Ch.10

Controlling Microbial Growth in the Body: Antimicrobial Drugs

Before You Begin

1. The microbial growth curve has four phases: lag, log, stationary, and death. A chemical that prevents microbial cell division would be most effective during the log phase, where cells are actively dividing.
2. Bacteria that stain either Gram-negative, Gram-positive, or with acid-fast stains rely most on the presence of peptidoglycan for strength. Gram-positive bacteria rely most on peptidoglycan due to the thickness of their peptidoglycan layer.
3. The function of tRNA during translation is to transport amino acids to the ribosome, where they are added to the growing polypeptide chain based on the sequence of codons in the mRNA.
4. Prokaryotic ribosomes (70S) are smaller and have different subunit compositions (30S and 50S) than eukaryotic ribosomes (80S, with 40S and 60S subunits). They also differ in the proteins and rRNA molecules they contain.

Micro in the Clinic: A Cure Leading to Disease?

• Thomas, a 63-year-old man, experienced a persistent cough for three weeks, which worsened after azithromycin treatment.
• A chest X-ray revealed consolidation, leading to a prescription of levofloxacin.
• After a week, his respiratory symptoms improved, but he developed abdominal cramping and diarrhea.
• The symptoms intensified, with fever and severe diarrhea, leading him to suspect food poisoning.

1. Are Thomas’s symptoms due to food poisoning?
   Not necessarily; other explanations are possible.
2. Are there other possible explanations for his illness?
   Yes, such as antimicrobial-associated diarrhea (AAD) caused by Clostridium difficile.

Mechanisms of Antimicrobial Action

• Drugs: Chemicals that affect physiology (e.g., caffeine, alcohol, tobacco).
• Chemotherapeutic agents: Drugs that act against diseases (e.g., insulin, anticancer drugs).
• Antimicrobial agents (antimicrobials): Drugs for treating infections.

The History of Antimicrobial Agents

Learning Outcomes

• Describe the contributions of Paul Ehrlich, Alexander Fleming, and Gerhard Domagk in the development of antimicrobials.

• Explain how semisynthetic and synthetic antimicrobials differ from antibiotics.

• In the early 20th century, medicine was limited in its ability to inhibit pathogens.

• One-third of children born in the early 1900s died from infectious diseases before age five.

Paul Ehrlich (1854–1915)

• Proposed chemotherapy: using chemicals to selectively kill pathogens without harming the patient.
• Envisioned "magic bullets" that would bind to receptors on germs, causing their death while ignoring host cells.
• Discovered arsenic compounds effective against syphilis.

Alexander Fleming (1881–1955)

• In 1928, reported the antibacterial action of penicillin released from Penicillium mold.

Gerhard Domagk (1895–1964)

• In 1932, discovered sulfanilamide, the first practical antimicrobial agent effective against a wide array of bacterial infections.

Selman Waksman (1888–1973)

• Discovered that other microorganisms, notably Streptomyces in soil, are sources of useful antimicrobials.
• Coined the term antibiotics: antimicrobial agents produced naturally by an organism.
• In common usage, antibiotic denotes an antibacterial agent, including synthetic compounds and excluding antiviral and antifungal agents.

Semisynthetic Antimicrobials

• Chemically altered antibiotics that are more effective, longer-lasting, or easier to administer than naturally occurring antibiotics.
• Synthetic drugs: Antimicrobials completely synthesized in a laboratory.
• Most commonly used antimicrobials are either natural or semisynthetic.

Mechanisms of Antimicrobial Action

Learning Outcomes

• Explain the principle of selective toxicity.

• List six mechanisms by which antimicrobial drugs affect pathogens.

• Key to successful chemotherapy against microbes is selective toxicity: an effective antimicrobial agent must be more toxic to a pathogen than to the pathogen’s host.

• Selective toxicity is possible because of differences in structure or metabolism between the pathogen and its host.

• Antibacterial drugs constitute the greatest number and diversity of antimicrobial agents due to many differences between the structure and metabolism of pathogenic bacteria and their eukaryotic hosts.

