Microbiology Week 13- Antimicrobials

Chemotherapeutic drug

A chemical used to treat a disease

Paul Ehrlich

• magic bullets

  • Salvarsan for syphilis

Gerhard Domagk

• Red Dye

  • Prontosil for streptococci

Antimicrobial drugs

Any antimicrobial drug regardless of its origin

Antibiotic

A substance produced by a microbe that, in small amounts, inhibits another microbe

• First discovered by Alexander Fleming

Alexander Fleming

• Dr. and bacteriologist working at St. Mary’s in London

• 1928: discovered penicillin, produced by Penicillium mold

Selective Toxicity

cause greater harm to microbes

• Interfere with structures or properties of microbes, but not human cells

Toxicity

relative

Therapeutic index

• Minimum total dose/ minimum effective dose

• Penicillin G has high index

• Medications too toxic for systemic use may be used topically

Bacteriostatic

chemicals inhibit bacterial growth

• patient’s defenses must still eliminate

Bactericidal

chemicals kill bacteria

Broad-spectrum antimicrobials

• Affect a wide range

  • For treating acute life-threatening diseases

  • No time for identification

Opportunistic infections

Can disrupt normal microbiota

Narrow-spectrum antimicrobials

• have limited range

  • Requires identification of pathogen & testings for sensitivity

  • Less disruptive to normal microbiota

Antagonistic combination

Medications interfere with each other

Synergistic combination

One medication enhances the activity of another

Additive combination

Taking two drugs together is the same as adding the effects of the drugs taken independently

Tissue Distribution, Metabolism, and Excretion

• Antimicrobials differ in behavior in body

  • Only some medications cross the brain-blood barrier

  • important in treating meningitis

  • If unstable at low pH, must be injected

Half life

Rate of elimination

• Dictates dose frequency

Adverse effects of antimicrobial medications

• Include allergic reactions and toxic effects

• Suppression of normal microbiota may allow dysbiosis

Bacterial cell wall

• Peptidoglycan is often a target for medications

  • often have high therapeutic index

Penicillins, Cephalosporins and other ß-Lactam Antibiotics

• Inhibit enzymes for cell wall synthesis

Drugs all have ß-Lactam ring

Penicillin-binding proteins (PBPs)

• Bind penicillin

• Interfere with peptidoglycan synthesis

• Prevent formation of peptide bridges between glycan strands

• Weakens cell walls—leads to cell lysis

• Only effective against actively growing cells

Differences in activity

• Peptidoglycan of Gram-positives is exposed

• Outer membrane of Gram-negatives blocks them

• PBPs are different in Gram-positive vs. Gram-negatives

ß-lactamase

breaks ß-lactam ring

Extended-spectrum ß-lactamases (ESBLs)

• inactivate a wide variety of ß-lactam medications

• Gram-negatives produce a greater variety than Gram-positives

Natural Penicillin antibiotics

Narrow-spectrum, act against Gram-positives and a few Gram-negatives

Penicillinase-resistant penicillins

• Developed in response to resistant S. aureus strains

• Some strains can produce altered PBPs so ß-lactam antibiotics do not bind

• MRSA

Broad-spectrum penicillin antibiotics

• act against Gram-positives and Gram-negatives

• Inactivated by ß-lactamases

Extended-spectrum penicillin antibiotics

• Greater activity against Pseudomonas species

  • Reduced activity against Gram-positives

  • destroyed by ß-lactamases

Penicillins+ß-lactamase inhibitor

includes inhibitor, clavulinic acid, which protects penicillin

• Augmentin

Cephalosporins

• Structure resistant to some ß-lactamases

• Some have low affinity for PBPs of Gram-positives

• Chemical modifications have led to four generations

Other ß-Lactam Antibiotics

• Carbapenems and monobactams resist ß-lactamases

Vancomycin

blocks peptidoglycan synthesis

• Poorly absorbed from intestines, usually administered via IV

• Often antibiotic of last resort

• Does not cross outer membrane of Gram-negatives

Bacitracin

Toxicity limits to topical applications

• Interferes with transport of peptidoglycan precursors across membrane;

• common in first-aid ointments

Differences between prokaryotic and eukaryotic ribosomes

Prokaryotes—70S

Eukaryotes—80S

Mitochondria—70S ribosomes

• accounts for some toxicity

Aminoglycosides

• Bind to 30S ribosomal subunit

  • Ineffective against anaerobes, enterococci, and streptococci because needs to enters cells via active transport process

