Revealing the biological significance of multiple metabolic pathways of chloramphenicol by Sphingobium sp. WTD-1

Chloramphenicol (CAP) is an antibiotic that commonly pollutes the environment, and microorganisms primarily drive its degradation and transformation. Although several pathways for CAP degradation have been documented in different bacteria, multiple metabolic pathways in the same strain and their potential biological significance have not been revealed. In this study, Sphingobium WTD-1, which was isolated from activated sludge, can completely degrade 100 mg/L CAP within 60 h as the sole energy source. UPLC-HRMS and HPLC analyses showed that three different pathways, including acetylation, hydroxyl oxidation, and oxidation (C1-C2 bond cleavage), are responsible for the metabolism of CAP. Importantly, acetylation and C3 hydroxyl oxidation reduced the cytotoxicity of the substrate to strain WTD-1, and the C1-C2 bond fracture of CAP generated the metabolite p-nitrobenzoic acid (PNBA) to provide energy for its growth. This indicated that the synergistic action of three metabolic pathways caused WTD-1 to be adaptable and able to degrade high concentrations of CAP in the environment. This study deepens our understanding of the microbial degradation pathway of CAP and highlights the biological significance of the synergistic metabolism of antibiotic pollutants by multiple pathways in the same strain.

Effects of chemical oxygen demand and chloramphenicol on attached microalgae growth: Physicochemical properties and microscopic mass transfer in biofilm

During the wastewater treatment and resource recovery process by attached microalgae, the chemical oxygen demand (COD) can cause biotic contamination in algal culture systems, which can be mitigated by adding an appropriate dosage of antibiotics. The transport of COD and additive antibiotic (chloramphenicol, CAP) in algal biofilms and their influence on algal physiology were studied. The results showed that COD (60 mg/L) affected key metabolic pathways, such as photosystem II and oxidative phosphorylation, improved biofilm autotrophic and heterotrophic metabolic intensities, increased nutrient demand, and promoted biomass accumulation by 55.9 %, which was the most suitable COD concentration for attached microalgae. CAP (5-10 mg/L) effectively stimulated photosynthetic pigment accumulation and nutrient utilization in pelagic microalgal cells. In conclusion, controlling the COD concentration (approximately 60 mg/L) in the medium and adding the appropriate CAP concentration (5-10 mg/L) are conducive to improving attached microalgal biomass production and resource recovery potential from wastewater.

Novel antibiotic susceptibility of an RNA polymerase α-subunit mutant in Pseudomonas aeruginosa

BACKGROUND

RNA polymerase (RNAP) is highly conserved and essential for prokaryotic housekeeping activities, making it an important target for the development of new antibiotics. The rpoB gene, encoding a β-subunit of bacterial RNAP, has a well-known association with rifampicin resistance. However, the roles of other RNAP component genes such as rpoA, encoding an α-subunit of RNAP, in antibiotic resistance remain unexplored.

OBJECTIVES

To characterize the antibiotic resistance-related role of RpoA.

METHODS

We measured the expression of the MexEF-OprN efflux pump in an RpoA mutant using a transcriptional reporter. The MICs of various antibiotics for this RpoA mutant were determined.

RESULTS

We uncover a novel role of antibiotic susceptibility for an RpoA mutant in Pseudomonas aeruginosa. We found that a single amino acid substitution in RpoA resulted in reduced activity of the MexEF-OprN efflux pump, which is responsible for the exportation of various antibiotics, including ciprofloxacin, chloramphenicol, ofloxacin and norfloxacin. This attenuated efflux pump activity, caused by the RpoA mutation, conferred the bacteria further susceptibility to antibiotics regulated by MexEF-OprN. Our work further revealed that certain clinical P. aeruginosa isolates also contained the same RpoA mutation, providing clinical relevance to our findings. Our results elucidate why this new antibiotic-susceptible function of RpoA mutants would have remained undetected in conventional screens for mutants involving antibiotic resistance.

CONCLUSIONS

The discovery of antibiotic susceptibility in an RpoA mutant implicates a new therapeutic approach for treating clinical isolates of P. aeruginosa with RpoA mutations, using specific antibiotics regulated by MexEF-OprN. More generally, our work suggests that RpoA could serve as a promising candidate target for anti-pathogen therapeutic purposes.

Biotransformation of chloramphenicol by white-rot-fungi Trametes versicolor under cadmium stress

The recalcitrant chloramphenicol (CAP) combined with heavy metals cadmium (Cd) commonly co-existed in the environment, posing threat to environment health. The capacity of Trametes versicolor to remove/biodegrade CAP in air-pulse fluidized-bed reactor was evaluated, even under Cd stress. T. versicolor could remove 44 % CAP of 5 mg/L in 15 days, even 51 % CAP under 1 mg/L Cd stress. Sustained Cd stress inhibited CAP biodegradation and Cd removal in a 5-batches of a 5-days cycle sequential batch reactor. Nine transformation products and two novel pathways were proposed, with initial multi-step transformation reaction into CP2 and allylic alcohol, respectively. Furthermore, the main mechanism of Cd removal by T. versicolor was extracellular surface bioadsorption and intracellular accumulation. This study filled the gap of the mechanism of simultaneous CAP removal/biodegradation and Cd removal by white-rot fungi T. versicolor, which offer a theoretical basis for future application of biological removal of CAP containing wastewater.

N-acyl-homoserine lactones in extracellular polymeric substances from sludge for enhanced chloramphenicol-degrading anode biofilm formation in microbial fuel cells

Exploring an efficient acclimation strategy to obtain robust bioanodes is of practical significance for antibiotic wastewater treatment by bioelectrochemical systems (BESs). This study investigated the effects of two acclimation conditions on chloramphenicol (CAP)-degrading anode biofilm formation in microbial fuel cells (MFCs). The one was continuously added the extracellular polymeric substances (EPS) extracted from anaerobic sludge and increasing concentrations of CAP after the first start-up phase, while the other was added the EPS-1 (N-acyl-homoserine lactones, namely AHLs were extracted from the EPS) at the same conditions. The results demonstrated that AHLs in the sludge EPS played a crucial role for enhanced CAP-degrading anode biofilm formation in MFCs. The AHL-regulation could not only maintain stable voltage outputs but also significantly accelerate CAP removal in the EPS MFC. The maximum voltage of 653.83 mV and CAP removal rate of 1.21 ± 0.05 mg/L·h were attained from the EPS MFC at 30 mg/L of CAP, which were 0.84 and 1.57 times higher than those from the EPS-1 MFC, respectively. These improvements were largely caused by the thick and 3D structured biofilm, strong and homogeneous cell viability throughout the biofilm, and high protein/polysaccharide ratio along with more conductive contents in the biofilm EPS. Additionally, AHLs facilitated the formation of a biofilm with rich biodiversity and balanced bacterial proportions, leading to more beneficial mutualism among different functional bacteria. More bi-functional bacteria (for electricity generation and antibiotic resistance/degradation) were specifically enriched by AHLs as well. These findings provide quorum sensing theoretical knowledge and practical instruction for rapid antibiotic-degrading electrode biofilm acclimation in BESs.

