Systematic Review of In Vitro Antimicrobial Susceptibility Testing for Bacillus anthracis, 1947-2019.

Bacillus anthracis, the causative agent of anthrax, is a high-consequence bacterial pathogen that occurs naturally in many parts of the world and is considered an agent of biowarfare or bioterrorism. Understanding antimicrobial susceptibility profiles of B. anthracis isolates is foundational to treating naturally occurring outbreaks and to public health preparedness in the event of an intentional release. In this systematic review, we searched the peer-reviewed literature for all publications detailing antimicrobial susceptibility testing of B. anthracis. Within the set of discovered articles, we collated a subset of publications detailing susceptibility testing that followed standardized protocols for Food and Drug Administration-approved, commercially available antimicrobials. We analyzed the findings from the discovered articles, including the reported minimal inhibitory concentrations. Across the literature, most B. anthracis isolates were reported as susceptible to current first-line antimicrobials recommended for postexposure prophylaxis and treatment. The data presented for potential alternative antimicrobials will be of use if significant resistance to first-line antimicrobials arises, the strain is bioengineered, or first-line antimicrobials are not tolerated or available.

A new genome-mining tool redefines the lasso peptide biosynthetic landscape.

Ribosomally synthesized and post-translationally modified peptide (RiPP) natural products are attractive for genome-driven discovery and re-engineering, but limitations in bioinformatic methods and exponentially increasing genomic data make large-scale mining of RiPP data difficult. We report RODEO (Rapid ORF Description and Evaluation Online), which combines hidden-Markov-model-based analysis, heuristic scoring, and machine learning to identify biosynthetic gene clusters and predict RiPP precursor peptides. We initially focused on lasso peptides, which display intriguing physicochemical properties and bioactivities, but their hypervariability renders them challenging prospects for automated mining. Our approach yielded the most comprehensive mapping to date of lasso peptide space, revealing >1,300 compounds. We characterized the structures and bioactivities of six lasso peptides, prioritized based on predicted structural novelty, including one with an unprecedented handcuff-like topology and another with a citrulline modification exceptionally rare among bacteria. These combined insights significantly expand the knowledge of lasso peptides and, more broadly, provide a framework for future genome-mining efforts.

Targeting Reactive Carbonyls for Identifying Natural Products and Their Biosynthetic Origins.

Natural products (NPs) serve important roles as drug candidates and as tools for chemical biology. However, traditional NP discovery, largely based on bioassay-guided approaches, is biased toward abundant compounds and rediscovery rates are high. Orthogonal methods to facilitate discovery of new NPs are thus needed, and herein we describe an isotope tag-based expansion of reactivity-based NP screening to address these shortcomings. Reactivity-based screening is a directed discovery approach in which a specific reactive handle on the NP is targeted by a chemoselective probe to enable its detection by mass spectrometry. In this study, we have developed an aminooxy-containing probe to guide the discovery of aldehyde- and ketone-containing NPs. To facilitate the detection of labeling events, the probe was dibrominated, imparting a unique isotopic signature to distinguish labeled metabolites from spectral noise. As a proof of concept, the probe was then utilized to screen a collection of bacterial extracts, leading to the identification of a new analogue of antipain, deimino-antipain. The bacterial producer of deimino-antipain was sequenced and the responsible biosynthetic gene cluster was identified by bioinformatic analysis and heterologous expression. These data reveal the previously undetermined genetic basis for a well-known family of aldehyde-containing, peptidic protease inhibitors, including antipain, chymostatin, leupeptin, elastatinal, and microbial alkaline protease inhibitor, which have been widely used for over 40 years.

Targeted Treatment for Bacterial Infections: Prospects for Pathogen-Specific Antibiotics Coupled with Rapid Diagnostics.

