Physical/ Chemical Control of Microorganisms

Introduction

• Until late 19th century, patients undergoing surgeries were at great risk of developing fatal infections

• Modern hospitals use strict procedures to avoid microbial contamination

Approaches to Control

1. Principles of Control

   • Sterilization: removal or destruction of all microorganisms and viruses

   • Disinfection: elimination of most or all pathogens

   • Disinfectants: chemicals used on inanimate objects

   • Antiseptics: chemicals used on living tissues

   • Decontamination

     • Reduces number of pathogens to a safe level

     • Washing, use of heat, or chemicals

   • Sanitization

     • Substantially reduces microbial population to meet accepted health standards

     • Minimizes spread of disease

   • Preservation

     • Delays spoilage of perishable products

     • Slows growth or adds preservatives

   • Pasteurization

     • Brief heating to reduce number of spoilage organisms

     • Destroy pathogens without changing characteristics of product

2. Daily Life

   • Washing and scrubbing with soaps and detergents

   • Cooking foods, cleaning surfaces, and refrigeration

3. Hospitals and Other Healthcare Facilities

   • Controlling microbes is important to prevent healthcare-associated infections (HAIs)

   • Standard Precautions and Transmission-Based Precautions are used

4. Microbiology Laboratories

   • Use rigorous methods of control

   • Aseptic technique used to prevent contamination

Selecting an Antimicrobial Procedure

• Selection depends on factors like type and number of microbes, environmental conditions, risk of infection, and composition of item to be treated

Types of Microbes

• Bacterial endospores: most resistant

  • Only destroyed by extreme heat or chemical treatment

• Protozoan cysts and oocysts: resistant to disinfectants

  • Excreted in feces, cause diarrheal disease if ingested

  • Destroyed by boiling

• Mycobacterium species: resistant to many chemical treatments

  • Wavy cell walls

• Pseudomonas species: resistant to some disinfectants

  • Can grow in some disinfectants

• Non-enveloped viruses: more resistant to disinfectants

  • Lack lipid envelope

Number of Microbes

• Removing some organisms by washing reduces time needed to sterilize or disinfect a product

• Scrubbing helps remove biofilms

• Decimal reduction time (D value) is time required to kill 90% of population

Environmental Conditions

• Dirt, grease, body fluids can interfere with heat penetration and action of chemicals

• Temperature and pH can influence effectiveness

Risk for Infection

• Medical instruments categorized according to risk for transmitting infectious agents

• Critical items must be sterile

• Semicritical instruments must be free of microorganisms and viruses

• Non-critical instruments and surfaces have low risk of transmission

Composition of an Item

• Some methods inappropriate for certain items

• Heat can damage plastics, irradiation damages some types of plastic, moisture-sensitive materials cannot be treated with moist heat or liquid chemical disinfectants

Physical Methods Used to Destroy or Remove Microorganisms and Viruses

• Heat treatment is reliable, safe, relatively fast, inexpensive, and non-toxic

• Filtration, irradiation, and high-pressure treatment can be used on materials that cannot withstand heat treatment

Moist Heat

• Moist heat: irreversibly denatures protein

• Boiling destroys most microorganisms and viruses, but not endospores

• Pasteurization destroys heat-sensitive pathogens and spoilage organisms

• High-temperature–short-time (HTST) method and ultra-high-temperature (UHT) method

Sterilization Using Pressurized Steam

• Autoclave used to sterilize using pressurized steam

• Increased pressure raises steam temperature and kills endospores

• Sterilization typically at 121 degrees Celsius and 15 pound per square inch in 15 minutes

Dry Heat

• Incineration destroys cell components by burning

• Hot air ovens kill microbes by destroying cell components and denaturing proteins

Filtration

• Membrane filtration used to remove organisms from heat-sensitive fluids

• Filtration of air using HEPA filters to remove microbes

Chemicals Methods Used to Destroy Microorganisms and Viruses

• Germicidal chemicals can disinfect and, in some cases, sterilize.

  • React irreversibly with proteins, DNA, cytoplasmic membranes, or viral envelopes.

  • Exact mechanisms of action are often poorly understood.

• Less reliable than heat but useful for treating large surfaces and heat-sensitive items.

• Some are sufficiently non-toxic to be used as antiseptics.

• In the United States, the FDA is responsible for ensuring that chemicals used to treat medical devices work and that drug products, including antiseptics, are safe and effective.