Functional and Structural Characterization of Diverse NfsB Chloramphenicol Reductase Enzymes from Human Pathogens

Phylogenetically diverse bacteria can carry out chloramphenicol reduction, but only a single enzyme has been described that efficiently catalyzes this reaction, the NfsB nitroreductase from Haemophilus influenzae strain KW20. Here, we tested the hypothesis that some NfsB homologs function as housekeeping enzymes with the potential to become chloramphenicol resistance enzymes. We found that expression of H. influenzae and Neisseria spp. nfsB genes, but not Pasteurella multocida nfsB, allows Escherichia coli to resist chloramphenicol by nitroreduction. Mass spectrometric analysis confirmed that purified H. influenzae and N. meningitides NfsB enzymes reduce chloramphenicol to amino-chloramphenicol, while kinetics analyses supported the hypothesis that chloramphenicol reduction is a secondary activity. We combined these findings with atomic resolution structures of multiple chloramphenicol-reducing NfsB enzymes to identify potential key substrate-binding pocket residues. Our work expands the chloramphenicol reductase family and provides mechanistic insights into how a housekeeping enzyme might confer antibiotic resistance. IMPORTANCE The question of how new enzyme activities evolve is of great biological interest and, in the context of antibiotic resistance, of great medical importance. Here, we have tested the hypothesis that new antibiotic resistance mechanisms may evolve from promiscuous housekeeping enzymes that have antibiotic modification side activities. Previous work identified a Haemophilus influenzae nitroreductase housekeeping enzyme that has the ability to give Escherichia coli resistance to the antibiotic chloramphenicol by nitroreduction. Herein, we extend this work to enzymes from other Haemophilus and Neisseria strains to discover that expression of chloramphenicol reductases is sufficient to confer chloramphenicol resistance to Es. coli, confirming that chloramphenicol reductase activity is widespread across this nitroreductase family. By solving the high-resolution crystal structures of active chloramphenicol reductases, we identified residues important for this activity. Our work supports the hypothesis that housekeeping proteins possessing multiple activities can evolve into antibiotic resistance enzymes.

AdpA Positively Regulates Morphological Differentiation and Chloramphenicol Biosynthesis in Streptomyces venezuelae

In members of genus Streptomyces, AdpA is a master transcriptional regulator that controls the expression of hundreds of genes involved in morphological differentiation, secondary metabolite biosynthesis, chromosome replication, etc. However, the function of AdpASv, an AdpA ortholog of Streptomyces venezuelae, is unknown. This bacterial species is a natural producer of chloramphenicol and has recently become a model organism for studies on Streptomyces. Here, we demonstrate that AdpASv is essential for differentiation and antibiotic biosynthesis in S. venezuelae and provide evidence suggesting that AdpASv positively regulates its own gene expression. We speculate that the different modes of AdpA-dependent transcriptional autoregulation observed in S. venezuelae and other Streptomyces species reflect the arrangement of AdpA binding sites in relation to the transcription start site. Lastly, we present preliminary data suggesting that AdpA may undergo a proteolytic processing and we speculate that this may potentially constitute a novel regulatory mechanism controlling cellular abundance of AdpA in Streptomyces. IMPORTANCEStreptomyces are well-known producers of valuable secondary metabolites which include a large variety of antibiotics and important model organisms for developmental studies in multicellular bacteria. The conserved transcriptional regulator AdpA of Streptomyces exerts a pleiotropic effect on cellular processes, including the morphological differentiation and biosynthesis of secondary metabolites. Despite extensive studies, the function of AdpA in these processes remains elusive. This work provides insights into the role of a yet unstudied AdpA ortholog of Streptomyces venezuelae, now considered a novel model organism. We found that AdpA plays essential role in morphological differentiation and biosynthesis of chloramphenicol, a broad-spectrum antibiotic. We also propose that AdpA may undergo a proteolytic processing that presumably constitutes a novel mechanism regulating cellular abundance of this master regulator.

Rhodococcus comparative genomics reveals a phylogenomic-dependent non-ribosomal peptide synthetase distribution: insights into biosynthetic gene cluster connection to an orphan metabolite

Natural products (NPs) are synthesized by biosynthetic gene clusters (BGCs), whose genes are involved in producing one or a family of chemically related metabolites. Advances in comparative genomics have been favourable for exploiting huge amounts of data and discovering previously unknown BGCs. Nonetheless, studying distribution patterns of novel BGCs and elucidating the biosynthesis of orphan metabolites remains a challenge. To fill this knowledge gap, our study developed a pipeline for high-quality comparative genomics for the actinomycete genus Rhodococcus , which is metabolically versatile, yet understudied in terms of NPs, leading to a total of 110 genomes, 1891 BGCs and 717 non-ribosomal peptide synthetases (NRPSs). Phylogenomic inferences showed four major clades retrieved from strains of several ecological habitats. BiG-SCAPE sequence similarity BGC networking revealed 44 unidentified gene cluster families (GCFs) for NRPS, which presented a phylogenomic-dependent evolution pattern, supporting the hypothesis of vertical gene transfer. As a proof of concept, we analysed in-depth one of our marine strains, Rhodococcus sp. H-CA8f, which revealed a unique BGC distribution within its phylogenomic clade, involved in producing a chloramphenicol-related compound. While this BGC is part of the most abundant and widely distributed NRPS GCF, corason analysis unveiled major differences regarding its genetic context, co-occurrence patterns and modularity. This BGC is composed of three sections, two well-conserved right/left arms flanking a very variable middle section, composed of nrps genes. The presence of two non-canonical domains in H-CA8f’s BGC may contribute to adding chemical diversity to this family of NPs. Liquid chromatography-high resolution MS and dereplication efforts retrieved a set of related orphan metabolites, the corynecins, which to our knowledge are reported here for the first time in Rhodococcus . Overall, our data provide insights to connect BGC uniqueness with orphan metabolites, by revealing key comparative genomic features supported by models of BGC distribution along phylogeny.

In Silico Identification of Chemicals Capable of Binding to the Ecdysone Receptor

The process of molting, known alternatively as ecdysis, is a feature integral in the life cycles of species across the arthropod phylum. Regulation occurs as a function of the interaction of ecdysteroid hormones with the arthropod nuclear ecdysone receptor-a process preceding the triggering of a series of downstream events constituting an endocrine signaling pathway highly conserved throughout environmentally prevalent insect, crustacean, and myriapod organisms. Inappropriate ecdysone receptor binding and activation forms the essential molecular initiating event within possible adverse outcome pathways relating abnormal molting to mortality in arthropods. Definition of the characteristics of chemicals liable to stimulate such activity has the potential to be of great utility in mitigation of hazards posed toward vulnerable species. Thus the aim of the present study was to develop a series of rule-sets, derived from the key structural and physicochemical features associated with identified ecdysone receptor ligands, enabling construction of Konstanz Information Miner (KNIME) workflows permitting the flagging of compounds predisposed to binding at the site. Data describing the activities of 555 distinct chemicals were recovered from a variety of assays across 10 insect species, allowing for formulation of KNIME screens for potential binding activity at the molecular initiating event and adverse outcome level of biological organization. Environ Toxicol Chem 2020;39:1438-1450. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Comparative Genomics Determines Strain-Dependent Secondary Metabolite Production in Streptomyces venezuelae Strains

Streptomyces venezuelae is well known to produce various secondary metabolites, including chloramphenicol, jadomycin, and pikromycin. Although many strains have been classified as S. venezuelae species, only a limited number of strains have been explored extensively for their genomic contents. Moreover, genomic differences and diversity in secondary metabolite production between the strains have never been compared. Here, we report complete genome sequences of three S. venezuelae strains (ATCC 10712, ATCC 10595, and ATCC 21113) harboring chloramphenicol and jadomycin biosynthetic gene clusters (BGC). With these high-quality genome sequences, we revealed that the three strains share more than 85% of total genes and most of the secondary metabolite biosynthetic gene clusters (smBGC). Despite such conservation, the strains produced different amounts of chloramphenicol and jadomycin, indicating differential regulation of secondary metabolite production at the strain level. Interestingly, antagonistic production of chloramphenicol and jadomycin was observed in these strains. Through comparison of the chloramphenicol and jadomycin BGCs among the three strains, we found sequence variations in many genes, the non-coding RNA coding regions, and binding sites of regulators, which affect the production of the secondary metabolites. We anticipate that these genome sequences of closely related strains would serve as useful resources for understanding the complex secondary metabolism and for designing an optimal production process using Streptomyces strains.

