Chapter 20 Microbiology - Exam 3

Antimicrobial drugs

Selective toxicity

selectively finding and destroying pathogens without damaging the host

Chemotherapy

the use of chemicals to treat a disease

Antimicrobial drugs

substances that in small maounts kill or inhibit the growth of microbes, including bacteria, fungi, viruses, and parasites 

Antibiotics

specific type of antimicrobial drug that target bacteria

Narrow-spectrum

drugs that affect a narrow range of microbial types

Isoniazid: mycobacteria (prokaryotes)

Ketoconazole: fungi (eukaryotes)

Mefloquine (malaria): protozoa (eukaryotes)

Acyclovir: viruses (herpes)

broad-spectrum

affect a broad range of gram-positive or gram-negative bacteria

Streptomycin: mycobacteria + gram-negative bacteria (prokaryotes)

Tetracycline: gram-negative, gram-positive, chlamydias rickettisias (prokaryotes)

Superinfection

overgrowth (flourish) of an opportunistic pathogen when the normal microbiota are disrupted, typically due to antibiotic use

Bacteriostatic

an agent or substance that inhibits growth without killing

• Ex. tetracyclines

bactericidal

an agent or substance that kills bacteria

• ex. penicillin, cephalosporins

Antibacterial drug actions: Inhibition of cell wall synthesis

Prevents the synthesis of peptidoglycan

Generally safe and effective (can have side effects)

• Penicillins, cephalosporins, bacitracin, vancomycin

Antibacterial drug actions: inhibition of protein synthesis

Target bacterial 70S ribsomes (not free ribsomes which are 80S)

Has side effects

• Chloramphenicol, erythromycin, streptomycin, tetracyclines

Antibacterial drug actions: inhibition of nucleic acid replication and transcription

Can have some effects

Quinolones: block topoisomerase (interferes with DNA replication)

Rifampin: block RNA polymerase (interferes with transcription)

Antibacterial drug actions: injury to plasma membrane

Polypeptide antibiotics change membrane permeability

antifungal drugs combine w/ membrane sterols

have side effects

• Polymyxin B

Antimicrobial drug actions: inhibition of essential metabolite synthesis

Antimetabolites compete with normal substrates for an enzyme

Sulfanilamide: competes with para-aminobenzoic acid (PABA), stopping synthesis of folic acid

Can have some efefcts

• sulfanilamide, trimethoprim

Antifungal (ergosterol)

Ketoconazole —> systemic fungal infections

Clotrimazole —> topical use

Antiprotozoan drugs

Chloroquine/mefloquine —> treats malaria

Metronidazole —> used to treat giardisis, amebiasis, trichomoniasis

• “d” for anti-parasite

Antihelminthic drugs

Mebendazole/albendazole —> inhibit absorption of nutrients, intestinal roundworms

Antiviral drugs

Acyclovir: treats herpes infection (nucleic acid synthesis inhibitor)

Oseltamivir: treats influenza virus (inhibits NA spike)

• ends with “vir”

Penicillin (inhibition of cell wall synthesis)

Prevents cross-linking of peptidoglycan, interfering with cell wall

Contains beta-lactam ring (types depend on side chains)

Natural penicillin, Semisynthetic penicillin

Natural Penicillin

Extracted from Penicillium fungi

Narrow spectrum of activity susceptible to penicillinases

Penicillin G —> requires injection

Penicillin V —> can be taken orally

Semisynthetic penicillins

Contains chemically added side chains

oxacillin: narrow spectrum, only gram-positive, but resistant to penicillinase

Ampicillin: extended spectrum, many gram-negatives

Penicillin categories

Penicillinase-resistant

• methicillin and oxacillin

Broad-spectrum (effective against gram-positive/negative)

• Amoxicillin/Clavulanate

Penicillins plus penicillinase inhibitors

• Contain clavulanic acid, a noncompetitive inhibitor of penicillinase —> makes them more effective against bacteria that produce enzymes capable of destroying penicillin

Penicillinase (beta-lactam lactamase)

an enzyme produced by some bacteria that destroys penicillin

• breaks the beta-lactam ring in penicillin

• can no longer bind to its enzyme in bacteria, making it ineffective

Cephalosporins

work similar to penicillins but has different beta-lactam ring

• grouped according to their generation of development

Disk-diffsion method (kirby-bauer test)

tests for effectiveness of chemotherapeutic agents

• paper disks placed with chemotherapeutic agent on agar w/ organism

• Zone of inhibition: determines the susceptibility of the organism to the antibiotic

Epsilometer test (E test)

a gradient diffusion method that determines antibiotic sensitivity and estimates the MIC

• lowest antibiotic concentration preventing bacterial growth

Broth dilution tests

determines the MIC and MBC of an antimicrobial drug

test organism is placed into the wells of a tray containing dilutions of a drug

subculture is used to determine whether bacteria have been killed or only inhibited

• lowest lethal concentration that kills the bacteria is MBC

Resistance to antimicrobial drugs

Superbugs: bacteria that are resistant to multiple antibiotics making them difficult to treat

• resistant genes are spread horizontally among bacteria on plasmids or transposons

Resistant pathogens to antibiotics

• Acinetobacter baumannii

• Pseudomonas aeruginosa

• Some members of the enterobactiaceae

Mechanisms of resistance

1. Ezymatic destruction or inactivation of the drug

2. Prevention of penetration to the target site within the microbe

3. Alteration of the drug’s target site

4. Rapid efflux of the antibiotic

5. Variations of mechanisms of resistance

Misuse of antibiotics

1. Failing to complete the prescribed regiment (most detrimental)

2. Availability of antibiotics w/o prescriptions in some countries

3. Using outdated or weakened antibiotics

4. Using antibiotics for the common cold and other inappropriate conditions 

5. Using antbiotics in animal feed

6. Using someone else’s leftover prescription

Synergism

the effect of two drugs together is greater than the effect of either alone

• penicillin and streptomycin are more effective when taken together in some situations

Antagonism

the effect of two drugs together is less than the effect of either alone

• tetracycline sometimes interferes with the activity of penicillin

(Future) Antimicrobial peptides: Bacteriocins

antimicrobial peptides produced by bacteria to inhibit the growth of closely related or competitive bacterial strains

Future of antimicrobial agents: Phage therapy

using bacteriophage to treat infections