• Various active ingredients previously allowed in non-prescription antiseptic products must now be proven safe and effective.

  • Antiseptic washes (antimicrobial soaps).

  • Antiseptic rubs (hand sanitizers).

  • Topical antiseptic products used by healthcare professionals to disinfect patients' skin in preparation for injections or surgery.

Categories of Germicidal Potency

• Germicidal Chemicals disinfect and, sometimes, sterilize.

• Sterilants destroy all microbes.

  • Heat-sensitive critical instruments.

• High-level disinfectants destroy viruses, vegetative cells.

  • Do not reliably kill endospores.

  • Semi-critical instruments.

• Intermediate-level disinfectants destroy vegetative bacteria, mycobacteria, fungi, and most viruses.

  • Disinfect non-critical instruments.

• Low-level disinfectants destroy fungi, vegetative bacteria except mycobacteria, and enveloped viruses.

  • Do not kill endospores, non-enveloped viruses.

  • Disinfect furniture, floors, walls.

Classes of Germicidal Chemicals

• Alcohols.

  • 60 to 80% aqueous solutions of ethyl or isopropyl alcohol.

  • Destroy vegetative bacteria and fungi.

  • Not reliable against endospores, non-enveloped viruses.

  • Denatures essential proteins, damages membranes.

  • Commonly used as antiseptic and disinfectant; non-toxic, inexpensive.

  • Limitations.

    • Evaporates quickly, limiting contact time.

    • Can damage rubber, some plastics.

  • Tincture: antimicrobial chemical dissolved in alcohol.

• Aldehydes.

  • Includes glutaraldehyde, formaldehyde, and ortho-phthalaldehyde (OPA).

  • Inactivates proteins and nucleic acids.

  • 2% alkaline glutaraldehyde common liquid sterilant.

    • Immersion of medical items for 10 to 12 hours.

    • Toxic; requires thorough rinsing after use.

  • Formaldehyde used as gas or as formalin (37% solution).

    • Effective germicide that kills most microbes quickly.

    • Used to kill bacteria and inactivate viruses for vaccines.

    • Used to preserve specimens.

    • Probable carcinogen.

• Biguanides.

  • Chlorhexidine most effective.

    • Extensive use as antiseptics.

    • Stays on skin, mucous membranes.

    • Relatively low toxicity.

    • Destroys vegetative bacteria, fungi, some enveloped viruses.

    • Common in many products: skin cream, prescription mouthwashes.

• Ethylene oxide.

  • Gaseous sterilant for heat- or moisture-sensitive items.

  • Destroys microbes, including endospores and viruses, by chemically modifying proteins and nucleic acids.

  • Penetrates fabrics, equipment, implantable devices.

    • Mattresses, electrical equipment, artificial hips.

  • Used for many disposable laboratory items.

    • Petri dishes, pipettes.

  • Applied in special chamber resembling autoclave.

  • Limitations: explosive, toxic, potentially carcinogenic.

    • Must be eliminated by heated forced air for 8 to 12 hours.

• Halogens: oxidizing agents which react with proteins, cellular components.

• Chlorine: Destroys all microorganisms, endospores and viruses.

  • Caustic to skin and mucous membranes.

  • 1:100 dilution of household bleach effective.

  • Very low levels disinfect drinking water.

  • Cryptosporidium oocysts, Giardia cysts survive.

  • Can react with organic compounds in water.

  • Disrupts germicidal activity.

  • Chlorine dioxide (ClO2) used as disinfectant and sterilant.

• Iodine: Kills vegetative cells, unreliable on endospores.

  • Used as tincture (in alcohol).

  • Used as iodophor.

    • Iodine slowly released from carrier molecule.

    • Less irritating.

  • Pseudomonas species can survive in stock solution.

• Metal Compounds

  • Combine with sulfhydryl groups (–SH) of proteins.

  • High concentrations too toxic to be used medically.

  • Silver used in creams, bandages.

  • Antibiotics replaced silver nitrate eye drops once given at birth to prevent Neisseria gonorrhoeae infections.

• Peroxygens

  • Powerful oxidizers used as sterilants.

  • Readily biodegradable, no residue.

  • Less toxic than ethylene oxide, glutaraldehyde.

  • Hydrogen peroxide: effectiveness depends on surface.

    • Aerobic cells (humans) produce enzyme catalase.

      • Breaks down.

    • More effective on inanimate object.

    • Doesn’t damage most materials, no residue.

