Microbiology and Immunology 2500: Antibiotics and Antibiotic Resistance

University Details

  • Location: Western University, Canada

Course Information

  • Course Title: Microbiology and Immunology 2500

  • Topic: Antibiotics and Antibiotic Resistance

  • Instructors: Dr. Idowu Olawoye (iolawoye@uwo.ca)

Learning Objectives

  • Understand the mechanism of how antibiotics function.

  • Recognize the causes and development of antibiotic resistance.

  • Identify major bacterial pathogens associated with antibiotic resistance.

  • Explain how antibiotic usage can result in pseudomembranous colitis.

Types of Antimicrobial Agents

  • Disinfectants: Antimicrobial agents used on inanimate objects (e.g., floors, tables, walls etc).

  • Antiseptics: Antimicrobial agents that are sufficiently non toxic to be applied to living tissue (e.g., hand sanitizers).

  • Antibiotics: antimicrobial agents produced by bacteria and fngi that are exploited by humans ( delivered topically and internally )

Importance and Problems of Antibiotics

  • Antibiotics are most effective therapeutic for treating bacterial infections and the availability of antibiotics enables cancer chemotherapy, organ transplants, all invasive surgeries, treatment of premature infants .

  • Major Issues:

    1. Decreasing interest from pharmaceutical companies in developing new antibiotics.

    2. Ongoing development of bacterial resistance to existing antibiotics.

Timeline of Antibiotic Deployment and Resistance

  • Historical Overview of Antibiotics:

    • 1930: Introduction of Penicillin, Streptomycin.

    • 1980: Emergence of Vancomycin, Daptomycin.

    • Increasingly observed antibiotic resistance alongside deployment of various antibiotics over the decades.

Misuse of Antibiotics

  • Contributing factors to antibiotic resistance include:

    • Empiric (blind) antibiotic use.

    • Increased use of broad-spectrum antibiotics.

    • pediatric use for viral infections

    • patients who do not complete course ( chronic disease, e.g. TB)

    • antibiotics in animal feeds

Measuring Antibiotic Activity

  • Minimum Inhibitory Concentration (MIC):

    • Tests using culture tubes with different antibiotic concentrations to determine the lowest concentration of agent needed to inhibit bacterial growth. ( check for visible growth )

    • Antibiotic strips provide a faster way to measure the MIC for multiple antibiotics.

MIC Measurement Data

  • Visual representation of MIC values across various antibiotic concentrations from a study.

Mechanism of Antibiotics

  • Antibiotics function by targeting essential bacterial components, including:

    • Cell wall synthesis.

    • Protein synthesis.

    • DNA/RNA synthesis.

    • Folate synthesis.

    • Cell membrane alterations.

  • These targets are typically absent or significantly different in eukaryotic cells.

β-Lactam Antibiotics

  • Example: Penicillin.

  • Contains a "β-lactam ring" that inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs).

    • PBPs are transpeptidases

    • no peptide corss-links = weak cell wall = cell death

  • Some bacteria produce β-lactamase, which destroys the β-lactam ring and thus the antibiotic's effectiveness.

Methicillin as a β-Lactam Antibiotic

  • Modified version of Penicillin that cannot be cleaved by β-lactamases.

  • Some bacteria produce “a different penicillin binding protein” PBP2a, which does not bind Methicillin or other β-lactams, leading to resistance.

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Visual of Bacterial Cell Wall Synthesis

  • Diagram illustrating the interaction between antibiotics like Methicillin and bacterial cell wall synthesis pathways.

Vancomycin Overview

  • a glycopeptide antibiotic

  • inhibits cell wall synthesis in gram positives

  • often a drug of “last resort” e.g. MRSA

  • Vancocymin binds the peptide linkage at terminal D-Ala-D-Ala residues and inhibits transpeptidation

  • resistance genes change these D-Ala-D-Lac and vancomycin can no longer bind

  • resistance is encoded by the van genes

Visual of Vancomycin Interaction

  • Diagram showing the binding site and changes in bacterial peptide chains conferring resistance.

Selection for Antibiotic Resistance

  • Paradoxically the use of antibiotics actively selects for antibiotics resistant bacteria

Bacterial Strategies Against Antibiotics

  • Strategies for antibiotic resistance include:

    • Prevention of antibiotic entry (e.g., Gram-negative outer membrane).

    • Antibiotic modification (e.g., β-lactamase production).

    • Efflux pumps to expel antibiotics.

    • Altered antibiotic targets (e.g., modified ribosomes).

    • Bypassing antibiotic action through environmental factors.