• Fewer antifungal, antiprotozoan, and anthelmintic drugs are available because fungi, protozoa, and helminths share many common features with their animal and human hosts.

• The number of effective antiviral drugs is limited because viruses utilize their host cells’ enzymes and ribosomes to metabolize and replicate.

General Groups of Antimicrobial Drugs According to Their Mechanisms of Action

1. Drugs that inhibit cell wall synthesis
   • Selectively toxic to certain fungal or bacterial cells, which have cell walls, but not to animals, which lack cell walls.
2. Drugs that inhibit protein synthesis (translation)
   • Target the differences between prokaryotic and eukaryotic ribosomes.
3. Drugs that disrupt unique components of a cytoplasmic membrane
4. Drugs that inhibit general metabolic pathways not used by humans
5. Drugs that inhibit nucleic acid synthesis
6. Drugs that block a pathogen’s recognition of or attachment to its host

Inhibition of Cell Wall Synthesis

Learning Outcome

• Describe the actions and give examples of drugs that affect the cell walls of bacteria or fungi.

• A cell wall protects a cell from the effects of osmotic pressure.

• Both pathogenic bacteria and fungi have cell walls, which animals and humans lack.

Inhibition of Synthesis of Bacterial Walls

• The major structural component of a bacterial cell wall is its peptidoglycan layer.
• Peptidoglycan is a huge macromolecule composed of polysaccharide chains:
  • Alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) molecules
  • Cross-linked by short peptide chains extending between NAM subunits.
• To enlarge or divide, a cell synthesizes more peptidoglycan by adding new NAG and NAM subunits to existing NAG-NAM chains.
  $\text{NAG}$
  $\text{NAM}$

Beta-Lactams

• Prevent the cross-linkage of NAM subunits; include penicillins, cephalosporins, and carbapenems.
  $\beta$-lactams

• Beta-lactam rings inhibit peptidoglycan formation by irreversibly binding to the enzymes that cross-link NAM subunits.
  Dorothy Hodgkin developed protein X-ray crystallography and used her technique to elucidate the structure of penicillin’s beta-lactam ring

• In the absence of correctly formed peptidoglycan, growing bacterial cells have weakened cell walls that are less resistant to the effects of osmotic pressure.

• The underlying cytoplasmic membrane bulges through the weakened portions of the cell wall as water moves into the cell, and eventually, the cell lyses.

• Chemists have made alterations to natural beta-lactams, such as penicillin G, to create semisynthetic derivatives (e.g., methicillin and imipenem).

  • More stable in the acidic environment of the stomach
  • More readily absorbed in the intestinal tract
  • Less susceptible to deactivation by bacterial enzymes
  • More active against more types of bacteria

Other Antimicrobials

• Lipoglycopeptides (e.g., vancomycin) and cycloserine: disrupt cell wall formation in a different manner.
• They directly interfere with particular alanine-alanine bridges that link the NAM subunits in many Gram-positive bacteria.
• Bacteria that lack this type of alanine-alanine crossbridge are naturally resistant to these drugs.

Bacitracin

• Blocks the transport of NAG and NAM across the cytoplasmic membrane to the wall.

Mycobacteria

• Mycobacterium (agents of leprosy and tuberculosis) are characterized by unique, complex cell walls that contain waxy lipids called mycolic acids in addition to the usual peptidoglycan of bacterial cells.

Isoniazid (INH)

• Disrupts the formation of mycolic acid.

Ethambutol

• Disrupts the formation of mycolic acid.
• Since mycobacteria typically reproduce only every 12 to 24 hours, antimicrobial agents that act against mycobacteria must be administered for months or even years.

Inhibition of Synthesis of Fungal Walls

• Fungal cell walls are composed of various polysaccharides containing a sugar—1,3-β-glucan—that is not found in mammalian cells.

Echinocandins

• New class of antifungal drugs (e.g., caspofungin) that inhibit the enzyme that synthesizes glucan.
• Without glucan, fungal cells cannot make cell walls, leading to osmotic rupture.

