ANTIMICROBIAL THERAPY

CHAPTER 13

Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.

Mad cow disease is caused by an “infectious” protein

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Mad cow disease-related death confirmed in TexasCNN) -- Mad cow disease has caused a fourth death in the United States, health officials say.Lab tests have confirmed that a patient in Texas who recently died had Variant Creutzfeldt-Jakob Disease. Variant CJD is a fatal brain disorder linked to eating beef from cattle with mad cow disease, the Centers for Disease Control and Prevention said in a press release.Variant CJD was first identified in the United Kingdom in 1996, according to the CDC. Worldwide more than 220 cases have been reported, the majority in Europe. CDC officials said this is only the fourth case to be reported in the United States, and that each U.S. infection is believed to have happened while the patient was traveling abroad.The Texan patient traveled "extensively" to Europe and the Middle East, the CDC said. Variant CJD is different than what the CDC calls classic CJD, which is not related to mad cow disease. Classic CJD strikes less than 400 Americans each year. It is also fatal.Case of CJD confirmed in New HampshireMad cow disease is officially called bovine spongiform encephalopathy, or BSE. Animals with BSE have infectious prions in their brain, spinal cord and some parts of the central nervous system. These prions can be spread to humans who eat those specific parts of the cow, or who eat meat that has come in contact with infected tissue or that has been processed in contaminated machinery.Eating meat contaminated with the disease is thought to be the cause of Variant CJD in humans. The first case of mad cow disease was confirmed in a cow in the United States in December 2003.

Drug-resistant TB may be treatable with new small molecule drug In a new study, scientists report how a new small molecule drug appears able to kill drug-resistant tuberculosis without toxic side effects. According to the World Health Organization (WHO), 9 million people around the world fell ill with TB in 2013 and 1.5 million died of it. Improper use of antibiotics has led to new strains of TB that are resistant to the two most powerful drugs used to treat it: isoniazid and rifampicin. Now, researchers at the University of Georgia (UGA) in Athens have developed a new small molecule drug that may serve as a treatment against multidrug-resistant TB that cannot be cured with conventional drugs. "Multidrug-resistant TB is spreading rapidly in many parts of the world. There is a tremendous need for new therapies, and we think our laboratory has developed a strong candidate that disrupts fundamental steps in the bacterium's reproduction process."Dr. Nair and colleagues were interested in one particular molecule - an enzyme that helps to produce TB RNA called RNA polymerase, or RNAP. Without this molecule, the TB bacterium cannot produce the proteins it needs to survive. The team developed a compound that interrupts the process through which RNAP produces TB RNA. The compound - which they refer to as "Compound 2" in their paper - is a small molecule that binds to specific amino acids and magnesium in the bacterial cells. The team was also surprised - when carrying out early tests on the new compound - that it shows strong anti-HIV properties. This could open the door to dual-purpose therapies, where the drug tackles more than one disease at the same time.A dual-purpose drug that tackles TB and HIV at the same time is a very exciting prospect because the risk of developing TB is 26-31 times higher in people infected with HIV, according to the WHO.Here we report a series of compounds containing a nitroimidazopyran nucleus that possess antitubercular activity. After activation by a mechanism dependent on M. tuberculosis F420 cofactor, nitroimidazopyrans inhibited the synthesis of protein and cell wall lipid. We conclude that nitroimidazopyrans offer the practical qualities of a small molecule with the potential for the treatment of tuberculosis.

Breastfeeding may influence immune system development in early life A series of studies set to be presented at the American Academy of Allergy, Asthma & Immunology's Annual Meeting in Houston, TX, claim an infant's immune system development and susceptibility to asthma and allergies may be influenced by a number of factors that shape what bacteria is in their gut, such as gestational age at birth, breastfeeding and delivery by Cesarean section. "The immune system is designed to be exposed to bacteria on a grand scale," she adds. "If you minimize those exposures, the immune system won't develop optimally."Other studies have supported this claim. In June 2014, for example, Medical News Today reported on a study published in the journal Allergy and Clinical Immunology, in which researchers found exposing babies to bacteria and allergens in the first year of life may reduce the risk of allergies, wheezing and asthma later in life. he results of their analysis revealed that a mother's race/ethnicity, an infant's gestational age at birth, prenatal and postnatal tobacco smoke exposure, the presence of pets in the home and whether a baby was born via Cesarean section or vaginal delivery influenced an infant's gut microbiome composition. They also found that babies who were breastfed at 1 and 6 months had specific gut microbiome compositions, compared with babies who were not breastfed, which the researchers say may affect immune system development. In addition, babies who were breastfed at 1 month were at lower risk of pet-related allergies. "The research is telling us that exposure to a higher and more diverse burden of environmental bacteria and specific patterns of gut bacteria appear to boost the immune system's protection against allergies and asthma."

