Benzylidene acylhydrazides inhibit chlamydial growth in a type III secretion- and iron chelation-independent manner

Acylpyrazoline-Based Third-Generation Selective Antichlamydial Compounds with Enhanced Potency

Chlamydiae are obligate intracellular Gram-negative bacteria and widespread pathogens in humans and animals. Broad-spectrum antibiotics are currently used to treat chlamydial infections. However, broad-spectrum drugs also kill beneficial bacteria. Recently, two generations of benzal acylhydrazones have been shown to selectively inhibit chlamydiae without toxicity to human cells and lactobacilli, which are dominating, beneficial bacteria in the vagina of reproductive-age women. Here, we report the identification of two acylpyrazoline-based third-generation selective antichlamydials (SACs). With minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) of 10-25 μM against Chlamydia trachomatis and Chlamydia muridarum, these new antichlamydials are 2- to 5-fold more potent over the benzal acylhydrazone-based second-generation selective antichlamydial lead SF3. Both acylpyrazoline-based SACs are well tolerated by Lactobacillus, Escherichia coli, Klebsiella, and Salmonella as well as host cells. These third-generation selective antichlamydials merit further evaluation for therapeutic application.

Inhibition of the futalosine pathway for menaquinone biosynthesis suppresses Chlamydia trachomatis infection

Chlamydia trachomatis, an obligate intracellular bacterium with limited metabolic capabilities, possesses the futalosine pathway for menaquinone biosynthesis. Futalosine pathway enzymes have promise as narrow-spectrum antibiotic targets, but the activity and essentiality of chlamydial menaquinone biosynthesis have yet to be established. In this work, menaquinone-7 (MK-7) was identified as a C. trachomatis-produced quinone through liquid chromatography-tandem mass spectrometry. An immunofluorescence-based assay revealed that treatment of C. trachomatis-infected HeLa cells with the futalosine pathway inhibitor docosahexaenoic acid (DHA) reduced inclusion number, inclusion size, and infectious progeny. Supplementation with MK-7 nanoparticles rescued the effect of DHA on inclusion number, indicating that the futalosine pathway is a target of DHA in this system. These results open the door for menaquinone biosynthesis inhibitors to be pursued in antichlamydial development.

Identification of a GrgA-Euo-HrcA Transcriptional Regulatory Network in Chlamydia

Chlamydia trachomatis is an obligate intracellular bacterium whose unique developmental cycle consists of an infectious elementary body and a replicative reticulate body. Progression of this developmental cycle requires temporal control of the transcriptome. In addition to the three chlamydial sigma factors (σ⁶⁶, σ²⁸, and σ⁵⁴) that recognize promoter sequences of genes, chlamydial transcription factors are expected to play crucial roles in transcriptional regulation. Here, we investigate the function of GrgA, a Chlamydia-specific transcription factor, in C. trachomatis transcriptomic expression. We show that 10 to 30 min of GrgA overexpression induces 13 genes, which likely comprise the direct regulon of GrgA. Significantly, σ⁶⁶-dependent genes that code for two important transcription repressors are components of the direct regulon. One of these repressors is Euo, which prevents the expression of late genes during early phases. The other is HrcA, which regulates molecular chaperone expression and controls stress response. The direct regulon also includes a σ²⁸-dependent gene that codes for the putative virulence factor PmpI. Furthermore, overexpression of GrgA leads to decreased expression of almost all tRNAs. Transcriptomic studies suggest that GrgA, Euo, and HrcA have distinct but overlapping indirect regulons. These findings, together with temporal expression patterns of grgA, euo, and hrcA, indicate that a transcriptional regulatory network of these three transcription factors plays critical roles in C. trachomatis growth and development.

IMPORTANCEChlamydia trachomatis is the most prevalent sexually transmitted bacterial pathogen worldwide and is a leading cause of preventable blindness in underdeveloped areas as well as some developed countries. Chlamydia carries genes that encode a limited number of known transcription factors. While Euo is thought to be critical for early chlamydial development, the functions of GrgA and HrcA in the developmental cycle are unclear. Activation of euo and hrcA immediately following GrgA overexpression indicates that GrgA functions as a master transcriptional regulator. In addition, by broadly inhibiting tRNA expression, GrgA serves as a key regulator of chlamydial protein synthesis. Furthermore, by upregulating pmpI, GrgA may act as an upstream virulence determinant. Finally, genes coregulated by GrgA, Euo, and HrcA likely play critical roles in chlamydial growth and developmental control.

