Antibacterial Activity of a Natural Clay Mineral against Burkholderia cepacia Complex and Other Bacterial Pathogens Isolated from People with Cystic Fibrosis.

There is an impending crisis in healthcare brought about by a new era of untreatable infections caused by bacteria resistant to all available antibiotics. Thus, there is an urgent need to identify novel antimicrobial agents to counter the continuing threat posed by formerly treatable infections. We previously reported that a natural mineral clay known as Kisameet clay (KC) is a potent inhibitor of the organisms responsible for acute infections. Chronic bacterial infections present another major challenge to treatment by antimicrobials, due to their prolonged nature, which results in repeated exposure to antibiotics and a constant selection for antimicrobial resistance. A prime example is bacteria belonging to the Burkholderia cepacia complex (Bcc), which particularly causes some of the most serious chronic lung infections in patients with cystic fibrosis (CF) associated with unpredictable clinical outcomes, poor prognosis, and high mortality rates. Eradication of these organisms from CF patients with limited effective antimicrobial options is a major challenge. Novel therapeutic approaches are urgently required. Here, we report the in vitro antibacterial activity of KC aqueous suspensions (1-10% w/v) and its aqueous extract (L100) against a collection of extensively and multi-drug resistant clinical isolates of Bcc, Pseudomonas aeruginosa, and Stenotrophomonas maltophilia isolated from patients with CF. These findings present a potential novel therapy for further investigation in the clinic.

Broad-Spectrum Antimicrobial and Antibiofilm Activity of a Natural Clay Mineral from British Columbia, Canada.

Worldwide increases in antibiotic resistance and the dearth of new antibiotics have created a global crisis in the treatment of infectious diseases. These concerns highlight the pressing need for novel antimicrobial agents. Natural clay minerals have a long history of therapeutic and biomedical applications and have lately received specific attention for their potent antimicrobial properties. In particular, Kisameet clay (KC) has strong antibacterial activity against a variety of multidrug-resistant (MDR) bacterial pathogens in vitro Here, we have extended the known spectrum of activity of KC by demonstrating its efficacy against two major fungal pathogens, Candida albicans and Cryptococcus neoformans In addition, KC also exhibits potent activity against the opportunistic bacterial pathogen Mycobacterium marinum, a model organism for M. ulcerans infection. Moreover, aqueous KC leachates (KC-L) exhibited broad-spectrum antibacterial activity, eradicated Gram-negative and Gram-positive biofilms, and prevented their formation. The mechanism(s) underlying KC antibacterial activity appears to be complex. Adjusting KC-L to neutral pH rendered it inactive, indicating a contribution of pH, although low pH alone was insufficient for its antibacterial activity. Treatment of KC minerals with cation-chelating agents such as EDTA, 2,2'-bipyridyl, and deferoxamine reduced the antibacterial activity, while supplementation of KC-L with these chelating agents eliminated the inhibitory activity. Together, the data suggest a positive role for divalent and trivalent cations, including iron and aluminum, in bacterial inhibition by KC. Collectively, these studies demonstrate the range of KC bioactivity and provide a better understanding of the mechanism underlying its antibacterial effects.IMPORTANCE The escalating emergence of multidrug-resistant (MDR) bacteria, together with the paucity of novel antimicrobial agents in antibiotic development, is recognized as a worldwide public health crisis. Kisameet clay (KC), found in British Columbia (BC), Canada, is a clay mineral with a long history of therapeutic applications among people of the First Nations. We previously reported the antibacterial activity of KC against a group of MDR clinical pathogens. Here, we demonstrate its activity against two major human-pathogenic fungal species, as well as against bacterial biofilms, which underlie many recalcitrant bacterial infections. In these studies, we also identified several geochemical characteristics of KC, such as metal ions and low pH, which are involved in its antibacterial activity. These findings provide a better understanding of the components of KC antibacterial activity and a basis for developing defined preparations of this clay mineral for therapeutic applications.

An ancient solution to a modern problem.

The dearth of new antibiotics and escalating emergence of multidrug resistant bacteria have created a global healthcare crisis and highlight the drastic need for novel antimicrobial agents. Complementary and alternative strategies including the investigation of ancient medicinals could address this problem. Natural clay minerals with a long history of medicinal and biomedical applications have become an interest due to their broad-spectrum antimicrobial activity. Such untapped natural sources may provide new therapeutic agents in the battle against infectious diseases in the post-antibiotic era.