CHAPTER 13 ANTIMICROBIAL THERAPYThe term chemotherapy was coined by Paul

Ehrlich to describe the use of chemical substances to kill pathogenic organisms without injuring the host. Ehrlich also coined the phrase “magic bullet” to describe chemotherapeutic agents.

E-mail I received from Consumers Report (I subscribe to CR):

Dear Sheldon Did you know that an antibiotic resistant strain of staph infection now kills an estimated 17,000 people each year—and most people contract this infection during a stay in a hospital?

Hospitals know how to prevent the spread of these dangerous infections—good hand washing, isolation of infected patients, and use of masks, gowns and gloves—but they haven't taken the necessary steps to make us all safe.

Tell your governor to direct hospitals in Kentucky to reduce the spread of these dangerous infectionsAnd tell you which hospitals do the best job.If you contract MRSA during a hospital stay, you are four times as likely to die, you will stay in the hospital far longer than you planned, and your charges will triple, according to a recent study.Some people who contract MRSA during their hospital stay live with it for years, undergo multiple surgeries and are permanently disabled. These dangerous infections were once rare, but are now epidemic.Your governor can help! Tell your governor to make MRSA prevention a top priority in KentuckyWhen you finish, be sure and forward this email to people in your address book who might also want to help us prevent the spread of dangerous hospital-acquired infections.Sincerely,Jim GuestPresident, Consumers Union of the U.S.101 Truman AvenueYonkers, NY 10703-1057\

Antibiosis- against lifeantibiotic- coined by Selman Waksman, “a chemical

substance produced by microorganisms which has the capacity to inhibit the growth of bacteria and even destroy bacteria and other microorganisms in dilute solution.”

Synthetic drugs are agents synthesized in the laboratory

Semi-synthetic drugs are antimicrobial agents made partly by laboratory and partly by microorganisms

Bacteriocidal- killing Bacteriostatic- growth inhibiting

Paul Erhlich- 1910 used salvarsan (heavy metal-arsenic salt) to treat syphilis. Although salvarsan did not withstand the test of time the concept of using a systemic “magic bullet” did.

Gerhard Domagk and Ernest Fourneau (1935)- discovered “sulfa” drugs (sulfonamides)

Alexander Fleming (1928) reported on his observation with penicillin- but did not realize the magic bullet possibilities

Ernst Chain (stumbled across Flemings research paper) and Howard Florey - brought penicillin to it “magic bullet” status. There was a great deal of money available for research because the findings were directed towards soldiers during WWII.

The combination of sulfonamides and penicillin began the modern drug era.

Selective Toxicity- must harm the microbes without causing significant damage to the host.

Therapeutic index- maximal tolerable dose per Kg body weight, divided by the minimum dose per Kg body weight.

If the max= 8 and the min= 2- the therapeutic index = 4If the max=6 and the min= 3 - the therapeutic index = 2

The top drug would be more effective and less toxic to the patient than the bottom drug.

The therapeutic dose (also known as thetherapeutic ratio =LD50 (lethal dose for 50% of the population)ED50 (effective dose for 50% of the population)

is a very important number in studies of new drugs. If the therapeutic index is too low (such as, Phase II studies) development of the drug will be discontinued.“the active ingredient in popular over-the-counter analgesic Tylenol, also has a narrow therapeutic range.” the active drug is acetaminophen. What does that statement mean to you? Paracetamol (acetominophen) toxicity is the foremost cause of acute liver failure in the Western world, and accounts for most drug overdoses in the United States, the United Kingdom, Australia and New Zealand. (from wikipidea- with the appropriate references).

Fig. 13.1 the spectrum of antibiotic activity

very broad spectrum

Penicillins fairly broad spectrum

fairly broad spectrum

Narrow spectr.

Broad spectrum drugs, such as tetracycline, affect a variety of different organisms. Narrow-spectrum drugs, such as isoniazid, affect only a few specific types of organisms.

Fig. 13.2 The five major modes of action of antibacterial drugs

You want to know the modes of action and examples

How penicillin inhibits cell wall synthesis

http://www.youtube.com/watch?v=4EJEr_lt5dM

1. Inhibition of cell wall synthesis- Antibiotics such as penicillin and cephalosporin contain a chemical structure called a beta-lactam ring which interferes with the cross-linking of the peptidoglycan (we will see the structure when we discuss resistance- Figure 13-7.