Antibiotics are a cornerstone of modern medicine and have significantly reduced the burden of infectious diseases. However, commonly used broad-spectrum antibiotics can cause major collateral damage to the human microbiome, causing complications ranging from antibiotic-associated colitis to the rapid spread of resistance. Employing narrower spectrum antibiotics targeting specific pathogens may alleviate this predicament as well as provide additional tools to expand an antibiotic repertoire threatened by the inevitability of resistance. Improvements in clinical diagnosis will be required to effectively utilize pathogen-specific antibiotics and new molecular diagnostics are poised to fulfill this need. Here we review recent trends and the future prospects of deploying narrower spectrum antibiotics coupled with rapid diagnostics. Further, we discuss the theoretical advantages and limitations of this emerging approach to controlling bacterial infectious diseases.

Identification of the minimal cytolytic unit for streptolysin S and an expansion of the toxin family.

BACKGROUND: Streptolysin S (SLS) is a cytolytic virulence factor produced by the human pathogen Streptococcus pyogenes and other Streptococcus species. Related "SLS-like" toxins have been characterized in select strains of Clostridium and Listeria, with homologous clusters bioinformatically identified in a variety of other species. SLS is a member of the thiazole/oxazole-modified microcin (TOMM) family of natural products. The structure of SLS has yet to be deciphered and many questions remain regarding its structure-activity relationships. RESULTS: In this work, we assessed the hemolytic activity of a series of C-terminally truncated SLS peptides expressed in SLS-deficient S. pyogenes. Our data indicate that while the N-terminal poly-heterocyclizable (NPH) region of SLS substantially contributes to its bioactivity, the variable C-terminal region of the toxin is largely dispensable. Through genome mining we identified additional SLS-like clusters in diverse Firmicutes, Spirochaetes and Actinobacteria. Among the Spirochaete clusters, naturally truncated SLS-like precursors were found in the genomes of three Lyme disease-causing Borrelia burgdorferi sensu lato (Bbsl) strains. Although unable to restore hemolysis in SLS-deficient S. pyogenes, a Bbsl SLS-like precursor peptide was converted to a cytolysin using purified SLS biosynthetic enzymes. A PCR-based screen demonstrated that SLS-like clusters are substantially more prevalent in Bbsl than inferred from publicly available genome sequences. CONCLUSIONS: The mutagenesis data described herein indicate that the minimal cytolytic unit of SLS encompasses the NPH region of the core peptide. Interestingly, this region is found in all characterized TOMM cytolysins, as well as the novel putative TOMM cytolysins we discovered. We propose that this conserved region represents the defining feature of the SLS-like TOMM family. We demonstrate the cytolytic potential of a Bbsl SLS-like precursor peptide, which has a core region of similar length to the SLS minimal cytolytic unit, when modified with purified SLS biosynthetic enzymes. As such, we speculate that some Borrelia have the potential to produce a TOMM cytolysin, although the biological significance of this finding remains to be determined. In addition to providing new insight into the structure-activity relationships of SLS, this study greatly expands the cytolysin group of TOMMs.

Genetic basis of clarithromycin resistance in Bacillus anthracis.

The high-consequence pathogen Bacillus anthracis causes human anthrax and often results in lethal infections without the rapid administration of effective antimicrobial treatment. Antimicrobial resistance profiling is therefore critical to inform post-exposure prophylaxis and treatment decisions, especially during emergencies such as outbreaks or where intentional release is suspected. Whole-genome sequencing using a rapid long-read sequencer can uncover antimicrobial resistance patterns if genetic markers of resistance are known. To identify genomic markers associated with antimicrobial resistance, we isolated B. anthracis derived from the avirulent Sterne strain with elevated minimal inhibitory concentrations to clarithromycin. Mutants were characterized both phenotypically through broth microdilution susceptibility testing and observations during culturing, as well as genotypically with whole-genome sequencing. We identified two different in-frame insertions in the L22 ribosomal protein-encoding gene rplV, which were subsequently confirmed to be involved in clarithromycin resistance through the reversion of the mutant gene to the parent (drug-susceptible) sequence. Detection of the rplV insertions was possible with rapid long-read sequencing, with a time-to-answer within 3 h. The mutations associated with clarithromycin resistance described here will be used in conjunction with known genetic markers of resistance for other antimicrobials to strengthen the prediction of antimicrobial resistance in B. anthracis.IMPORTANCEThe disease anthrax, caused by the pathogen Bacillus anthracis, is extremely deadly if not treated quickly and appropriately. Clarithromycin is an antibiotic recommended for the treatment and post-exposure prophylaxis of anthrax by the Centers for Disease Control and Prevention; however, little is known about the ability of B. anthracis to develop resistance to clarithromycin or the mechanism of that resistance. The characterization of clarithromycin-resistant isolates presented here provides valuable information for researchers and clinicians in the event of a release of the resistant strain. Additionally, knowledge of the genetic basis of resistance provides a foundation for susceptibility prediction through rapid genome sequencing to inform timely treatment decisions.