High-efficiency biodegradation of chloramphenicol by enriched bacterial consortia: Kinetics study and bacterial community characterization

The risk of environmental pollution caused by chloramphenicol has necessitated special attention. Biodegradation has tremendous potential for chloramphenicol removal in the environment. Six chloramphenicol-degrading consortia were acclimated under different culture conditions to investigate their chloramphenicol biodegradation behaviors, and the bacterial community structures were comprehensively characterized. The enriched consortia CL and CH which utilized chloramphenicol as their sole carbon and energy source could thoroughly degrade 120 mg/L chloramphenicol within 5 days, and the mineralization rate reached up to 90%. Chloramphenicol biodegradation kinetics by different enriched consortia fit the modified Gompertz model or the first-order decay model (R²≥0.97). Consortia CL could almost completely degrade 1-500 mg/L CAP with a final mineralization rate of 87.8-91.7%. Chloramphenicol 3-acetate was identified to be a major intermediate of CAP biodegradation by metabolite analysis and enzyme activity assay. 16S rRNA sequencing analysis revealed that the diversities and abundances of the main genera in the enriched consortia were distinct from each other. Forty-one core OTUs belonging to 18 genera were the core bacteria which might be related to chloramphenicol biodegradation. Among them, the genera Sphingomonas, Chryseobacterium, Cupriavidus, Bradyrhizobium, Burkholderia, and Afipia with high abundance may play potential roles for chloramphenicol biodegradation.

Fabrication of novel MXene (Ti3C2)/polyacrylamide nanocomposite hydrogels with enhanced mechanical and drug release properties

A highly stretchable nanocomposite (NC) hydrogel was fabricated via in situ free radical polymerization of acrylamide. In particular, an exfoliated two-dimensional MXene (Ti₃C₂) nanosheet was utilized as a crosslinker instead of traditional organic crosslinkers. The exfoliated Ti₃C₂ nanosheets were confirmed by atomic force microscopy (AFM) and dynamic light scattering (DLS) measurements. Compared with traditional organic crosslinked N,N-methylene bisacrylamide (BIS)/polyacrylamide (PAM) hydrogels (fracture strength of 32.0 kPa and elongation of 109.6%), the synthesized Ti₃C₂/PAM NC hydrogels exhibited greatly improved mechanical properties with fracture strengths of 66.5 to 102.7 kPa, compressive strengths of 400.6 to 819.4 kPa and elongations at break of 2158.6% to 3047.5% as the Ti₃C₂ content increases from 0.0145% to 0.0436%. The enhanced mechanical performances can be attributed to the honeycomb-like fine structure with uniform pores as well as more flexible polymer chains in NC hydrogel networks. When loaded with drugs, Ti₃C₂/PAM NC hydrogels exhibited good sustained-release performance, higher drug loading amounts (97.5-127.7 mg g^-1) and higher percentage releases (62.1-81.4%), greatly superior to those of the BIS/PAM hydrogel (46.4 mg g^-1, 45.0%). Our work reveals the application of MXene materials in the fabrication of NC hydrogels with enhanced mechanical and drug release behaviors.

Aeration-assisted sulfite activation with ferrous for enhanced chloramphenicol degradation

In this study, an Fe(Ⅱ)/S(IV) system was designed for the degradation of chloramphenicol (CAP). The pseudo-first-order rate constants for CAP degradation under typical conditions with and without air purging were investigated. The greatly enhanced rate of 0.0099 min^-1 with air purging compared with 0.0006 min^-1 with no air purging indicated that aeration was significant to the degradation of CAP in Fe(Ⅱ)/S(Ⅳ) system. Radical scavenging experiments revealed that SO₄^- was the primary oxidant generated from the activation of S(IV) with Fe(II), accounting for around 70% of degradation under weak acidic and neutral conditions. Increasing Fe(II) and S(IV) doses promoted the degradation of CAP, whereas the overdose of them led to a decreased degradation rate by scavenging radicals. Owing to the participation of oxygen in the formation of ferric sulfite complex and SO₅^-, the increase of dissolved oxygen improved the removal efficiency of CAP. The removal efficiency of CAP was also found to be pH dependent, decreasing from acid condition (initial pH = 4) to basic condition (initial pH = 8). The presence of coexisting anions and water matrix was found inhibiting CAP degradation in Fe(Ⅱ)/S(Ⅳ) system. This work provides an understanding on the working mechanism and possible applications of Fe(Ⅱ)/S(Ⅳ) system in organic compound degradation in wastewater.

Characterization of an efficient chloramphenicol-mineralizing bacterial consortium

Obtaining efficient antibiotic-mineralizing consortium or pure cultures is a central issue for the deep elimination of antibiotic-contaminated environments. However, the antibiotic chloramphenicol (CAP) mineralizing consortium has not yet been reported. In this study, an efficient CAP-mineralizing consortium was successfully obtained with municipal activated sludge as the initial inoculum. This consortium is capable of aerobically subsisting on CAP as the sole carbon, nitrogen and energy sources and completely degrading 50 mg L^-1 CAP within 24 h. After 5 d, 71.50 ± 2.63% of CAP was mineralized and Cl^- recovery efficiency was 90.80 ± 7.34%. Interestingly, the CAP degradation efficiency obviously decreased to 18.22 ± 3.52% within 12 h with co-metabolic carbon source glucose. p-nitrobenzoic acid (p-NBA) was identified as an intermediate product during CAP biodegradation. The consortium is also able to utilize p-NBA as the sole carbon and nitrogen sources and almost completely degrade 25 mg L^-1p-NBA within 24 h. Microbial community analysis indicated that the dominant genera in the CAP-mineralizing consortium all belong to Proteobacteria (especially Sphingobium with the relative abundance over 63%), and most bacteria could degrade aromatics including p-NBA, suggesting these genera involved in the upstream and downstream pathway of CAP degradation. Although the acclimated consortium has been successively passaged 152 times, the microbial community structure and core genera were not obviously changed, which was consistent with the stable CAP degradation efficiency observed under different generations. This is the first report that the acclimated consortium is able to mineralize CAP through an oxidative pathway with p-NBA as an intermediate product.

Safety Assessment of Compounds after In Vitro Metabolic Conversion Using Zebrafish Eleuthero Embryos

Zebrafish-based platforms have recently emerged as a useful tool for toxicity testing as they combine the advantages of in vitro and in vivo methodologies. Nevertheless, the capacity to metabolically convert xenobiotics by zebrafish eleuthero embryos is supposedly low. To circumvent this concern, a comprehensive methodology was developed wherein test compounds (i.e., parathion, malathion and chloramphenicol) were first exposed in vitro to rat liver microsomes (RLM) for 1 h at 37 °C. After adding methanol, the mixture was ultrasonicated, placed for 2 h at −20 °C, centrifuged and the supernatant evaporated. The pellet was resuspended in water for the quantification of the metabolic conversion and the detection of the presence of metabolites using ultra high performance liquid chromatography-Ultraviolet-Mass (UHPLC-UV-MS). Next, three days post fertilization (dpf) zebrafish eleuthero embryos were exposed to the metabolic mix diluted in Danieau’s medium for 48 h at 28 °C, followed by a stereomicroscopic examination of the adverse effects induced, if any. The novelty of our method relies in the possibility to quantify the rate of the in vitro metabolism of the parent compound and to co-incubate three dpf larvae and the diluted metabolic mix for 48 h without inducing major toxic effects. The results for parathion show an improved predictivity of the toxic potential of the compound.

