Effects of quinolones on nucleoid segregation in Escherichia coli

Classification of antimicrobial mechanism of action using dynamic bacterial morphology imaging

Antimicrobial resistance is a major threat to human health. Basic knowledge of antimicrobial mechanism of action (MoA) is imperative for patient care and for identification of novel antimicrobials. However, the process of antimicrobial MoA identification is relatively laborious. Here, we developed a simple, quantitative time-lapse fluorescence imaging method, Dynamic Bacterial Morphology Imaging (DBMI), to facilitate this process. It uses a membrane dye and a nucleoid dye to track the morphological changes of single Bacillus subtilis cells in response to antimicrobials for up to 60 min. DBMI of bacterial cells facilitated assignment of the MoAs of 14 distinct, known antimicrobial compounds to the five main classes. We conclude that DBMI is a simple method, which facilitates rapid classification of the MoA of antimicrobials in functionally distinct classes.

Antibacterial mechanism of rhodomyrtone involves the disruption of nucleoid segregation checkpoint in Streptococcus suis

Rhodomyrtone has been recently demonstrated to possess a novel antibiotic mechanism of action against Gram-positive bacteria which involved the multiple targets, resulting in the interference of several bacterial biological processes including the cell division. The present study aims to closely look at the downstream effect of rhodomyrtone treatment on nucleoid segregation in Streptococcus suis, an important zoonotic pathogen. The minimum inhibition concentration (MIC) and the minimum bactericidal concentration (MBC) values of rhodomyrtone against the recombinant S. suis ParB-GFP, a nucleoid segregation reporter strain, were 0.5 and 1 µg/ml, respectively, which were equivalent to the potency of vancomycin. Using the fluorescence live-cell imaging, we demonstrated that rhodomyrtone at 2× MIC caused incomplete nucleoid segregation and septum misplacement, leading to the generation of anucleated cells. FtsZ immune-staining of rhodomyrtone-treated S. suis for 30 min revealed that the large amount of FtsZ was trapped in the region of high fluidity membrane and appeared to be able to polymerize to form a complete Z-ring. However, the Z-ring was shifted away from the midcell. Transmission electron microscopy further confirmed the disruption of nucleoid segregation and septum misplacement at 120 min following the rhodomyrtone treatment. Asymmetric septum formation resulted in either generation of minicells without nucleoid, septum formed over incomplete segregated nucleoid (guillotine effect), or formation of multi-constriction of Z-ring within a single cell. This finding spotlights on antibacterial mechanism of rhodomyrtone involves the early stage in bacterial cell division process.

The multiple antibiotic resistance operon of enteric bacteria controls DNA repair and outer membrane integrity

The multiple antibiotic resistance (mar) operon of Escherichia coli is a paradigm for chromosomally encoded antibiotic resistance in enteric bacteria. The locus is recognised for its ability to modulate efflux pump and porin expression via two encoded transcription factors, MarR and MarA. Here we map binding of these regulators across the E. coli genome and identify an extensive mar regulon. Most notably, MarA activates expression of genes required for DNA repair and lipid trafficking. Consequently, the mar locus reduces quinolone-induced DNA damage and the ability of tetracyclines to traverse the outer membrane. These previously unrecognised mar pathways reside within a core regulon, shared by most enteric bacteria. Hence, we provide a framework for understanding multidrug resistance, mediated by analogous systems, across the Enterobacteriaceae. Transcription factors MarR and MarA confer multidrug resistance in enteric bacteria by modulating efflux pump and porin expression. Here, Sharma et al. show that MarA also upregulates genes required for lipid trafficking and DNA repair, thus reducing antibiotic entry and quinolone-induced DNA damage.

Structural and biochemical analysis of the pentapeptide repeat protein EfsQnr, a potent DNA gyrase inhibitor

The chromosomally encoded Qnr homolog protein from Enterococcus faecalis (EfsQnr), when expressed, confers to its host a decreased susceptibility to quinolones and consists mainly of tandem repeats, which is consistent with belonging to the pentapeptide repeat family of proteins (PRPs). EfsQnr was cloned with an N-terminal 6× His tag and purified to homogeneity. EfsQnr partially protected DNA gyrase from fluoroquinolone inhibition at concentrations as low as 20 nM. EfsQnr inhibited the ATP-dependent supercoiling activity of DNA gyrase with a 50% inhibitory concentration (IC(50)) of 1.2 μM, while no significant inhibition of ATP-independent relaxation activity was observed. EfsQnr was cytotoxic when overexpressed in Escherichia coli, resulting in the clumping of cells and a loss of viability. The X-ray crystal structure of EfsQnr was determined to 1.6-Å resolution. EfsQnr exhibits the right-handed quadrilateral beta-helical fold typical of PRPs, with features more analogous to MfpA (mycobacterium fluoroquinolone resistance pentapeptide) than to the PRPs commonly found in cyanobacteria.