Fluorescent ribonucleoside as a FRET acceptor for tryptophan in native proteins.

A new fluorescent ribonucleoside analogue, containing 5-aminoquinazoline-2,4(1H,3H)-dione, acts as a Forster resonance energy transfer acceptor for tryptophan (R(0) = 22 A) and displays visible emission (440 nm). As tryptophan is frequently found at or near the recognition domains of RNA binding proteins, this FRET pair facilitates the study of RNA binding to native proteins and peptides, which is demonstrated here for the HIV-1 Rev association with the Rev Response Element (RRE).

Fluorescent nucleoside analogue displays enhanced emission upon pairing with guanine.

A fluorescent nucleobase analogue, 7-aminoquinazoline-2,4-(1H,3H)-dione, is incorporated into a DNA oligonucleotide and senses mismatched pairing by displaying G-specific fluorescence enhancement.

Rapid antibiotic susceptibility testing from blood culture bottles with species agnostic real-time polymerase chain reaction.

Development and implementation of rapid antimicrobial susceptibility testing is critical for guiding patient care and improving clinical outcomes, especially in cases of sepsis. One approach to reduce the time-to-answer for antimicrobial susceptibility is monitoring the inhibition of DNA production, as differences in DNA concentrations are more quickly impacted compared to optical density changes in traditional antimicrobial susceptibility testing. Here, we use real-time PCR to rapidly determine antimicrobial susceptibility after short incubations with antibiotic. Application of this assay to a collection of 144 isolates in mock blood culture, covering medically relevant pathogens displaying high rates of resistance, provided susceptibility data in under 4 hours. This assay provided categorical agreement with a reference method in 96.3% of cases across all species. Sequencing of a subset of PCR amplicons showed accurate genus level identification. Overall, implementation of this method could provide accurate susceptibility results with a reduced time-to-answer for a number of medically relevant bacteria commonly isolated from blood culture.

Semi-quantitative MALDI-TOF for antimicrobial susceptibility testing in Staphylococcus aureus.

Antibiotic resistant bacterial infections are a significant problem in the healthcare setting, in many cases requiring the rapid administration of appropriate and effective antibiotic therapy. Diagnostic assays capable of quickly and accurately determining the pathogen resistance profile are therefore crucial to initiate or modify care. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) is a standard method for species identification in many clinical microbiology laboratories and is well positioned to be applied towards antimicrobial susceptibility testing. One recently reported approach utilizes semi-quantitative MALDI-TOF MS for growth rate analysis to provide a resistance profile independent of resistance mechanism. This method was previously successfully applied to Gram-negative pathogens and mycobacteria; here, we evaluated this method with the Gram-positive pathogen Staphylococcus aureus. Specifically, we used 35 strains of S. aureus and four antibiotics to optimize and test the assay, resulting in an overall accuracy rate of 95%. Application of the optimized assay also successfully determined susceptibility from mock blood cultures, allowing both species identification and resistance determination for all four antibiotics within 3 hours of blood culture positivity.

Plantazolicin is an ultra-narrow spectrum antibiotic that targets the Bacillus anthracis membrane.