Bacitracin is another antibiotic that blocks cell wall synthesis- it block the basic synthesis of the cell wall by blocking the movement of key wall components from the cytoplasm through the membrane into the wall.

http://www.youtube.com/watch?v=Ov3G9GQo0fg&feature=relmfu

Spread of methicillin resistant Staphylococcus aureeus

Video below shows the mode of action of vancomycin

http://www.youtube.com/watch?v=UK7TaxmxOQ0

2. Disruption of cell membrane function- certain polypeptide antibiotics such as polymyxins act as detergents and disorder bacterial membranes such that it leaks and cannot carry out its function. Particularly active against Gram negative organisms.

Polyene antibiotics, such as amphotericin B, bind to

particular sterols present in the membrane of fungi (and animal cells) and do not act on bacteria (except Mycoplasma) and polymyxins do not act on fungi.

Amphotericin B liposome delivery system

http://www.youtube.com/watch?v=JzP2K60VDmc&feature=related

3. Inhibition of protein synthesis. Differences between bacterial (70S) and animal (80S) ribosomes allow antimicrobial agents to attack bacterial cells without significantly damaging animal cells-selective toxicity.

Aminoglycoside antibiotics, such as streptomycin, act on the 30S portion of bacterial ribosomes by interfering with the accurate reading (translation) of the mRNA message.

Chloramphenicol and erythromycin act on the 50S portion of bacterial ribosomes, inhibiting the formation of the growing polypeptide.

Antibiotics targeting ribosomes

http://www.youtube.com/watch?v=mM_HK3nSmsw&feature=relmfu

Streptococcus pneumoniae-Resistance to erythromycin

http://www.youtube.com/watch?v=oC21vLFtsjo

4. Inhibition of Nucleic acid synthesis- Differences between the enzymes used by bacterial and animal cells to synthesize nucleic acids provide a means for selective action of antimicrobial agents. Antibiotics of the rifamycin family bind to bacterial RNA polymerase and inhibit RNA synthesis. This is due to the high affinity of rifamycins to prokaryotic RNA polymerase.

5. Action as antimetabolites -Antimetabolites are substances that affect the utilization of metabolites and therefore prevent a cell from carrying out necessary metabolic reactions.

Antimetabolites function in two ways: 1) by competitively inhibiting enzymes

2) by being erroneously incorporated into important molecules such as nucleic acids.

Classic example of competitive inhibition is the use of sulfanilamide which is structurally similar to para-aminobenzoic acid.

http://www.youtube.com/watch?v=GpYMgwXoy7Y

Developing resistance to sulfa drugs

Fig. 13.4 Competitive inhibition

a) PABA (para-aminobenzoic acid) a metabolite required by many bacteria for the synthesis of folic acid among other things b) sulfanilamide, a sulfa drug c) para-aminosalicylic acid (PAS) another competitive inhibitor or PABA. Animal cells require folic acid (vitamin) and therefore their metabolism is not disturbed by these competitive inhibitors

Para-aminobenzoic acid (PABA)

sulfanilamide

para-aminosalicylic acid (PAS)

PABA-is a metabolite in the synthesis of Folic Acid

Polyene antibiotics, e.g., amphotericin B, function by blocking ergosterol synthesis in fungal membranes

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Parkinson's trial suggests growth factor has regenerative effects in brain The first trial to test the effect of a growth-factor compound in humans with Parkinson's disease has shown promising results. A report in the Journal of Clinical Investigation raises the hope that one day we will see regenerative treatments for Parkinson's disease that use the brain's own protective mechanisms to halt the loss of brain cells and restore dopamine levels. Research into the effects of platelet-derived growth factor or PDGF started over 10 years ago with studies in animals. These found that the growth factor reduces symptoms of Parkinson's, improves motor skills, and restores levels of dopamine in the brain. They also suggest PDGF repairs neurons and nerve fibers. he treatment - PDGF or placebo - was delivered via a pump surgically implanted in the abdomen, with an internal catheter that went up into the brain. The pump delivered the drug for 12 days, and patients were followed for another 73 days, during which the pump delivered an infusion of saline. The results show that PDGF does not cause serious, unresolvable side effects. The researchers were also encouraged by other results, which they discovered when they examined PET scans of the patients' brains taken 4 months after treatment.The PET scan images showed that in the patients who received the active treatment - as opposed to placebo - levels of dopamine signaling were not only maintained, but even increased. "Although we still have a long way to go our study suggests that it may be possible to stimulate the brain's built-in protective mechanisms in order to slow or halt disease progression."