Investigating the promiscuity of the chloramphenicol nitroreductase from Haemophilus influenzae towards the reduction of 4-nitrobenzene derivatives

Chloramphenicol nitroreductase (CNR), a drug-modifying enzyme from Haemophilus influenzae, has been shown to be responsible for the conversion of the nitro group into an amine in the antibiotic chloramphenicol (CAM). Since CAM structurally bears a 4-nitrobenzene moiety, we explored the substrate promiscuity of CNR by investigating its nitroreduction of 4-nitrobenzyl derivatives. We tested twenty compounds containing a nitrobenzene core, two nitropyridines, one compound with a vinylogous nitro group, and two aliphatic nitro compounds. In addition, we also synthesized twenty-eight 4-nitrobenzyl derivatives with ether, ester, and thioether substituents and assessed the relative activity of CNR in their presence. We found several of these compounds to be modified by CNR, with the enzyme activity ranging from 1 to 150% when compared to CAM. This data provides insights into two areas: (i) chemoenzymatic reduction of select compounds to avoid harsh chemicals and heavy metals routinely used in reductions of nitro groups and (ii) functional groups that would aid CAM in overcoming the activity of this enzyme.

The health risk for seafood consumers under future ocean acidification (OA) scenarios: OA alters bioaccumulation of three pollutants in an edible bivalve species through affecting the in vivo metabolism

The current knowledge about the effect of pCO₂-driven ocean acidification on the bioaccumulation of pollutants in marine species is still scarce, as only limited types of pollutants have been investigated. Therefore, to obtain a better understanding of the effect of ocean acidification on the process of bioaccumulation and subsequent food safety, the accumulation of benzo[a]pyrene (B[a]P), chloramphenicol (CAP), and nitrofurazone (NFZ) in an edible bivalve species, Tegillarca granosa, under present and near-future ocean acidification scenarios was investigated in the present study. The health risks associated with consuming contaminated blood clams were also assessed using target hazard quotient (THQ), lifetime cancer risk (CR), or margin of exposure (MoE). To explain the alterations in bioaccumulation of these pollutants, the expressions of genes encoding corresponding key metabolic proteins were analyzed as well. The results obtained showed that ocean acidification exerted a significant effect on the accumulation of B[a]P, NFZ, and CAP in the clams. After four-week exposure to B[a]P, NFZ, or CAP contaminated seawater acidified with CO₂ at pH 7.8 and 7.4, significantly greater amounts of B[a]P and lower amounts of NFZ and CAP were accumulated in the clams compared to that in the control. Although no non-carcinogenic risk of consuming B[a]P-contaminated blood clams was detected using the THQ values obtained, the CR values obtained indicated a high life-time risk in all groups. In addition, according to the MoE values obtained, the health risks in terms of consuming NFZ- and CAP-contaminated clams were significantly reduced under ocean acidification scenarios but still cannot be ignored, especially for children. The gene expression results showed that the ability of clams to eliminate B[a]P may be significantly constrained, whereas the ability to eliminate NFZ and CAP may be enhanced under ocean acidification scenarios, indicating that the changes in the accumulation of these pollutants may be due to the altered in vivo metabolism.

Crystal structure of chloramphenicol-metabolizing enzyme EstDL136 from a metagenome

Metagenomes often convey novel biological activities and therefore have gained considerable attention for use in biotechnological applications. Recently, metagenome-derived EstDL136 was found to possess chloramphenicol (Cm)-metabolizing features. Sequence analysis showed EstDL136 to be a member of the hormone-sensitive lipase (HSL) family with an Asp-His-Ser catalytic triad and a notable substrate specificity. In this study, we determined the crystal structures of EstDL136 and in a complex with Cm. Consistent with the high sequence similarity, the structure of EstDL136 is homologous to that of the HSL family. The active site of EstDL136 is a relatively shallow pocket that could accommodate Cm as a substrate as opposed to the long acyl chain substrates typical of the HSL family. Mutational analyses further suggested that several residues in the vicinity of the active site play roles in the Cm-binding of EstDL136. These results provide structural and functional insights into a metagenome-derived EstDL136.

Accurate Quantitation and Analysis of Nitrofuran Metabolites, Chloramphenicol, and Florfenicol in Seafood by Ultrahigh-Performance Liquid Chromatography-Tandem Mass Spectrometry: Method Validation and Regulatory Samples

We developed and validated a method for the extraction, identification, and quantitation of four nitrofuran metabolites, 3-amino-2-oxazolidinone (AOZ), 3-amino-5-morpholinomethyl-2-oxazolidinone (AMOZ), semicarbazide (SC), and 1-aminohydantoin (AHD), as well as chloramphenicol and florfenicol in a variety of seafood commodities. Samples were extracted by liquid-liquid extraction techniques, analyzed by ultrahigh-performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS), and quantitated using commercially sourced, derivatized nitrofuran metabolites, with their isotopically labeled internal standards in-solvent. We obtained recoveries of 90-100% at various fortification levels. The limit of detection (LOD) was set at 0.25 ng/g for AMOZ and AOZ, 1 ng/g for AHD and SC, and 0.1 ng/g for the phenicols. Various extraction methods, standard stability, derivatization efficiency, and improvements to conventional quantitation techniques were also investigated. We successfully applied this method to the identification and quantitation of nitrofuran metabolites and phenicols in 102 imported seafood products. Our results revealed that four of the samples contained residues from banned veterinary drugs.

Cometabolic degradation of chloramphenicol via a meta-cleavage pathway in a microbial fuel cell and its microbial community

The performance of a microbial fuel cell (MFC) in terms of degradation of chloramphenicol (CAP) was investigated. Approximately 84% of 50mg/L CAP was degraded within 12h in the MFC. A significant interaction of pH, temperature, and initial CAP concentration was found on removal of CAP, and a maximum degradation rate of 96.53% could theoretically be achieved at 31.48°C, a pH of 7.12, and an initial CAP concentration of 106.37mg/L. Moreover, CAP was further degraded through a ring-cleavage pathway. The antibacterial activity of CAP towards Escherichia coli ATCC 25922 and Shewanella oneidensis MR-1 was largely eliminated by MFC treatment. High-throughput sequencing analysis indicated that Azonexus, Comamonas, Nitrososphaera, Chryseobacterium, Azoarcus, Rhodococcus, and Dysgonomonas were the predominant genera in the MFC anode biofilm. In conclusion, the MFC shows potential for the treatment of antibiotic residue-containing wastewater due to its high rates of CAP removal and energy recovery.

Response of anodic bacterial community to the polarity inversion for chloramphenicol reduction

Chloramphenicol (CAP) is a frequently detected environmental pollutant. In this study, an electroactive biofilm for CAP reduction was established by initially in the anode and then inverting to the cathode. The established biocathode could enhance the reduction of CAP to the nitro-group reduced CAP (AMCl2) and further dechlorinated form (AMCl), both had lost the antibacterial activity. The phylogenetic diversity of the acclimated biofilm was decreased after the polar inversion. Proportions of functional bacterial genera, including Geobacter, Desulfovibrio and Pseudomonas responsible for the bidirectional electron transfer and nitroaromatics reduction, had increased 28%, 104% and 43% in the cathode. The relatively high abundances (over 50%) of Geobacter in anode and cathode were rarely detected for the nitroaromatics reduction. This study provides new insights into the electroactive biofilm structure improvement by the polarity inversion strategy for refractory antibiotics degradation.