  • Sometimes used synergistically with ß-lactams

    • Allows the aminoglycoside to enter cells

Tetracyclines

Bind to 30S ribosomal subunit

• Bacteriostatic

• Actively transported into prokaryotic but not animal cells

• Resistance arises from decreased uptake or increased excretion

Macrolides

Bind to the 50S ribosomal subunit

• Prevents translation

• Used for patients allergic to penicillin

• Bacteriostatic against many Gram-positives

  • e.g. erthromycin

Chloramphenicol

Bind to the 50S ribosomal subunit

• Blocks translation

• Active against wide range of bacteria

Fluoroquinolones

• Inhibits DNA gyrase that breaks and rejoins DNA so it can uncoil DNA

• Bactericidal against wide variety of bacteria

Rifamycins

block prokaryotic RNA polymerase

• Prevents initiation of transcription

• Bactericidal against Gram-positives, some Gram-negatives, Mycobacterium

Metronidazole

toxic only in anaerobic organisms

• Anaerobic metabolism required to convert to active forms

  • Binds DNA, interferes with synthesis, causes breaks

Antibiotics that interfere with metabolism

• Folate inhibitors

• Sulfonamides, trimethoprim inhibit different steps in synthesis of folic acid and coenzyme required for nucleotide synthesis

Antimicrobials that damage bacterial membranes

• Cause cells to leak leading to cell death

• Limited to topical applications

Daptomycin

Inserts into cytoplasmic membrane

• Used against Gram-positives resistant to other antibiotics

Polymyxin B

Binds to membranes of Gram-negatives

MIC (Minimum Inhibitory Concentration)

Lowest concentration that prevents growth in vitro

• Serial dilutions of chemical in a growth medium

  • cultures added, incubated, examined for turbidity

• Microbes are either

  • Susceptible

  • Resistant

  • Intermediate

MBC—Minimum Bacterial Concentrations

Lowest concentrations that kills 99.9% of cells in vitro; determined from plate count from MIC

Kirby-Bauer disc diffusion test

• Determines the susceptibility of a bacterial strain to antibiotics

• Measures the ability of a drug to kill bacteria

• Zones of inhibition are measured & compared with charts

  • susceptible

  • intermediate

  • resistant

• Drug characteristics are taken into account

  • molecular weight, stability, amount

Resistance to Antimicrobials

• Certain bacteria have innate or intrinsic resistance

• E.g. Mycoplasma lack cell wall, so resist penicillin etc.

• Outer membrane of Gram-negatives blocks many medications

Acquired resistance

• Spontaneous mutations

• Horizontal gene transfer

• Developed by bacteria

Antibiotic-Inactivating Enzymes

Penicillinase

Alteration in Target Molecule

• Minor structural changes can prevent binding

• PBPs, ribosomal RNA (macrolides)

Decreased Uptake

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

Increased Elimination of Medication

• Efflux pumps remove compounds from cell

• Increased production of pumps allows faster removal

Spontaneous mutations

Occur during replication at a low rate

Combination Therapy

unlikely cells will simultaneously develop resistance

Gene Transfer

• Genes encoding resistance can spread to different strains, species, even genera

  • commonly through transfer of R plasmids

• May also originate from the soil microbes that naturally produce the antibiotic

ESKAPE Pathogens—E 1/2

Entercoccus faecium

ESKAPE Pathogens—S

Staphylococcus aureus

ESKAPE Pathogens—K

Klebsiella pneumoniae

ESKAPE Pathogens—A

Acinetobacter baumannii

ESKAPE Pathogens—P

Pseudomonas aeruginosa

ESKAPE Pathogens—E 2/2

Enterobacter spp

Mycobacterium tuberculosis

requires long treatment

• Combination treatment required to minimize developing resistance

Multidrug-resistant M. tuberculosis (MDR-TB)

resist two first-line antibiotics

Extensively drug-resistant M. tuberculosis (XDR-TB)

Also resist three or more second-line antibiotics

Neisseria gonorrheae

• was susceptible to penicillin

• Resistance developed—combination therapy used now

Staphylococcus aureus

Increasingly resistant

• Most strains resistant to penicillin

• New strains have PBPs with low affinity to all ß-lactam antibiotics

• MRSA