Chapter+10 copy

Antimicrobial Treatment

• Learning changes everything.®
• Chapter 10 in Microbiology: A Clinical Approach, Fourth Edition, by Marjorie Kelly Cowan, Heidi Smith, and Jennifer Lusk.
• © 2022 McGraw Hill, LLC.

Learning Outcomes

• Section 10.1:
  • State the main goal of antimicrobial treatment.
  • Identify the sources for the most commonly used antimicrobials.
  • Describe two methods for testing antimicrobial susceptibility.
  • Define therapeutic index and identify whether a high or a low index is preferable.
• Section 10.2:
  • Explain the concept of selective toxicity.
  • List the five major targets of antimicrobial agents.
  • Identify which categories of drugs are most selectively toxic and why.
  • Distinguish between broad-spectrum and narrow-spectrum antimicrobials, and explain the significance of the distinction.
  • Identify the microbes against which the various penicillins are effective.
  • Explain the mode of action of penicillinases and their role in treatment decisions.
  • Identify two antimicrobials that act by inhibiting protein synthesis.
  • Explain how drugs targeting folic acid synthesis work.
  • Identify one example of a fluoroquinolone.
  • Describe the mode of action of drugs that target the cytoplasmic or cell membrane.
  • Discuss how treatments of biofilm and nonbiofilm infections differ.
  • Name the four main categories of antifungal agents.
  • Explain why antiprotozoal and antihelminthic drugs are likely to be more toxic than antibacterial drugs.
  • List the three major targets of action of antiviral drugs.
• Section 10.3:
  • Discuss two main ways that microbes acquire antimicrobial resistance.
  • List five cellular or structural mechanisms that microbes use to resist antimicrobials.
  • Discuss at least two novel antimicrobial strategies that are under investigation.
• Section 10.4:
  • Distinguish between drug toxicity and allergic reactions to drugs.
  • Explain what a superinfection is and how it occurs.

Principles of Antimicrobial Therapy

• The introduction of modern drugs to control infections was a medical revolution in the 1940s.
• Antimicrobial drugs have reduced the incidence of certain infections, but they have not eradicated infectious diseases and probably never will.
• Today, doctors are worried that we are dangerously close to a post-antibiotic era, where the drugs we have are no longer effective.

Antimicrobial Chemotherapy

• Goal: Administer a drug to an infected person that destroys the infective agent without harming the host’s cells.
• A drug must be able to:
  • Be easy to administer and able to reach the infectious agent anywhere in the body.
  • Be absolutely toxic to the infectious agent and absolutely nontoxic to the host.
  • Remain active in the body as long as needed and be safely and easily broken down and excreted.

Characteristics of the Ideal Antimicrobial Drug

• Toxic to the microbe but nontoxic to host cells.
• Microbicidal rather than microbistatic.
• Relatively soluble; functions even when highly diluted in body fluids.
• Remains potent long enough to act and is not broken down or excreted prematurely.
• Does not lead to the development of antimicrobial resistance.
• Complements or assists the activities of the host’s defenses.
• Remains active in tissues and body fluids.
• Readily delivered to the site of infection.
• Does not disrupt the host’s health by causing allergies or predisposing the host to other infections.

Terminology of Antimicrobials

• Prophylaxis: Use of a drug to prevent infection of a person at risk.
• Antimicrobial Chemotherapy: The use of drugs to control infection.
• Antimicrobials: All-inclusive term for any antimicrobial drug, regardless of its origin.
• Antibiotics: Substances produced by the natural metabolic processes of some microorganisms that can inhibit or destroy other microorganisms; generally, the term is used for drugs targeting bacteria and not other types of microbes.
• Semisynthetic Drugs: Drugs that are chemically modified in the laboratory after being isolated from natural sources.
• Synthetic Drugs: Drugs produced entirely by chemical reactions.
• Narrow-Spectrum (Limited Spectrum): Antimicrobials effective against a limited array of microbial types—for example, a drug effective mainly against gram-positive bacteria.
• Broad-Spectrum (Extended Spectrum): Antimicrobials effective against a wide variety of microbial types—for example, a drug effective against both gram-positive and gram-negative bacteria.

