Antibacterial Agents

Chemistry, Mode of Action, Mechanisms of Resistance and Clinical Applications

Rosaleen Anderson, Paul W. Groundwater, Adam Todd, Alan Worsley

• Publisher: Wiley
• Year: 2012
• ISBN:
  9781118325445
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

Antibacterial agents act against bacterial infection either by killing the bacterium or by arresting its growth. They do this by targeting bacterial DNA and its associated processes, attacking bacterial metabolic processes including protein synthesis, or interfering with bacterial cell wall synthesis and function. Antibacterial Agents is an essential guide to this important class of chemotherapeutic drugs. Compounds are organised according to their target, which helps the reader understand the mechanism of action of these drugs and how resistance can arise. The book uses an integrated "lab-to-clinic" approach which covers drug discovery, source or synthesis, mode of action, mechanisms of resistance, clinical aspects (including links to current guidelines, significant drug interactions, cautions and contraindications), prodrugs and future improvements. Agents covered include:

• agents targeting DNA - quinolone, rifamycin, and nitroimidazole antibacterial agents
• agents targeting metabolic processes - sulfonamide antibacterial agents and trimethoprim
• agents targeting protein synthesis - aminoglycoside, macrolide and tetracycline antibiotics, chloramphenicol, and oxazolidinones
• agents targeting cell wall synthesis - ?-Lactam and glycopeptide antibiotics, cycloserine, isonaizid, and daptomycin

Antibacterial Agents will find a place on the bookshelves of students of pharmacy, pharmacology, pharmaceutical sciences, drug design/discovery, and medicinal chemistry, and as a bench reference for pharmacists and pharmaceutical researchers in academia and industry.

Section 1

Introduction to Microorganisms and Antibacterial Chemotherapy

Chapter 1.1

Microorganisms

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Key Points

• Bacteria can be classified according to their staining by the Gram stain (Gram positive, Gram negative, and mycobacteria) and their shape.
• Most bacterial (prokaryotic) cells differ from mammalian (eukaryotic) cells in that they have a cell wall and cell membrane, have no nucleus or organelles, and have different biochemistry.
• Bacteria can be identified by microscopy, or by using chromogenic (or fluorogenic) media or molecular diagnostic methods (e.g. real-time polymerase chain reaction (PCR)).
• Bacterial resistance to an antibacterial agent can occur as the result of alterations to a target enzyme or protein, alterations to the drug structure, and alterations to an efflux pump or porin.
• Antibiotic stewardship programmes are designed to optimise antimicrobial prescribing in order to improve individual patient care and slow the spread of antimicrobial resistance.

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1.1.1 Classification

There are two basic cell types: prokaryotes and eukaryotes, with prokaryotes predating the more complex eukaryotes on earth by billions of years. Bacteria are prokaryotes, while plants, animals, and fungi (including yeasts) are eukaryotes. For our purposes in the remainder of this book, we will further subdivide bacteria into Gram positive, Gram negative, and mycobacteria (we will discuss prokaryotic cell shapes a little later).

As you are probably already aware, we can use the Gram stain to distinguish between groups of bacteria, with Gram positive being stained dark purple or violet when treated with Gentian violet then iodine/potassium iodide (Figures 1.1.1 and 1.1.2). Gram negative bacteria do not retain the dark purple stain, but can be visualised by a counterstain (usually eosin or fuschin, both of which are red), which does not affect the Gram positive cells. Mycobacteria do not retain either the Gram stain or the counterstain and so must be visualised using other staining methods. Hans Christian Joachim Gram developed this staining technique in 1884, while trying to develop a new method for the visualisation of bacteria in the sputum of patients with pneumonia, but the mechanism of staining, and how it is related to the nature of the cell envelopes in these different classes of bacteria, is still unclear.

Figure 1.1.1 Dyes used in the Gram stain

Figure 1.1.2 Example of a Gram stain showing Gram positive (Streptococcus pneumoniae) and Gram negative bacteria (Image courtesy of Public Health Image Library, Image ID 2896, Online, [http://phil.cdc.gov/phil/home.asp, last accessed 26th March 2012].)

Some of the Gram positive and Gram negative bacteria, as well as some mycobacteria, which we shall encounter throughout this book, are listed in Table 1.1.1.

Table 1.1.1 Examples of Gram positive and Gram negative bacteria, and mycobacteria.