    • Hot solutions or vapor-phase used as sterilant.

  • Peracetic acid: more potent than H2O2.

    • Sterilizes in less than 1 hour.

    • Effective in presence of organic compounds, no residue, used on wide range of materials.

• Phenolic Compounds

  • Phenol (carboic acid) one of earliest disinfectants.

    • Has unpleasant odor, irritates skin.

    • Destroy cytoplasmic membranes, denature proteins.

  • Phenolics kill most vegetative bacteria; not reliable on all virus groups.

  • Wide activity range, reasonable cost, remain effective in presence of detergents and organic contaminants.

    • Leave antimicrobial residue.

  • Hexachlorophene and triclosan have been widely used in medical and personal care products.

• Preservation of Perishable Products

  • Low-Temperature Storage.

  • Refrigeration inhibits growth of pathogens and spoilage organisms by slowing or stopping enzyme reactions.

    • Psychrotrophs, psychrophilic organisms can still grow.

  • Freezing preserves by stopping all microbial growth.

    • Some microbial cells killed by ice crystal formation, but many survive and can grow once thawed.

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Antimicrobial Medications

History and Development of Antimicrobial Medications

• Salvarsan (Paul Ehrlich, 1910) first documented case.

• Red dye Prontosil (Gerhard Domagk, 1932) used to treat streptococcal infections in animals.

  • No effect in test tubes; enzymes in blood split to produce sulfanilamide, the first sulfa drug.

• Both are chemotherapeutic agents.

  • Chemicals used to treat disease.

  • Antimicrobial medications, antimicrobial drugs, or antimicrobials.

Discovery of Antibiotics

• In 1928, Fleming identified mold Penicillium secreting compound toxic to Staphylococcus (penicillin).

  • Showed effective in killing many bacterial species.

    • Unable to purify, he later abandoned research.

  • Chain and Florey purified, tested compounds in 1941 on police officer with Staphylococcus aureus infection.

    • Patient improved but supply of purified penicillin ran out and he later died.

    • WWII spurred research and development; penicillin G, the first antibiotic (naturally-produced antimicrobial).

Discovery of Antibiotics

• Selman Waksman purified streptomycin from soil bacterium Streptomyces griseus.

  • Researchers began screening hundreds of thousands of microbes for antibiotics.

  • Pharmaceutical companies today examine soil samples from around the world.

Development of Antimicrobial Medications

• Most antibiotics come from microorganisms that normally live in the soil including Streptomyces and Bacillus (bacteria), and Penicillium and Cephalosporium (fungi).

• Altering structure of antibiotics such as penicillin yields new medications (ampicillin, methicillin).

  • Development of new antimicrobial drugs is financially risky because FDA demands strict and expensive testing

  • Resistance will likely develop soon after drug is used

  • Drug may be held as last resort to avoid resistance

• GAIN (Generating Antibiotic Incentives Now) is a U.S. law to encourage companies to develop new antimicrobials against certain pathogens.

  • If a drug is promising, it may be designated a Qualified Infectious Disease Product (QIDP).

    • QIDPs receive high priority for review.

    • The company that develops a QIDP has exclusive marketing rights for 5 years.

Characteristics of Antimicrobial Medications

• Antimicrobial medications have selective toxicity, causing greater harm to microbes than to human hosts.

  • They interfere with essential structures or properties in microbes but not human cells.

  • Antiviral medications are difficult because viruses rely on human cells for replication.

• Toxicity is relative and expressed as therapeutic index.

  • Therapeutic index is calculated as the lowest dose toxic to the patient divided by the dose used for therapy.

  • Penicillin G is an example of a medication with a high therapeutic index.

  • Penicillin G interferes with cell wall synthesis, a process not present in humans.

• Medications that are too toxic for systemic use may be used topically.

Antimicrobial Action

• Bacteriostatic chemicals inhibit bacterial growth.

  • Sulfa drugs are an example of bacteriostatic antimicrobials that require the patient's defenses to eliminate the pathogen.

• Bactericidal chemicals kill bacteria.

Spectrum of Activity

• Broad-spectrum antibiotics affect a wide range of pathogens.

  • They are important for treating acute life-threatening diseases when there is no time to culture for identification.

  • However, broad-spectrum antibiotics can disrupt the microbiome that helps keep out other pathogens.

• Narrow-spectrum antibiotics affect a limited range of pathogens.

  • They require identification and susceptibility testing of the pathogen.

