Exam 2 will be on Wednesday March 27

Exam 2 will be on Wednesday, March 27, 2019. Place: If your last name starts with A – Pe: please take exam in room 112 Gregory If your last name starts with Ph – Z: please take exam in room 213 Gregory Covers chapters: 5, 6, 5, 7, 5, 9 &10 and 5 Review Session: 7: 00 – 8: 20 pm, Monday, March 25 Place: Room 124 Burrill Hall

MCB 100 Introductory Microbiology March 25, 2019 Chapter 10 – Antibiotics Kirby-Bauer Test (also known as the antibiotic diffusion test)

CHAPTER 10 ANTIBIOTICS & ANTIMICROBIAL DRUGS Characteristics of an Ideal Antimicrobial Drug - Selective Toxicity - Microbicidal (vs. microbistatic) Soluble in Water Potent Stable (just long enough) Unlikely for microbial resistance to develop Complements host’s defenses Not inactivated by other metabolites Does not cause allergies Doesn’t increase host susceptibility to further infection

General Mode of Action of Antibiotics An antibiotic is a poison that kills a microorganism but does not harm the host. Most poisons are in one way or another enzyme inhibitors. Antibiotics kill undesirable bacteria by inhibiting enzymes that are important for the life cycle of the bacteria but are not found in the cells of the host organism. General Classes of Antibiotics Based On Specific Modes of Action Inhibitors of peptidoglycan synthesis – stop growth of bacterial cell wall Inhibitors of 70 S ribosomes – stop bacterial protein synthesis Inhibitors of folic acid biosynthesis Inhibitors of bacterial RNA polymerase or DNA polymerase Disruption of cell membranes

Penicillins, Cephalosporins, Vancomycin and Bacitracin are antibiotics that disrupt bacterial cell wall synthesis. Bacterial cell walls are made of peptidoglycan. Peptidoglycan is a netlike bag that is made of polysaccharide chains that are cross-linked by peptide chains. Peptidoglycan protects the bacterial cell from osmotic lysis.

Antibiotics That Inhibit Peptidoglycan Biosynthesis Example: Penicillins and Cephalosporins (b-lactam drugs) Penicillin binding protein is a bacterial enzyme that forms the peptide bond that cross-links one amino acid side chain in the peptidoglycan to another peptide chains. b-lactam drugs are competitive inhibitors of this enzyme. If the synthesis of peptidoglycan is inhibited, the bacterial cell wall becomes weaker with each round of cell division, eventually the bacteria burst due to osmotic pressure. Other Examples of peptidoglycan synthesis inhibitors: Bacitracin interferes with the movement of peptidoglycan precursors (NAG-NAM-peptide subunits) through the cytoplasmic membrane to the wall Vancomycin interferes with the incorporation of NAG-NAM-peptide subunits into peptidoglycan

The enzyme that forms the peptide bond that cross-links amino acid side chains is called: “Penicillin Binding Protein”. Bacteria do not make this protein for the purpose of binding penicillin.

Which one of the following statements about bacterial cell wall synthesis inhibitors like penicillin, vancomycin and bacitracin is TRUE? A. Penicillin, vancomycin and bacitracin are bacteriostatic, not bacteriocidal. B. All types of bacteria can be killed by penicillin, vancomycin and bacitracin. C. Penicillin, vancomycin and bacitracin are very useful for treating patients that have mold or yeast infections. D. Penicillin, vancomycin and bacitracin are very useful for treating patients that have viral infections. E. Penicillin, vancomycin and bacitracin do not kill bacterial cells that are not growing.

Antibiotics That Inhibit Protein Synthesis by Bacterial Ribosomes - Aminoglycosides (Example: streptomycin) These agents bind to the 30 S subunit of the 70 S ribosome and cause the m. RNA to be misread. They cause the ribosome to slip, sometimes moving 4 bases when changing from one codon to the next when it needs to move exactly 3 bases. This causes the incorporation of incorrect amino acids into the growing protein chain. - Tetracyclines These block binding of t. RNAs to ribosome-m. RNA complex. - Chloramphenicol This antibiotic blocks the formation of peptide bonds between the amino acids of the growing protein chain. - Macrolides (Example: erythromycin) These stop movement of the ribosome down the m. RNA.

. Tetrahydrofolate is an enzyme cofactor that acts as a donor of 1 -C units in a variety of biosynthetic reactions, such as the synthesis of amino acids or nucleotides. Top: THF a carrier of 1 C units Bottom: 5 , 10 methenyl THF with 1 more carbon atom .

THF is involved in two steps of the purine synthesis pathway. Source: library. med. utah. edu/Net Biochem

Which one of the following antibiotics kills bacterial cells by inhibiting the synthesis of folic acid derivatives? A. streptomycin B. bacitracin C. vancomycin D. sulfonamide E. penicillin

Which one of the following antibiotics kills bacterial cells by inhibiting the synthesis of proteins by 70 S ribosomes? A. streptomycin B. bacitracin C. vancomycin D. sulfonamide E. penicillin

Other Antibiotics - Quinolones (Especially Fluoroquinolones) These drugs inhibit bacterial gyrase, which is involved in controlling the amount of supertwisting in DNA. Blocking the action of DNA gyrase blocks replication of bacterial DNA. The Fluoroquinolones are broad-spectrum antibiotics useful against both Gram + and Gram – bacteria. Example: Ciprofloxacin (Cipro) - Polymyxin A detergent-like drug that disrupts bacterial cell membranes - Nucleotide Analogs Block DNA replication and cause mutations

Polymyxin structure Polymyxin disrupts the cytoplasmic membrane.