Plantazolicin (PZN) is a ribosomally synthesized and post-translationally modified natural product from Bacillus methylotrophicus FZB42 and Bacillus pumilus. Extensive tailoring to twelve of the fourteen amino acid residues in the mature natural product endows PZN with not only a rigid, polyheterocyclic structure, but also antibacterial activity. Here we report a remarkably discriminatory activity of PZN toward Bacillus anthracis, which rivals a previously-described gamma (γ) phage lysis assay in distinguishing B. anthracis from other members of the Bacillus cereus group. We evaluate the underlying cause of this selective activity by measuring the RNA expression profile of PZN-treated B. anthracis, which revealed significant upregulation of genes within the cell envelope stress response. PZN depolarizes the B. anthracis membrane like other cell envelope-acting compounds but uniquely localizes to distinct foci within the envelope. Selection and whole-genome sequencing of PZN-resistant mutants of B. anthracis implicate a relationship between the action of PZN and cardiolipin (CL) within the membrane. Exogenous CL increases the potency of PZN in wild type B. anthracis and promotes the incorporation of fluorescently tagged PZN in the cell envelope. We propose that PZN localizes to and exacerbates structurally compromised regions of the bacterial membrane, which ultimately results in cell lysis.

Crystal structure and absolute configuration of (3S,4aS,8aS)-N-tert-butyl-2-[(S)-3-(2-chloro-4-nitro-benzamido)-2-hy-droxy-prop-yl]deca-hydro-isoquinoline-3-carboxamide and (3S,4aS,8aS)-N-tert-butyl-2-{(S)-2-[(S)-1-(2-chloro-4-nitro-benzoyl)pyrrolidin-2-yl]-2-hy-droxy-eth-yl}deca-hydro-iso-quinoline-3-carboxamide.

The crystal structure and absolute configuration of the two new title nelfinavir analogs, C24H35ClN4O5, (I), and C27H39ClN4O5, (II), have been determined. Each of these mol-ecules exhibits a number of disordered moieties. There are intra-molecular N-H⋯O hydrogen bonds in both (I) and (II). In (I) it involves the two carboxamide groups, while in (II) it involves the N-tert-butyl carboxamide group and the 2-hydroxyl O atom. The inter-molecular hydrogen bonding in (I) (O-H⋯O and N-H⋯O) leads to two-dimensional sheets that extend parallel to the ac plane. The inter-molecular hydrogen bonding in (II) (O-H⋯O) leads to chains that extend parallel to the a axis.

HIV protease inhibitors block streptolysin S production.

Streptolysin S (SLS) is a post-translationally modified peptide cytolysin that is produced by the human pathogen Streptococcus pyogenes. SLS belongs to a large family of azole-containing natural products that are biosynthesized via an evolutionarily conserved pathway. SLS is an important virulence factor during S. pyogenes infections, but despite an extensive history of study, further investigations are needed to clarify several steps of its biosynthesis. To this end, chemical inhibitors of SLS biosynthesis would be valuable tools to interrogate the various maturation steps of both SLS and biosynthetically related natural products. Such chemical inhibitors could also potentially serve as antivirulence therapeutics, which in theory may alleviate the spread of antibiotic resistance. In this work, we demonstrate that FDA-approved HIV protease inhibitors, especially nelfinavir, block a key proteolytic processing step during SLS production. This inhibition was demonstrated in live S. pyogenes cells and through in vitro protease inhibition assays. A panel of 57 nelfinavir analogs was synthesized, leading to a series of compounds with improved anti-SLS activity while illuminating structure-activity relationships. Nelfinavir was also found to inhibit the maturation of other azole-containing natural products, namely those involved in listeriolysin S, clostridiolysin S, and plantazolicin production. The use of nelfinavir analogs as inhibitors of SLS production has allowed us to begin examining the proteolysis event in SLS maturation and will aid in further investigations of the biosynthesis of SLS and related natural products.