cholerae aphBdeletion mutants containing pBAD-aphB(WT and C235S) were grown aerobically and anaerobically to OD600 0

cholerae aphBdeletion mutants containing pBAD-aphB(WT and C235S) were grown aerobically and anaerobically to OD600 0.2. absence of reducing agents in vitro. Mass spectrometry analysis suggests that under aerobic conditions, AphB is modified at the C235residue. This modification is reversible between oxygen-rich aquatic environments and oxygen-limited human hosts, suggesting thatV. choleraemay use a thiol-based switch mechanism to sense intestinal signals and activate virulence. Keywords:thiol-modification, virulence activators The Gram-negative bacteriumVibrio cholerae, the causative agent of the acute, dehydrating diarrheal disease cholera, has figured prominently in the history of infectious diseases as a cause of periodic, deadly pandemics.V. choleraeresides in aquatic environments between epidemics, and human infection normally starts with the ingestion Microtubule inhibitor 1 of contaminated food or water.Vibriocells surviving passage through the acidic gastric environment enter the Microtubule inhibitor 1 small intestine, where they must produce an array of virulence factors including cholera toxin (CT) and the toxin co-regulated pilus (TCP) that are transcriptionally regulated by multiple systems (1). The primary, direct transcriptional activator of virulence genes is ToxT, whose transcription is regulated by the ToxRS and TcpPH proteins. Two additional activators encoded by unlinked genes, AphA and AphB, regulate the transcription oftcpPH. The environmental cues within the host and their effect on the expression of virulence genes inV. choleraein vivo remain poorly characterized. It has been shown that anaerobiosis serves as one of the host environmental factors that modulate virulence factor production (2). This is not surprising because it is generally presumed that the oxygen concentration in the intestine is low (3). A recent report showed that under anaerobic conditions,tcpPexpression is higher and this effect depends on AphB (4). However, whether and how this AphB-mediatedtcpPexpression contributes to anaerobic virulence induction is unclear. In this study, we found that under anaerobic conditions, AphB proteins are more active than under aerobic conditions. This leads to higher expression of TcpP, which plays a key role in virulence factor production under anaerobic conditions. We further demonstrated that one of three cysteine residues (C235) of AphB is critical for sensing O2concentration and modulating AphB activity. == Results == == Virulence Gene Expression Requires Oxygen-Limiting Conditions. == WhenV. choleraeenters host intestines, genes related toV. choleraepathogenesis, such as thetcpgene clusters, are strongly induced (5). In vitro, virulence genes can be induced using a set of artificial conditions, such as liquid AKI medium for El Tor strains or 30 C and pH 6.5 for classical strains (6,7). Culturing under AKI conditions requires an initial phase in the absence of shaking, suggesting that oxygen levels may have an effect on virulence gene induction in El Tor strains. To further investigate the relationship between ambient oxygen levels and virulence factor expression, we examined the transcription and translation of the major virulence determinant TcpA of El Tor strain C6706 containing a PtcpA-luxCDABEreporter plasmid in AKI under aerobic (shaking, without any stationary culturing), microaerophilic (stationary), and anaerobic (anaerobic chamber) conditions. We determined that O2levels did not affect Lux activity itself by comparing luciferase production inV. choleraecells containing a PBAD-luxCDABEplasmid grown under various conditions. We found COL18A1 that under microaerophilic and anaerobic conditions,tcpAwas highly induced (Fig. 1A) and TcpA protein could readily be detected by Western blot (Fig. 1B). However,tcpAexpression was dramatically reduced under aerobic conditions, and little TcpA was detected. == Fig. 1. == The effect of oxygen on virulence factor production.V. choleraeC6706 containing Microtubule inhibitor 1 a PtcpA-luxCDABEplasmid was grown under aerobic (shaking), microaerophilic (stationary), or anaerobic (anaerobic chambers) conditions in AKI medium (6) at 37 C until OD600reached 0.2. (A) Transcription oftcpA.The units were calculated from Lux units (arbitrary light units/OD600). Results are the mean of three experiments SD. (B) Whole-cell extracts (normalized by the total protein amounts) were subjected to Western blot analysis using anti-TcpA antiserum (Upper). The corresponding cell-free culture fluids were assayed for CT production (Lower). (C) GFP-labeled WT ortcpAmutants were inoculated on the surface of mouse small intestinal tissues and incubated at 37 C aerobically or anaerobically for 4 h. Rinsed tissue samples were then visualized using a fluorescence confocal laser microscope. Representative micrographs are shown. (Scale bar: 10 m.) Oxygen had a similar effect on the production of CT, the other major virulence determinant, which is under the control of the same regulatory cascade as Microtubule inhibitor 1 TCP (Fig. 1B). We also examined the effect of oxygen on virulence gene expression in O395, a classical strain, and MO10, an O139 strain, and found that TcpA was also greatly induced under oxygen-limiting conditions in these strains using the AKI medium (Fig. S1A), suggesting that.