Small-molecule inhibitor of Vibrio cholerae virulence and intestinal colonization

Identification of residues critical for the function of the Vibrio cholerae virulence regulator ToxT by scanning alanine mutagenesis

Virulence factor expression in Vibrio cholerae is controlled by the transcriptional regulatory protein ToxT. ToxT activates transcription of the genes encoding cholera toxin (ctx) and the toxin co-regulated pilus (tcp), as well as accessory colonization factor (acf) genes. Previous studies of ToxT, a member of the AraC family of proteins, have revealed that it consists of two domains, an N-terminal dimerization and environmental sensing domain, and a C-terminal DNA binding domain. In this study, comprehensive scanning alanine mutagenesis was utilized to identify amino acids critical for the function of ToxT. Forty-eight proteins with Ala substitutions (of 267 total) exhibited defects in ToxT-dependent activation (>90% reduction) in both a V. cholerae acfA-phoA reporter strain and a Salmonella typhimurium ctxAp-lacZ reporter strain. Most of these mutant proteins also caused reductions in cholera toxin (CT) and toxin coregulated pilus (TCP) expression in a DeltatoxT V cholerae strain under in vitro virulence factor inducing conditions. Further analysis with a LexA-based reporter system revealed that one of the 20 Ala substitutions in the N terminus (F151A) diminishes dimerization, and this residue is located in a region of predicted alpha-helical structure, thus identifying a putative dimer interface. Ala substitutions in two putative helix-turn-helix (HTH) recognition helices that caused differential promoter activation (K203A and S249A) did not appear to alter specific DNA binding, suggesting these residues contribute to other aspects of transcriptional activation. A number of Ala substitutions were also found that result in a higher level of ToxT transcriptional activity, and these mutations were almost exclusively found within the N terminus, consistent with this domain being involved in modulation of ToxT activity. This study illuminates the contribution of specific amino acids to the dimerization, DNA binding, and transcriptional activity of ToxT.

Pseudolipasin A is a specific inhibitor for phospholipase A2 activity of Pseudomonas aeruginosa cytotoxin ExoU

A number of bacterial pathogens utilize the type III secretion pathway to deliver effector proteins directly into the host cell cytoplasm. Certain strains of Pseudomonas aeruginosa associated with acute infections express a potent cytotoxin, exoenzyme U (ExoU), that is delivered via the type III secretion pathway directly into contacting host cells. Once inside the mammalian cell, ExoU rapidly lyses the intoxicated cells via its phospholipase A(2) (PLA(2)) activity. A high-throughput cell-based assay was developed to screen libraries of compounds for those capable of protecting cells against the cytotoxic effects of ExoU. A number of compounds were identified in this screen, including one group that blocks the intracellular activity of ExoU. In addition, these compounds specifically inhibited the PLA(2) activity of ExoU in vitro, whereas eukaryotic secreted PLA(2) and cytosolic PLA(2) were not inhibited. This novel inhibitor of ExoU-specific PLA(2) activity, named pseudolipasin A, may provide a new lead for virulence factor-based therapeutic design.

MarA, SoxS and Rob function as virulence factors in an Escherichia coli murine model of ascending pyelonephritis

MarA, SoxS and Rob are transcription factors belonging to the AraC family. While these proteins have been associated historically with control of multiple antibiotic resistance, and tolerance to oxidative stress agents and organic solvents, only a paucity of experimental data support a role in regulating virulence. Clinical Escherichia coli isolates, and isogenic strains lacking marA, soxS and rob, were studied in a murine model of ascending pyelonephritis, which is a clinically relevant model of urinary tract infection. Organisms lacking all three transcription factors (triple knockouts) were significantly less virulent than parental strains, and complementation studies demonstrated that the addition of marA, soxS and rob individually restored wild-type virulence in the triple-knockout strain. Deletion of soxS or rob alone was more detrimental than the removal of marA. Thus, all three proteins contribute to virulence in vivo.

Rationally designed small compounds inhibit pilus biogenesis in uropathogenic bacteria

A chemical synthesis platform with broad applications and flexibility was rationally designed to inhibit biogenesis of adhesive pili assembled by the chaperone-usher pathway in Gram-negative pathogens. The activity of a family of bicyclic 2-pyridones, termed pilicides, was evaluated in two different pilus biogenesis systems in uropathogenic Escherichia coli. Hemagglutination mediated by either type 1 or P pili, adherence to bladder cells, and biofilm formation mediated by type 1 pili were all reduced by approximately 90% in laboratory and clinical E. coli strains. The structure of the pilicide bound to the P pilus chaperone PapD revealed that the pilicide bound to the surface of the chaperone known to interact with the usher, the outer-membrane assembly platform where pili are assembled. Point mutations in the pilicide-binding site dramatically reduced pilus formation but did not block the ability of PapD to bind subunits and mediate their folding. Surface plasmon resonance experiments confirmed that the pilicide interfered with the binding of chaperone-subunit complexes to the usher. These pilicides thus target key virulence factors in pathogenic bacteria and represent a promising proof of concept for developing drugs that function by targeting virulence factors.

