Video Challenge
The Challenge
Learn
Participate
Submit
Judges
Rules
Resources
2017 Awards
2016 Awards
2015 Awards
2014 Awards

2018 RCSB PDB Video Challenge for High School Students

Mechanisms of Bacterial Resistance
to Beta-lactam Antibiotics

 

Learn about the action of beta-lactam antibiotics and the mechanisms of resistance

 

Before you begin

In order to comprehend the information on antibiotic action and resistance in this section, you should be familiar with the biomolecular concepts listed below. Click on the links to access the PDB-101 resources for each topic.

Learn about action of beta-lactam antibiotics and the mechanisms of resistance

The information in this section will help you become familiar with the challenge topic, and provides links to further learning resources. You will also find links to related PDB structures that you can explore in 3D with visualization tips and resources that can be used to create molecular images/animations for your video. The information is divided into 4 sections:

  1. Introduction
  2. How beta-lactam antibiotics work
  3. Mechanisms of bacterial resistance
  4. Fighting the resistance mechanisms

I. Introduction

Antibiotics are small molecules, originally discovered as defensive molecules made by fungi and other organisms, and now created by medical scientists. They bind to essential bacterial molecules, such as enzymes and ribosomes, blocking the growth of the bacteria or killing them. Because these bacterial proteins are structurally different from their animal equivalents or are non-existent in them, they cause no harm to the organism treated for bacterial infection.

Figure 1 presents main classes of antibiotics and their target protein/molecule organized by the biochemical process that they interrupt in bacteria. In this video challenge, we will be focusing only on the action of and resistance to the beta-lactam class.


Figure 1

Main classes of antibiotics and their target protein/molecule organized by the biochemical process that they interrupt in bacteria. Download as PDF


II. How beta-lactam antibiotics work

Bacteria protect themselves from osmotic pressure by building multiple layers of peptidoglycans. By the way the peptidoglycans are organized, we can distinguish between 2 types of bacteria: Gram-positive and Gram-negative. Gram-positive bacteria build a thick glycan lattice on the outside of the lipid bilayer membrane, while the Gram-negative bacteria build a thinner layer of peptidoglycan sheets in between 2 lipid bilayer membranes (Figure 2a ). Peptidoglycan monomers are synthesized in the bacterial cytosol and each consists of two sugars connected to a short chain of amino acids ending with two D-alanines. The peptide has also a pentaglycine bridge extending to the side (Figure 2b).

Once synthesized, the monomers are moved across the cell membrane where they are inserted into the existing peptidoglycan sheet. First, the sugars are assembled into chains, which are then crosslinked via the pentaglycine bridges. These processes are catalyzed by enzymes called penicillin-binding proteins (PBPs). Most of them have a DD-transpeptidase domain that is responsible for the crosslinking of peptidoglycans using a reactive serine residue in the active site (Figure 2c). The serine cleaves the terminal D-alanine and connects the penultimate D-alanine to the pentaglycine bridge of another glycan (Figure 2d).


Figure 2

Figure 2a: Schematic view of outer membrane arrangement in Gram-positive (left) and Gram-negative (right) bacteria.


Figure 2b: Peptidoglycan monomer in schematic representation (left and in space filling representation (right).


Figure 2c: Penicillin binding protein (PDB) with DD-transpeptidase domain and reactive serine residue highlighted. PDB structure 5hl9


Figure 2d: PBP creating a link between two peptidoglycan chains. PDB structure: 5hl9



The penicillin binding proteins, as the name suggests, have high affinity to bind penicillin and other beta-lactam antibiotics. The name beta-lactam antibiotics is derived from the core part of their chemical structure, the beta-lactam ring (Figure 3a). Beta-lactam antibiotics bind covalently to the active serine reside in the PBPs (Figure 3b), interrupting the crosslinking process and leaving the bacteria with an incomplete cell wall. As a result, bacteria become permeated with water and eventually explode.


Figure 3

Figure 3a: Penicillin antibiotic in schematic representation (left) and ball-and-stick representation (right) with the beta-lactam ring highlighted in yellow.


Figure 3b: Ampicillin binding covalently to penicillin binding protein. PDB structure 5hl9



Table 1: Major groups of beta-lactam antibiotics

 

III. Mechanisms of bacterial resistance

In the face of overuse and misuse of antibiotics, new strains of bacteria have emerged that can proliferate even in the presence of the newest antibiotics. The resistance to beta-lactam antibiotics is possible due to 2 mechanisms:

  1. Inactivation by a bacterial enzyme, beta-lactamase

    Beta-lactamases are enzymes expressed by resistant bacteria that can cleave the beta-lactam ring in antibiotics rendering them ineffective. In the presence of beta-lactamases, PBPs can continue their cross-linking activity and bacteria can thrive.

    Depending on the sequence similarity, the beta lactamases are divided into classes A, B, C, and D. Class A, C, and D have a reactive serine in the active site, while class B has a zinc ion in the active site.

  2. Alteration to the target site

    Bacteria can develop mutations in the PBPs drug binding site that are of no consequence to their crosslinking activity, but render them immune to antibiotics action. The Methicillin-resistant Staphylococcus aureus (MRSA) is an example of bacteria that developed this type of resistance.


Table 2: Mechanisms of bacterial resistance


There are 2 ways in which the resistance genes can be passed on:

  • Via Chromosome: the gene is transferred from generation to generation (vertical transfer)
  • Via Plasmid: passed from bacterial species to species (horizontal transfer)


IV. Fighting the resistance mechanisms

Researchers design inhibitors of beta-lactamases that can be given along with beta-lactam antibiotics. They inhibit the beta-lactamases leaving the antibiotic free to bind to PBPs.

Table 3: Competitive inhibitors for beta-lactamases



Visualization Resources

  1. The user guide for NGL can be found here. The NGL is the default 3D viewer accessible from each structure summary page, from the tab “3D View”.
  2. You can download and open these sessions using UCSF Chimera. Use the tutorials available here and here to edit the sessions, create animations or save pictures.
    Color coding for PBPs sessions: DD-transpeptidase domain is highlighted in darker orange, the reactive serine residue is highlighted in dark red, antibiotic bound has carbons highlighted in green.
    Color coding for beta-lactamases sessions: reactive serine residue is highlighted in red (classes A, C, D), zinc (class B) in purple, antibiotic (or inhibitor in the active site) has carbons highlighted in green.

 

References

  • Sung Joon Kim et al. (2015) Peptidoglycan Architecture of Gram-positive Bacteria by Solid-State NMR. Biochim Biophys Acta. 1848(0): 350–362.
  • Dustin T. King et al. (2017) Structural Insights into Inhibition of Escherichia coli Penicillin-binding Protein 1B. The Journal of Biological Chemistry 292: 979-993.
  • Wenlin Lee et al. (2001) A 1.2-Å snapshot of the final step of bacterial cell wall biosynthesis. PNAS 98 (4): 1427-1431