  • Narrow-spectrum antibiotics are less disruptive to the microbiome.

• Patients may be started on broad-spectrum antimicrobials and later switched to narrow-spectrum once more is known about the pathogen.

Effects of Antimicrobial Combinations

• Some medications can interfere with each other (antagonistic).

  • For example, bacteriostatic antimicrobials that prevent cell division can interfere with bactericidal antimicrobials that kill only dividing cells.

• Some medications can enhance each other (synergistic).

• Other combinations are additive.

Tissue Distribution, Metabolism, and Excretion of the Medication

• Antimicrobial behaviors differ in the body.

  • Only some medications can access the brain, while only some can withstand stomach acid.

  • If a medication is poorly absorbed from the intestinal tract, it must be administered by injection.

• The half-life of a medication is the time it takes for the serum concentration to decrease by 50%.

  • The half-life dictates the frequency of doses required to maintain an effective level in the body.

• Patients with kidney or liver dysfunction excrete or metabolize medications more slowly, so dosage adjustments are necessary to avoid toxic levels.

Adverse Effects

• Antimicrobials have saved countless lives when properly prescribed and used.

  • Allergic reactions can occur and may be life-threatening.

  • Some antimicrobials have toxic effects, so monitoring is necessary for those taking low therapeutic index drugs.

    • Some side effects of antimicrobials are life-threatening, such as aplastic anemia caused by chloramphenicol.

  • Dysbiosis, an imbalance in the microbiome, can occur due to broad-spectrum antimicrobials allowing the growth of Clostridium difficile without competition, resulting in diarrhea or colitis.

Resistance to Antimicrobials

• Certain bacteria have intrinsic (innate) resistance to antimicrobials.

  • For example, Mycoplasma lacks a cell wall and is resistant to penicillin.

• Bacteria can also develop acquired resistance through spontaneous mutations or horizontal gene transfer.

Mechanism of Action of Antibacterial Medications

• Antibacterial medications target specific bacterial processes and structures.

  • These include cell wall synthesis, protein synthesis, nucleic acid synthesis, metabolic pathways, and cell membranes.

Inhibit Cell Wall Synthesis

• Antibacterial medications can inhibit cell wall synthesis.

  • Bacterial cell walls contain peptidoglycan, which is targeted by β-lactam antibiotics, glycopeptide antibiotics, and bacitracin.

• β-lactam antibiotics

  • Have a β-lactam ring and a high therapeutic index.

    • Examples include penicillins, cephalosporins, carbapenems, and monobactams.

  • They competitively inhibit penicillin-binding proteins (PBPs) that catalyze the formation of peptide bridges between adjacent glycan strands, disrupting cell wall synthesis.

  • Only effective against actively growing cells.

  • Vary in activity.

    • Gram-positive bacteria have exposed peptidoglycan, while the outer membrane of Gram-negative bacteria blocks the antibiotics.

  • Some bacteria produce β-lactamase, which breaks the β-lactam ring and destroys the activity of the antibiotic.

    • Penicillinase inactivates members of the penicillin family, while extended-spectrum β-lactamases (ESBLs) inactivate a wide variety of β-lactam medications.

    • Gram-negative bacteria produce a more extensive array of β-lactamases than Gram-positive bacteria.

  • Penicillins are grouped by modifications in their side chain.

  • Natural penicillins are narrow-spectrum and act against Gram-positives and a few Gram-negatives but are deactivated by penicillinases.

  • Penicillinase-resistant penicillins have been developed in response to penicillinase from S. aureus strains.

    • Some bacteria can produce altered PBPs to which β-lactam antibiotics do not bind well, such as methicillin-resistant S. aureus (MRSA).

  • Broad-spectrum penicillins, such as ampicillin and amoxicillin, act against Gram-positives and many Gram-negatives but are inactivated by many β-lactamases.

  • Extended-spectrum penicillins have greater activity against Enterobacteriaceae and Pseudomonas species but reduced activity against Gram-positives and are destroyed by many β-lactamases.

  • Penicillins combined with a β-lactamase inhibitor, such as Augmentin, include an inhibitor to protect the penicillin.

  • Cephalosporins have a structure that makes them resistant to some β-lactamases.

    • Some cephalosporins have low affinity for PBPs of Gram-positives.

    • Chemical modifications have led to five generations of cephalosporins.

    • Later generations are more effective against Gram-negatives and resist β-lactamases.

    • Fifth-generation cephalosporins are effective against MRSA.