Inverse Temperature Dependence in Static Quenching versus Calorimetric Exploration: Binding Interaction of Chloramphenicol to β-Lactoglobulin

The binding interaction between the whey protein bovine β-lactoglobulin (βLG) with the well-known antibiotic chloramphenicol (Clp) is explored by monitoring the intrinsic fluorescence of βLG. Steady-state and time-resolved fluorescence spectral data reveal that quenching of βLG fluorescence proceeds through ground state complex formation, i.e., static quenching mechanism. However, the drug-protein binding constant is found to vary proportionately with temperature. This anomalous result is explained on the basis of the Arrhenius theory which states that the rate constant varies proportionally with temperature. Thermodynamic parameters like ΔH, ΔS, ΔG, and the stoichiometry for the binding interaction have been estimated by isothermal titration calorimetric (ITC) study. Thermodynamic data show that the binding phenomenon is mainly an entropy driven process suggesting the major role of hydrophobic interaction forces in the Clp-βLG binding. Constant pressure heat capacity change (ΔCp) has been calculated from enthalpy of binding at different temperatures which reveals that hydrophobic interaction is the major operating force. The inverse temperature dependence in static quenching is however resolved from ITC data which show that the binding constant regularly decreases with increase in temperature. The modification of native protein conformation due to binding of drug has been monitored by circular dichroism (CD) spectroscopy. The probable binding location of Clp inside βLG is explored from AutoDock based blind docking simulation.

The finite dose technique as a valid in vitro model for the study of percutaneous absorption in man

An in vitro model of percutaneous absorption has been developed which permits close stimulation of conditions commonly associated with topical drug use in living man. Quantitative comparison of the absorption of selected compounds in the model and in living man was made to test the validity of the model. Excellent agreement has been found between the two sets of data, both with respect to the total amount absorbed and the kinetics of absorption.

An unusual peroxo intermediate of the arylamine oxygenase of the chloramphenicol biosynthetic pathway

Streptomyces venezuelae CmlI catalyzes the six-electron oxygenation of the arylamine precursor of chloramphenicol in a nonribosomal peptide synthetase (NRPS)-based pathway to yield the nitroaryl group of the antibiotic. Optical, EPR, and Mössbauer studies show that the enzyme contains a nonheme dinuclear iron cluster. Addition of O(2) to the diferrous state of the cluster results in an exceptionally long-lived intermediate (t(1/2) = 3 h at 4 °C) that is assigned as a peroxodiferric species (CmlI-peroxo) based upon the observation of an (18)O(2)-sensitive resonance Raman (rR) vibration. CmlI-peroxo is spectroscopically distinct from the well characterized and commonly observed cis-μ-1,2-peroxo (μ-η(1):η(1)) intermediates of nonheme diiron enzymes. Specifically, it exhibits a blue-shifted broad absorption band around 500 nm and a rR spectrum with a ν(O-O) that is at least 60 cm(-1) lower in energy. Mössbauer studies of the peroxo state reveal a diferric cluster having iron sites with small quadrupole splittings and distinct isomer shifts (0.54 and 0.62 mm/s). Taken together, the spectroscopic comparisons clearly indicate that CmlI-peroxo does not have a μ-η(1):η(1)-peroxo ligand; we propose that a μ-η(1):η(2)-peroxo ligand accounts for its distinct spectroscopic properties. CmlI-peroxo reacts with a range of arylamine substrates by an apparent second-order process, indicating that CmlI-peroxo is the reactive species of the catalytic cycle. Efficient production of chloramphenicol from the free arylamine precursor suggests that CmlI catalyzes the ultimate step in the biosynthetic pathway and that the precursor is not bound to the NRPS during this step.

Characterization of cefalexin degradation capabilities of two Pseudomonas strains isolated from activated sludge

Pharmaceuticals have recently been regarded as contaminants of emerging concern. To date, there is limited knowledge about antibiotic-degrading microorganisms in conventional activated sludge treatment systems and their characteristics toward antibiotic degradation especially in the presence of a pharmaceutical mixture. As such, antibiotic-degrading microorganisms were investigated and isolated from the activated sludge, and their degradation capabilities were evaluated. Two strains of cefalexin-degrading bacteria CE21 and CE22 were isolated and identified as Pseudomonas sp. in the collected activated sludge. Strain CE22 was able to degrade over 90% of cefalexin, while CE21 was able to remove 46.7% of cefalexin after incubation for 24h. The removal efficiency of cefalexin by CE22, different from that of CE21, was not significantly affected by an increase in cefalexin concentration, even up to 10ppm, however the presence of 1ppm of other pharmaceuticals had a significant effect on the degradation of cefalexin by CE22, but no significant effect on CE21. The degradation product of cefalexin by the two strains was identified to be 2-hydroxy-3-phenyl pyrazine. Our results also indicated that CE21 and CE22 were able to degrade caffeine, salicylic acid and chloramphenicol. Moreover, CE21 was found to be capable of eliminating sulfamethoxazole and naproxen.

Reductive degradation of chloramphenicol using bioelectrochemical system (BES): a comparative study of abiotic cathode and biocathode

Reductive degradation of choramphenicol (CAP) using Bioelectrochemical system (BES) with both abiotic cathode and biocathode was investigated. It was found that the CAP reduction efficiency during the first 24 h reached 86.3% of the biocathode group, while which was only 62.9% in the case of abiotic cathode. Except for the cathode potential, other indicators of the cathode performance as the cathode current, the current response of the cyclic voltammetry, the ohm resistance, and the polarization resistance of the biocathode group were all better than those of the abiotic group. Moreover, specific CAP reductive rate of the biocathode with sludge fermentation liquid (0.199 h(-1)) as carbon source was close to that of the glucose (0.215 h(-1)), but was about 3.2 times of the abiotic cathode group (0.062 h(-1)). It suggested that the introduction of biocathode would better the cathode performance, and then further increase the CAP reduction.

Accelerated reduction of chlorinated nitroaromatic antibiotic chloramphenicol by biocathode

Chlorinated nitroaromatic antibiotic chloramphenicol (CAP) is a priority pollutant in wastewaters. A fed-batch bioelectrochemical system (BES) with biocathode with applied voltage of 0.5 V (served as extracellular electron donor) and glucose as intracellular electron donor was applied to reduce CAP to amine product (AMCl2). The biocathode BES converted 87.1 ± 4.2% of 32 mg/L CAP in 4 h, and the removal efficiency reached 96.0 ± 0.9% within 24 h. Conversely, the removal efficiency of CAP in BES with an abiotic cathode was only 73.0 ± 3.2% after 24 h. When the biocathode was disconnected (no electrochemical reaction but in the presence of microbial activities), the CAP removal rate was dropped to 62.0% of that with biocathode BES. Acetylation of one hydroxyl of CAP was noted exclusive in the biocatalyzed process, while toxic intermediates, hydroxylamino (HOAM), and nitroso (NO), from CAP reduction were observed only in the abiotic cathode BES. Electrochemical hydrodechlorination and dehalogenase were responsible for dechlorination of AMCl2 to AMCl in abiotic and microbial cathode BES, respectively. The cyclic voltammetry (CV) highlighted higher peak currents and lower overpotentials for CAP reduction at the biocathode compared with abiotic cathode. With the biocathode BES, antibacterial activity of CAP was completely removed and nitro group reduction combined with dechlorination reaction enhanced detoxication efficiency of CAP. The CAP cathodic transformation pathway was proposed based on intermediates analysis. Bacterial community analysis indicated that the dominate bacteria on the biocathode were belonging to α, β, and γ-Proteobacteria. The biocathode BES could serve as a potential treatment process for CAP-containing wastewater.

