More than a century ago in the United States, one out of three children was expected to die of an infectious disease before the age of 5. Early death or severe lifelong debilitation from scarlet fever, diphtheria, tuberculosis, meningitis, and many other bacterial diseases was a fearsome yet undeniable fact of life to most of the world’s population.
The introduction of modern drugs to control infections in the 1930s was a medical revolution that has added significantly to the life span and health of humans. It is no wonder that for many years, antibiotics in particular were regarded as miracle drugs. Although antimicrobial drugs have greatly reduced the incidence of certain infections, they have definitely not eradicated infectious disease and probably never will. In fact, in some parts of the world, mortality rates from infectious diseases are as high as before the arrival of antimicrobial drugs. Nevertheless, humans have been taking medicines to try to control diseases for thousands of years.
The goal of antimicrobial chemotherapy1 is deceptively simple: administer to an infected person a drug that destroys the infectious agent without harming the host’s cells. In actuality this goal is rather difficult to achieve, because many often-contradictory factors must be taken into account. The ideal drug should be easily administered and able to reach the infectious agent anywhere in the body. It should be able to kill the infectious agent rather than merely inhibit its growth, and remain active in the body as long as needed, yet be safely and easily broken down and excreted. In short, the perfect drug does not exist, but by balancing drug characteristics against one another, a satisfactory compromise can usually be achieved (table 1).
Table1. Characteristics of the Ideal Antimicrobial Drug
The Origins of Antimicrobial Drugs
Modern antimicrobial drugs are highly varied in their origins. The drugs traditionally termed antibiotics are substances produced by the natural metabolic processes of certain microorganisms that can inhibit or destroy other microorganisms (figure 1a). Nature is a prolific producer of antimicrobial drugs. Many thousands of antibiotics have been discovered (although, surprisingly, only a relatively small number have actually been used clinically). Antibiotics are made primarily by aerobic spore-forming bacteria and fungi. The antibiotics appear to have some survival value for these microbes, because the genes for antibiotic production are preserved in evolution. Some experts theorize that microorganisms synthesize antibiotics to inhibit or destroy nearby competitors or predators (antagonism); others propose that antibiotics are connected to the formation of spores.
Fig1. Mining for antibiotic-producing microbes. (a) One strategy is to seed a plate with a sample (here, soil) and then spray its surface with a fine mist of bacteria and incubate it. Zones of inhibition (clear areas with no growth) surrounding several colonies indicate colonies that produce antibiotics. (b) Two colonies with antibiotic secretions on their surfaces. Left is Penicillium chrysogenum, with yellow droplets of penicillin; right is Streptomyces coelicolor, with blue droplets of actinorhodin, an experimental antibiotic. This release of the antibiotics is an important factor in the isolation and production of the drugs. (a, b left): McGraw Hill; (b right): Dr. Ramón Santamaría/IMB. CSIC/USAL. Salamanca. Spain
The greatest numbers of antibiotics are derived from bacteria in the genera Streptomyces and Bacillus and molds in the genera Penicillium and Cephalosporium (figure 1b and table 2). Of these, species of Streptomyces are the most fertile producers. Nearly 9,000 different bioactive compounds have been isolated from them, and they are currently the source of one-third of all prescribed antibiotics. The pharmaceutical industry has harnessed the power of these microbes by growing them in vast quantities and harvesting their products to be sold as drugs. Researchers have facilitated the work of nature by selecting mutant species that yield more abundant or effective products, by varying the growth medium, or by altering the procedures for large-scale industrial production.
Table2. Selected Microbial Sources of Antibiotics
A few classes of antimicrobial drugs, termed synthetic, are not considered antibiotics because they are synthesized entirely through chemical processes and not secreted by microorganisms. Synthetic drugs include sulfa drugs, quinolones, quinines, azoles, and most antiviral drugs.
Probably the most common approach to drug production is the semisynthetic method, combining both natural and synthetic meth ods. Think of it as improving on nature by using chemical reactions to combine the basic antibiotic made by microorganisms with various preselected functional groups that tailor the action of the drug. Drugs produced in this way are designed to have new and improved functions over the original antibiotic. We can see examples of this technology in the design of the semisynthetic penicillins, cephalosporins, and tetracyclines. The potential for using bioengineering techniques to design drugs seems almost limitless, and, indeed, several drugs have already been produced by manipulating the genes of antibiotic producers.
Antimicrobial drugs can be organized by their origin, range of effectiveness, and whether they are naturally produced or chemically synthesized. A few of the more important terms you will encounter are found in table 3.