Nonoptimal microbial response to antibiotics underlies suppressive drug interactions

Suppressive drug interactions, in which one antibiotic can actually help bacterial cells to grow faster in the presence of another, occur between protein and DNA synthesis inhibitors. Here, we show that this suppression results from nonoptimal regulation of ribosomal genes in the presence of DNA stress. Using GFP-tagged transcription reporters in Escherichia coli, we find that ribosomal genes are not directly regulated by DNA stress, leading to an imbalance between cellular DNA and protein content. To test whether ribosomal gene expression under DNA stress is nonoptimal for growth rate, we sequentially deleted up to six of the seven ribosomal RNA operons. These synthetic manipulations of ribosomal gene expression correct the protein-DNA imbalance, lead to improved survival and growth, and completely remove the suppressive drug interaction. A simple mathematical model explains the nonoptimal regulation in different nutrient environments. These results reveal the genetic mechanism underlying an important class of suppressive drug interactions.

Mechanism of action of NB2001 and NB2030, novel antibacterial agents activated by beta-lactamases

Two potent antibacterial agents designed to undergo enzyme-catalyzed therapeutic activation were evaluated for their mechanisms of action. The compounds, NB2001 and NB2030, contain a cephalosporin with a thienyl (NB2001) or a tetrazole (NB2030) ring at the C-7 position and are linked to the antibacterial triclosan at the C-3 position. The compounds exploit beta-lactamases to release triclosan through hydrolysis of the beta-lactam ring. Like cephalothin, NB2001 and NB2030 were hydrolyzed by class A beta-lactamases (Escherichia coli TEM-1 and, to a lesser degree, Staphylococcus aureus PC1) and class C beta-lactamases (Enterobacter cloacae P99 and E. coli AmpC) with comparable catalytic efficiencies (k(cat)/K(m)). They also bound to the penicillin-binding proteins of S. aureus and E. coli, but with reduced affinities relative to that of cephalothin. Accordingly, they produced a cell morphology in E. coli consistent with the toxophore rather than the beta-lactam being responsible for antibacterial activity. In biochemical assays, they inhibited the triclosan target enoyl reductase (FabI), with 50% inhibitory concentrations being markedly reduced relative to that of free triclosan. The transport of NB2001, NB2030, and triclosan was rapid, with significant accumulation of triclosan in both S. aureus and E. coli. Taken together, the results suggest that NB2001 and NB2030 act primarily as triclosan prodrugs in S. aureus and E. coli.

beta-Lactamase hydrolysis of cephalosporin 3'-quinolone esters, carbamates, and tertiary amines

The beta-lactam hydrolysis of five cephalosporin 3'-quinolones (dual-action cephalosporins) by three gram-negative beta-lactamases was examined. The dual-action cephalosporins tested were the ester Ro 23-9424; the carbamates Ro 25-2016, Ro 25-4095, and Ro 25-4835; and the tertiary amine Ro 25-0534. Also tested were cephalosporins with similar side chains (cefotaxime, desacetylcefotaxime, cephalothin, cephacetrile, and Ro 09-1227 [SR 0124]) and standard beta-lactams (penicillin G, cephaloridine). The beta-lactamases used were the plasmid-mediated TEM-1 and TEM-3 enzymes and the chromosomal AmpC. The cephacetrile-related compounds Ro 25-4095 and Ro 25-4835 were hydrolyzed by all three beta-lactamases with catalytic efficiencies (relative to penicillin G) ranging from approximately 5 (TEM-1, AmpC) to approximately 25 (TEM-3). The cephalothin-related Ro 25-2016 was also hydrolyzed by all three beta-lactamases, particularly the AmpC enzyme (relative catalytic efficiency, 110). The cefotaxime-related compounds Ro 25-0534 and Ro 23-9424 were hydrolyzed to any significant extent only by the TEM-3 enzyme (relative catalytic efficiencies, 1.2 and 4.7, respectively.