The Rgg regulator of Streptococcus pyogenes influences utilization of nonglucose carbohydrates, prophage induction, and expression of the NAD-glycohydrolase virulence operon

The expression of many virulence-associated genes in Streptococcus pyogenes is controlled in a growth phase-dependent manner. Unlike the model organisms Escherichia coli and Bacillus subtilis, such regulation is apparently not dependent upon alternative sigma factors but appears to rely on complex interactions among several transcriptional regulators, including Rgg. The purpose of this study was to identify changes in gene expression associated with inactivation of the rgg gene in S. pyogenes strain NZ131 (serotype M49). To this end, the transcriptomes of wild-type and rgg mutant strains were analyzed during both the exponential and postexponential phases of growth using Affymetrix NimbleExpress gene chips. Genomewide differences in transcript levels were identified in both phases of growth. Inactivation of rgg disrupted coordinate expression of genes associated with the metabolism of nonglucose carbon sources, such as fructose, mannose, and sucrose. The changes were associated with an inability of the mutant strain to grow using these compounds as the primary carbon source. Bacteriophage transcript levels were also altered in the mutant strain and were associated with decreased induction of at least one prophage. Finally, transcripts encoding virulence factors involved in cytolysin-mediated translocation of NAD-glycohydrolase, including the immunity factor IFS and the cytolysin (streptolysin O [SLO]), were more abundant in the mutant strain, which correlated with the amount of NADase and SLO activities in culture supernatant fluids. The results provide further evidence that Rgg contributes to growth phase-dependent gene regulation in strain NZ131.

The fallacies of hope: will we discover new antibiotics to combat pathogenic bacteria in time?

While newly developed technologies have revolutionized the classical approaches to combating infectious diseases, the difficulties associated with developing novel antimicrobials mean that these technologies have not yet been used to introduce new compounds into the market. The new technologies, including genomics and structural biology, open up exciting possibilities for the discovery of antibiotics. However, a substantial effort to pursue research, and moreover to incorporate the results into the production chain, is required in order to bring new antimicrobials to the final user. In the current scenario of emerging diseases and the rapid spread of antibiotic resistance, an active policy to support these requirements is vital. Otherwise, many valuable programmes may never be fully developed for lack of "interest" and funds (private and public). Will we react in time to avoid potential disaster?

Antibiotics--past, present, and future

The drug therapy for diseases other than those caused by microbial agents involves treating the host. In infectious diseases therapy, the goal is to rid the host of the pathogen. Hence, drug therapy is aimed at the pathogen. Because a second living agent is involved in the triangle, drug therapy is affected by the pathogen's nature, its tissue specificity, and, most importantly, the changes it undergoes to survive. The history of antimicrobial therapy has clearly demonstrated that the drugs that are used to treat infections are also responsible for making them more difficult to treat in future. The only way to keep antimicrobial agents useful is to use them appropriately and judiciously.

The pathogen-associated iroA gene cluster mediates bacterial evasion of lipocalin 2

Numerous bacteria cope with the scarcity of iron in their microenvironment by synthesizing small iron-scavenging molecules known as siderophores. Mammals have evolved countermeasures to block siderophore-mediated iron acquisition as part of their innate immune response. Secreted lipocalin 2 (Lcn2) sequesters the Escherichia coli siderophore enterobactin (Ent), preventing E. coli from acquiring iron and protecting mammals from infection by E. coli. Here, we show that the iroA gene cluster, found in many pathogenic strains of Gram-negative enteric bacteria, including E. coli, Salmonella spp., and Klebsiella pneumoniae, allows bacteria to evade sequestration of Ent by Lcn2. We demonstrate that C-glucosylated derivatives of Ent produced by iroA-encoded enzymes do not bind purified Lcn2, and an iroA-harboring strain of E. coli is insensitive to the growth inhibitory effects of Lcn2 in vitro. Furthermore, we show that mice rapidly succumb to infection by an iroA-harboring strain of E. coli but not its wild-type counterpart, and that this increased virulence depends on evasion of host Lcn2. Our findings indicate that the iroA gene cluster allows bacteria to evade this component of the innate immune system, rejuvenating their Ent-mediated iron-acquisition pathway and playing an important role in their virulence.