    • Some cephalosporins are available with a β-lactamase inhibitor, such as Zerbaxa.

  • Carbapenems are effective against a wide range of Gram-positive and Gram-negative bacteria and are not inactivated by extended-spectrum β-lactamases (ESBL).

    • They are often reserved as a last resort against ESBL-producing organisms but can be inactivated by carbapenemases.

  • Monobactam, such as aztreonam, is used against the family Enterobacteriaceae.

• Glycopeptide antibiotics

  • Bind to the amino acid side chain of NAM molecules, blocking peptidoglycan synthesis.

  • Effective only against Gram-positive bacteria and do not cross the outer membrane of Gram-negatives.

  • Side effects give low therapeutic index.

  • Vancomycin is the most widely used glycopeptide antibiotic and is usually administered via IV except for intestinal infections.

    • Often used as a last resort to treat Gram-positives resistant to β-lactam antibiotics.

• Bacitracin

  • Toxicity limits and is usually used topically.

  • It interferes with the transport of peptidoglycan precursors across the membrane and is common in first-aid skin ointments.

Inhibit Protein Synthesis

• They are generally bacteriostatic and can exploit differences between prokaryotic and eukaryotic ribosomes.

  • Prokaryotes have 70S ribosomes, while eukaryotes have 80S ribosomes.

  • Mitochondria also have 70S ribosomes, which may

• Aminoglycosides

  • Irreversibly bind to 30S ribosomal subunit, causing it to malfunction; bacteriocidal

    • Blocks initiation of translation; causes misreading of mRNA by ribosomes past initiation

  • Often toxic; generally used when alternatives unavailable

  • Generally ineffective against anaerobes, enterococci, and streptococci because cannot enter cells

    • Sometimes used synergistically with a penicillin that allows the aminoglycoside to enter cells

  • Inhaled form of tobramycin treats Pseudomonas lung infections in cystic fibrosis patients

  • Neomycin too toxic for systemic use; common in first-aid skin ointments

• Tetracyclines and Glycylcyclines

  • Tetracyclines reversibly bind to 30S ribosomal subunit

    • Block tRNA attachment; prevent translation

    • Effective against certain Gram-positives and Gram-negatives

    • Some have longer half-life meaning less frequent doses

    • Resistance from decreased uptake or increased excretion

  • The Glycylcyclines are related to the tetracyclines

    • Wider activity

    • Effective against bacteria resistant to the tetracyclines

    • Relatively new, so acquired resistance is rare

      • Tigecycline is only one currently approved

• Macrolides

  • Reversibly bind to 50S subunit; prevent translation from continuing

  • Often antibiotic of choice for patients allergic to penicillin

  • Bacteriostatic against many Gram-positives and most common causes of atypical pneumonia

    • Outer membrane of Enterobacteriaceae blocks

  • Resistance occurs from modification of ribosomal RNA target, enzyme that modifies chemical, and decreased uptake

• Chloramphenicol

  • Binds to 50S ribosomal subunit; blocks translation

    • Active against wide range of bacteria

    • Used as last resort due to rare, but lethal side effect

      • May cause aplastic anemia, inability to form white, red blood cells

• Lincosamides

  • Bind 50S ribosomal subunit; block translation from continuing

  • Inhibit variety of Gram-negatives and Gram-positives

  • Useful against Bacteroides fragilis (resists antibiotics)

    • Increases risk of developing C. difficile infection; most strains are resistant to lincosamides

• Oxazolidinones

  • Bind 50S ribosomal subunit; interfere with initiation of translation

  • Useful against variety of Gram-positives strains resistant to β- lactams, vancomycin

• Pleuromutilins

  • Bind to 50s ribosomal subunit; prevent formation of peptide bonds during translation

  • Active against many types of Gram-positives

  • Once used only in animals; a topical now approved in humans

• Streptogramins

  • Bind to 50S ribosomal subunit; inhibit translation

  • Quinupristin and dalfopristin each are bacteriostatic, but bactericidal when given together

    • Effective against variety of Gram-positives, but often reserved for treating infections caused by strains that are resistant to other antimicrobials

Inhibit Nucleic Acid Synthesis

• Fluoroquinolones

  • Inhibit topoisomerases, enzymes that maintain supercoiling of DNA; bactericidal against wide variety of bacteria

  • DNA gyrase breaks, rejoins strands to relieve strain from localized unwinding of DNA; function is essential