Mechanisms of action of cephalosporin 3'-quinolone esters, carbamates, and tertiary amines in Escherichia coli

Cephalosporin 3'-quinolone esters, carbamates, and tertiary amines are potent antibiotics whose antibacterial activities reflect the action of both the beta-lactam and the quinolone components. The biological properties of representative compounds from each class were compared in Escherichia coli. All compounds bound to the essential PBP 3, inhibited DNA gyrase, and caused filamentation in growing cells. To distinguish between cephalosporin- and quinolone-induced filaments, nucleoid segregation was also examined, as quinolones disrupt nucleoid segregation while the beta-lactams do not (N. H. Georgopapadakou and A. Bertasso, Antimicrob. Agents Chemother. 35:2645-2648, 1991). The cephalosporin quinolone esters Ro 23-9424 and Ro 24-6392, at concentrations causing filamentation in E. coli ATCC 25922, did not affect nucleoid segregation after 1 h of incubation (cephalosporin response) but did not affect it after 2 h (quinolone response), indicating the release of free quinolone. Accordingly, only the quinolone response was produced in a strain possessing TEM-3, an expanded-spectrum beta-lactamase. The cephalosporin carbamate Ro 24-4383 and the tertiary amine Ro 24-8138 produced a quinolone response in E. coli ATCC 25922, though they produced a cephalosporin response in a quinolone-resistant strain. Carbamate and tertiary amine linkages are chemically more stable than the ester linkage, and both cephalosporin 3'-quinolone carbamates and tertiary amines are more potent inhibitors of DNA gyrase than are the corresponding esters. The results suggest that, while intact cephalosporin 3'-quinolone esters act as cephalosporins, carbamates and amines may possess both cephalosporin and quinolone activity in the intact molecule.

Quinolone mode of action--new aspects

The interactions of quinolones with the complex of DNA gyrase and DNA have been elucidated by the sequencing of additional mutant gyrA and gyrB genes that produce altered quinolone susceptibility. Strong patterns have emerged in Escherichia coli in which amino acids between positions 67 and 106 of the gyrase A subunit (GyrA) and at positions 426 and 447 of the gyrase B subunit (GyrB) have been consistently identified as important for quinolone action. The susceptibility patterns and changes in amino acids 426 and 447 in mutant resistant GyrB proteins suggest direct electrostatic interactions with quinolones at these positions. The small size and the polar nature of the serine at position 83 of the E. coli GyrA protein are particularly important for determining enzyme sensitivity and bacterial susceptibility to quinolones. Norfloxacin and ciprofloxacin bind most stably to a complex of DNA gyrase and DNA rather than to either component alone, and reduction of norfloxacin binding to complexes containing resistant GyrA proteins confirms the biological relevance of this direct measure of quinolone interaction with the gyrase-DNA complex. Although recent crystallographic studies have expanded and refine information about gyrase structure at the atomic level, direct determination of the sites of quinolone binding within the gyrase-DNA complex awaits further studies. Although quinolones have little activity against E. coli topoisomerases I and III, topoisomerase IV, a recently described enzyme thought to be involved in chromosome segregation into daughter cells, has homology with GyrA and GyrB, particularly in regions important for quinolone action, and is inhibited by some quinolones in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of macromolecular biosynthesis to define the quinolone-induced postantibiotic effect in Escherichia coli

Quinolones inhibit DNA gyrase, and the major effects of this inhibition are on replication and transcription of DNA. The postantibiotic effect (PAE) refers to continued inhibition of cell division, in terms of the viable count, following transient exposure to an antibiotic. Previous work has shown that quinolone-treated cells have not fully recovered by the time the classically defined PAE has ended. We describe the PAE of the quinolones CI-960, enoxacin, and ciprofloxacin on macromolecular biosynthesis in the clinical isolate Escherichia coli J96 in an attempt to relate the PAE to the time that it actually takes for the cells to recover fully. DNA synthesis was inhibited immediately upon exposure to these quinolones at 0.5x or 0.75x the MIC. This inhibition continued for several hours following quinolone removal. The effects of these quinolones on RNA and protein synthesis varied; enoxacin treatment at 0.5x the MIC resulted in an increase of over 60% in both RNA and protein synthesis per unit of cell mass, while ciprofloxacin and CI-960 at that level had no significant effects on either RNA or protein synthesis. The effects of enoxacin and ciprofloxacin on bacterial protein profiles were also distinguishable, and these changes corresponded to their PAE on DNA synthesis. Throughout the study, all measures of the physiological status of the cells returned to normal by the time DNA synthesis per unit of cell mass did so. These results suggest that DNA synthesis per unit of cell mass provides an accurate measure of the time required for quinolone-treated cells to recover fully.