GrgA overexpression inhibits Chlamydia trachomatis growth through sigma66- and sigma28-dependent mechanisms

The obligate intracellular bacterium Chlamydia trachomatis is an important human pathogen with a biphasic developmental cycle comprised of an infectious elementary body (EB) and a replicative reticulate body (RB). Whereas σ⁶⁶, the primary sigma factor, is necessary for transcription of most chlamydial genes throughout the developmental cycle, σ²⁸ is required for expression of some late genes. We previously showed that the Chlamydia-specific transcription factor GrgA physically interacts with both of these sigma factors and activates transcription from σ⁶⁶- and σ²⁸-dependent promoters in vitro. Here, we investigated the organismal functions of GrgA. We show that overexpression of GrgA slows EB-to-RB conversion, decreases RB proliferation, and reduces progeny EB production. In contrast, overexpression of a GrgA variant without the σ²⁸-binding domain shows significantly less severe inhibitory effects, while overexpression of a variant without the σ⁶⁶-binding domain demonstrates no adverse effects. These findings indicate that GrgA plays important roles in the expression regulation of both σ⁶⁶-dependent genes and σ²⁸-dependent genes during the chlamydial developmental cycle.

A 2-pyridone amide inhibitor of transcriptional activity in Chlamydia trachomatis

Chlamydia trachomatis is a strict intracellular bacterium that causes sexually transmitted infections and eye infections that can lead to life-long sequelae. Treatment options are limited to broad-spectrum antibiotics that disturb the commensal flora and contribute to selection of antibiotic-resistant bacteria. Hence, development of novel drugs that specifically target C. trachomatis would be beneficial. 2-pyridone amides are potent and specific inhibitors of Chlamydia infectivity. The first generation compound KSK120, inhibits the developmental cycle of Chlamydia resulting in reduced infectivity of progeny bacteria. Here, we show that the improved, highly potent second-generation 2-pyridone amide KSK213 allowed normal growth and development of C. trachomatis and the effect was only observable upon re-infection of new cells. Progeny elementary bodies (EBs) produced in the presence of KSK213 were unable to activate transcription of essential genes in early development and did not differentiate into the replicative form, the reticulate body (RB). The effect was specific to C. trachomatis since KSK213 was inactive in the closely related animal pathogen C. muridarum and in C. caviae The molecular target of KSK213 may thus be different in C. trachomatis or non-essential in C. muridarum and C. caviae Resistance to KSK213 was mediated by a combination of amino acid substitutions in both DEAD/DEAH RNA helicase and RNAse III, which may indicate inhibition of the transcriptional machinery as the mode of action. 2-pyridone amides provide a novel antibacterial strategy and starting points for development of highly specific drugs for C. trachomatis infections.

Inhibitory Activity of Pyrroloisoxazolidine Derivatives against Chlamydia trachomatis

The obligate intracellular bacterium Chlamydia trachomatis is a group of worldwide human pathogens that can lead to serious reproductive problems. The frequent clinical treatment failure promoted the development of novel antichlamydial agents. Here, we firstly reported a group of pyrroloisoxazolidine-inhibited C. trachomatis in a dose-dependent manner in vitro. Among them, compounds 1 and 2 exhibited the strongest inhibitory activity with IC₅₀ values from 7.25 to 9.73 μM. The compounds disturbed the whole intracellular life cycle of C. trachomatis, mainly targeting the middle reticulate body proliferation stages. Besides, the compounds partially inhibited the chlamydial infection by reducing elementary body infectivity at high concentration. Our findings suggest the potential of pyrroloisoxazolidine derivatives as promising lead molecules for the development of antichlamydial agents.

Synthesis and Assessment of 3-Substituted Phenazines as Novel Antichlamydial Agents

BACKGROUND

In the past century, many phenazines were isolated from the marine microorganism, and some of these phenazines possessed potent antibacterial activities. We found that a few of the synthesized 4-substituted phenazines could block the infectivity of chlamydiae without host cell toxicity.

