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Selection for in Vivo Regulators of Bacterial Virulence Selection for in vivo regulators of bacterial virulence Sang Ho Lee, Susan M. Butler, and Andrew Camilli* Tufts University School of Medicine, Department of Molecular Biology and Microbiology, 136 Harrison Avenue, Boston, MA 02111 Edited by John J. Mekalanos, Harvard Medical School, Boston, MA, and approved April 6, 2001 (received for review December 8, 2000) We devised a noninvasive genetic selection strategy to identify regulatory pathways controlling these complex patterns of expres- positive regulators of bacterial virulence genes during actual infection sion, we used a RIVET selection strategy to identify in vivo of an intact animal host. This strategy combines random mutagenesis regulators of toxT and of ctxA. A number of these regulators are with a switch-like reporter of transcription that confers antibiotic described, some of which affect virulence gene induction only resistance in the off state and sensitivity in the on state. Application during infection. The roles in gene regulation and colonization of of this technology to the human intestinal pathogen Vibrio cholerae infant mice of one major class of regulators controlling chemotaxis identified several regulators of cholera toxin and a central virulence signaling were further investigated by defined mutational analysis. gene regulator that are operative during infection. These regulators function in chemotaxis, signaling pathways, transport across the cell Materials and Methods envelope, biosynthesis, and adherence. We show that phenotypes Bacterial Strains and in Vitro Growth Conditions. AlloftheV. cholerae that appear genetically independent in cell culture become interre- strains used in this study are isogenic derivatives of El Tor biotype lated in the host milieu. strain, C6709–1, containing the res-tet-res substrate for TnpR resolvase integrated within the endogenous lacZ gene (strain ibrio cholerae, the causative agent of the epidemic disease AC-V66) (10). Bacterial strains were either grown in Luria-Bertani Vcholera, requires the coordinated expression of multiple viru- (LB) broth with aeration at 37°C (virulence gene-noninducing lence factors. Two of the most well studied factors are cholera toxin condition) or AKI broth grown statically for 4 h followed by growth (CT) and toxin-coregulated pilus (TCP). CT is an ADP-ribosylating with aeration at 37°C (inducing condition) (11). Antibiotics were ␮ ͞ toxin largely responsible for eliciting the profuse diarrhea charac- used at the following concentrations: ampicillin, 50 g ml; strep- ␮ ͞ ␮ ͞ ␮ ͞ teristic of this disease, and TCP is a type IV bundle-forming pilus tomycin, 100 g ml; kanamycin, 50 g ml; and Tc, 3 g ml. that is essential for intestinal colonization in humans and in animal MICROBIOLOGY models of cholera (1, 2). Regulation of these factors involves the Transposon Mutagenesis and RIVET Selection. A promoterless re- concerted actions of three proteins, ToxR, TcpP, and ToxT, which combinase gene, tnpR, was inserted in a nonpolar manner down- together form the V. cholerae virulence gene regulatory cascade stream of ctxA or toxT by allelic exchange into the strain AC-V66. (3–5). ToxR and TcpP are inner-membrane-associated transcrip- In each fusion strain, random mutagenesis was performed with tional regulators that act cooperatively to induce the expression of mini-Tn5Kn2 (mTn5) to generate 15,000 independent strains in 10 ToxT in response to particular environmental signals. ToxT is a pools of 1,500 each. Each pool was grown in the presence of Tc and 6 cytoplasmic transcriptional activator responsible for directly acti- 10 colony-forming units (cfu) were intragastrically inoculated into vating the transcription of the structural genes for CT, TCP, and 5-day-old CD-1 mice (2 mice per pool). After 7 h, the small other virulence factors (3). Investigation of the regulatory networks intestines were removed and homogenized. The bacteria within coordinating expression of these genes has been predominately each homogenate were grown overnight in the presence of Tc to R limited to in vitro culture conditions largely because of the inability enrich for Tc mutant strains. The animal passage and Tc enrich- to monitor regulatory processes during infection of an intact host. ment were repeated three times. After each animal passage, but Therefore, the true nature of the in vivo regulation of these before Tc enrichment, a portion of the recovered bacteria was virulence factors within a host remains unclear. examined for previous induction of the toxT::tnpR or ctxA::tnpR To extend our knowledge of virulence gene regulation during fusion during infection by replica plating colonies onto LB agar R infection, we developed a noninvasive genetic selection that incor- containing Tc. The percentage of Tc cfu was calculated by dividing R porates the use of the recombination-based in vivo expression the number of Tc cfu by the total cfu. technology (RIVET) and transposon mutagenesis to identify pos- itive regulators of virulence genes during infection. RIVET relies Mutant