  • Resistance usually due to alteration in DNA gyrase target

  • Used extensively in the past; severe side effects limit their use

• Rifamycins

  • Block prokaryotic RNA polymerase; prevents initiation of transcription

  • Rifampin is bactericidal against Gram-positives, some Gram-negatives, Mycobacterium

    • Resistance develops quickly due to mutation in RNA polymerase gene

• Fidaxomicin

  • Binds to RNA polymerase; interferes with transcription

  • Relatively new; bactericidal

  • Not absorbed in intestinal tract; effective in treating C. difficile infections

• Metronidazole (Flagyl)

  • Anaerobic metabolism required to convert to active form

  • Active form binds DNA, interferes with synthesis, causes breaks

  • Used to treat bacterial vaginosis and C. difficile infection

Interfere with Metabolic Pathways

• Folate inhibitors

  • Inhibit steps in synthesis of folate and ultimately synthesis of coenzyme required for nucleotide biosynthesis

  • Animals lack enzymes to synthesize folate; required in diet

  • Sulfonamides, trimethoprim inhibit different steps in synthesis

• Sulfonamides and related: called sulfa drugs

  • Inhibit many Gram-positives and Gram-negatives

  • Structurally similar to PABA, so enzyme binds chemical

    • Example of competitive inhibition

    • Human cells lack enzyme

• Trimethoprim inhibits enzyme in later step

  • Has little effect on enzyme’s counterpart in human cells

  • Combination of trimethoprim and sulfonamide has synergistic effect; co-trimoxazole (sulfamethoxazole and trimethoprim)

Interfere with Cell Membrane Integrity

• Daptomycin inserts into cytoplasmic membrane

  • Used against Gram-positives resistant to other antibiotics

  • Ineffective against Gram-negatives; cannot penetrate outer membrane

• Polymyxins bind to membranes of Gram-negatives

  • Generally limits usefulness to topical applications

  • Also bind to eukaryotic cells, though to a lesser extent

• Newest glycopeptide antibiotics disrupt cell membranes (albavancin, oritavancin)

Effective against Mycobacterium Tuberculosis

• Waxy cell wall prevents entry of many chemicals; slow growth

• First-line drugs are most effective, least toxic

  • Combination therapy decreases chance of development of resistant mutants

• Second-line drugs given for strains resistant to first-line drugs

  • Some target unique cell wall of mycobacteria

• Isoniazid inhibits mycolic acid synthesis; ethambutol inhibits enzymes required for synthesis of other cell wall components; pyrazinamide interferes with protein synthesis

Antimicrobial Susceptibility Testing

• Kirby-Bauer disc diffusion test routinely used to determine susceptibility of bacterial strain to antibiotics

  • Standard sample of strain uniformly spread on agar plate; discs containing different antibiotics placed on surface

  • Drugs diffuse outward, establish concentration gradients

    • Resulting zone of inhibition compared with specially prepared charts to determine whether strain is susceptible, intermediate, or resistant

Resistance to Antimicrobial Medications

• Increasing use, misuse selects for resistant microorganisms

• Only 3% of Staphylococcus aureus originally resistant to penicillin G; now more than 90% are resistant

• Antimicrobial resistance alarming

  • Impact on cost, complications, and outcomes of treatment

• Dealing with problem requires understanding of mechanisms and spread of resistance

• Mechanisms of Acquired Resistance:

  • Medication-inactivating enzymes

    • Bacteria produce enzymes that interfere with drug

    • Penicillinase, extended spectrum β-lactamases

  • Alteration in target molecule

    • Minor structural changes can prevent binding

    • PBPs (β-lactam antibiotics), ribosomal RNA (macrolides, lincosamides, streptogramins)

  • Decreased uptake of the medication

    • Changes in porin proteins of outer membrane of Gram-negatives

  • Increased elimination of medication

    • Efflux pumps remove compounds from cell

    • Increased production or structural changes of pumps allows faster removal

    • Resistance to range of antimicrobials

  • Spontaneous mutations

    • Mutations happen at low rate during replication, but can have significant effect

    • Just a single base-pair change in gene encoding a ribosomal protein yields resistance to streptomycin

    • Combination therapy of multiple antibiotics is often used

      • Unlikely cells will simultaneously develop resistance

  • Gene transfer

    • Genes encoding resistance are often carried by conjugative R plasmids, which can carry multiple resistance genes.