OBJECTIVE

The aim of this study was to design and synthesize two series of novel 3-substituted phenazines to find novel antichlamydial agents.

METHODS

The 3-substituted phenazines were synthesized via Buchwald-Hartwig cross coupling reaction and Suzuki reaction from 3-bromo-1-methoxyphenazine. The antichlamydial activity of these synthesized compounds was evaluated by determining their effect on the yield of infectious progeny EBs. Cytotoxicity of these compounds on host cells was assessed by the treatment of uninfected HeLa cells using WST-1 method.

RESULTS

Most of the 3-substituted phenazines possessed potent antichlamydial activity with IC50 values from 0.15 to 12.08 μM against Chlamydia trachomatis L2, C. muridarum MoPn and C. pneumoniae AR39. Among them, 7d and 9a exhibited better antichlamydial activity with IC50 values from 0.20 to 1.01 μM while they have no apparent cytotoxicity to host cells. Biological assay disclosed that both 7d and 9a inhibited chlamydial infection by reducing elementary body infectivity and disturbing chlamydial growth during the whole chlamydial developmental cycle.

CONCLUSION

Our findings suggested that 3-substituted phenazine derivatives might be a promising class of therapeutic agents for chlamydial infections. More effective phenazines with low toxicity could be acquired through further chemical modification on C-3 position rather than C-4 position of phenazine.

Optimal cultivation of Chlamydia requires testing of serum on individual species

OBJECTIVE

This report is a side product of experiments aimed at identifying serum for culturing obligate intracellular bacteria Chlamydia trachomatis and C. muridarum in mouse fibroblast L929 cells.

RESULTS

Of five commercial serum samples tested, two showed optimal efficiencies at supporting growth of the human pathogen Chlamydia trachomatis as control fetal bovine serum, whereas two showed modest ~ 40% inhibitions in progeny production, and the remaining one showed a 20% inhibition. Three of the six sera poorly supported growth of the murine pathogen Chlamydia muridarum, resulting in 73-90% reduction in progeny formation. Most significantly, the one with the strongest (90%) C. muridarum inhibition activity showed optimal C. trachomatis-supporting efficiency. These findings indicate that in laboratories that study multiple Chlamydia species, serum samples should be prescreened on a species basis. Considering Chlamydial biology and epidemiology, it may even be necessary to perform serum tests on a serovar- or strain-basis for studying some animal chlamydiae.

Nonspecific toxicities of Streptococcus pyogenes and Staphylococcus aureus dCas9 in Chlamydia trachomatis

Chlamydiae are common, important pathogens for humans and animals alike. Despite recent advancement in genetics, scientists are still searching for efficient tools to knock out or knock down the expression of chromosomal genes. We attempted to adopt a dCas9-based CRISPR interference (CRISPRi) technology to conditionally knock down gene expression in Chlamydia trachomatis using an anhydrotetracycline (ATC)-inducible expression system. Surprisingly, expression of the commonly used Streptococcus pyogenes dCas9 in C. trachomatis causes strong inhibition in the absence of any guide RNA (gRNA). Staphylococcus aureus dCas9 also shows strong toxicity in the presence of only an empty gRNA scaffold. Toxicity of the S. pyogenes dCas9 is readily observed with as little as 0.2 nM ATC. Growth inhibition by S. aureus dCas9 is evident starting at 1.0 nM ATC. In contrast, C. trachomatis growth was not affected by methionine-tRNA ligase overexpression induced with 10 nM ATC. We conclude that S. pyogenes and S. aureus dCas9 proteins in their current forms have limited utility for chlamydial research and suggest strategies to overcome this problem.