Table3. Terminology of Chemotherapy
Interactions between Drugs and Microbes
The goal of antimicrobial drugs is either to disrupt the cell processes or structures of bacteria, fungi, and protozoa or to inhibit the virus multiplication cycle. Most of the drugs used in chemotherapy interfere with the function of enzymes required to synthesize or assemble macromolecules, but a few destroy structures already formed in the cell. Above all, drugs should be selectively toxic, which means they can kill or inhibit the growth of microbes without simultaneously damaging host tissues. The best drugs are those that act specifically on microbial structures or functions not found in vertebrate cells. Examples of drugs with excellent selective toxicity are those that block the synthesis of the cell wall in bacteria (penicillins). They have low toxicity and few direct effects on human cells, because human cells lack a cell wall. Among the most toxic to human cells are drugs that act upon a structure common to both the infectious agent and the host cell, such as the cell membrane (for example, amphotericin B used to treat fungal infections). As the characteristics of the pathogen become more similar to those of the host cell, selective toxicity becomes more difficult to achieve and undesirable side effects are more likely to occur.
Mechanisms of Drug Action
The primary basis of chemotherapy is to target an infectious agent with a chemical that irreversibly damages or inhibits it. This knowledge guides drug development as well. Most classes of antimicrobial drugs are active against some unique feature of microbial structure or metabolism. Observe table 4 and take note of the diversity of cell types and possible effects of drug action. Overall, drugs may work in five ways (figure 2) to slow or stop the growth of an actively dividing cell:
1. inhibition of cell wall synthesis,
2. breakdown of the cell membrane structure or function,
3. interference with functions of DNA and RNA,
4. inhibition of protein synthesis, and
5. blockage of key metabolic pathways.
These targets are not completely discrete, and some have overlap ping effects. For example, if DNA transcription is inhibited, protein synthesis will be as well. Viruses are a special case and have very specialized targets for drugs.
Table4. General Actions of Drugs on Microbial Groups
Fig2. Major targets of drugs acting on bacterial cells.
The Spectrum of an Antimicrobial Drug One result of drugs acting upon a particular microbial structure or function is that each drug has a given range of activity. This has been termed the drug’s spectrum. Traditionally, narrow-spectrum drugs are effective over a limited range of cell types. This is usually because they target a specific component that is found only in certain bacteria. For instance, bacitracin blocks the elongation of the peptidoglycan in gram-positive bacteria but has no effect on most gram-negative bacteria because of the outer membrane on their cell wall that serves as a barrier to the drug. Polymyxin breaks down the outer membrane on gram-negative cells but does not target gram-positive cells because they lack this structure.
Drugs that are effective against a wider range of microbes are termed extended or broad spectrum, depending on the particular drug. For example, the extended-spectrum drug ampicillin is effective on both gram-positive and gram-negative bacteria, but not a wide range of them. Broad-spectrum drugs, such as tetracycline, have the greatest range of activity. They work on most gram- negative and gram-positive bacteria, rickettsias, mycoplasmas, and spirochetes. The broad-spectrum drugs usually exert their effects on common cell components such as ribosomes, which are found in all cells and mitochondria. Consult table 5 to see an array of antibacterial drugs and their spectra.
Table5. Spectrum of Effectiveness for Antibacterial Drugs
Antimicrobial Drugs that Affect the Bacterial Cell Wall The cell walls of most bacteria contain a rigid girdle of peptidoglycan, which protects the cell against rupture from hypotonic environments. Active cells must constantly synthesize new peptidoglycan and transport it to its proper place in the cell envelope. Drugs such as penicillins and cephalosporins react with one or more of the enzymes required to complete this process, causing the cell to develop weak points at growth sites and to become osmotically fragile (figure 3). Antibiotics that produce this effect are considered bactericidal, because the weakened cell usually undergoes lysis. However, cells that are old, inactive, or dormant and are not synthesizing peptidoglycan may resist these effects. (One exception is a class of antibiotics abbreviated as “-penems.”)
Fig3. Effects of penicillin on gram-positive cell walls. photos: (Top): Janice Carr/CDC; (Bottom): CNRI/Science Source
Penicillins and cephalosporins bind and block peptidases that cross-link the glycan molecules, thereby interrupting the completion of the cell wall (figure 3b). Narrow-spectrum penicillins do not penetrate the outer membrane and are therefore less effective against gram-negative bacteria. Many broad-spectrum penicillins (ampicillin) and cephalosporins (ceftriaxone) can pass into the cell walls of gram-negative species. Cycloserine inhibits the formation of the basic peptidoglycan subunits, and vancomycin hinders the elongation of the peptidoglycan.
Antimicrobial Drugs that Disrupt Cell Membrane Function A cell with a damaged membrane invariably dies from disruptions in metabolism or complete breakdown and lysis. It does not even have to be actively dividing to be destroyed. The antibiotic classes that damage cell membranes usually have specificity for particular microbial groups, based on differences in the types of lipids in their cell membranes.
Polymyxins interact with membrane phospholipids and cause leakage of proteins and nucleic acids, particularly in gram- negative bacteria (figure 4). Daptomycin shows selectivity for gram- positive cells, but occasionally causes serious side effects that limit its use. The polyene antifungal antibiotics (amphotericin B and nystatin) form complexes with the sterols on fungal membranes, which causes abnormal openings and seepage of small ions. Unfortunately this selectivity is not exact, and the similarities in membranes of microbial and animal cells means that most of these antibiotics have greater toxicity to human cells.
