Vibrio choleraeis a gram-negative bacterium that is the causative agent of cholera, a severe diarrheal illness. The two biotypes ofV. choleraeO1 capable of causing cholera, classical and El Tor, require different in vitro growth conditions for induction of virulence gene expression. Growth under the inducing conditions or infection of a host initiates a complex regulatory cascade that results in production of ToxT, a regulatory protein that directly activates transcription of the genes encoding cholera toxin (CT), toxin-coregulated pilus (TCP), and other virulence genes. Previous studies have shown that sodium bicarbonate induces CT expression in theV. choleraeEl Tor biotype. However, the mechanism for bicarbonate-mediated CT induction has not been defined. In this study, we demonstrate that bicarbonate stimulates virulence gene expression by enhancing ToxT activity. Both the classical and El Tor biotypes produce inactive ToxT protein when they are cultured statically in the absence of bicarbonate. Addition of bicarbonate to the culture medium does not affect ToxT production but causes a significant increase in CT and TCP expression in both biotypes. Ethoxyzolamide, a potent carbonic anhydrase inhibitor, inhibits bicarbonate-mediated virulence induction, suggesting that conversion of CO₂into bicarbonate by carbonic anhydrase plays a role in virulence induction. Thus, bicarbonate is the first positive effector for ToxT activity to be identified. Given that bicarbonate is present at high concentration in the upper small intestine whereV. choleraecolonizes, bicarbonate is likely an important chemical stimulus thatV. choleraesenses and that induces virulence during the natural course of infection.

▪ Abstract Transfer-messenger RNA (tmRNA, or SsrA), found in all eubacteria, has both transfer and messenger RNA activity. Relieving ribosome stalling by a process called trans-translation, tmRNA^ala enters the ribosome and adds its aminoacylated alanine to the nascent polypeptide. The original mRNA is released and tmRNA becomes the template for translation of a 10-amino-acid tag that signals for proteolytic degradation. Although essential in a few bacterial species, tmRNA is nonessential in Escherichia coli and many other bacteria. Proteins known to be associated with tmRNA include SmpB, ribosomal protein S1, RNase R, and phosphoribosyl pyrophosphate. SmpB, having no other known function, is essential for tmRNA activity. trans-translation operates within ribosomes stalled both at the end of truncated mRNAs and at rare codons and some natural termination sites. Both the release of stalled ribosomes and the subsequent degradation of tagged proteins are important consequences of trans-translation.

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The human diarrheal disease cholera is caused by the aquatic bacterium Vibrio cholerae . V. cholerae in the environment is associated with several varieties of aquatic life, including insect egg masses, shellfish, and vertebrate fish. Here we describe a novel animal model for V. cholerae , the zebrafish. Pandemic V. cholerae strains specifically colonize the zebrafish intestinal tract after exposure in water with no manipulation of the animal required. Colonization occurs in close contact with the intestinal epithelium and mimics colonization observed in mammals. Zebrafish that are colonized by V. cholerae transmit the bacteria to naive fish, which then become colonized. Striking differences in colonization between V. cholerae classical and El Tor biotypes were apparent. The zebrafish natural habitat in Asia heavily overlaps areas where cholera is endemic, suggesting that zebrafish and V. cholerae evolved in close contact with each other. Thus, the zebrafish provides a natural host model for the study of V. cholerae colonization, transmission, and environmental survival.