GrgA as a potential target of selective antichlamydials

Chlamydia is a common pathogen that can causes serious complications in the reproductive system and eyes. Lack of vaccine and other effective prophylactic measures coupled with the largely asymptomatic nature and unrare clinical treatment failure calls for development of new antichlamydials, particularly selective antichlamydials without adverse effects on humans and the beneficial microbiota. We previously reported that benzal-N-acylhydrazones (BAH) can inhibit chlamydiae without detectable adverse effects on host cells and beneficial lactobacilli that dominate the human vaginal microbiota among reproductive-age women. However, the antichlamydial mechanism of BAH is not known. Whereas 4 single nucleotide polymorphisms (i.e., SNP1-4) were identified in a rare Chlamydia variant with a low level of BAH resistance, termed MCR, previous studies failed to establish a causal effect of any particular SNP(s). In the present work, we performed recombination to segregate the four SNPs. Susceptibility tests indicate that the R51G GrgA allele is both necessary and sufficient for the low level of BAH resistance. Thus, the Chlamydia-specific transcription factor GrgA either is a direct target of BAH or regulates BAH susceptibility. We further confirm an extremely low rate of BAH resistance in Chlamydia. Our findings warrant exploration of GrgA as a therapeutic and prophylactic target for chlamydial infections.

Red Fluorescent Chlamydia trachomatis Applied to Live Cell Imaging and Screening for Antibacterial Agents

In this study, we describe the application of a transformed Chlamydia trachomatis strain constitutively expressing the red fluorescent protein mCherry, to allow real-time monitoring of the infection cycle and screening for agents that block replication of C. trachomatis. The red fluorescent C. trachomatis strain was detected autonomously without antibody staining and was equally susceptible to doxycycline as the wild type strain. A high-throughput screening assay was developed using the transformed strain and automated fluorescence microscopy. The assay was used in a pilot screen of a 349 compound library containing natural products from Australian flora and fauna. Compounds with anti-chlamydial activity were tested for dose response and toxicity to host cells and two non-toxic compounds had 50% effective concentration (EC₅₀) values in the low micromolar range. Natural products are valuable sources for drug discovery and the identified Chlamydia growth inhibition may be starting points for future drug development. Live cell imaging was used to visualize growth of the red fluorescent C. trachomatis strain over time. The screening assay reduced workload and reagents compared to an assay requiring immunostaining and could further be used to monitor the development of Chlamydia inclusions and anti-chlamydial effect in real time.

Synthesis and Antichlamydial Activity of Novel Phenazines

Results:

C-4 morpholinyl 8a and C-4 phenyl phenazine 9c exhibited stronger antichlamydial activity with no apparent cytotoxicity. Both phenazine derivatives inhibited chlamydial inclusions formation and growth in a dose-dependent manner. They inhibited Chlamydia infection by reducing elementary body infectivity and disturbing Chlamydia growth at the mid-stage of the chlamydial developmental cycle.

Conclusion:

Our findings suggest C-4 aryl and C-4 amino phenazine derivatives as promising lead molecules for antichlamydials development.

Pyocyanin Inhibits Chlamydia Infection by Disabling Infectivity of the Elementary Body and Disrupting Intracellular Growth

The obligate intracellular bacterium Chlamydia is a widespread human pathogen that causes serious problems, including (but not limited to) infertility and blindness. Our search for novel antichlamydial metabolites from marine-derived microorganisms led to the isolation of pyocyanin, a small compound from Pseudomonas aeruginosa Pyocyanin is an effective antichlamydial for all three Chlamydia spp. tested. It has a 50% inhibitory concentration (IC50) of 0.019 to 0.028 μM, which is comparable to the IC50 of tetracycline. At concentrations as low as 0.0039 μM, pyocyanin disables infectivity of the chlamydial elementary body (EB). At 0.5 μM or higher concentrations, the continuous presence of pyocyanin also inhibits chlamydial growth in the inclusion during later stages of the developmental cycle. Oxidative stress, a major known antimicrobial mechanism of pyocyanin, appears to be responsible only for the inhibition of bacterial growth and not for the disinfection of EBs. Pyocyanin is well-tolerated by probiotic vaginal Lactobacillus spp. Our findings suggest that pyocyanin is of therapeutic value for chlamydial infections and can serve as a valuable chemical probe for studying chlamydial biology.

In vitro Antichlamydial Activity of 1,2,3,5-Tetrasubstituted Pyrrole Derivatives

BACKGROUND

Chlamydia is a group of bacterial pathogens distributed worldwide that can lead to serious reproductive and other health problems. The rise of antibiotic-resistant pathogens promotes the development of novel antichlamydial agents. The aim of this study is to assess in vitro antichlamydial activity of our previously synthesized 1,2,3,5- tetrasubstituted pyrroles.