Inhibition of Protein Synthesis

Learning Outcomes

• Describe the actions and give examples of six antimicrobial drugs that interfere with ribosomes.

• Describe how mupirocin interferes with protein synthesis.

• Cells use proteins for structure and regulation, as enzymes in metabolism, and as channels and pumps to move materials across cell membranes.

• A consistent supply of proteins is vital for the active life of a cell.

• There are differences between prokaryotic and eukaryotic ribosomes as well as differences in the enzymes that "load" amino acids onto tRNA molecules.

Interference with Prokaryotic Ribosomes

• Prokaryotic ribosomes differ from eukaryotic ribosomes in structure and size:
  • Prokaryotic ribosomes are 70S and composed of 30S and 50S subunits.
  • Eukaryotic ribosomes are 80S with 60S and 40S subunits.
• Many antimicrobial agents take advantage of the differences between ribosomes to selectively target bacterial protein translation without significantly affecting eukaryotes.
• Because some of these drugs affect eukaryotic mitochondria, which also contain 70S ribosomes like those of prokaryotes, such drugs may be harmful to animals and humans, especially very active cells in the liver and bone marrow.

Aminoglycosides and Tetracyclines

• Target the 30S ribosomal subunit.
• Aminoglycosides (e.g., streptomycin and gentamicin) change the shape of the 30S subunit, making it impossible for the ribosome to read the codons of mRNA correctly.
• Tetracyclines block the tRNA docking site (A site), which then prevents the incorporation of additional amino acids into a growing polypeptide.

Chloramphenicol, Lincosamides, Streptogramins, and Macrolides

• Interfere with the function of the 50S subunit.
• Chloramphenicol blocks the enzymatic site of the 50S subunit, which prevents translation.
• Lincosamides, streptogramins, and macrolides (e.g., erythromycin) bind to a different portion of the 50S subunit, preventing movement of the ribosome from one codon to the next.
  • Translation is frozen, and protein synthesis is halted.

Antisense Nucleic Acids

• These are DNA or single-stranded DNA molecules that are designed to be complementary to specific mRNA molecules of pathogens.
• They block ribosomal subunits from attaching to that mRNA with no effect on human mRNA.
• Fomivirsen: inactivates cytomegalovirus and is used to treat eye infections.

Oxazolidinones

• Antimicrobial drugs that work to stop protein synthesis by blocking the initiation of translation.
• Used as a last resort in treating infections of Gram-positive bacteria resistant to other antimicrobials (e.g., vancomycin- and methicillin-resistant Staphylococcus aureus).

Interference with Charging of Transfer RNA Molecules

• Each transfer RNA molecule has an anticodon composed of three nucleotides complementary to the nucleotides of a codon that codes for a specific amino acid.
• Enzymes called aminoacyl-tRNA synthetases “charge” or “load” amino acids onto tRNA molecules.
• Mupirocin: selectively binds to isoleucyl-tRNA synthetase, the enzyme that loads isoleucine in Gram-positive bacteria, particularly Staphylococcus and Streptococcus.
• The drug is selectively toxic because it does not bind to any eukaryotic tRNA molecules.
• In the presence of the drug, Gram-positive cells cannot load isoleucine, so protein synthesis cannot proceed past isoleucine codons; thus, mupirocin cripples a bacterium’s protein production.

Disruption of Cytoplasmic Membranes

Learning Outcome

• Describe the action of antimicrobial drugs that interfere with cytoplasmic membranes.

• Some antibacterial drugs, such as the short polypeptide gramicidin, disrupt the cytoplasmic membrane of a targeted cell, often by forming a channel through the membrane, damaging its integrity.

Polyenes

• A group of antifungal and antiprotozoan drugs.
• Nystatin and amphotericin B are fungicidal because they attach to ergosterol, a lipid constituent of fungal membranes, in the process disrupting the membrane and causing lysis of the cell.
• The cytoplasmic membranes of humans are somewhat susceptible to amphotericin B because they contain cholesterol, which is similar to ergosterol, though cholesterol does not bind amphotericin B as well as does ergosterol.

