Cholera toxin expression by El Tor Vibrio cholerae in shallow culture growth conditions.
Vibrio cholerae O1 classical, El Tor and O139 are the primary biotypes that cause epidemic cholera, and they also express cholera toxin (CT). Although classical V. cholerae produces CT in various settings, the El Tor and O139 strains require specific growth conditions for CT induction, such as the so-called AKI conditions, which consist of growth in static conditions followed by growth under aerobic shaking conditions. However, our group has demonstrated that CT production may also take place in shallow static cultures. How these type of cultures induce CT production has been unclear, but we now report that in shallow culture growth conditions, there is virtual depletion of dissolved oxygen after 2.5 h of growth. Concurrently, during the first three to 4 h, endogenous CO2 accumulates in the media and the pH decreases. These findings may explain CT expression at the molecular level because CT production relies on a regulatory cascade, in which the key regulator AphB may be activated by anaerobiosis and by low pH. AphB activation stimulates TcpP synthesis, which induces ToxT production, and ToxT directly stimulates ctxAB expression, which encodes CT. Importantly, ToxT activity is enhanced by bicarbonate. Therefore, we suggest that in shallow cultures, AphB is activated by initial decreases in oxygen and pH, and subsequently, ToxT is activated by intracellular bicarbonate that has been generated from endogenous CO2. This working model would explain CT production in shallow cultures and, possibly, also in other growth conditions.
Facultad de Ciencias UAEM, Cuernavaca, Morelos, Mexico.
Mayra
M
Cobaxin
Facultad de Ciencias UAEM, Cuernavaca, Morelos, Mexico.
Haydee
H
Martínez
Centro de Investigación en Enfermedades Infecciosas, INSP, Cuernavaca, Morelos, Mexico.
Guadalupe
G
Ayala
Institute of Biomedicine, Department of Microbiology and Immunology, Sahlgrenska Academy, University of Gothenburg, Göteborg, Sweden.
Jan
J
Holmgren
Institute of Biomedicine, Department of Microbiology and Immunology, Sahlgrenska Academy, University of Gothenburg, Göteborg, Sweden.
Asa
A
Sjöling
Facultad de Medicina, UAEM, Cuernavaca, Morelos, Mexico. Electronic address: joaquinsanchez5@gmail.com.
Joaquín
J
Sánchez
D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002245:Carbon Dioxide / Q000737:chemistry; D002772:Cholera Toxin / Q000235:genetics / Q000378:metabolism*; D003470:Culture Media / Q000737:chemistry; D015964:Gene Expression Regulation, Bacterial; D006863:Hydrogen-Ion Concentration; D010100:Oxygen / Q000737:chemistry; D014734:Vibrio cholerae / Q000235:genetics / Q000378:metabolism*
D001426:Bacterial Proteins; D003470:Culture Media; D002245:Carbon Dioxide; D002772:Cholera Toxin; D010100:Oxygen
El Tor vibrios; Endogenous bicarbonate; Low oxygen and AphB regulation; Model for cholera toxin expression; Shallow cultures; ToxT regulator