METHODS

The derivatives were screened for their antichlamydial activity against three Chlamydia strains by calculating IC50 values using concentration-response inhibition data between 1 and 32 μM. The action of the compounds on Chlamydia elementary body (EB) infectivity and the impact of the chemicals' administration time on their antichlamydial effect were evaluated to reveal the inhibitory mechanism.

RESULTS

Some of the compounds moderately inhibited the Chlamydia strains. Compound 10 exhibited the strongest inhibitory activity, with IC50 values from 4.34 to 5.83 μM. These pyrrole derivatives inhibited Chlamydia infection by reducing EB infectivity during the early stage and disturbing Chlamydia growth by targeting the early-to-middle stage prior to 12 h of the chlamydial life cycle.

CONCLUSION

Our findings highlight the potential of 1,2,3,5-tetrasubstituted pyrrole derivatives as promising lead molecules for the development of antichlamydial agents.

Identification of a strong and specific antichlamydial N-acylhydrazone

Sexually transmitted Chlamydia trachomatis is an extremely common infection and often leads to serious complications including infertility and pelvic inflammatory syndrome. Several broad-spectrum antibiotics are currently used to treat C. trachomatis. Although effective, they also kill beneficial vaginal lactobacilli. Two N-acylhydrazones, CF0001 and CF0002, have been shown previously to inhibit chlamydial growth without toxicity to human cells and Lactobacillus spp. Of particular significance, the rate of random mutation leading to resistance of these inhibitors appears to be extremely low. Here, we report three analogs of CF0001 and CF0002 with significantly stronger inhibitory effects on chlamydiae. Even though the new compounds (termed SF1, SF2 and SF3) displayed slightly decreased inhibition efficiencies for a rare Chlamydia variant selected for CF0001 resistance (Chlamydia muridarum MCR), they completely overcame the resistance when used at concentrations of 75–100 μM. Importantly, SF1, SF2 and SF3 did not shown any toxic effect on lactobacilli, whereas SF3 was also well tolerated by human host cells. An effort to isolate SF3-resistant variants was unsuccessful. By comparison, variants resistant to rifampin or spectinomycin were obtained from smaller numbers of chlamydiae. Our findings suggest that SF3 utilizes an antichlamydial mechanism similar to that of CF0001 and CF0002, and will be more difficult for chlamydiae to develop resistance to, potentially making it a more effective antichlamydial agent.

Antichlamydial Dimeric Indole Derivatives from Marine Actinomycete Rubrobacter radiotolerans

Chlamydiae are widely distributed pathogens of human populations, which can lead to serious reproductive and other health problems. In our search for novel antichlamydial metabolites from marine derived-microorganisms, one new (1) and two known (2, 3) dimeric indole derivatives were isolated from the sponge-derived actinomycete Rubrobacter radiotolerans. The chemical structures of these metabolites were elucidated by NMR spectroscopic data as well as CD calculations. All three metabolites suppressed chlamydial growth in a concentration-dependent manner. Among them, compound 1 exhibited the most effective antichlamydial activity with IC₅₀ values of 46.6 ~ 96.4 µM in the production of infectious progeny. Compounds appeared to target the mid-stage of the chlamydial developmental cycle by interfering with reticular body replication, but not directly inactivating the infectious elementary body.

Chlamydial Antibiotic Resistance and Treatment Failure in Veterinary and Human Medicine

The Chlamydiaceae are widespread pathogens of both humans and animals. Chlamydia trachomatis infection causes blinding trachoma and reproductive complications in humans. Chlamydia pneumoniae causes human respiratory tract infections and atypical pneumonia. Chlamydia suis infection is associated with conjunctivitis, diarrhea, and failure to gain weight in domestic swine. Chlamydial infections in humans and domesticated animals are generally controlled by antibiotic treatment—particularly macrolides (usually azithromycin) and tetracyclines (tetracycline and doxycycline). Tetracycline-containing feed has also been used to limit infections and promote growth in livestock populations, although its use has decreased because of growing concerns about antimicrobial resistance development. Because Sandoz and Rockey published an elegant review of chlamydial anti-microbial resistance in 2010, we will review the following: (i) antibiotic resistance in C. suis, (ii) recent evidence for acquired resistance in human chlamydial infections, and (iii) recent non-genetic mechanisms of antibiotic resistance that may contribute to treatment failure.