Azoles and Allylamines

• Two other classes of antifungal and antiprotozoan drugs that disrupt cytoplasmic membranes.
• Act by inhibiting the synthesis of ergosterol; without ergosterol, a fungal cell’s cytoplasmic membrane does not remain intact, and the cell dies.
• Generally harmless to humans because human cells do not manufacture ergosterol.

Polymyxin

• Produced by Bacillus polymyxa.
• Effective against Gram-negative bacteria, particularly Pseudomonas, but because it is toxic to human kidneys, it is usually reserved for use against external pathogens that are resistant to other antibacterial drugs.

Inhibition of Metabolic Pathways

Learning Outcomes

• Explain the action of antimicrobials that disrupt synthesis of folic acid.

• Define the term analog as it relates to antimicrobial drugs.

• Describe the action of antiviral drugs that interfere with metabolism.

• Metabolism can be defined simply as the sum of all chemical reactions that take place within an organism.

• Whenever differences exist between the metabolic processes of a pathogen and its host, antimetabolic agents can be effective.

Various Kinds of Antimetabolic Agents

• Atovaquone, which interferes with electron transport in protozoa and fungi
• Heavy metals (e.g., arsenic, mercury, and antimony), which inactivate enzymes
• Agents that disrupt tubulin polymerization and glucose uptake by many protozoa and parasitic worms
• Drugs that block the activation of viruses
• Metabolic antagonists, such as sulfanilamide, the first commercially available antimicrobial agent

Sulfanilamide and Similar Compounds

• Collectively called sulfonamides or sulfa drugs; act as antimetabolic drugs because they are structural analogs of—that is, are chemically very similar to—para-aminobenzoic acid (PABA).

Para-aminobenzoic acid (PABA)

• Crucial in the synthesis of nucleotides required for DNA and RNA synthesis.

• Many organisms, including some pathogens, enzymatically convert PABA into dihydrofolic acid and then dihydrofolic acid into tetrahydrofolic acid (THF).

  • A form of folic acid that is used as a coenzyme in the synthesis of purine and pyrimidine nucleotides.

• As analogs of PABA, sulfonamides compete with PABA molecules for the active site of the enzyme involved in the production of dihydrofolic acid.

• This competition leads to a decrease in the production of THF and thus of DNA and RNA.

• The end result of sulfonamide competition with PABA is the cessation of cell metabolism, which leads to cell death.

• Humans do not synthesize THF from PABA; instead, we take simple folic acids found in our diets and convert them into THF.

• As a result, human metabolism is unaffected by sulfonamides.

Trimethoprim

• Interferes with nucleic acid synthesis.
• Binds to the enzyme involved in the conversion of dihydrofolic acid to THF, the second step in this metabolic pathway.

Amantadine, Rimantadine, and Weak Organic Bases

• Can neutralize the acid of phagolysosomes and thereby prevent viral uncoating; thus, these are antiviral drugs.
• The Centers for Disease Control and Prevention (CDC) showed that amantadine is no longer effective against influenza type A virus, so physicians currently do not prescribe this drug for use against type A influenzaviruses.

Protease Inhibitors

• Interfere with the action of protease—an enzyme that HIV needs near the end of its replication cycle.
• These drugs, when used as part of a “cocktail” of drugs including reverse transcriptase inhibitors. Have revolutionized treatment of AIDS patients in industrialized countries.

Inhibition of Nucleic Acid Synthesis

Learning Outcome

• Describe the antimicrobial action of nucleotide and nucleoside analogs, quinolones, drugs that bind to RNA or DNA, and reverse transcriptase inhibitors.

• The nucleic acids DNA and RNA are built from purine and pyrimidine nucleotides and are critical to the survival of cells.

• Several drugs function by blocking either the replication of DNA or its transcription into RNA.