Resistance Genes

  • many mechanisms of antibiotic resistance are genetically encoded (e.g. mec, β lactamase, efflux pumps)

  • can produce very high levels of antibiotic resistance

  • often encoded on mobile genetic elements (e.g. plasmids) allowing for horizontal gene transfer à "superbugs"

Horizontal gene transfer

  • rather than alter gene function through mutations, new genes are acquired from another source

CDC biggest antibiotic resistant threats

Serious Threats

  • Multidrug-resistant Acinetobacter

  • Drug-resistant Campylobacter

  • Fluconazole-resistant Candida

  • Extended-spectrum Beta-lactamase producing Enterobacteriaceae

  • Vancomycin-resistant Enterococcus (VRE)

  • Multidrug-resistant Pseudomonas aeruginosa

  • Drug-resistant non-typhoidal Salmonella

  • Drug-resistant Salmonella Serotype Typhi

  • Drug-resistant Shigella

  • Methicillin-resistant Staphylococcus aureus (MRSA)

  • Drug-resistant Streptococcus pneumoniae

  • Drug-resistant Tuberculosis

Urgent Threats

  • Clostridioides difficile

  • Carbapenem-resistant Enterobacteriaceae (CRE)

  • Drug-resistant Neisseria gonorrhoeae

Concerning Threats

  • Vancomycinresistant Staphylococcus aureus (VRSA)

  • Erythromycin-Resistant Group A Streptococcus

  • Clindamycin-resistant Group B Streptococcus

Klebsiella pneumoniae

  • Gram negative

  • an important cause of nosocomial pneumonia

  • produces a capsule and is commonly resistant to multiple antibiotics

  • first documented source of "NDM-1"

    • New Delhi Metallo-beta-lactamase-1

    • also known as a carbapenemase

  • carbapenem antibiotics are β-lactamase resistant β-lactams with broad spectrum activity

  • NDM-1 is now widespread in other Gram negatives = CRE (carbapenem resistant Enterobacteriaceae)

Clostridia

  • Gram-positive, rod shaped, endospore-formers

  • Strict anaerobes, vegetative cells killed by O2

  • Generally found in soil and intestinal tracts of animals

  • Important human pathogens:

    • Clostridioides difficile-pseudomembranous colitis

    • Clostridium tetani - tetanus

    • Clostridium botulinum - botulism

    • Clostridium perfringens – food-borne illness and gas gangrene

      (can cause life threatening diseases mediated by exotoxins)

Clostridioides difficile (“C. diff”)

  • can exist as:

    • asymptomatic carrier state in the large intestine

    • cause of mild to moderate diarrhea

    • cause of life-threatening pseudomembranous colitis

  • often found in nursing homes and hospital environments

    • a nosocomial pathogen

  • endospores can be very difficult to eradicate from the environment

    • cultured from floor, bed pans, toilets, hands and clothing of medical personnel

  • mode of transmission is through the spore: fecal-oral route

  • cause of pseudomembranous colitis, also called antibioticassociated diarrhea

Pseudomembranous colitis

  • an inflammatory condition of the large intestine

  • the most important risk factor is having recently received an antimicrobial agent

  • diarrhea, abdominal pain, fever, nausea, dehydration

  • symptoms may occur 1-2 days after antibiotics or several weeks after the antibiotic is discontinued

  • endoscopy can show characteristic lesions

  • lesions can enlarge to cover substantial portions of inflamed mucosa and can be stripped off à the pseudomembrane

Pseudomembranous colitis

  • antibiotics are used to cure infections, but they also kill the normal microbiota

  • suppression of normal microbiota + persistence of C. difficile endospores

  • after the antibiotic is stopped, spores germinate, overgrowth of C. difficile occurs with production of toxins

  • C. difficile is not considered an invasive bacterium, but the exotoxins cause damage and inflammation to the intestinal lining of the large intestine

Clostridioides difficile (“C. diff”)

  • C. difficile produces A-B toxins called the large clostridial cytotoxins Clostridioides difficile (“C. diff”)

  • “A-B” serves to designate two domains

    • The A domain denotes the active portion of the toxin that carries the enzymatic activity

    • The B domain denotes the portion of the toxin molecule responsible for binding and uptake by the host cell

  • “A” domain functions to inactivate key regulatory proteins of host cells à causes dysregulation of multiple cell processes including cytoskeletal rearrangements, cell death and inflammation

Diagnosis and Treatment

  • history (antibiotic use), symptoms and laboratory tests to confirm C. difficile

  • endoscopy and toxin detection assays

  • discontinue inciting antibiotic if still being used

  • fluids

  • antibiotics more specific for “C. diff” – oral vancomycin or I.V. metronidazole

  • avoid antidiarrheal agents – would cause decreased toxin clearance