Origins of Antimicrobial Drugs

• Antibiotics are originally metabolic products of bacteria and fungi.
  • Produced to inhibit the growth of competing microbes in their habitat.
• Greatest numbers derived from:
  • Bacteria in the genera Streptomyces and Bacillus.
  • Molds in the genera Penicillium and Cephalosporium.

Before Therapy Can Begin

• Three factors must be considered:
  • The identity of the microorganism causing the infection.
  • The degree of the microorganism’s susceptibility (also called sensitivity) to various drugs.
  • The overall medical condition of the patient.

Identifying the Agent

• Identification of infectious agents should begin as soon as possible.
  • Should occur before antimicrobial drugs are given, before their numbers are reduced.
• Direct examination of body fluids, sputum, or stool samples is a rapid method for detection.
• Doctors often begin therapy on the basis of immediate findings and informed guesses.
• Epidemiological statistics may be required.

Testing for Drug Susceptibility

• Testing is necessary for the following organisms:
  • Staphylococcus species
  • Neisseria gonorrhoeae
  • Enterococcus faecalis
  • Aerobic, gram-negative intestinal bacilli
• If treatment is ineffective for fungal or protozoal infections initially, drug testing is essential.
• In general, these tests involve exposing a pure culture of the microbe to several different drugs and observing the effects of the drugs on growth.

Kirby-Bauer Technique

• Surface of an agar plate is spread with test bacterium (for example).
• Small discs containing a prepared amount of antibiotic are placed on the plate.
• Zone of inhibition surrounding the discs is measured and compared with a standard for each drug.

Results of a Sample Kirby-Bauer Test

• R=resistant, I=intermediate, S=sensitive

Tube Dilution Test

• More sensitive and quantitative than the Kirby-Bauer test.
• Antimicrobial is diluted serially in tubes of broth.
• Each tube is inoculated with a small uniform sample of pure culture, incubated, and examined.
• Minimum inhibitory concentration (MIC): the smallest concentration (highest dilution) of drug that visibly inhibits growth.
  • Useful in determining the smallest effective dosage and providing a comparative index against other antimicrobials.

Response to Treatment

• In vitro activity of a drug is not always correlated with the in vivo effect.
• Failure of antimicrobial treatment is due to:
  • The inability of the drug to diffuse into that body compartment (brain, joints, skin); this can include the possibility that the microbes are in a biofilm.
  • Resistant microbes in the infection that did not make it into the sample collected for testing.
  • An infection caused by more than one pathogen (mixed), some of which are resistant to the drug.
  • In outpatient situations you have to also consider the possibility that the patient did not take the antimicrobials correctly.

Therapeutic Index

• The ratio of the dose of the drug that is toxic to humans compared to its minimum effective (therapeutic) dose.
  • The smaller the ratio, the greater the potential for toxic drug reactions.
  • $TI = <br /> umerator{Toxic Dose}{\text{Minimum Effective Dose}}$
  • TI = 1.1 is a risky choice.
  • TI = 10 is a safer choice.
  • The drug with the highest therapeutic index has the widest margin of safety.

Before Prescribing an Antibiotic

• The physician must take a careful history:
  • Preexisting conditions that might influence the activity of the drug or the response of the patient.
  • History of allergy to a certain class of drugs.
  • Underlying liver or kidney disease.
  • Infants, the elderly, and pregnant women require special precautions.
  • Intake of other drugs can result in increased toxicity or failure of one or more drugs.
  • Some drug combinations have synergistic effects and may allow for reduced dosages.

The Art and Science of Choosing an Antimicrobial Drug

• Even when all of the information is in, the final choice of a drug is not always easy or straightforward.
• Two hypothetical cases:
  • Elderly alcoholic patient with pneumonia caused by Klebsiella and complicated by diminished liver and kidney function, requiring parenteral drug administration, and history of allergy to penicillins.
  • Cancer patient with severe systemic Candida infection.