Pyrazinamide

• Disrupts transport across the cytoplasmic membrane of Mycobacterium tuberculosis.
• The pathogen uniquely activates and accumulates the drug.
• Unlike many other antimicrobials, pyrazinamide is most effective against intracellular, nonreplicating bacterial cells.
• Some antiparasitic drugs also act against cytoplasmic membranes.
  • Praziquantel and ivermectin change the permeability of cell membranes of several types of parasitic worms.

Nucleotide Analogs or Nucleoside Analogs

• Molecules with structural similarities to the normal nucleotide building blocks of nucleic acids.

• Nucleotide analogs are often incorporated into the DNA or RNA of pathogens, where they distort the shapes of the nucleic acid molecules and prevent further replication, transcription, or translation.

• Nucleotide and nucleoside analogs are most often used against viruses because viral DNA polymerases are tens to hundreds of times more likely to incorporate these nonfunctional nucleotides into nucleic acids than is human DNA polymerase.

• Other antimicrobial agents function by binding to and inhibiting the action of RNA polymerases during the synthesis of RNA from a DNA template.

Rifampin

• Binds more readily to prokaryotic RNA polymerase than to eukaryotic RNA polymerase; as a result, rifampin is more toxic to prokaryotes than to eukaryotes.

Clofazimine

• Binds to bacterial DNA and prevents normal transcription (and DNA replication).
• Rifampin and clofazimine are used primarily against species of mycobacteria such as M. tuberculosis and M. leprae, the causative agents of tuberculosis and leprosy, respectively.

Pentamidine and Propamidine Isethionate

• Bind to protozoan DNA, inhibiting DNA replication and RNA transcription.
• The result is that the protozoa cannot reproduce or metabolize.

Reverse Transcriptase Inhibitors

• Act against reverse transcriptase, which is an enzyme HIV uses early in its replication cycle to make DNA copies of its RNA genome (reverse transcription).
• Since people lack reverse transcriptase, the inhibitor does not harm patients.

Prevention of Virus Attachment, Entry, or Uncoating

Learning Outcome

• Describe the action of antimicrobial attachment antagonists and of drugs that inhibit viral entry.

• Many pathogens, particularly viruses, attach to their host’s cells via the chemical interaction between attachment proteins on the pathogen and complementary receptor proteins on a host cell.

Attachment Antagonists

• Block these sites, viruses can neither attach to nor enter their host’s cells.
• Pleconaril is a synthetic antagonist of the receptors of picornaviruses such as some cold viruses, polioviruses, and coxsackievirus.
  • The drug blocks viral attachment and deters infections.
• Arildone is a synthetic antiviral that prevents removal of poliovirus capsids (uncoating), thereby interrupting the viral replication cycle.

Clinical Considerations in Prescribing Antimicrobial Drugs

Learning Outcome

• List six characteristics of an ideal, hypothetical, antimicrobial drug.

• The ideal antimicrobial agent to treat an infection or disease would be one that has all these characteristics:

  • Readily available
  • Inexpensive
  • Chemically stable (so that it can be transported easily and stored for long periods of time)
  • Easily administered
  • Nontoxic and nonallergenic
  • Selectively toxic against a wide range of pathogens

• Doctors and medical laboratory technicians must evaluate antimicrobials with respect to several characteristics:

  • The types of pathogens against which they are effective
  • Their effectiveness, including the dosages required to be effective
  • The routes by which they can be administered
  • Their overall safety
  • Any side effects they produce.

Spectrum of Action

Learning Outcome

• Distinguish narrow-spectrum drugs from broad-spectrum drugs in terms of their targets and side effects.

• The number of different kinds of pathogens a drug acts against is known as its spectrum of action.

  • Drugs that work against only a few kinds of pathogens are narrow-spectrum drugs.
  • Those that are effective against many different kinds of pathogens are broad-spectrum drugs.

• Tetracycline acts against many different kinds of bacteria, including Gram-negatives, Gram-positives, chlamydias, and rickettsias, it is considered a broad-spectrum antibiotic.

