Microbiology Chapter 20

Flashcard set to review for Exam 3

Define antimicrobial drug

Substances that, in small amounts, kill or inhibit the growth of microbes.

What type of microbes does antimicrobial drugs kill/inhibit?

• Bacteria

• Fungi

• Viruses

• Parasites

Difference between antibiotic and antimicrobial drug?

Antibiotics are a specific type of antimicrobial drug that targets bacteria.

What is selective toxicity?

Selectively finding and destroying pathogens without damaging the host.

Broad-spectrum vs narrow-spectrum drugs

Narrow-spectrum drugs affect a narrow range of microbes; broad-spectrum drugs affect a broad range of Gram-positive or Gram-negative bacteria.

What is a superinfection?

Overgrowth of an opportunistic pathogen when normal microbiota is disrupted by antibiotic use.

Examples of superinfection organisms?

• Candida albicans (cause yeast infections)

• Clostridioides difficile (cause GI infections)

Bactericidal drugs

Drugs that directly kill bacteria.

Examples of bactericidal drugs?

• Penicillin

• Cephalosporins

Bacteriostatic drugs

Prevent bacterial growth but do not directly kill bacteria.

Example of a bacteriostatic drug?

• Tetracyclines

What are the 5 major modes of action that antibiotics have on bacterial cells?

• Mode 1: Inhibit Cell wall synthesis

• Mode 2: Inhibit Protein synthesis

• Mode 3: Injure Plasma Membrane

• Mode 4: Inhibit Nucleic Acid synthesis

• Mode 5: Inhibit Synthesis of essential metabolites

Drugs associated with Mode 1: Inhibit Cell Wall Synthesis?

• Natural Penicillins - Penicillin G & V

• Semisynthetic Penicillins - Oxacillin, Ampicillin, Amoxicillin

• Carbapenems

• Cephalosporins

• Polypeptide Antibiotics - Bacitracin, Vancomycin

• Antimycobacterial Antibiotics - Isoniazid

Drugs associated with Mode 2: Inhibit Protein Synthesis?

• Chloramphenicol

• Aminoglycosides - Streptomycin, Neomycin, Gentamicin

• Macrolides - Erythromycin

• Tetracyclines - Tetracyclines

Drugs associated with Mode 3: Injure Plasma Membrane?

• Polymyxin B

Drugs associated with Mode 4: Inhibit Nucleic Acid Synthesis?

• Quinolones and Fluoroquinolones - Nalidixic acid, Ciprofloxacin

• Rifamycin - Rifampin

Drugs associated with Mode 5: Inhibit Synthesis of Essential Metabolites?

• Sulfonamides - Sulfamethoxazole-trimethoprim

How does penicillin work?

Prevents cross-linking of peptidoglycan, interfering with cell wall construction.

How do protein synthesis inhibitors work?

Target bacterial 70S ribosomes.

What does polymyxin B do?

Damages bacterial plasma membranes by changing membrane permeability.

What does ciprofloxacin inhibit?

Topoisomerase

What does rifampin inhibit?

the synthesis of mRNA; RNA polymerase, preventing transcription.

How does sulfanilamide work?

Competes with PABA and stops folic acid synthesis.

Neosporin components?

• Polymyxin B (damages membrane)

• Neomycin (blocks protein synthesis)

• Bacitracin (damages cell wall)

Natural penicillins (β-lactam antibiotics)

• Penicillin G

• Penicillin V

Semisynthetic penicillins (β-lactam antibiotics)

• Oxacillin

• Ampicillin

• Amoxicillin

Carbapenem Example?

Imipenem → β-lactam antibiotic reserved for severe multidrug-resistant infections.

Cephalosporin examples? (β-lactam antibiotics)

• Cephalothin (1st generation)

• Ceftaroline (5th generation, active against MRSA).

Polypeptide antibiotics

• Bacitracin

• Vancomycin

Antimycobacterial antibiotics

• Isoniazid

• Ethambutol

Aminoglycosides

• Streptomycin

• Neomycin

• Gentamicin

Macrolide example?

Erythromycin

Nucleic acid synthesis inhibitor examples?

• Rifampin

• Nalidixic acid

• Ciprofloxacin

Widely used sulfonamide combination?

Sulfamethoxazole-trimethoprim (Bactrim, TMP-SMX)

Why are fungal infections difficult to treat?

Fungi are eukaryotic and selective toxicity is harder to achieve.

Fungal membrane sterol?

Ergosterol; human membranes use cholesterol.

Fungal cell wall component highlighted?

β-glucan

Important antifungal drug example?

Ketoconazole

Why are viral infections difficult to treat?

Viruses reside inside host cells.

What is AZT?

Zidovudine (AZT); inhibits HIV reverse transcriptase.

Important antiviral against herpesviruses?

Acyclovir

What are interferons?

Produced by virus-infected cells to inhibit further spread of infection.

How does the Kirby-Bauer test work?

Antibiotic disks are placed on inoculated agar. Zone of inhibition indicates susceptibility.

What determines susceptibility in Kirby-Bauer?

The size of the zone of inhibition around the disk.

What is the E-test?

A gradient diffusion method that determines antibiotic sensitivity and estimates MIC.

Define MIC.

Lowest antibiotic concentration preventing bacterial growth.

How is MIC observed on the E-test?

At the tip of the zone of inhibition.

How is MIC determined by dilution testing?

Lowest drug concentration that prevents visible growth in broth.

How is MBC determined?

Subculture onto antibiotic-free agar and find the lowest concentration that kills bacteria.

Define acquired antibiotic resistance.

An acquired ability of bacteria to survive and multiply in the presence of a previously effective antibiotic.

Define intrinsic resistance.

Natural lack of susceptibility due to the organism’s structure.

Five mechanisms of resistance?

1. Enzymatic inactivation

2. Prevention of penetration

3. Target alteration

4. Efflux pumps

5. Variations/overproduction strategies

How do β-lactamases cause resistance?

They enzymatically inactivate β-lactam antibiotics.

How does penicillinase inactivate penicillin?

It cleaves the β-lactam ring, converting penicillin into inactive penicilloic acid.

Example of target alteration?

MRSA with mecA gene/PBP2a

What are efflux pumps?

Active transport systems that eject antibiotics from the cell.

Examples of antibiotic misuse?

• Using antibiotics without a prescription.

• Taking outdated antibiotics.

• Treating viral colds.

• Using them in animal feed.

• Not finishing prescriptions.

• Using leftover prescriptions.

What can happen if a patient does not finish antibiotics?

Selects for resistant bacteria and promotes antibiotic resistance.

General antibiotic safety concerns

• Therapeutic index

• Drug interactions

• Organ damage

• Fetal risk during pregnancy

Define synergism

Two drugs together have a greater effect than either one alone.

Example of synergism?

Penicillin and streptomycin.

Define antagonism

Two drugs together have a less effective result than either one alone.

Example of antagonism?

Tetracycline interfering with penicillin activity.