Study on the binding of chloroamphenicol with bovine serum albumin by fluorescence and UV-vis spectroscopy

The binding of chloroamphenicol (CPC) to bovine serum albumin (BSA) at 296 K, 303 K, and 310 K by fluorescence and UV-visible absorption spectroscopy were investigated under imitated physiological conditions. The experimental results showed that the fluorescence quenching mechanism between CPC and BSA was combined quenching (dynamic and static quenching) procedure at low CPC concentration, or a dynamic quenching procedure at high concentrations. The binding constants, binding sites and the corresponding thermodynamic parameters of the interaction system were calculated. According to Förster non-radiation energy transfer theory, the binding distance between CPC and BSA was calculated to be 3.02 nm. Both synchronous fluorescence and FT-IR spectra confirmed the interaction, and indicated the conformational changes of BSA. The effects of some common metal ions Ca(2+), Ni(2+), Mg(2+), Fe(2+), and Cu(2+) on the binding constant between CPC and BSA were examined. Furthermore, we investigated the possible sub-domains on BSA that bind CPC by displacement experiments.

[Reductive degradation of chloramphenicol in bioelectrochemical system]

In this study, we investigated reductive degradation of nitroaromatic antibiotic chloramphenicol to non-effective antibacterial amine product in fed-batch biocatalyzed electrolysis systems (BES) (applied voltage was 0.5 V) under low temperature (12 +/- 2 degrees C). The ohm resistance of the whole BES reactor increased when the phosphate buffer solution concentrations decreased. Efficiencies (ErCAP) of chloramphenicol reduction with biocathode (PBS, 25 mmol/L) in presence of glucose was (86.3 +/- 1.69)% within 24 h and sludge fermentation liquor was (74.1 +/-1.44)% within 24 h. While the ErCAP of abiotic cathode under the same condition was only (57.9 +/- 1.94)% within 24 h. It suggested that biocathode could be a promising technology for reductive biodegradation of nitroaromatic antibiotics-containing wastewater in areas with relatively low annual mean temperature.

Antibiotics that bind to the A site of the large ribosomal subunit can induce mRNA translocation

In the absence of elongation factor EF-G, ribosomes undergo spontaneous, thermally driven fluctuation between the pre-translocation (classical) and intermediate (hybrid) states of translocation. These fluctuations do not result in productive mRNA translocation. Extending previous findings that the antibiotic sparsomycin induces translocation, we identify additional peptidyl transferase inhibitors that trigger productive mRNA translocation. We find that antibiotics that bind the peptidyl transferase A site induce mRNA translocation, whereas those that do not occupy the A site fail to induce translocation. Using single-molecule FRET, we show that translocation-inducing antibiotics do not accelerate intersubunit rotation, but act solely by converting the intrinsic, thermally driven dynamics of the ribosome into translocation. Our results support the idea that the ribosome is a Brownian ratchet machine, whose intrinsic dynamics can be rectified into unidirectional translocation by ligand binding.

Isolation and characterization of Aeromonas schubertii from diseased snakehead, Channa maculata (Lacepède)

Pure bacterial cultures were isolated from diseased snakeheads, Channa maculata (Lacepède), suffering high mortality in a farm in Zhongshan, southern China. Three isolates, namely ZS20100725, ZS20100725-1 and ZS20100725-2, were identified as Aeromonas schubertii. All the isolates showed high 16S rRNA sequence similarities with A. schubertii. The isolates exhibited strong virulence to snakeheads in experimental challenges with LD(50) ranging between 1.4 × 10(4) and 6.4 × 10(6) CFU g(-1). Two of the isolates were positive for haemolysin, elastase, lipase and lecithinase by phenotypic determination, which was further confirmed by PCR amplification of the haemolysin and elastase genes. In sterile liquid medium, the best growth conditions of strain ZS20100725 were 30 °C, pH 7 and 0.5% salinity (w/v). Antibiotic susceptibility tests showed that strain ZS20100725 was susceptible to cefoxitin, cefoperazone and chloramphenicol. Furthermore, histopathology of diseased snakeheads infected with A. schubertii showed necrosis and congestion in liver, kidney and spleen and also damage to the cardiac muscle, intestine and gills.

Uncovering toxicological complexity by multi-dimensional screenings in microsegmented flow: modulation of antibiotic interference by nanoparticles

The technique of microsegmented flow was applied for the generation of two- and higher dimensional concentration spaces for the screening of toxic effects of selected substances on the bacterium Escherichia coli at the nanolitre scale. Up to about 5000 distinct experiments with different combinations of effector-concentrations could be realized in a single experimental run. This was done with the help of a computer program controlling the flow rates of effector-containing syringe pumps and resulted in the formation of multi-dimensional concentration spaces in segment sequences. Prior to the application of this technique for toxicological studies on E. coli the accuracy of this method was tested by simulation experiments with up to five dissolved dyes with different spectral properties. Photometric microflow-through measurement of dye distribution inside the concentration spaces allowed the monitoring of microfluid segment compositions. Finally, we used this technique for the investigation of interferences of the antibiotics ampicillin and chloramphenicol towards E. coli cultures and their modulation by silver nanoparticles by measuring bacterial autofluorescence. Each concentration point in this three-dimensional concentration space was represented by 4 or 5 single segments. Thus, a high reliability of the measured dose/response relations was achieved. As a result, a complex response pattern was discovered including synergistic and compensatory effects as well as the modulation of the range of stimulation of bacterial growth by a sublethal dose of chloramphenicol by silver nanoparticles.

Jadomycins derived from the assimilation and incorporation of norvaline and norleucine

Streptomyces venezuelae ISP5230 is recognized for the production of chloramphenicol and the jadomycin family of natural products. The jadomycins are angucycline natural products containing a unique oxazolone ring incorporating an amino acid present in the minimal culture media. Substitution of different amino acids results in products of varying biological activity. Analysis of cultures of S. venezuelae ISP5230 incubated with l- and d-norvaline and l- and d-norleucine indicated that only the d-configured amino acids were incorporated into the natural products. Subsequently, jadomycin DNV and jadomycin DNL were isolated and characterized (titers 4 and 9 mg L(-1), respectively). The compounds were evaluated in the National Cancer Institute cell line cancer growth inhibition and cytotoxicity screens, for antimicrobial activity against selected Gram-positive and Gram-negative bacteria, and as DNA-cleavage agents in vitro.

Dissipation of carbendazim and chloramphenicol alone and in combination and their effects on soil fungal:bacterial ratios and soil enzyme activities

The dissipation of carbendazim and chloramphenicol alone and in combination and their effects on soil fungal:bacterial ratios and soil enzyme activities were investigated. The results revealed that carbendazim dissipation was little affected by chloramphenicol, whereas chloramphenicol dissipation was found to be retarded significantly by the presence of carbendazim. The inhibitory effect of carbendazim on the fungal:bacterial ratios was increased by the presence of chloramphenicol, and the inhibitory effect of chloramphenicol on neutral phosphatase was increased by the presence of carbendazim. Carbendazim increased soil catalase and urease activities, but this increase was partially diminished by the presence of chloramphenicol. Little interaction was observed between carbendazim and chloramphenicol with regard to their influence on soil invertase. The results obtained in this study suggest that combinations of fungicides and antibiotics may alter the compounds' individual behaviors in soil and their effects on soil enzymes.