Strain Construction. The desired point and deletion muta- on the construction of transcriptional fusions to a promoterless tions were constructed by using PCR, and the mutant alleles were reporter gene, tnpR, encoding a site-specific DNA recombinase. cloned into the suicide shuttle vector pCVD442 (12). The deletion After it is produced, TnpR functions in trans to permanently excise mutations of motY and flaA were of the entire coding region fusing from the bacterial genome a tetracycline resistance (TcR) marker the putative ATG start codon with the putative stop codon for each (tet) that is flanked by recombinase recognition sequences (res). gene. For construction of the motAB deletion, the putative ATG start codon of motA was fused to the putative stop codon of motB. The subsequent conversion to tetracycline (Tc) sensitivity serves as ␭ an ex post facto indicator of increased transcription of the gene Escherichia coli strain, SM10 pir carrying the plasmids was mated fusion. RIVET has been used as a promoter-trap method to with V. cholerae. The desired mutation was introduced into the identify bacterial and fungal genes induced during infection (6–8) genome by allelic exchange after appropriate counterselection for and as a tool to analyze the spatiotemporal patterns of induction of loss of the integrated plasmid. PCR, Southern blot, and DNA such genes (9). Using RIVET, we recently found that the patterns sequence analyses were used to confirm the correct mutations. The of transcriptional induction of genes encoding CT and TCP fun- mTn5 insertion junctions for the isolated mutants were determined damentally differ during in vitro growth vs. during infection with respect to timing and interdependence (9). During infection, tcpA, This paper was submitted directly (Track II) to the PNAS office. encoding the pilin subunit of TCP, was transcriptionally induced in Abbreviations: CT, cholera toxin; TCP, toxin-coregulated pilus; RIVET, recombination-based a biphasic pattern with an early induction occurring at1hafter in vivo expression technology; Tc, tetracycline; TcR, Tc resistance; cfu, colony-forming units. inoculation and a second more robust induction occurring at 4 h *To whom reprint requests should be addressed. E-mail: [email protected]. after inoculation. Meanwhile, ctxA, encoding the enzymatic subunit The publication costs of this article were defrayed in part by page charge payment. This of CT, was not induced until4hafterinoculation and was article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. dependent on previous expression of TCP. To learn more about the §1734 solely to indicate this fact. www.pnas.org͞cgi͞doi͞10.1073͞pnas.111581598 PNAS ͉ June 5, 2001 ͉ vol. 98 ͉ no. 12 ͉ 6889–6894 Downloaded by guest on September 25, 2021 by arbitrary-primed PCR and DNA sequencing as described (13). The genes disrupted and sites of mTn5 insertion relative to the first base of the predicted coding regions are: sltA (1048), prpB (519), vps70͞capK (816), rtxB (1817), cheA-2 (1977), cheY-3 (178), vieS (22), mfrha (239), cheZ (659), and mcpX (1774). Swarm Assay and Electron Microscopy. Chemotactic ability of the mutants were determined by stabbing cells into swarm agar (1% tryptone͞0.5% NaCl͞0.3% agar) and incubating overnight at 30°C. Production of the polar flagellum was determined by electron microscopy after negative staining with uranyl acetate. In Vitro and in Vivo Transcription Assays. Strains were grown either in LB broth or in AKI broth for7hat37°C, and Ϸ150–200 cfu were plated on LB agar containing streptomycin. After growth overnight at 37°C, these plates were then replica plated onto LB agar containing Tc and the extent of toxT::tnpR or ctxA::tnpR induction was measured as the percent Tc-sensitive (TcS) cfu (TcS cfu divided by the total cfu). For the in vivo temporal gene fusion induction experiments, 105 cfu were intragastrically inoculated into 5-day-old CD-1 mice. At each hour after inoculation, the small intestines were removed and homogenized, and the percent TcS cfu was deter- mined as described above. Competition Assays of Mutant Strains and Population Dynamics Study. The competitive index was determined by mixing a LacZϪ mutant strain with a LacZϩ wild-type strain at approximately a 1:1 ratio, and 105 cfu were intragastrically inoculated into 5-day-old CD-1 mice and also into 2 ml of LB broth to be grown overnight at 37°C. Five mice were used per mutant strain tested. At 24 h after inoculation, the small intestines were removed, homogenized, and then plated on LB agar supplemented with streptomycin and ϩ 5-bromo-4-chloro-3-indolyl ␤-D-galactopyranoside. Blue (LacZ ) and white (LacZϪ) colonies were enumerated. The competitive index was calculated as the ratio of mutant to wild type recovered after 24 h of infection divided by the ratio of mutant to wild-type after 24 h of growth in LB broth, with both ratios corrected for deviations of the input from a value of 1:1. The population dynamics study was performed as follows: 106 cfu of wild-type or cheY-3D52N mutant were intragastrically inoculated into 5-day-old CD-1 mice.
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