival and dissemination of V. cholerae O1 in aquatic environments, e.g., the seventh pandemic strains mostly. In this study, tilapia were exposed to high concentrations of V. cholerae to ensure initial uptake and follow-up studies with lower doses resembling natural concentrations of V. cholerae in the aquatic environment are needed to confirm our findings.
Highlights
Vibrio cholerae is one of the longest recognized human infectious pathogens, yet there is still much to clarify on the emergence and transmission of cholera, the disease for which V. cholerae is the causative agent
Survival of V. cholerae in Tilapia Kept in Stagnant Water and Given Live Feed In Experiment 1, tilapia were exposed to 5 × 107 cfu/mL of V. cholerae as described in the Section “Materials Methods.”
Tilapia juveniles exposed to V. cholerae were found to be colonized by V. cholerae within 24 h after exposure, with average intestinal counts for all test strains varying between 107 cfu/intestine on day 1 post infection to 105 cfu/intestine 14 days after infection
Summary
Vibrio cholerae is one of the longest recognized human infectious pathogens, yet there is still much to clarify on the emergence and transmission of cholera, the disease for which V. cholerae is the causative agent. V. cholerae O1 and O139 are the only serogroups causing cholera, with the leading strains being the toxigenic V. cholerae O1 El Tor and Classical biotypes (Dalsgaard et al, 2001). The intestinal colonization of V. cholerae in humans requires production of the cholera toxin co-regulated pilus (TCP), whose main transcription activator is ToxT (Faruque et al, 1998; Sanchez and Holmgren, 2011). V. cholerae O1 biotype El Tor that lacks active toxT can be regarded as non-toxigenic. V. cholerae non-O1/O139 strains are ubiquitous in aquatic environments and rarely produce cholera toxin, but can cause sporadic diarrhea (Hounmanou et al, 2016). A toxT mutant of V. cholerae O1 El Tor served to assess whether the lack of transcription of tcpA (regulated by ToxT) would affect colonization in tilapia
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