Concept Check (1)

• Which of the following antibiotics would be the safest choice for a patient with no exceptional medical history? Why?
  • Drug A: Zone of inhibition: 30 mm, TI: 1.2
  • Drug B: Zone of inhibition: 20 mm, TI: 12

Goal of Antimicrobial Drugs

• Disrupt cell processes or structures of bacteria, fungi, or protozoa.
• Inhibit virus replication.
• Interfere with the function of enzymes required to synthesize or assemble macromolecules.
• Destroy structures already formed in the cell.
• Selectively toxic: kill or inhibit microbial cells without damaging host tissues.

Interactions Between Drug and Microbe

• Drugs with excellent selective toxicity block the synthesis of the bacterial cell wall (penicillins).
  • Human cells lack the chemical peptidoglycan and are unaffected by the drug.
• Drugs most toxic to humans:
  • Drugs that act upon a structure common to both the infective agent and the host cell (cytoplasmic membrane).
  • As characteristics of the infectious agent are more and more similar to the host cell, selective toxicity becomes more difficult to achieve.

Mechanisms of Drug Action

• Goals of chemotherapy: disrupt the structure or function of an organism to the point where it can no longer survive.
• Antimicrobial drug categories:
  • Inhibition of cell wall synthesis.
  • Inhibition of nucleic acid (RNA and DNA) structure and function.
  • Inhibition of the ribosome in protein synthesis.
  • Interference with cytoplasmic membrane structure or function.
  • Inhibition of folic acid synthesis.

Drugs That Target the Cell Wall

• Penicillins:
  • Penicillins G and V
  • Ampicillin, carbenicillin, amoxicillin
  • Nafcillin, cloxacillin
  • Clavulanic acid
• Cephalosporins:
  • Cefazolin
  • Cefaclor
  • Cephalexin, cefotaxime
  • Ceftriaxone
  • Cefepime
  • Cegtaroline
• Carbapenems:
  • Doripenem, imipenem
  • Aztreonam
• Miscellaneous drugs that target the cell wall:
  • Bacitracin
  • Isoniazid
  • Vancomycin
  • Fosfomycin tromethamine

Drugs That Target Protein Synthesis

• Aminoglycosides: insert on sites on the 30S subunit and cause the misreading of the mRNA, leading to abnormal proteins.
  • Streptomycin
• Tetracyclines: block the attachment of tRNA on the A acceptor site and stop further protein synthesis.
  • Tetracycline
• Glycylcyclines
  • Tigecycline
• Macrolides: inhibit translocation of the subunit during translation (erythromycin).
  • Erythromycin, clarithromycin, azithromycin
• Miscellaneous drugs that target protein synthesis:
  • Clindamycin
  • Quinupristin + dalfopristin (Synercid)
  • Linezolid

Drugs That Target Folic Acid Synthesis

• Sulfonamides: interfere with folate metabolism by blocking enzymes required for the synthesis of tetrahydrofolate, which is needed by the cells for folic acid synthesis and eventual production of DNA, RNA, and amino acids.
  • Sulfamethoxazole
  • Silver sulfadiazine
  • Trimethoprim

Drugs That Target DNA or RNA

• Fluoroquinolones: inhibit DNA unwinding enzymes or helicases, thereby stopping DNA transcription.
  • Ciprofloxacin, ofloxacin
  • Levofloxacin
• Miscellaneous Drugs That Target DNA or RNA
  • Rifampin

Drugs That Target Cytoplasmic or Cell Membranes

• Polymyxins (colistins): interact with membrane phospholipids; distort the cell surface and cause leakage of protein and nitrogen bases, particularly in gram-negative bacteria.
  • Polymyxin B
  • Daptomycin

Concept Check (2)

• Which of the following antibiotic modes of action will have the least toxic effect on a human cell?
  • Antibiotic A: acts on DNA replication
  • Antibiotic B: acts on the cell membrane
  • Antibiotic C: acts on the peptidoglycan cell wall

Spectrum of Activity

• Broad-spectrum drugs: effective against more than one group of bacteria.
  • Tetracycline antibiotics
• Narrow-spectrum drugs: target a specific group.
  • Polymyxin
• Penicillins:
  • Original penicillin was narrow-spectrum and susceptible to microbial counterattacks.
  • Molecule has been altered and improved upon over the years.
  • Later penicillins overcome the limitations of the original molecule.