Repression of Salmonella Host Cell Invasion by Aromatic Small Molecules from the Human Fecal Metabolome.

The human microbiome is a collection of microorganisms that inhabit every surface of the body that is exposed to the environment, generally coexisting peacefully with their host. These microbes have important functions, such as producing vitamins, aiding in maturation of the immune system, and protecting against pathogens. We have previously shown that a small-molecule extract from the human fecal microbiome has a strong repressive effect on Salmonella enterica serovar Typhimurium host cell invasion by modulating the expression of genes involved in this process. Here, we describe the characterization of this biological activity. Using a series of purification methods, we obtained fractions with biological activity and characterized them by mass spectrometry. These experiments revealed an abundance of aromatic compounds in the bioactive fraction. Selected compounds were obtained from commercial sources and tested with respect to their ability to repress the expression of hilA, the gene encoding the master regulator of invasion genes in Salmonella We found that the aromatic compound 3,4-dimethylbenzoic acid acts as a strong inhibitor of hilA expression and of invasion of cultured host cells by Salmonella Future studies should reveal the molecular details of this phenomenon, such as the signaling cascades involved in sensing this bioactive molecule.IMPORTANCE Microbes constantly sense and adapt to their environment. Often, this is achieved through the production and sensing of small extracellular molecules. The human body is colonized by complex communities of microbes, and, given their biological and chemical diversity, these ecosystems represent a platform where the production and sensing of molecules occur. In previous work, we showed that small molecules produced by microbes from the human gut can significantly impair the virulence of the enteric pathogen Salmonella enterica Here, we describe a specific compound from the human gut that produces this same effect. The results from this work not only shed light on an important biological phenomenon occurring in our bodies but also may represent an opportunity to develop drugs that can target these small-molecule interactions to protect us from enteric infections and other diseases.

Leupeptazin, a highly modified tripeptide isolated from cultures of a Streptomyces sp. inhibits cathepsin K.

Using a human cathepsin K-targeting inhibitor screen, a new leupeptin analogue, leupeptazin (1), containing an unprecedented piperidinotriazine moiety, was isolated from a liquid culture of soil Streptomyces sp. IS2-4 collected in northern Italy. The structure of leupeptazin was established using HRESIMS as well as 1D and 2D NMR data. The inhibitory activity of the compound towards the collagenase cathepsin K was tested in vitro to reveal moderate activity with an inhibition constant, Ki, of 44µM.

Identification of synergists that potentiate the action of polymyxin B against Burkholderia cenocepacia.

Burkholderia cenocepacia and other members of the Burkholderia cepacia complex (BCC) are highly multidrug-resistant bacteria that cause severe pulmonary infections in patients with cystic fibrosis. A screen of 2686 compounds derived from marine organisms identified molecules that could synergise with polymyxin B (PMB) to inhibit the growth of B. cenocepacia. At 1 µg/mL, five compounds synergised with PMB and inhibited the growth of B. cenocepacia by ≥70% compared with growth in PMB alone. Follow-up testing revealed that one compound from the screen, the aminocoumarin antibiotic novobiocin, synergised with PMB and colistin against tobramycin-resistant clinical isolates of B. cenocepacia and Burkholderia multivorans. In parallel, we show that novobiocin sensitivity is common among BCC species and that these bacteria are even more susceptible to an alternative aminocoumarin, clorobiocin, which also had an additive effect with PMB against B. cenocepacia. These studies support using aminocoumarin antibiotics to treat BCC infections and show that synergisers can be found to increase the efficacy of antimicrobial peptides and polymyxins against BCC bacteria.

Discovery, isolation, and structure elucidation of dretamycin.

The Candida albicans fitness test is a whole cell screening platform that utilizes a mixed-pool of C. albicans mutants, each of which carries a heterozygous deletion of a particular gene. In the presence of an antifungal inhibitor, a subset of these mutants exhibits a growth phenotype of hypersensitivity or hyposensitivity. Collectively these mutants reflect aspects of the mechanism of action of the compound in question. In the course of screening natural products a culture of Streptomyces sp. MS-1-4 was discovered to produce a compound, dretamycin, which yielded a fitness profile exhibiting significant hypersensitivity of the DRE2 heterozygote and hyposensitivity of the DIP5 heterozygote. Herein we report the production, isolation, and structure elucidation of dretamycin.

