bio 205 chapter 24 powerpoint

8/14/2019 BIO 205 Chapter 24 Powerpoint

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BIO 205

Chapter 24


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Alexander Flemings Serendipitous Discovery of PenicillinUshered in the Era of Antibiotics

Penicillium mold produces a substance that kills gram-positive

bacteria


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Selective toxicity means that a drug should harm the pathogen but not thehost

The toxic dose of a drug is the concentration causing harm to the host

The therapeutic dose is the concentration eliminating pathogens in thehost

Together, the toxic and therapeutic doses are used to formulate thechemotherapeutic index


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The Synthetic Antibacterial Agents


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Sulfanilamide and Other Sulfonamides Target SpecificMetabolic Reactions

Sulfonamides out compete essential folic acid components

for binding sites in a bacterial enzyme

They prevent nucleic acid synthesis and DNA replication


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Other Synthetic Antimicrobials Have Additional Bacterial CellTargets

Isoniazid interferes with cell wall synthesis in species of

Mycobacterium

Quinolones block DNA synthesis in bacteria


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The Beta-Lactam Family of Antibiotics


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Penicillin Has Remained the Most Widely Used Antibiotic

Penicillins are active against many gram-positive andsome gram-negative bacteria

They interfere with cell wall synthesis, causing the cell toburst


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Some individuals experience an anaphylactic allergic reaction

Many penicillin-resistant species produce beta-lactamasesthat inactivate penicillin

Numerous semisynthetic penicillins have been developed


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Other Beta-Lactam Antibiotics Also Inhibit Cell Wall Synthesis

Cephalosporins are broader spectrum alternatives topenicillins

Monobactams are active against aerobic, gram-negative rods

Carbapenems are broad spectum drugsFor example, Imipenem


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Vancomycin Also Inhibits Cell Wall Synthesis

It is effective against gram-positive bacteria such asstaphylococci

Side effects include damage to the ears and kidneys


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Chloramphenicol is used against a wise variety of bacteria

and some rickettsiae and fungi

It is reserved for serious infections like:

meningitis

choleratyphoid and typhus feversRocky Mountain spotted fever

Severe side effects include aplastic anemia and gray

syndrome


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Tetracyclines have a similar antimicrobial range tochloramphenicol

They have a benzene ring formation

They can destroy intestinal microbiota and cause stainingof the teeth


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Some Antibiotics Inhibit Nucleic Acid Synthesis

Rifampin interferes with RNA synthesis

It is effective against tuberculosis, leprosy, andmeningitis

It can cause liver damage


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Targets for antibacterial agents


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Antibiotic Assays and Resistance


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The agar disk diffusion method involves different antibioticsdiffusing from paper disks in a bacterial colony


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There Are Four Mechanisms of Antibiotic Resistance

Many species of bacteria have evolved resistance to certainantibiotics and synthetic agents

The evolution of strains ofS. aureus resistant to multiple drugs isparticularly alarming

1. Resistance to sulfonamides may develop if the bacterialenzyme changes or if the bacteria evolves an alternate

metabolic pathway

3. Bacteria may evolve the ability to enzymatically inactivate anantibiotic

5. Bacteria may evolve the ability to prevent drug entry into thecytoplasm or to pump the drug out of the cytoplasm

7. Bacteria can evolve changes in drug targets like ribosomesor enzymes involved in replication


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Unnecessarily large antibiotic doses allow resistant strains toovergrow susceptible ones

If resistant strains spread to other patients, a superinfectionoccurs

Antibiotics are available over the counter in developing

countries, allowing for overuse and incorrect use

Antibiotic use is widespread in livestock feeds

They can be transmitted to humans through meat

consumption

Bacterial cells can pass resistance genes to other bacterialcells


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New Approaches to Antibiotic Therapy Are Needed

Scientists work to find new antibiotic targets in pathogens

The lipopolysaccharide in gram-negative membranescontains several possible targets

Interference with DNA adenine methylase disables:DNA structurereplicationrepair

Discovery of new and unique antibiotics is necessary


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End of Chapter 24

bio 205 chapter 24 powerpoint

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