The Journal of Experimental Vol 3:50-56 Microbiology & Immunology+ Macrolide Structures Can Confer Differential Susceptibility in Escherichia coli K30 Deletions of Group 1 Capsule Assembly Genes Jady Chiu, Gloria Han, Kevin McCrystal, Michelle Zuo Department of Microbiology and Immunology, University of British Columbia The Wzy-dependent biosynthesis system in Escherichia coli K30 is responsible for the assembly and transport of extracellular capsule polysaccharides. The Wzy system has three key components: Wza, a transport channel located on the outer membrane; Wzc, an autokinase in the inner membrane coupled to Wza; and Wzb, a cytosolic phosphatase that regulates Wzc activity. Knock-outs of this assembly system have been observed to correlate with increased resistance to some macrolide antibiotics, including erythromycin, clarithromycin, and roxithromycin, but not to telithromycin, which is distinct from other macrolides with additional ketone groups and aromatic rings. We hypothesize that the route of entry of macrolides is partially dependent on the Wzy capsular transport system, and that distinct molecular structures of macrolides contribute to differences in bacterial susceptibility. Disc diffusion assays were conducted using Wza knock-outs, Wzc knock-outs, and Wza-Wzb-Wzc triple knock-outs of the E. coli K30 strain to test their resistance against erythromycin, clarithromycin, azithromycin, and telithromycin. We observed that a Wza deletion confers greatest resistance to erythromycin and clarithromycin, while the absence of Wzc resulted in mildly increased resistance against these two macrolides. All knock-out strains were observed to have increased but incomplete resistance to telithromycin, and only the Wza single knock-out strain was more resistant to azithromycin. We conclude that Wza is essential for outer membrane penetration of erythromycin and clarithromycin, while telithromycin entry is equally dependent on Wza and Wzc. Azithromycin entry is largely dependent on Wza. However, the antibiotic susceptibilities of the triple knockout results contrasted the Wza single knockout susceptibilities, indicating that there may be other factors involved in susceptibility. Macrolides are a class of widely-used antibiotics that pumps, enzymatic inactivation, and modification of the inhibit bacterial protein synthesis. Their mechanism of ribosomal target site (5). Semi-synthetic derivatives of action involves binding to the 50s ribosomal subunit and erythromycin such as ketolides and azalides have dissociating peptidyl-tRNA from the ribosome to halt structures that improve membrane permeability, acid polypeptide elongation, resulting in a bacteriostatic effect stability, or ribosomal binding to overcome resistance (1). Most macrolides contain a lactone ring composed of 14 issues and result in increased activity against certain to 16 atoms linked to at least one sugar moiety, as they are Gram-negative strains (2). derived from the lead optimization of erythromycin (Fig. Capsular polysaccharides are extracellular structures 1) (2, 3). that shield bacteria from the environment and assist in Macrolides are effective against infections caused by biofilm formation (6). In Escherichia coli K30, the assembly Gram-positive bacteria, but their hydrophobicity means and transport of Group 1 capsular polysaccharides are that Gram-negative bacterial strains are rarely susceptible performed by the Wzy-dependent biosynthesis system to macrolides (4). The negatively-charged core region of (Fig. 2), which includes Wza, Wzb, and Wzc (6). Wza is an lipopolysaccharides (LPS) in the outer membrane (OM) of outer membrane protein that forms a periplasm-spanning Gram-negative bacteria prevents the passage of large, capsular polysaccharide transport channel with Wzc. Wzb hydrophobic, and structurally complex molecules, thereby providing intrinsic resistance to nonpolar antibiotics (4). Received: 06/29 2016 Accepted: 06/01 2017 Published: 07/10 2017 Other resistance mechanisms include antibiotic efflux Author Contributions: Worked as a team in UBC MICB 421. July 2017 Volume 3 Undergraduate Research Article 50 telithromycin may be utilised to enter Δwza-wzb-wzc cells due to several key differences in the chemical structure of telithromycin that render it comparatively more polar (Fig. 1) (13). Therefore, increased macrolide polarity may result in an improved ability to diffuse across the polar core domain of the outer membrane independently of the Wza- Wzc channel (13). The prevention of drug entry into a bacterial cell is a significant source of antibiotic resistance (5). Researching the potential mechanism of macrolide resistance due to the absence of the Wzy-dependent biosynthesis system, or necessary constituent proteins may identify