Separate mechanisms are involved in rifampicin upmodulated and downmodulated gene expression in Salmonella Typhimurium.

Sub-MIC antibiotics differentially modulate transcription of subsets of genes by unknown mechanisms. Paradoxically, the RNA polymerase inhibitor rifampicin is able to both upmodulate as well as downmodulate transcription when present at sub-MIC levels. In this study, we analyzed DNA sequences required for transcription modulation. For three downmodulated promoters, the necessary sequences were within those contacted by the RNA polymerase during transcription initiation. Thus hypersensitivity is a characteristic of the RNA polymerase promoter complexes. The sequences needed for upmodulation included both upstream and downstream sequences in one case, only upstream sequences for another promoter and only downstream sequences for the third. Thus, there appear to be multiple mechanisms of transcription modulation by rifampicin.

Chemical signaling in the gastrointestinal tract.

Chemical signaling via the production of small molecules such as hormones has been studied in detail in higher organisms. These molecules have important functions in maintaining physiological homeostasis as well as allowing organisms to respond to external insults. Virtually every living cell produces hormone-like diffusible small molecules that can be used to convey messages to neighboring cells-a vital step in adaptation, development, and survival within populations. Although most of our knowledge on cellular chemical communication comes from studies of multicellular eukaryotes, it is now understood that bacteria can also communicate using sophisticated signaling systems, in a way analogous to those used by higher organisms. Many of these microbes live in close association with higher eukaryotes, in mutualistic or commensal relationships. We suggest that there may be a wealth of unidentified bioactive small molecules in the human body, originating from both microbial and human cells and that have important biological functions. Because chemical signaling has important roles for the biology of both microbes and humans, detecting, identifying, and studying these chemical signals can further our understanding of the chemical interplay between microbiota and their hosts and provide us with an unexplored source of molecules that could be used for human benefit.

Confronting the challenges of natural product-based antifungal discovery.

Starting with the discovery of penicillin, the pharmaceutical industry has relied extensively on natural products (NPs) as an unparalleled source of bioactive small molecules suitable for antibiotic development. However, the discovery of structurally novel and chemically tractable NPs with suitable pharmacological properties as antibiotic leads has waned in recent decades. Today, the repetitive "rediscovery" of previously known NP classes with limited antibiotic lead potential dominates most industrial efforts. This limited productivity, exacerbated by the significant financial and resource requirements of such activities, has led to a broad de-emphasis of NP research by most pharmaceutical companies, including most recently Merck. Here we review our strategies--both technological and philosophical--in addressing current antifungal discovery bottlenecks in target identification and validation and how such efforts may improve NP-based antimicrobial discoveries when aligned with NP screening and dereplication.

Modulation of Salmonella gene expression by subinhibitory concentrations of quinolones.

Approximately 2.7% of a collection of Salmonella enterica var. Typhimurium promoter-lux reporter strains showed altered transcriptional patterns when exposed to low concentrations of nine different fluoroquinolones (FQs). Even at the subinhibitory concentrations employed, all nine FQs upregulated genes involved in the SOS response, umuD, lexA, sbmC and dinP. In addition, transcriptional regulators, genes putatively associated with membrane integrity (spr), virulence (sicA) and metabolism (plsB) were affected. Using the Ames test with Salmonella strain TA102, increased mutagenicity was demonstrated in response to all the FQs tested: ciprofloxacin, moxifloxacin, levofloxacin and gatifloxacin. Transcriptional effects were largely specific to the FQ antimicrobials. Such responses are consistent with the primary mechanism of action of this class of inhibitor, namely, the introduction of DNA damage. This work provides support for the notion that small molecules can have functions other than growth inhibition that may affect the establishment and maintenance of community dynamics in complex environments.

Occurrence of highly fluoroquinolone-resistant and methicillin-resistant Staphylococcus aureus in domestic animals.

We describe phenotypic and genotypic analyses carried out on multidrug-resistant Staphylococcus aureus isolated from domestic animals. The sequence type ST239 methicillin-resistant Staphylococcus aureus isolated from dogs were highly resistant to fluoroquinolones, and new combinations of GyrA and GrlA mutations were identified. These findings are consistent with a role for animal carriage in the dissemination of important human pathogens in the community.