Spectrum of Activity for Antibiotics

Characteristics of Selected Penicillin Drugs

Bacteria in Biofilms

• Bacteria in biofilms behave differently than when they are free-living:
  • Often unaffected by the same antimicrobials that work against them.
  • Antibiotics often cannot penetrate the sticky extracellular material surrounding biofilms.
  • Bacteria in biofilms express a different phenotype and have different antibiotic susceptibility profiles than free-living bacteria.

Antibiotics and Biofilms

• Biofilm treatment strategies:
  • Interrupting quorum sensing pathways
  • Daptomycin: shown success
  • Adding DNAse to antibiotics aids penetration through extracellular debris
  • Impregnating devices with antibiotics prior to insertion to prevent colonization
• Some antibiotics cause biofilms to form at a higher rate than they normally would.

Agents to Treat Fungal Infections

• Fungal cells are eukaryotic, present special problems in drug treatment:
  • Drugs designed to act on bacteria are ineffective against fungi.
  • Similarities between fungal and human cells mean that drugs toxic to fungi will harm human tissues.
  • Only a few agents with special antifungal properties have been developed.

Agents Used to Treat Fungal Infections

Antimalarial Drugs

• Quinine:
  • Principal treatment of malaria for hundreds of years
  • Has been replaced by less toxic synthesized quinolones, chloroquine and primaquine
  • Several species of Plasmodium and many stages in its life cycle mean that no single drug is universally effective.
• Artemisinin:
  • Has become the staple for malaria treatment.

Anti-Protozoal Drugs

• Metronidazole: widely used amoebicide:
  • Treats intestinal infections and hepatic disease caused by Entamoeba histolytica
  • Also treats Giardia lamblia and Trichomonas vaginalis
• Other drugs with antiprotozoal activities:
  • Quinacrine
  • Sulfonamides
  • Tetracyclines

Challenges of Antihelminthic Drug Therapy

• Flukes, tapeworms, and roundworms are larger parasites
• Their physiology is much more similar to humans
• Blocking reproduction does not usually affect adult worms
• Most effective drugs immobilize, disintegrate, or inhibit the metabolism of all stages of the life cycle

Agents to Treat Helminthic Infections

• Albendazole inhibits microtubules of worms, eggs, and larvae
• Pyrantel paralyzes the muscles of intestinal roundworms
• Praziquantel:
  • Tapeworm and fluke infections
• Ivermectin:
  • Used for strongyloidiasis and oncocerosis in humans

Agents to Treat Viral Infections

• Treatment of viral infections presents unique problems
• Infectious agent relies on a host cell for the vast majority of its metabolic functions
• Disrupting viral metabolism requires disruption of cellular metabolism of host
• Measles, mumps, and hepatitis are prevented through the use of vaccines
• AIDS, influenza, and the common cold attest to the need for more effective medications for the treatment of viral pathogens

Actions of Antiviral Drugs

• Inhibition of virus entry: Receptor/fusion/uncoating inhibitors
  • Examples: Enfuvirtide (Fuzeon®), amantadine (Symmetrel®)
• Inhibition of nucleic acid synthesis
  • Examples: Acyclovir (Zovirax®), other “cyclovirs,” vidarabine, ribavirin, Remdesivir, zidovudine (AZT), lamivudine (3TC), didanosine (ddI), zalcitabine (ddC), stavudine (d4T), nevirapine, efavirenz, delavirdine
• Inhibition of viral assembly/release
  • Examples: Indinavir, saquinavir, zanamivir (Relenza®), oseltamivir (Tamiflu®)

Concept Check (3)

• For each of the drugs listed below, determine if each is antibacterial, antifungal, antiprotozoal, antihelminthic, or antiviral.
  • A. Ribavirin
  • B. Oxacillin
  • C. Metronidazole
  • D. Mebendazole
  • E. Amphotericin B

Antimicrobial Resistance

• Drug resistance:
  • An adaptive response in which microorganisms begin to tolerate an amount of drug that would normally be inhibitory
  • Due to the genetic versatility and adaptability of microbial populations
  • Can be intrinsic as well as acquired

How Does Resistance Develop?