Antibiotic Spectrum A broad spectrum antibiotic can be used to kill or inhibit the growth of a wide variety of bacteria. A narrow spectrum antibiotic is useful against only a few types of bacteria. Minimum Inhibitory Concentration MIC is a measure of how much of a given antibiotic is required to treat a patient. Therapeutic Index The therapeutic index of a drug is a measure of the margin of safety one has when using that drug to treat a patient. Antibiotic Resistance This is a property of bacteria that is seen when a strain of bacteria acquires the ability to avoid being killed by an antibiotic.

Minimum Inhibitory Concentration (MIC) MIC is the lowest concentration of an antibiotic that will inhibit a pathogen. A low MIC indicates a potent drug for inhibiting a certain microorganism. For a given drug, the MIC varies with the species and strain of the microorganism and the environmental conditions. The Kirby-Bauer test is a standard laboratory test that can be used to predict the effectiveness of antibiotics in the treatment of a patient.

Minimum Inhibitory Concentration The size of the zone of inhibition around an antibiotic disc is in part determined by the MIC. The tube dilution assay is a more quantitative way of measuring the MIC.

Therapeutic Index (T. I. ) The therapeutic index of a drug is the ratio of the toxic dose divided by the effective dose. T. I. = __(toxic dose)__ (effective dose) A higher therapeutic index indicates a safer drug. A low therapeutic index indicates there is a smaller margin for error as a physician tries to give the patient enough of the drug to kill the infectious microorganism but not so much that it hurts the patient.

Antibiotic-Resistant Strains of Pathogenic Bacteria A Growing Problem Source: U. S. A. Center for Disease Control

Source: U. S. A. Center for Disease Control

Mechanisms of Resistance to Antibiotics Seen in Some Bacteria 1) Bacteria may produce an enzyme that degrades or modifies the antibiotic. Genes for drug resistance factors are often located on plasmids. Drug resistance plasmids can often be transferred from species to species. 2) Altered pore proteins may slow diffusion of the drug into the bacterial cell. 3) The gene for the drug receptor may be mutated so the receptor no longer binds to the antibiotic. 4) Resistant cells may alter their metabolism so they can by-pass the inhibited step. 5) Resistant cells may acquire a gene for an active transporter protein that pumps the antibiotic out of the cell using energy from ATP. This type of resistance gene is often found on a plasmid or a transposon.

Bacteria can become resistant to penicillin by acquiring a plasmid that includes a gene that encodes the enzyme beta-lactamase. Source: www. blad. co. in/images

Bacteria that make beta-lactamase acquire resistance to penicillin, derivatives of penicillin and cephalosporins.

Anti fungal, Antihelminthic and Antiprotozoan Drugs Antibiotics are drugs that kill bacteria. Antibiotics inhibit metabolic processes that occur in bacteria but not in mammalian cells. Most are natural products made by a microorganism or are derivatives of natural products. Fungi, worms and protozoa are eukaryotic organisms like us. They have 80 S ribosomes rather than 70 S ribosomes so antibiotics that inhibit 70 S ribosomes like the tetracyclines, macrolides and aminoglycosides are not effective against them. They do not have cell walls made of peptidoglycan so the penicillins and cephalosporins are not effective against them. Drugs that are effective against eukaryotic microorganisms often have toxic side effects & rather low therapeutic index. Examples: Allyamines and Azoles, such as Miconazole, inhibit ergosterol synthesis in fungi. Quinine inhibits metabolism in Malaria parasites (a protozoa).

Antiviral Drugs Because viruses grow inside our cells and use the metabolic pathways of our own cells it is more difficult to find substances that interfere with their life cycle that do not harm the host. Antiviral compounds are usually very specific inhibitors of certain enzymes that are found in virally infected cells that are not found in uninfected cells. Because there is a lot of diversity seen in the pathways used by various viruses to replicate their genomic material, compounds that affect the reproduction or maturation of one type of virus may have no effect on other types of viruses. Example: HIV protease inhibitor is useful in the treatment of AIDS but it is very specific for a protein found in HIV infected cells – the HIV protease. This compound is not useful against the majority of other viruses because they do not make a protease enzyme like the one found in HIV infected cells. Invirase

Another Neat Trick In the future, antisense RNA may be introduced into human cells to inhibit the expression of specific genes that are found in microbial pathogens. The main challenges are to safely introduce the antisense RNA into infected cells and to design sequences that will inhibit the pathogen but not harm the host. How Antisense RNA Works Antisense RNA AG CGU UUG AGU CCG CAA CGC AAG AC II III II III III II II III AUG CUC GCA AAC UCA GGC GUU GCG UUC UGU CGA Viral or Bacterial m. RNA Antisense RNA can block binding of a ribosome to a specific m. RNA molecule.

Antiviral Drugs - continued Another Example: Acyclovir inhibits an enzyme that is involved in the cytoplasmic replication of the DNA genomes of some viruses such as herpes and chickenpox. It is not effective against RNA viruses or DNA viruses that do not use a similar virally encoded kinase enzyme. Acyclovir Another Example: Zanamivir inhibits an enzyme found on the surface of influenza virus called neuraminidase which is involved in the entry and exit of the virus from host cells. Influenza Virus