      • Resistance genes on R plasmids can originate from spontaneous mutations or from microbes that naturally produce the antibiotic.

    • An example is the gene coding for an enzyme that modifies aminoglycoside, which likely originated from the Streptomyces species that produces the antibiotic.

Examples of Emerging Resistance

• Enterococci, which are part of the normal intestinal microbiota, can cause healthcare-associated infections.

  • They are intrinsically less susceptible to many antimicrobials.

    • Enterococci often have R plasmids, some of which code for resistance to vancomycin, leading to the emergence of vancomycin-resistant enterococci (VRE).

    • Vancomycin is often the antibiotic of last resort.

• Mycobacterium tuberculosis, the causative agent of tuberculosis, can become resistant to first-line antibiotics through mutation.

  • Due to the large numbers of cells found in active infection, it is likely that at least one cell has developed resistance.

  • Combination therapy is required for the treatment of tuberculosis, as it requires a long treatment period.

  • Multi-drug-resistant tuberculosis (MDR-TB) is resistant to two favored first-line antibiotics: isoniazid and rifampin.

  • Extensively drug-resistant tuberculosis (XDR-TB) is even more concerning, as it can resist three or more second-line anti-TB medications.

• Neisseria gonorrhoeae, the causative agent of gonorrhea, was once susceptible to penicillin.

  • Some strains developed resistance through mutation, while others acquired a plasmid that encoded the production of penicillinase.

  • Only certain cephalosporins are reliably effective against gonorrhea.

  • Newer combination therapy, such as an intramuscular dose of ceftriaxone with an oral dose of azithromycin, minimizes resistance.

• Staphylococcus aureus, a common cause of healthcare-associated infections, is increasingly resistant.

  • Most strains of S. aureus are resistant to penicillin and encode penicillinase.

  • New strains have emerged with penicillin-binding proteins (PBPs) that have low affinity for most β-lactam antibiotics.

    • Methicillin-resistant Staphylococcus aureus (MRSA) is a major concern, especially healthcare-associated (HA-MRSA) strains that are resistant to a wide range of antibiotics.

  • Community-acquired (CA-MRSA) strains are currently treatable.

• Streptococcus pneumoniae, historically susceptible to antibiotics, has recently acquired penicillin resistance.

  • It produces PBPs with lower affinity, likely through DNA-mediated transformation involving other Streptococcus species.

Preventing Resistance to Antimicrobial Medications

• Preventing resistance will require cooperation from everyone globally.

• Physicians and healthcare workers have responsibilities in increasing efforts to identify the cause of infection and only prescribing suitable antimicrobials when appropriate.

• Patients have responsibilities in carefully following instructions, even if inconvenient, to maintain adequate blood levels of antibiotics.

  • Failure to complete treatment may allow less-sensitive organisms to survive and spread.

• The public should be educated about the ineffectiveness of antibiotics against viruses and the selection of antibiotic-resistant bacteria in the normal microbiota.

• The global use of antimicrobial medications has a significant impact on resistance.

  • Overuse of antibiotics is a worldwide concern, especially in regions where they are available without prescription.

  • The use of antimicrobial antibiotics in animal feeds at low levels to enhance growth can select for antibiotic-resistant microbes.

Mechanisms of Action of Antiviral Medications

• Viruses are difficult to target selectively as they rely on the host cell's metabolic machinery.

  • Antiviral medications often target virally encoded polymerases, which are enzymes involved in viral replication.

  • Antivirals are only effective against replicating viruses.

  • Examples of antivirals include those targeting HIV and SARS-CoV-2 (causes COVID-19).

• Antiviral medications can interfere with viral entry, uncoating, nucleic acid synthesis, and viral particle assembly and release.

  • Different antivirals target specific types of viruses.

  • Examples of antiviral medications include protease inhibitors, nucleoside and nucleotide analogs, and non-nucleoside polymerase inhibitors.

Antivirals that Prevent Viral Entry

• Some HIV medications prevent viral entry by targeting specific steps in the process.

• Post-attachment inhibitors

  • Iblizumab, a monoclonal antibody (mAb)

  • Binds to HIV receptor CD4 and prevents HIV particle from undergoing a change required for the virus to bind to a co-receptor

• CCR5 antagonist

  • Maraviroc (MCV) blocks the HIV co-receptor CCR5

• Fusion inhibitor

  • Enfuvirtide (ENF) binds to an HIV protein that promotes fusion of the viral envelope with the cell membrane

Antivirals that Interfere with Viral Uncoating

• Uncoating is the process by which the nucleic acid of a viral particle is released from the protein coat.