• Resistance to penicillin developed in some bacteria as early as 1940
• In the 1980s and 1990s scientists and physicians witnessed treatment failures on a large scale
• Microbes become newly resistant to a drug after one of the following occurs:
  • Spontaneous mutations in critical chromosomal genes
  • Acquisition of entire new genes or sets of genes via horizontal transfer from another species
  • Slowing or stopping of metabolism so that the microbe cannot be harmed by the antibiotic (“Persisters”)

Development of Drug Resistance

• Chromosomal drug resistance:
  • Usually results from spontaneous random mutation
  • Slight changes in drug sensitivity can be overcome with larger doses of the drug

Resistance Through Horizontal Transfer

• Resistance (R) factors: plasmids containing antibiotic resistance genes
• Can be transferred through conjugation, transformation, or transduction
• Transposons also duplicate and insert genes for drug resistance into plasmids
• Sharing of resistance genes accounts for the rapid proliferation of drug-resistant species

Mechanisms of Drug Resistance

Natural Selection and Drug Resistance

• Development of resistance and its long-term therapeutic consequences:
  • Any large population of microbes will contain a few individual cells that are already drug resistant
  • If the population is exposed to drugs, the drug-resistant population will have a selective advantage.
  • Offspring inherit the drug resistance
  • Replacement populations evolve to have the drug-resistant form as the dominant species

An Urgent Problem

• “Threat Report” issued by the CDC in 2013 outlines a “potentially catastrophic” antibiotic resistance situation:
  • We may enter a postantibiotic era where some infections will be untreatable
• New and effective antibiotics have been slow to come to market:
  • Antibiotics not economically lucrative
  • Time-consuming and expensive to develop

Threats

• Urgent threats:
  • Clostridioides difficile (C. diff)
  • Carbapenem-resistant Enterobacteriaceae (CRE)
  • Drug-resistant Neisseria gonorrhoeae
• Serious threats:
  • Multidrug-resistant Acinetobacter
  • Drug-resistant Campylobacter
  • Fluconazole-resistant Candida (a fungus)
  • Many more
• Concerning threats:
  • Vancomycin-resistant Staphylococcus aureus (VRSA)
  • Erythromycin-resistant Group A Streptococcus
  • Clindamycin-resistant Group B Streptococcus

New Approaches to Antimicrobial Therapy

• Using RNA interference strategies:
  • Small pieces of RNA that regulate the expression of genes
  • Used to shut down the metabolism of pathogenic microbes
  • Drug trials have begun to evaluate the effectiveness of synthetic RNAs in treating hepatitis C and respiratory syncytial virus.
• Mimicking defense peptides:
  • Peptides of 20 to 50 amino acids secreted as part of the mammalian innate immune system called defensin, magainins, and protegrins
  • Bacteria also produce defense peptides called bacteriocins and lantibiotics.
  • Insert into membranes and target other structures in cells
  • May be more effective than narrowly targeted drugs and less likely to foster resistance
• CRISPR:
  • System found in bacteria that can cause very specific cuts in genes
  • May treat antibiotic-resistant infections, together with an antibiotic
• Drugs from noncultivable bacteria:
  • 99% of all microbes are noncultivable
  • Scientists are developing new ways to grow and harvest their antimicrobial substances
• Bacteriophages:
  • The former Soviet Union and other regions used mixtures of bacteriophages as medicine
  • Biophage-PA used to treat ear infections caused by Pseudomonas aeruginosa biofilms
  • Other researchers are incorporating bacteriophages into wound dressings
  • Advantage to bacteriophage is their narrow specificity; only infect one species of bacterium
• Probiotics:
  • Preparations of live microorganisms fed to animals and humans to improve intestinal biota
  • Can replace microbes lost during antimicrobial therapy
  • Augment biota already there
  • Safe and, in some cases, effective
  • Useful in the management of food allergies
• Prebiotics:
  • Nutrients that encourage the growth of beneficial microbes in the intestine
  • Fructans encourage the growth of Bifidobacterium in the large intestine and discourage the growth of potential pathogens
• Fecal transplants:
  • Used to treat recurrent Clostridioides difficile infection and ulcerative colitis
  • Transfer of feces from a healthy patient via colonoscopy
  • Work is underway to develop a pill containing the species to re-colonize the colon