Relationships between chemical oxygen demand (COD) components and toxicity in a sequential anaerobic baffled reactor/aerobic completely stirred reactor system treating Kemicetine

In this study the interactions between toxicity removals and Kemicetine, COD removals, intermediate products of Kemicetine and COD components (CODs originating from slowly degradable organics, readily degradable organics, inert microbial products and from the inert compounds) were investigated in a sequential anaerobic baffled reactor (ABR)/aerobic completely stirred tank reactor (CSTR) system with a real pharmaceutical wastewater. The total COD and Kemicetine removal efficiencies were 98% and 100%, respectively, in the sequential ABR/CSTR systems. 2-Amino-1 (p-nitrophenil)-1,3 propanediol, l-p-amino phenyl, p-amino phenol and phenol were detected in the ABR as the main readily degradable inter-metabolites. In the anaerobic ABR reactor, the Kemicetin was converted to corresponding inter-metabolites and a substantial part of the COD was removed. In the aerobic CSTR reactor the inter-metabolites produced in the anaerobic reactor were completely removed and the COD remaining from the anerobic reactor was biodegraded. It was found that the COD originating from the readily degradable organics did not limit the anaerobic degradation process, while the CODs originating from the slowly degradable organics and from the inert microbial products significantly decreased the anaerobic ABR reactor performance. The acute toxicity test results indicated that the toxicity decreased from the influent to the effluent of the aerobic CSTR reactor. The ANOVA test statistics showed that there was a strong linear correlation between acute toxicity, CODs originating from the slowly degradable organics and inert microbial products. A weak correlation between acute toxicity and CODs originating from the inert compounds was detected.

A structural study of the interaction between the Dr haemagglutinin DraE and derivatives of chloramphenicol

Dr adhesins are expressed on the surface of uropathogenic and diffusely adherent strains of Escherichia coli. The major adhesin subunit (DraE/AfaE) of these organelles mediates attachment of the bacterium to the surface of the host cell and possibly intracellular invasion through its recognition of the complement regulator decay-accelerating factor (DAF) and/or members of the carcinoembryonic antigen (CEA) family. The adhesin subunit of the Dr haemagglutinin, a Dr-family member, additionally binds type IV collagen and is inhibited in all its receptor interactions by the antibiotic chloramphenicol (CLM). In this study, previous structural work is built upon by reporting the X-ray structures of DraE bound to two chloramphenicol derivatives: chloramphenicol succinate (CLS) and bromamphenicol (BRM). The CLS structure demonstrates that acylation of the 3-hydroxyl group of CLM with succinyl does not significantly perturb the mode of binding, while the BRM structure implies that the binding pocket is able to accommodate bulkier substituents on the N-acyl group. It is concluded that modifications of the 3-hydroxyl group would generate a potent Dr haemagglutinin inhibitor that would not cause the toxic side effects that are associated with the normal bacteriostatic activity of CLM.

[Diffusion and diffusion-osmosis models of the charged macromolecule transfer in barriers of biosystems]

Mathematical models of the transfer of charged macromolecules have been constructed on the basis of the classical equations of electromigration diffusion of Helmholtz-Smolukhovskii, Goldman, and Goldman-Hodgkin-Katz. It was shown that ion transfer in placental (mimicking lipid-protein barriers) and muscle barriers occurs by different mechanisms. In placental barriers, the electromigration diffusion occurs along lipid-protein channels formed due to the conformational deformation of phospholipid and protein molecules with the coefficients of diffusion D = (2.6-3.6) x 10(-8) cm2/s. The transfer in muscle barriers is due to the migration across charged interfibrillar channels with the negative diffusion activation energy, which is explained by changes in the structure of muscle fibers and expenditures of thermal energy for the extrusion of Cl- from channel walls with the diffusion coefficient D = (6.0-10.0) x 10(-6) cm2/s.

Random dissection to select for protein split sites and its application in protein fragment complementation

To identify protein split sites quickly, a selection procedure by using chloramphenicol acetyl transferase (CAT) as reporter was introduced to search for folded protein fragments from libraries generated by random digestion and reassembly of the target gene, which yielded an abundant amount of DNA fragments with controllable lengths. Experimental results of tryptophan synthase alpha subunit (TSalpha) and TEM-1 beta-lactamase agreed well with what the literature has reported. The solubility of these fragments correlated roughly with the minimum inhibitory concentrations of the CAT fusions. The application of this dissection protocol to protein fragment complementation assay (PCA) was evaluated using aminoglycoside-3'-phosphotransferase I (APH(3')-I) as a model protein. Three nearly bisectional sites and a number of possible split points were identified, and guided by this result, four novel pairs of fragments were tested for complementation. Three out of four pairs partially restored the APH activity with the help of leucine zippers, and a truncated but active APH(3')-I (Delta1-25) was also found. Finally, the weakly active APH(3')-I-(1-253)NZ/CZ (254-271) containing a short 18 residue tag was further improved by error-prone PCR, and a best mutant was obtained showing a fourfold improvement after just one round of evolution. These results demonstrate that protein random dissection based on the CAT selection can provide an efficient search for protein breakage points and guide the design of fragments for protein complementation assay. Furthermore, more active fragment pairs can be achieved with the classical directed evolution approach.

Identification and evolution of drug efflux pump in clinical Enterobacter aerogenes strains isolated in 1995 and 2003

Background

The high mortality impact of infectious diseases will increase due to accelerated evolution of antibiotic resistance in important human pathogens. Development of antibiotic resistance is a evolutionary process inducing the erosion of the effectiveness of our arsenal of antibiotics. Resistance is not necessarily limited to a single class of antibacterial agents but may affect many unrelated compounds; this is termed ‘multidrug resistance’ (MDR). The major mechanism of MDR is the active expulsion of drugs by bacterial pumps; the treatment of Gram negative bacterial infections is compromised due to resistance mechanisms including the expression of efflux pumps that actively expel various usual antibiotics (ß-lactams, quinolones, …).

Methodology/Principal Findings

Enterobacter aerogenes has emerged among Enterobacteriaceae associated hospital infections during the last twenty years due to its faculty of adaptation to antibiotic stresses. Clinical isolates of E. aerogenes belonging to two strain collections isolated in 1995 and 2003 respectively, were screened to assess the involvement of efflux pumps in antibiotic resistance. Drug susceptibility assays were performed on all bacterial isolates and an efflux pump inhibitor (PAßN) previously characterized allowed to decipher the role of efflux in the resistance. Accumulation of labelled chloramphenicol was monitored in the presence of an energy poison to determine the involvement of active efflux on the antibiotic intracellular concentrations. The presence of the PAßN-susceptible efflux system was also identified in resistant E. aerogenes strains.

Conclusions/Significance

For the first time a noticeable increase in clinical isolates containing an efflux mechanism susceptible to pump inhibitor is report within an 8 year period. After the emergence of extended spectrum ß-lactamases in E. aerogenes and the recent characterisation of porin mutations in clinical isolates, this study describing an increase in inhibitor-susceptible efflux throws light on a new step in the evolution of mechanism in E. aerogenes.

Chloramphenicol is a substrate for a novel nitroreductase pathway in Haemophilus influenzae

The p-nitroaromatic antibiotic chloramphenicol has been used extensively to treat life-threatening infections due to Haemophilus influenzae and Neisseria meningitidis; its mechanism of action is the inhibition of protein synthesis. We found that during incubation with H. influenzae cells and lysates, chloramphenicol is converted to a 4-aminophenyl allylic alcohol that lacks antibacterial activity. The allylic alcohol moiety undergoes facile re-addition of water to restore the 1,3-diol, as well as further dehydration driven by the aromatic amine to form the iminoquinone. Several Neisseria species and most chloramphenicol-susceptible Haemophilus species, but not Escherichia coli or other gram-negative or gram-positive bacteria we examined, were also found to metabolize chloramphenicol. The products of chloramphenicol metabolism by species other than H. influenzae have not yet been characterized. The strains reducing the antibiotic were chloramphenicol susceptible, indicating that the pathway does not appear to mediate chloramphenicol resistance. The role of this novel nitroreductase pathway in the physiology of H. influenzae and Neisseria species is unknown. Further understanding of the H. influenzae chloramphenicol reduction pathway will contribute to our knowledge of the diversity of prokaryotic nitroreductase mechanisms.