Gram positive	Gram negative	Mycobacteria
Bacillus subtilis	Burkholderia cenocepacia	Mycobacterium africanum
Enterococcus faecalis	Citrobacter freundii	Mycobacterium avium complex (MAC)
Enterococcus faecium	Enterobacter cloacae	Mycobacterium bovis
Staphylococcus epidermis	Escherichia coli	Mycobacterium leprae
Staphylococcus aureus	Morganella morganii	Mycobacterium tuberculosis
Meticillin-resistant Staphylococcus aureus (MRSA)	Pseudomonas aeruginosa
Streptococcus pyogenes	Salmonella typhimurium
Listeria monocytogenes	Yersinia enterocolitica

1.1.2 Structure

The ultimate aim of all antibacterial drugs is selective toxicity – the killing of pathogenic¹ bacteria (bactericidal agents) or the inhibition of their growth and multiplication (bacteriostatic agents), without affecting the cells of the host. In order to understand how antibacterial agents can achieve this desired selectivity, we must first understand the differences between bacterial (prokaryote) and mammalian (eukaryote) cells.

The name ‘prokaryote’ means ‘pre-nucleus’, while eukaryote cells possess a true nucleus, so one of the major differences between bacterial (prokaryotic) and mammalian (eukaryotic) cells is the presence of a defined nucleus (containing the genetic information) in mammalian cells, and the absence of such a nucleus in bacterial cells. Except for ribosomes, prokaryotic cells also lack the other cytoplasmic organelles which are present in eukaryotic cells, with the function of these organelles usually being performed at the bacterial cell membrane.

A schematic diagram of a bacterial cell is given in Figure 1.1.3, showing the main features of the cells and the main targets for antibacterial agents. As eukaryotic cells are much more complex, we will not include a schematic diagram for them here, and will simply list the major differences between the two basic cell types:

• Bacteria have a cell wall and plasma membrane (the cell wall protects the bacteria from differences in osmotic pressure and prevents swelling and bursting due to the flow of water into the cell, which would occur as a result of the high intracellular salt concentration). The plasma membrane surrounds the cytoplasm and between it and the cell wall is the periplasmic space. Surrounding the cell wall, there is often a capsule (there is also an outer membrane layer in Gram negative bacteria). Mammalian eukaryotic cells only have a cell membrane, whereas the eukaryotic cells of plants and fungi also have cell walls.
• Bacterial cells do not have defined nuclei (in bacteria the DNA is present as a circular double-stranded coil in a region called the ‘nucleoid’, as well as in circular DNA plasmids), are relatively simple, and do not contain organelles, whereas eukaryotic cells have nuclei containing the genetic information, are complex, and contain organelles,² such as lysosomes.
• The biochemistry of bacterial cells is very different to that of eukaryotic cells. For example, bacteria synthesise their own folic acid (vitamin B9), which is used in the generation of the enzyme co-factors required in the biosynthesis of the DNA bases, while mammalian cells are incapable of folic acid synthesis and mammals must acquire this vitamin from their diet.

Figure 1.1.3 Simplified representation of a prokaryotic cell, showing a cross-section through the layers surrounding the cytoplasm and some of the potential targets for antibacterial agents

Whenever we discuss the mode of action of a drug, we will be focussing on the basis of any selectivity. As you will see from the section headings, we have classified antibacterial agents into those which target DNA (Section 2), metabolic processes (Section 3), protein synthesis (Section 4), and cell-wall synthesis (Section 5). In some cases, the reasons for antibacterial selectivity are obvious, for example mammalian eukaryotic cells do not have a peptidoglycan-based cell wall, so the agents we will discuss in Section 5 (which target bacterial cell-wall synthesis) should have no effect on mammalian cells. In other cases, however, the basis for selectivity is not as obvious, for example agents targeting protein synthesis act upon a process which is common to both prokaryotic and eukaryotic cells, so that in these cases selective toxicity towards the bacterial cells must be the result of a more subtle difference between the ribosomal processes in these cells.

We will now look at these antibacterial targets in detail, in preparation for our in-depth study of the modes of action of antibacterial agents and bacterial resistance in the remaining sections.

1.1.3 Antibacterial targets

1.1.3.1 DNA Replication

DNA replication is a complex process, during which the two strands of the double helix separate and each strand acts as a template for the synthesis of complementary DNA strands. This process occurs at multiple, specific locations (origins) along the DNA strand, with each region of new DNA synthesis involving many proteins (shown in italics below), which catalyse the individual steps involved in this process (Figure 1.1.4):

• The separation of the two strands at the origin to...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• Preface
• Section 1: Introduction to Microorganisms and Antibacterial Chemotherapy
• Section 2: Agents Targeting DNA
• Section 3: Agents Targeting Metabolic Processes
• Section 4: Agents Targeting Protein Synthesis
• Section 5: Agents Targeting Cell-wall Synthesis
• Index