Fig4. Effects of drugs on membranes. Polymyxin binds to the outer and cell membranes of gram-negative bacteria and creates abnormal openings that cause leakage and lysis.
Antimicrobial Drugs that Affect Nucleic Acid Synthesis As you learned in chapter 9, the pathway that generates DNA and RNA involves an extensive series of enzyme-catalyzed reactions. Like any complicated process, it is subject to breakdown at many different points along the way, and inhibition at any point in the sequence can block subsequent events. Antimicrobial drugs interfere with nucleic acid synthesis by blocking synthesis of nucleotides, inhibiting replication, or stopping transcription. Because functioning DNA and RNA are required for proper translation as well, the production of proteins is also usually inhibited.
Several antimicrobials inhibit DNA synthesis. The broad- spectrum quinolones inhibit DNA-unwinding enzymes or helicases, thereby stopping DNA replication and repair. Certain antiviral drugs are analogs of purines and pyrimidines that will insert in the viral nucleic acid in place of a normal base. When this abnormal base stops synthesis, viral replication will be blocked. The antiviral drug Remdesivir (used to treat infection with the virus that causes COVID-19) is an analog of adenine that creates dysfunctional RNA, causing the RNA polymerase to stop transcription. With no RNA being produced, the virus cannot multiply.
Antimicrobial Drugs that Block Protein Synthesis Most drugs that inhibit protein synthesis react with the ribosome-mRNA complex. Although human cells also have ribosomes, the ribosomes of eukaryotes are different in size and structure from those of prokaryotes, so these antimicrobials usually have a selective action against bacteria. One potential therapeutic consequence of drugs that bind to the prokaryotic ribosome is the damage they can do to eukaryotic mitochondria, which contain a prokaryotic type of ribosome.
Recall that ribosomes consist of a large (50S) subunit and a smaller (30S) subunit, each of which can serve as a target for antimicrobics (figure 5). Aminoglycosides, like streptomycin or gentamicin, bind to sites on the 30S subunit and cause misreading of the mRNA, leading to abnormal proteins. Tetracyclines and glycylcyclines also bind to the 30S subunit, blocking the attachment of tRNA on the A site and stopping further protein synthesis. The 50S subunit similarly serves as a target. Chloramphenicol prevents the formation of peptide bonds and blocks protein synthesis, while erythromycin inhibits translocation of the ribosome along the mRNA. The oxizolidinones also bind to the 50S subunit but do so in such a way that the two ribosomal subunits cannot come together, preventing translation from beginning.
Fig5. Sites of inhibition on the prokaryotic ribosome and major antibiotics that act on these sites. All have the general effect of blocking protein synthesis. Blockage actions are indicated by X.
Antimicrobial Drugs that Affect Metabolic Pathways Some drugs act by mimicking the normal substrate of an enzyme through a process called competitive inhibition. These drugs, termed metabolic analogs, are structurally similar to the natural substrate and compete with it for the active site on the enzyme. When the drug is supplied to a target cell in high concentrations, it will occupy the active site and displace the normal substrate. But the metabolic analog drug is a “dead end” and cannot function as required. As the enzyme is no longer able to produce a needed product, cellular metabolism slows or stops. Antibacterial drugs called sulfonamides and trimethoprim provide an example of this mode of action.
Sulfonamides and trimethoprim interfere with folate metabolism by blocking enzymes required for the synthesis of tetrahydrofolate, which is needed by bacterial cells for the synthesis of folic acid and the eventual production of DNA and RNA and amino acids. Trimethoprim and sulfonamides are often given simultaneously to achieve a synergistic effect so that a lower dosage of each drug can be used. Figure6 illustrates sulfonamides’ competition with PABA (p-aminobenzoic acid) for the active site of the enzyme that synthesizes the folic acid precursor. The selective toxicity of these com pounds is explained by the fact that mammals derive folic acid from their diet and so do not possess this enzyme system. This makes it possible to inhibit bacterial and protozoan parasites, which must synthesize folic acid, while leaving the human host unaffected.
Fig6. Competitive inhibition as a mode of action. (a) This example shows how sulfa drugs block a metabolic pathway used by bacteria to synthesize folic acid. Two different enzymes in the metabolic pathway are blocked, meaning that even if a bacterium could bypass one enzyme-catalyzed reaction, its progress would still be inhibited. (b) Formula of a sulfanilamide molecule compared alongside one of PABA. Note that, despite a similarity in overall shape, sulfa drugs cannot be used to make folic acid. (c) The sulfa drug molecules can insert into the active site in the enzyme that would ordinarily bind PABA. This will work only when the sulfa molecules are more prevalent and can outcompete PABA for available enzyme binding sites (keeping sites on the enzyme filled). Since little or no PABA can bind, the synthesis of folic acid will be inhibited. (a): Barry Chess/McGraw Hill