No single irreplaceable acidic residues in the Escherichia coli secondary multidrug transporter MdfA

The largest family of solute transporters (major facilitator superfamily [MFS]) includes proton-motive-force-driven secondary transporters. Several characterized MFS transporters utilize essential acidic residues that play a critical role in the energy-coupling mechanism during transport. Surprisingly, we show here that no single acidic residue plays an irreplaceable role in the Escherichia coli secondary multidrug transporter MdfA.

Surgical removal of intraocular antibiotic ointment after routine cataract phacoemulsification

We report surgical removal of a droplet of intraocular chloramphenicol ointment in a 70-year-old man who had routine small-incision phacoemulsification. A spherical droplet of ointment adherent to the intraocular lens was noted 2 months postoperatively. The source was considered to be the immediate postoperative conjunctival fornix chloramphenicol ointment. Gonioscopy revealed tiny droplets adherent to the peripheral iris and angle. The droplet was surgically explanted using a minimally traumatic technique with a lens glide and ophthalmic viscosurgical device through an enlarged clear corneal incision and sent intact for laboratory biochemical analysis. The patient maintained good vision without evidence of uveitis or secondary glaucoma at 1 year of follow-up. This uncommon case also raises issues about the need for immediate postoperative antibiotic ointment for endophthalmitis prophylaxis and phacoemulsification thermal wound effects on the integrity of self-sealing clear corneal incisions.

Design, synthesis and ribosome binding of chloramphenicol nucleotide and intercalator conjugates

Molecular modelling based on X-ray structures of the antibiotic drug chloramphenicol bound in a bacterial ribosome has been used for design of chloramphenicol derivatives. Conjugates of the chloramphenicol amine through appropriate linkers to either a pyrene moiety or to a mono- or dinucleotide moiety were designed to improve binding to ribosomes by providing specific interactions in the peptidyl transferase site or to the P-loop in the ribosome. Specific binding of the conjugates were investigated by footprinting analysis using chemical modifications of accessible nucleotides in ribosomal RNA. The pyrene chloramphenicol conjugate shows enhanced binding to the chloramphenicol binding site compared to the native chloramphenicol, whereas the four nucleotide conjugates could not be shown to bind to the chloramphenicol binding site or to the P-loop.

Indole-3-carbinol inhibition of androgen receptor expression and downregulation of androgen responsiveness in human prostate cancer cells

Indole-3-carbinol (I3C), a naturally occurring compound found in vegetables of the Brassica genus, such as broccoli and cabbage, is a promising anticancer agent previously shown to induce a G(1) cell-cycle arrest in the cells of human lymph node carcinoma of prostate (LNCaP) through regulation of specific G(1)-acting cell-cycle components. Since the androgen receptor (AR) mediates proliferation and differentiation in the prostate and is expressed in nearly all human prostate cancers, the effects of I3C on AR expression and function were examined in LNCaP cells. Immunoblot and quantitative RT-PCR assays revealed that I3C inhibited the expression of AR protein and mRNA levels within 12 h of indole treatment. I3C downregulated the reporter activity of LNCaP cells transiently transfected with an AR promoter-luciferase plasmid, demonstrating that a unique response to I3C is the inhibition of AR promoter activity. In contrast to I3C, the natural I3C dimerization product 3,3'-diindolylmethane, which acts as an androgen antagonist, had no effect on AR expression. To determine the functional significance of the I3C-inhibited expression of AR, the AR-regulated prostate specific antigen (PSA) was utilized as a downstream indicator. I3C downregulated the expression of PSA transcripts and protein levels and inhibited PSA promoter activity, as well as that of a minimal androgen responsive element containing reporter plasmid. Expression of exogenous AR prevented the I3C disruption of androgen-induced PSA expression. Taken together, our results demonstrate that I3C represses AR expression and responsiveness in LNCaP cells as a part of its antiproliferative mechanism.

Inhibition of [14C]chloramphenicol binding to Escherichia coli ribosomes by erythromycin derivatives

The effect of erythromycin A and 35 analogues of erythromycin A on [(14)C]chloramphenicol binding to Escherichia coli ribosomes was evaluated. Substitutions on various portions of the erythromycin molecule were made with retention of ability to bind to ribosomes. Specifically, substantial activity in interference with [(14)C]chloramphenicol binding was retained upon removal of the cladinose and various substitutions on the 3-hydroxyl, the oxime, and 2-hydroxyl groups. Erythromycin analogues with relatively poor binding activity to ribosomes could be detected. This assay can be used alone or in conjunction with microbiological assays for screening of active analogues. It permits an estimate of the general binding activity of compounds rapidly and directly. The assay reflects the ability of the compounds to interact with their target organelle, the ribosome, and may serve as a useful adjunct in developing new compounds.

Mutations in the chloramphenicol acetyltransferase (S61G, Y105C) increase accumulated amounts and resistance in Pseudomonas aeruginosa

A chloramphenicol acetyltransferase (catB7) gene containing two point mutations, 181A/G and 314A/G, has been recently reported to be a determinant for high-level chloramphenicol resistance phenotype in a Pseudomonas aeruginosa strain PAhcr1. The mutant CATB7 was further characterized in vitro and in vivo to elucidate the molecular basis of high-level resistance. CAT assay demonstrated that the mutant and wild-type recombinant CATB7 had similar specific activities. Dot blotting revealed that the accumulated amounts of CATB7 in P. aeruginosa strains PAO1 and PAhcr1 were proportionate to the respective anti-chloramphenicol level. Site-directed mutagenesis showed that G61S and Y105C contributed synergistically to the PAhcr1 resistance phenotype. It could be proposed that the mutant CATB7 was more structurally stable than catalytically efficient as a chloramphenicol resistance determinant in PAhcr1.

High Resolution Studies of the Afa/Dr Adhesin DraE and Its Interaction with Chloramphenicol

Pathogenic Escherichia coli expressing Afa/Dr adhesins are able to cause both urinary tract and diarrheal infections. The Afa/Dr adhesins confer adherence to epithelial cells via interactions with the human complement regulating protein, decay accelerating factor (DAF or CD55). Two of the Afa/Dr adhesions, AfaE-III and DraE, differ from each other by only three residues but are reported to have several different properties. One such difference is disruption of the interaction between DraE and CD55 by chloramphenicol, whereas binding of AfaE-III to CD55 is unaffected. Here we present a crystal structure of a strand-swapped trimer of wild type DraE. We also present a crystal structure of this trimer in complex with chloramphenicol, as well as NMR data supporting the binding position of chloramphenicol within the crystal. The crystal structure reveals the precise atomic basis for the sensitivity of DraE-CD55 binding to chloramphenicol and demonstrates that in contrast to other chloramphenicol-protein complexes, drug binding is mediated via recognition of the chlorine "tail" rather than via intercalation of the benzene rings into a hydrophobic pocket.

Enzymatic inactivation and reactivation of chloramphenicol byMycobacterium tuberculosisandMycobacterium bovis

Mycobacterium tuberculosis and Mycobacterium bovis are inhibited by chloramphenicol. Chloramphenicol acetyltransferase (CAT) converts chloramphenicol to inactive diacetyl chloramphenicol, but a mycobacterial carboxylesterase hydrolyzes the diacetyl product to active chloramphenicol. The esterase activity was eliminated by proteinase K and heat treatment. Protein extracts of M. tuberculosis and M. bovis hydrolyzed four other ester substrates. cat was inserted into the chromosome of both M. tuberculosis and M. bovis resulting in a level of chloramphenicol resistance that could be used to select for transformants. CAT assays in the resistant strain of M. tuberculosis showed interference due to esterase activity. This interference could be eliminated with the addition of a heating step.