• Penicillin cannot easily penetrate the outer membrane of a Gram-negative bacterium to reach and prevent the formation of peptidoglycan, so its efficacy is limited largely to Gram-positive bacteria.

  • Thus, penicillin has a narrower spectrum of action than tetracycline.

• The use of broad-spectrum antimicrobials is not always as desirable as it might seem.

  • Broad-spectrum antimicrobials can also open the door to serious secondary infections by transient pathogens or superinfections by members of the normal microbiota unaffected by the antimicrobial.

• This results because the killing of normal microbiota reduces microbial antagonism, the competition between normal microbes and pathogens for nutrients and space.

Effectiveness

Learning Outcome

• Compare and contrast diffusion susceptibility tests, Etest, MIC test, and MBC test.

• To effectively treat infectious diseases, physicians must know which antimicrobial agent is most effective against a particular pathogen.

• To ascertain the efficacy of antimicrobials, microbiologists conduct a variety of tests, including diffusion susceptibility tests, the minimum inhibitory concentration test, and the minimum bactericidal concentration test.

Diffusion Susceptibility Test

• Also known as Kirby-Bauer tests
• Involve uniformly inoculating a Petri plate with a standardized amount of the pathogen in question.
• Then small disks of paper containing standard concentrations of the drugs to be tested are firmly arranged on the surface of the plate.
• The plate is incubated, and the bacteria grow and reproduce to form a “lawn” everywhere except the areas where effective antimicrobial drugs diffuse through the agar.
• After incubation, the plates are examined for the presence of a zone of inhibition.
• The zone of inhibition is measured as the diameter (to the closest millimeter) of the clear region.
• If all antimicrobials diffused at the same rate, then a larger zone of inhibition would indicate a more effective drug.
• The size of a zone of inhibition must be compared to a standard table for that particular drug before accurate comparisons can be made.
• Diffusion susceptibility tests enable scientists to classify pathogens as susceptible, intermediate, or resistant to each drug.

Minimum Inhibitory Concentration (MIC) Test

• MIC is the smallest amount of the drug that will inhibit growth and reproduction of the pathogen.
• Turbidity (cloudiness) indicates bacterial growth; lack of turbidity indicates that the bacteria were either inhibited or killed by the antimicrobial agent.

Etest

• Combines aspects of an MIC test and a diffusion susceptibility test.
• Involves placing a plastic strip containing a gradient of the antimicrobial agent being tested on a plate uniformly inoculated with the organism of interest.
• After incubation, an elliptical zone of inhibition indicates antimicrobial activity, and the minimum inhibitory concentration can be noted where the zone of inhibition intersects a scale printed on the strip.

Minimum Bactericidal Concentration (MBC) Test

• Similar to the MIC test is a minimum bactericidal concentration (MBC) test, though an MBC test determines the amount of drug required to kill the microbe rather than just the amount to inhibit it, as the MIC does.
• In an MBC test, samples taken from clear MIC tubes (or, alternatively, from zones of inhibition from a series of diffusion susceptibility tests) are transferred to plates containing a drug-free growth medium.
• The appearance of bacterial growth in these subcultures after appropriate incubation indicates that at least some bacterial cells survived that concentration of the antimicrobial drug and were able to grow and multiply once placed in a drug-free medium.
• Any drug concentration at which growth occurs in subculture is bacteriostatic, not bactericidal, for that bacterium.
• The lowest concentration of drug for which no growth occurs in the subcultures is the minimum bactericidal concentration (MBC).

Routes of Administration

Learning Outcome

• Discuss the advantages and disadvantages of the different routes of administration of antimicrobial drugs.

• For an antimicrobial agent to be effective, an adequate amount of it must reach a site of infection.