molecular trends in antibiotic resistance and susceptibility and identify protein characteristics that permit entry of other commonly available drugs, and avoid excessive stress on new drug design. FIG. 1 Chemical structures of macrolide antibiotics used in this Telithromycin is a member of the ketolide subgroup of experiment with differences from erythromycin highlighted in macrolides that is characterized by several key changes in colour. Black denotes the unchanged erythromycin chemical its chemical structure as compared to erythromycin. First, structure. Red denotes substitution of a hydroxyl group for a a sugar and a hydroxyl group on the main lactone ring of methoxy group. Green denotes the addition of a ketone group. Blue erythromycin are respectively replaced with a ketone denotes the addition of nitrogen containing groups such as a functional group and a cyclic carbamate group in tertiary amine, imine, or imidazole with pyridine. Original picture telithromycin (14). The addition of a large aromatic alkyl- modified from Gomes et al. (13). aryl extension group also increases the molecular affinity is a phosphatase which regulates Wzc activity (6). Wzc is a to the 50s ribosomal subunits (15). These changes increase n inner membrane tyrosine autokinase (6, 7). The structure the molecular polarity and binding strength, resulting in of Wza consists of eight alpha-helices forming a barrel crossing the OM and is required for capsule polysaccharide export (7, 8). The interior surface of Wza is polar and exposed to the environment, and Wza activity is regulated by the occlusion of the periplasmic region by a flexible loop domain that is manipulated by Wzc (7, 9). Interestingly, the simultaneous deletion of wza, wzb, and wzc from the wzy cassette in the Gram-negative strain E. coli K30 (E69) appears to confer resistance to macrolides (10). Using disc diffusion assays, Botros et al. found that a triple deletion of Wza, Wzb, and Wzc in CWG655 conferred increased resistance to erythromycin, clarithromycin, and roxithromycin, but the strain remained sensitive to telithromycin (10). Follow-up projects studying single knockouts have found that both wza and wzc single deletions conferred erythromycin resistance, while a wzb deletion did not (11, 12, 18). The telithromycin sensitivity of CWG655 (Δwza-wzb-wzc) observed by Botros et al. shows that the macrolide resistance phenotype does not extend to all antibiotics in this class (10). Erythromycin entry was speculated by Jazdarehee et al. to be via the transport channel formed by FIG. 2 Diagram of Wza, Wzb and Wzc arranged in a section of the Wza and Wzc, so that the deletion of the genes encoding Wzy-dependent biosynthesis system. Capsular polysaccharides either one of those two membrane proteins reduces the are transported across the inner membrane where they interact permeability of the cell to erythromycin (12). with Wza, Wzb, and Wzc, and are transferred through the OM (6). Based on the model that Wza and Wzc form a channel for Wza forms a channel that spans the OM and transports polysaccharides to the cell surface (6). Wzc is a tyrosine autokinase macrolide entry, other mechanisms of entry for that is regulated by Wzb, which is a phosphatase (6). July 2017 Volume 3 Undergraduate Research Article 51 more effective bacteriostatic potency of ketolides when concentration was diluted to 1 OD660 measured using a Spectronic compared to other macrolides (15). 20+ spectrophotometer. 100 µL of the 1 OD660 culture was spread onto LB plates. Each plate received three antibiotic discs that Based on the premise that telithromycin is more effective contained 15 µg of erythromycin, clarithromycin, telithromycin, against Δwza-wzb-wzc strains than erythromycin due to or azithromycin, which were placed equally spaced apart on one modifications that make the antibiotic more polar, we half of the plate using a sterilized forceps. Three blank Whatman hypothesized that other macrolide antibiotics that have paper discs were placed on the other half to serve as the negative higher polarity should also be effective against the Δwza- control. The plates were incubated for 18 hours at 37°C. wzb-wzc strain. To investigate this, we performed disc We initially used hand-poured LB agar plates, which had an diffusion assays with a range of macrolide antibiotics. average volume of 21.4 mL per plate. To control for variation in Alongside telithromycin, erythromycin and agar volume, we used an automated dispenser to pour plates that clarithromycin were also tested. Additionally, we included contained exactly 20 mL of LB agar per plate. A total of seven azithromycin in our experiments, which is a macrolide replicates of each antibiotic and strain combination was performed, yielding 21 measurements for