Concept Check (4)

• Which of the following mechanisms of antibiotic resistance act specifically on penicillins and cephalosporins?
  • A. Decreased permeability or uptake of the drug
  • B. Enzymes are synthesized, inactivating the drug
  • C. Binding sites for drugs are decreased
  • D. Alternative metabolic pathways are used
  • E. Drug is immediately eliminated through a transport pump

Interactions Between Drug and Host

• Drug is administered to the host even though its target is a microbe:
  • The effect of the drug on the host must be considered
• Selective toxicity is the ideal, but chemotherapy involves contact with foreign chemicals that can harm the host:
  • 5% of all people taking an antimicrobial drug will experience an adverse side effect

Toxicity to Organs

• Drugs can adversely affect the following organs:
  • Liver (hepatotoxic)
  • Kidneys (nephrotoxic)
  • Gastrointestinal tract
  • Cardiovascular system and blood-forming tissue (hemotoxic)
  • Nervous system (neurotoxic)
  • Respiratory tract
  • Skin
  • Bones and teeth

Major Adverse Toxic Reactions to Antibacterials

• Antibacterials:
  • Penicillin G: Rash, hives, watery eyes
  • Carbenicillin: Abnormal bleeding
  • Ampicillin: Diarrhea and enterocolitis
  • Cephalosporins: Inhibition of platelet function, decreased circulation of white blood cells; nephritis
  • Sulfonamides: Formation of crystals in kidney, blockage of urine flow, hemolysis, reduction in the number of red blood cells
  • Polymyxin (colistin): Kidney damage, weakened muscular responses
  • Quinolones (ciprofloxacin, norfloxacin): Headache, dizziness, tremors, GI distress
  • Rifampin: Damage to hepatic cells, dermatitis

Major Adverse Toxic Reactions to Other Common Drug Groups

• Antifungals:
  • Amphotericin B: Disruption of kidney function
  • Flucytosine: Decreased number of white blood cells
• Antiprotozoal drugs:
  • Metronidazole: Nausea, vomiting
• Antihelminthics:
  • Pyrantel: Intestinal irritation, headache, dizziness
• Antivirals:
  • Acyclovir: Seizures, confusion, rash
  • Amantadine: Nervousness, light-headedness, nausea
  • AZT: Immunosuppression, anemia

Allergic Responses to Drugs

• Allergy:
  • Drug acts as an antigen that stimulates an allergic response.
  • Can be provoked by the intact molecule or by substances that develop from the body’s metabolic alteration of the drug
  • Allergies have been reported for every major type of drug, but an allergy to penicillin is most common
  • Sensitization occurs during the first contact with the drug
  • Second exposure can lead to hives, respiratory inflammation, or anaphylaxis

Suppression and Alteration of the Microbiota by Antimicrobials

• Biota:
  • Normal microbial colonists of healthy body surfaces
  • Normally consist of harmless or beneficial bacteria
  • A few may be pathogens
• Broad-spectrum antimicrobials destroy “good” biota, along with pathogens
• Superinfection: microbes that were once small in number can begin to overgrow and cause disease

Examples of Superinfection

• Urinary tract infection caused by E. coli treated with antibiotics:
  • Lactobacilli in the female vagina are killed by the broad-spectrum cephalosporin used to treat the UTI
  • Overgrowth of Candida albicans occurs, causing a vaginal yeast infection or oral thrush
• Antibiotic-associated colitis:
  • Oral therapy with some antimicrobials is associated with a serious and potentially fatal condition known as antibiotic-associated colitis (pseudomembranous colitis)
  • Overgrowth of Clostridioides difficile invades the intestinal lining and releases toxins that cause diarrhea, fever, and abdominal pain

The Antimicrobial Drug Dilemma

• Physicians have used a “shotgun” approach, using broad-spectrum antimicrobial therapy for minor infections
  • This has led to superinfections and other adverse reactions
  • Caused the development of resistance in “bystander” microbes (normal biota) that were exposed to the drug as well, leading to the spread of resistant pathogens
  • Growing awareness has led to the reduction of this practice
• Tons of excess antimicrobial drugs in the U.S. are exported to countries where controls are not as