1. For external infections (e.g., athlete’s foot):
   • Drugs can be applied directly--Topical or local administration.
2. For internal infections, drugs can be administered:
   • Orally
   • Intramuscularly (IM)
   • Intravenously (IV)

• The oral route is simplest (it requires no needles and is self-administered).
  • The drug concentrations achieved in the body are lower than occur via other routes of administration.
• IM administration via a hypodermic needle allows a drug to diffuse slowly into the many blood vessels within muscle tissue, but the concentration of the drug in the blood is never as high as that achieved by IV administration.
• IV administration delivers the drug directly into the bloodstream through either a needle or a catheter (a plastic or rubber tube).
  • Initially very high for the IV route, but unless the drug is continuously administered, the concentration can rapidly diminish as the liver and kidneys remove the drug from circulation.
• Physicians can administer non-antimicrobial chemicals that prolong an antimicrobial’s life span in the body; for example, cilastatin inhibits a kidney enzyme that would destroy imipenem—a type of beta-lactam.

Safety and Side Effects

Learning Outcomes

• Identify three main categories of side effects of antimicrobial therapy.

• Define therapeutic index and therapeutic window.

• Another aspect of chemotherapy that physicians must consider is the possibility of adverse side effects.

These Fall Into Three Main Categories

1. Toxicity
2. Allergies
3. Disruption of normal microbiota

Toxicity

• Though antimicrobial drugs are ideally selectively toxic against microbes and harmless to humans, many antimicrobials in fact have toxic side effects.
• Drugs may be toxic to the kidneys, the liver, or nerves.
  Polymyxin and aminoglycosides can have fatally toxic effects on kidneys.
  Metronidazole (Flagyl), a drug effective against a variety of anaerobic protozoa and bacteria, may cause a harmless temporary condition called “black hairy tongue,”

Doctors Must Be Especially Careful When Prescribing Drugs for Pregnant Women

• Tetracyclines combine with calcium that is incorporated into bones and developing teeth, causing malformation of the skull and stained, weakened tooth enamel.

Therapeutic Index (TI) Margin of Safety

• Estimates the safety of an antimicrobial drug; a ratio of the dose of a drug that is toxic to 50% of a patient population to the dose of the drug that is therapeutic in 50% of patients.
• The higher the TI, the safer the drug. Caregivers must be careful when administering drugs with a low TI and must monitor the patient closely for signs of drug toxicity.

Therapeutic Window

• The range of concentrations of the drug that are effective without being excessively toxic.

Allergies

• In addition to toxicity, some drugs trigger allergic immune responses in sensitive patients.
• Anaphylactic shock: an immediate, violent reaction-life threatening.

Disruption of Normal Microbiota

• Drugs that disrupt the normal microbiota and their microbial antagonism may allow opportunistic pathogens to proliferate as secondary infections.
• Long-term use of broad-spectrum antibacterials often results in explosions in the growth rate of Candida albicans in the vagina (vaginitis) or mouth (thrush) and the multiplication of Clostridium difficile in the colon, causing a potentially fatal condition called pseudomembranous colitis.

Resistance to Antimicrobial Drugs

Learning Outcomes

• Describe how populations of resistant microbes can arise.
• Describe the relationship between R plasmids and resistant cells.
• Define and describe persister cells.

The Development of Resistance and Persister Cells

• Antimicrobial resistance is a global crisis.
• Not all pathogens are equally sensitive to a given therapeutic agent; a population may contain a few individuals that are naturally either partially or completely resistant.
• Individual cells can acquire such resistance in two ways:
  • Through new mutations of chromosomal genes
  • By acquiring resistance genes on extrachromosomal pieces of DNA called R plasmids (or R factors) via the processes of horizontal gene transfer—transformation, transduction, or conjugation.
• In the absence of an antimicrobial drug, resistant cells are usually less efficient than their normal neighbors because they must expend extra energy to maintain resistance genes and proteins.
  • Resistant individuals reproduce more slowly, resistant individuals remain the minority in a population.
• When an antimicrobial agent is present, the majority of microbes, which are sensitive to the antimicrobial, are inhibited or die while the resistant microbes continue to grow and multiply, often more rapidly because they then face less competition.

Persister Cells

• Cells of a clonal population that resist the effects of antimicrobial