Chlamydia trachomatis In Vivo to In Vitro Transition Reveals Mechanisms of Phase Variation and Down-Regulation of Virulence Factors

Research on the obligate intracellular bacterium Chlamydia trachomatis demands culture in cell-lines, but the adaptive process behind the in vivo to in vitro transition is not understood. We assessed the genomic and transcriptomic dynamics underlying C. trachomatis in vitro adaptation of strains representing the three disease groups (ocular, epithelial-genital and lymphogranuloma venereum) propagated in epithelial cells over multiple passages. We found genetic features potentially underlying phase variation mechanisms mediating the regulation of a lipid A biosynthesis enzyme (CT533/LpxC), and the functionality of the cytotoxin (CT166) through an ON/OFF mechanism. We detected inactivating mutations in CT713/porB, a scenario suggesting metabolic adaptation to the available carbon source. CT135 was inactivated in a tropism-specific manner, with CT135-negative clones emerging for all epithelial-genital populations (but not for LGV and ocular populations) and rapidly increasing in frequency (~23% mutants per 10 passages). RNA-sequencing analyses revealed that a deletion event involving CT135 impacted the expression of multiple virulence factors, namely effectors known to play a role in the C. trachomatis host-cell invasion or subversion (e.g., CT456/Tarp, CT694, CT875/TepP and CT868/ChlaDub1). This reflects a scenario of attenuation of C. trachomatis virulence in vitro, which may take place independently or in a cumulative fashion with the also observed down-regulation of plasmid-related virulence factors. This issue may be relevant on behalf of the recent advances in Chlamydia mutagenesis and transformation where culture propagation for selecting mutants/transformants is mandatory. Finally, there was an increase in the growth rate for all strains, reflecting gradual fitness enhancement over time. In general, these data shed light on the adaptive process underlying the C. trachomatis in vivo to in vitro transition, and indicates that it would be prudent to restrict culture propagation to minimal passages and check the status of the CT135 genotype in order to avoid the selection of CT135-negative mutants, likely originating less virulent strains.

Deep comparative genomics among Chlamydia trachomatis lymphogranuloma venereum isolates highlights genes potentially involved in pathoadaptation

Lymphogranuloma venereum (LGV) is a human sexually transmitted disease caused by the obligate intracellular bacterium Chlamydia trachomatis (serovars L1-L3). LGV clinical manifestations range from severe ulcerative proctitis (anorectal syndrome), primarily caused by the epidemic L2b strains, to painful inguinal lymphadenopathy (the typical LGV bubonic form). Besides potential host-related factors, the differential disease severity and tissue tropism among LGV strains is likely a function of the genetic backbone of the strains. We aimed to characterize the genetic variability among LGV strains as strain- or serovar-specific mutations may underlie phenotypic signatures, and to investigate the mutational events that occurred throughout the pathoadaptation of the epidemic L2b lineage. By analyzing 20 previously published genomes from L1, L2, L2b and L3 strains and two new genomes from L2b strains, we detected 1497 variant sites and about 100 indels, affecting 453 genes and 144 intergenic regions, with 34 genes displaying a clear overrepresentation of nonsynonymous mutations. Effectors and/or type III secretion substrates (almost all of those described in the literature) and inclusion membrane proteins showed amino acid changes that were about fivefold more frequent than silent changes. More than 120 variant sites occurred in plasmid-regulated virulence genes, and 66% yielded amino acid changes. The identified serovar-specific variant sites revealed that the L2b-specific mutations are likely associated with higher fitness and pointed out potential targets for future highly discriminatory diagnostic/typing tests. By evaluating the evolutionary pathway beyond the L2b clonal radiation, we observed that 90.2% of the intra-L2b variant sites occurring in coding regions involve nonsynonymous mutations, where CT456/tarp has been the main target. Considering the progress on C. trachomatis genetic manipulation, this study may constitute an important contribution for prioritizing study targets for functional genomics aiming to dissect the impact of the identified intra-LGV polymorphisms on virulence or tropism dissimilarities among LGV strains.