A small-molecule inhibitor of type III secretion inhibits different stages of the infectious cycle of Chlamydia trachomatis

The intracellular pathogen Chlamydia trachomatis possesses a type III secretion (TTS) system believed to deliver a series of effector proteins into the inclusion membrane (Inc-proteins) as well as into the host cytosol with perceived consequences for the pathogenicity of this common venereal pathogen. Recently, small molecules were shown to block the TTS system of Yersinia pseudotuberculosis. Here, we show that one of these compounds, INP0400, inhibits intracellular replication and infectivity of C. trachomatis at micromolar concentrations resulting in small inclusion bodies frequently containing only one or a few reticulate bodies (RBs). INP0400, at high concentration, given at the time of infection, partially blocked entry of elementary bodies into host cells. Early treatment inhibited the localization of the mammalian protein 14-3-3beta to the inclusions, indicative of absence of the early induced TTS effector IncG from the inclusion membrane. Treatment with INP0400 during chlamydial mid-cycle prevented secretion of the TTS effector IncA and homotypic vesicular fusions mediated by this protein. INP0400 given during the late phase resulted in the detachment of RBs from the inclusion membrane concomitant with an inhibition of RB to elementary body conversion causing a marked decrease in infectivity.

New perspectives for a new century: implications of pathogen responses for the future of antimicrobial therapy

Although the discovery of new classes of antibiotics has lagged behind in the last three decades, the incidence of life-threatening nosocomial infections that are resistant to multiple antibacterial agents has increased steadily. Recent advances in bacterial pathogenicity through the identification of a number of virulence factors and the bacterial genetics behind it have opened the way to a clearer understanding of the pathogen-host relationship. Bacteria communicate with each other through specific signaling chemicals to act as a community rather than individual cells to achieve a critical density or a "quorum." Establishment of quorum is the initiating signal for turning on a variety of virulence factors essential for the pathogenicity and dissemination of pathogens through the host. Pathogenic bacteria use a variety of biochemical mediators, collectively called "virulence factors," to invade and attack host tissues and to avoid detection and elimination by the host immune system. Delineating the specific responses the host immune system elicits in response to specific virulence factors and quorum-sensing molecules is essential to the development of new diagnostic methods for early detection of an infection and the prognosis to a given antibacterial therapy. Identification of inhibitors of virulence factors will represent new antimicrobial therapeutic modalities, and this can be used synergistically with current antibiotic therapy because they act through independent prokaryotic pathways to inhibit bacterial growth and survival.

Chemical biology and bacteria: not simply a matter of life or death

Chemical biological approaches to understanding bacteria have largely been confined to screening for antibiotics. More complex phenotypes, such as virulence, have largely been studied using bacterial genetics. However, it has recently become clear that these two methods are complementary and that combining chemical biologic and genetic approaches to studying bacteria brings new power to old problems.

Identification of novel antimicrobials using a live-animal infection model

The alarming increase of antibiotic-resistant bacterial pathogens points to the need for novel therapeutic approaches to combat infection. To discover novel antimicrobials, we devised a screen to identify compounds that promoted the survival of the model laboratory nematode Caenorhabditis elegans infected with the human opportunistic pathogen Enterococcus faecalis. E. faecalis colonizes the nematode intestinal tract, forming a persistent lethal infection. Infected nematodes were rescued by antibiotic treatment in a dose-dependent manner, and antibiotic treatment markedly reduced the number of bacteria colonizing the nematode intestine. To facilitate high throughput screening of compound libraries, we adapted a previously developed agar-based C. elegans-E. faecalis infection assay so that it could be carried out in liquid medium in standard 96-well microtiter plates. We used this simple infection system to screen 6,000 synthetic compounds and 1,136 natural product extracts. We identified 16 compounds and 9 extracts that promoted nematode survival. Some of the compounds and extracts inhibited E. faecalis growth in vitro, but, in contrast to traditional antibiotics, the in vivo effective dose of many of these compounds was significantly lower than the minimum inhibitory concentration needed to prevent the growth of E. faecalis in vitro. Moreover, many of the compounds and extracts had little or no affect on in vitro bacterial growth. Our findings indicate that the whole-animal C. elegans screen identifies not only traditional antibiotics, but also compounds that target bacterial virulence or stimulate host defense.