• Medications like amantadine and rimantadine can interfere with the uncoating of influenza A virus, but they are not currently used due to widespread resistance.

Antivirals that Interfere with Nucleic Acid Synthesis

• Many effective antivirals target virally encoded enzymes involved in the replication of viral nucleic acid.

• Nucleoside and nucleotide analogs, which have a structure similar to building blocks of DNA and RNA, can act as chain terminators when incorporated into the growing nucleotide chain.

  • These analogs selectively target virally encoded enzymes, causing more damage to the viral genome than the host cell's polymerases.

• Nucleoside and nucleotide analogs like acyclovir are converted by virally encoded enzymes in infected cells, causing little harm to uninfected cells.

• Sofosbuvir is highly effective against hepatitis C when used with another anti-HCV medication.

  • Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs) are used to treat HBV and HIV.

• Ganciclovir is used to treat life- or sight-threatening cytomegalovirus (CMV) infections in immunocompromised individuals.

• Non-nucleoside polymerase inhibitors

  • Inhibitor viral polymerases by binding to site other than nucleotide-binding site

  • Dasabuvir is a component of fixed-dose combination to treat HCV

  • Foscarnet used to treat resistant herpesviruses

• Non-nucleoside reverse transcriptase inhibitors (NNRTIs)

  • Inhibit reverse transcriptase by binding to site other than nucleotide-binding site (doravirine, efavirenz, etravirine)

• NS5A inhibitors are a relatively new option for treating HCV, as they inhibit an HCV-encoded protein required for viral genome replication (NS5A).

Antivirals that Prevent Genome Integration

• Integrase inhibitors interfere with action of HIV-encoded enzyme integrase

  • Prevent virus from inserting DNA copy of genome into host cell

• Prevent assembly and release of viral particles

  • Protease inhibitors

    • Several proteins translated as a polyprotein that must be cleaved by a protease

    • Virus-specific

    • Examples: atazanavir (anti-HIV) and grazoprevir (anti-HCV)

  • Neuraminidase inhibitors

    • Enzyme encoded by influenza viruses, needed for release

    • Several available, ingested, inhaled, or injected

Mechanism of Action of Antifungal Medications

• Eukaryotic pathogens difficult to target

  • Few targets for antifungals

  • Acquired resistance is a significant concern

• Antifungal medications can interfere with:

  • Fungal cytoplasmic membrane

  • Cell wall synthesis

  • Cell division

  • Nucleic acid synthesis

  • Protein synthesis

Antifungals that Interfere with Cytoplasmic Membrane Synthesis and Function

• Most antifungal chemicals target ergosterol, which humans lack

• Azoles inhibit ergosterol synthesis, membrane leaks

  • Newer less toxic triazoles used to treat systemic infections and nail infections

  • C. auris strains are resistant to at least one triazole; healthcare professionals are concerned about its spread

• Polyenes produced by Streptomyces, bind to ergosterol, cause membrane to leak

• Allylamines, tolnaftate, butenafine

  • Inhibit enzyme in ergosterol synthesis pathway

    • Most applied to skin for dermatophyte infections

Antifungals that Interfere with Cell Wall Synthesis

• Fungal cell walls have some components animals lack

• Echinocandins interfere with synthesis of β-1, 3 glucan

  • Causes cells to burst

  • Caspofungin treats systemic Candida, invasive aspergillosis

  • Some C. auris strains are resistant

Antifungals that Interfere with Cell Division

• Griseofulvin targets cell division, interferes with tubulin

  • Tubulin found in eukaryotic cells; selective toxicity may be due to greater uptake by fungal cells

  • Treats skin and nail infections

Antifungals that Interfere with Nucleic Acid Synthesis

• Common to all eukaryotes, generally poor chemical target

• Flucytosine taken up by yeast cells, converted to active form, inhibits nucleic acid synthesis

• Significant side effects

Antifungals that Interfere with Protein Synthesis

• New antifungal inhibits protein synthesis by preventing enzyme to add amino acid to tRNA (tavaborole)

  • Used topically to treat nail infections

Mechanism of Action of Antiprotozoan and Antihelminthic Medications

• Relatively little research and development

• Most parasitic diseases concentrated in poorer areas of the world; medications unaffordable

• Most chemicals interfere with biosynthetic pathways of protozoan parasites or neurom