A gene cluster encoding cholesterol catabolism in a soil actinomycete provides insight into Mycobacterium tuberculosis survival in macrophages.

Rhodococcus sp. strain RHA1, a soil bacterium related to Mycobacterium tuberculosis, degrades an exceptionally broad range of organic compounds. Transcriptomic analysis of cholesterol-grown RHA1 revealed a catabolic pathway predicted to proceed via 4-androstene-3,17-dione and 3,4-dihydroxy-9,10-seconandrost-1,3,5(10)-triene-9,17-dione (3,4-DHSA). Inactivation of each of the hsaC, supAB, and mce4 genes in RHA1 substantiated their roles in cholesterol catabolism. Moreover, the hsaC(-) mutant accumulated 3,4-DHSA, indicating that HsaC(RHA1), formerly annotated as a biphenyl-degrading dioxygenase, catalyzes the oxygenolytic cleavage of steroid ring A. Bioinformatic analyses revealed that 51 rhodococcal genes specifically expressed during growth on cholesterol, including all predicted to specify the catabolism of rings A and B, are conserved within an 82-gene cluster in M. tuberculosis H37Rv and Mycobacterium bovis bacillus Calmette-Guérin. M. bovis bacillus Calmette-Guérin grew on cholesterol, and hsaC and kshA were up-regulated under these conditions. Heterologously produced HsaC(H37Rv) and HsaD(H37Rv) transformed 3,4-DHSA and its ring-cleaved product, respectively, with apparent specificities approximately 40-fold higher than for the corresponding biphenyl metabolites. Overall, we annotated 28 RHA1 genes and proposed physiological roles for a similar number of mycobacterial genes. During survival of M. tuberculosis in the macrophage, these genes are specifically expressed, and many appear to be essential. We have delineated a complete suite of genes necessary for microbial steroid degradation, and pathogenic mycobacteria have been shown to catabolize cholesterol. The results suggest that cholesterol metabolism is central to M. tuberculosis's unusual ability to survive in macrophages and provide insights into potential targets for novel therapeutics.

Transcriptomic analysis reveals a bifurcated terephthalate degradation pathway in Rhodococcus sp. strain RHA1.

Phthalate isomers and their esters are important pollutants whose biodegradation is not well understood. Rhodococcus sp. strain RHA1 is notable for its ability to degrade a wide range of aromatic compounds. RHA1 was previously shown to degrade phthalate (PTH) and to have genes putatively encoding terephthalate (TPA) degradation. Transcriptomic analysis of 8,213 genes indicated that 150 were up-regulated during growth on PTH and that 521 were up-regulated during growth on TPA. Distinct ring cleavage dioxygenase systems were differentially expressed during growth on PTH and TPA. Genes encoding the protocatechuate (PCA) pathway were induced on both substrates, while genes encoding the catechol branch of the PCA pathway were additionally induced only on TPA. Accordingly, protocatechuate-3,4-dioxygenase activity was induced in cells grown on both substrates, while catechol-1,2-dioxygenase activity was induced only in cells grown on TPA. Knockout analysis indicated that pcaL, encoding 3-oxoadipate enol-lactone hydrolase and 4-carboxymuconolactone decarboxylase, was required for growth on both substrates but that pcaB, encoding beta-carboxy-cis,cis-muconate lactonizing enzyme, was required for growth on PTH only. These results indicate that PTH is degraded solely via the PCA pathway, whereas TPA is degraded via a bifurcated pathway that additionally includes the catechol branch of the PCA pathway.

The complete genome of Rhodococcus sp. RHA1 provides insights into a catabolic powerhouse.

Rhodococcus sp. RHA1 (RHA1) is a potent polychlorinated biphenyl-degrading soil actinomycete that catabolizes a wide range of compounds and represents a genus of considerable industrial interest. RHA1 has one of the largest bacterial genomes sequenced to date, comprising 9,702,737 bp (67% G+C) arranged in a linear chromosome and three linear plasmids. A targeted insertion methodology was developed to determine the telomeric sequences. RHA1's 9,145 predicted protein-encoding genes are exceptionally rich in oxygenases (203) and ligases (192). Many of the oxygenases occur in the numerous pathways predicted to degrade aromatic compounds (30) or steroids (4). RHA1 also contains 24 nonribosomal peptide synthase genes, six of which exceed 25 kbp, and seven polyketide synthase genes, providing evidence that rhodococci harbor an extensive secondary metabolism. Among sequenced genomes, RHA1 is most similar to those of nocardial and mycobacterial strains. The genome contains few recent gene duplications. Moreover, three different analyses indicate that RHA1 has acquired fewer genes by recent horizontal transfer than most bacteria characterized to date and far fewer than Burkholderia xenovorans LB400, whose genome size and catabolic versatility rival those of RHA1. RHA1 and LB400 thus appear to demonstrate that ecologically similar bacteria can evolve large genomes by different means. Overall, RHA1 appears to have evolved to simultaneously catabolize a diverse range of plant-derived compounds in an O(2)-rich environment. In addition to establishing RHA1 as an important model for studying actinomycete physiology, this study provides critical insights that facilitate the exploitation of these industrially important microorganisms.

Transcriptomic assessment of isozymes in the biphenyl pathway of Rhodococcus sp. strain RHA1.

Rhodococcus sp. RHA1 grows on a broad range of aromatic compounds and vigorously degrades polychlorinated biphenyls (PCBs). Previous work identified RHA1 genes encoding multiple isozymes for most of the seven steps of the biphenyl (BPH) pathway, provided evidence for coexpression of some of these isozymes, and indicated the involvement of some of these enzymes in the degradation of BPH, ethylbenzene (ETB), and PCBs. To investigate the expression of these isozymes and better understand how they contribute to the robust degradative capacity of RHA1, we comprehensively analyzed the 9.7-Mb genome of RHA1 for BPH pathway genes and characterized the transcriptome of RHA1 growing on benzoate (BEN), BPH, and ETB. Sequence analyses revealed 54 potential BPH pathway genes, including 28 not previously reported. Transcriptomic analysis with a DNA microarray containing 70-mer probes for 8,213 RHA1 genes revealed a suite of 320 genes of diverse functions that were upregulated during growth both on BPH and on ETB, relative to growth on the control substrate, pyruvate. By contrast, only 65 genes were upregulated during growth on BEN. Quantitative PCR assays confirmed microarray results for selected genes and indicated that some of the catabolic genes were upregulated over 10,000-fold. Our analysis suggests that up to 22 enzymes, including 8 newly identified ones, may function in the BPH pathway of RHA1. The relative expression levels of catabolic genes did not differ for BPH and ETB, suggesting a common regulatory mechanism. This study delineated a suite of catabolic enzymes for biphenyl and alkyl-benzenes in RHA1, which is larger than previously recognized and which may serve as a model for catabolism in other environmentally important bacteria having large genomes.

Phenylacetate catabolism in Rhodococcus sp. strain RHA1: a central pathway for degradation of aromatic compounds.

In gram-negative bacteria, a pathway for aerobic degradation of phenylacetic acid (PAA) that proceeds via phenylacetyl-coenzyme A (CoA) and hydrolytic ring fission plays a central role in the degradation of a range of aromatic compounds. In contrast, the PAA pathway and its role are not well characterized in gram-positive bacteria. A cluster including 13 paa genes encoding enzymes orthologous to those of gram-negative bacteria was identified on the chromosome of Rhodococcus sp. strain RHA1. These genes were transcribed during growth on PAA, with 11 of the genes apparently in an operon yielding a single transcript. Quantitative proteomic analyses revealed that at least 146 proteins were more than twice as abundant in PAA-grown cells of RHA1 than in pyruvate-grown cells. Of these proteins, 29 were identified, including 8 encoded by the paa genes. Knockout mutagenesis indicated that paaN, encoding a putative ring-opening enzyme, was essential for growth on PAA. However, paaF, encoding phenylacetyl-CoA ligase, and paaR, encoding a putative regulator, were not essential. paaN was also essential for growth of RHA1 on phenylacetaldehyde, phenylpyruvate, 4-phenylbutyrate, 2-phenylethanol, 2-phenylethylamine, and l-phenylalanine. In contrast, growth on 3-hydroxyphenylacetate, ethylbenzene, and styrene was unaffected. These results suggest that the range of substrates degraded via the PAA pathway in RHA1 is somewhat limited relative to the range in previously characterized gram-negative bacteria.