Peptide developed that may slow Parkinson's disease New research suggests it may be possible to slow the progression of Parkinson's disease using a man-made peptide that stops the formation of faulty protein fibrils that kill the brain cells that produce dopamine. The main reason behind the death of dopamine-producing cells in patients with Parkinson's disease is thought to be a fault in a common cell protein called α-synuclein. When faulty, the protein forms the wrong shape and clumps into long toxic fibrils that stop the cells functioning properly. Now, a new study - funded by Parkinson's UK and led by the University of Bath in the UK - shows how a peptide they designed may offer a way to slow the progression of Parkinson's disease. In this study, the researchers found that their peptide sticks to misshapen α- synuclein and stops it stacking into the fibrils that kill off the dopamine cells. The peptide matches a region of α-synuclein that is mutated in Parkinson'sTo make their peptide - which comprises 10 amino acids - the team searched a library of peptides for candidates that match the region of α-synuclein that is mutated in patients with early-onset Parkinson's. In their paper, the team notes that the technique may also apply to other neurodegenerative diseases like Alzheimer's disease, which also arises as a result of misshapen proteins clumping together in the brain.

PurinePyrimidine

Vidarabine idoxuridine

Antimetabolites such as the purine analogue vidarabine and the pyrimidine analogue

idoxuridine are erroneously incorporated into nucleic acids. Vidarabine is an anti-viral drug which is active against herpes simplex and varicella zoster viruses. Idoxuridine is an herpesvirus antiviral drug.

Fig. 13.5 Base analogues

Kinds of side effectsSide effects of antimicrobial agents on infected persons fall into

three general categories: 1) Toxicity, 2) allergy and 3) disruption of normal microflora.

1) toxicity- some antibiotics exert toxic effects on the host (will discuss as we deal with the individual antibiotics).

2) Allergy- An allergy is a condition in which the bodys’ immune system responds to a foreign substance, usually a protein. Allergic reactions can be limited to mild skin rashes or they can be life-threatening. Breakdown products of penicillins can combine with proteins in the body to trigger an anaphylactic type reaction which is life threatening.

3) Disruption of normal microflora- long term use of penicillins or aminoglycosides can abolish natural microbiota and allow colonization of the gut with resistant Gram negative bacteria, Clostridium difficile (very common and nasty Inflammatory bowel disease) and fungi such as Candida. Oral and vaginal superinfections with species of Candida yeasts are common after prolonged use of antimicrobial agents such as cephalosporins, tetracyclines, and chloramphenicol.

The Resistance of Microorganisms-resistance of a microorganism to an antibiotic means that a microorganism formerly susceptible to the action of the antibiotic is no longer affected by it.

How resistance is acquiredMicroorganisms generally acquire antibiotic resistance by genetic

changes, but sometimes they arise by nongenetic means. Resistance of the latter sort occurs when organisms evade antibiotics by remaining out of reach. TB in the lungs and sometimes E. coli in the urinary tract..Another type of resistance is the shift to L forms that lack most of their cell walls (L-forms are typically derived from Gram negative bacteria and similar to spheroplasts)

Genetic resistance to antimicrobial agents develops from genetic changes followed by natural selection. In most bacterial populations mutations occur spontaneously at a rate of approx. 1 in 107 to 1 in 1010.

Because bacteria multiple so rapidly it is easy to have 1010 cells and so there will typically be a number of resistant cells in a population of bacteria.. If the resistant organism is in an environment in which the antibiotic is present its progeny will have a selective advantage such that after a number of generations it will be the predominant organisms. Antibiotics do not induce mutations but they create an environment in which resistant organisms predominate (such as seen in the fluctuation test (repeated on the next slide).

Genetic resistance can be due to changes in the bacterial chromosome or to extrachromosomal material.

There are basically five mechanisms associated with bacterial resistance to antibiotics:

1. Alteration of targets- the target protein is changed due to mutation. Resistance to erythromycin, rifamycin and antimetabolites has developed by this mechanism.

2. Alteration of membrane permeability- antibiotic can no longer cross the membrane or is transported out of the cell- In bacteria, resistance to tetracyclines, quinolones and some aminoglycosides has occurred by this mechanism.

3. Development of enzymes that destroy or inactivate the antibiotic. One enzyme of this type is beta-lactamase which breaks the beta-lactam ring of penicillin and some cephalosporins. Similarly, enzymes can destroy various aminoglycosides and chloramphenicol.

4. Alteration of an enzyme- that works better with the substrate than the antibiotic. Example is with sulfonamide-resistant bacteria. These organisms have developed an enzyme that has a very high affinity for PABA and a very low affinity for sulfonamide.

5. Alteration of a metabolic pathway- This mechanism by- passes a reaction inhibited by an antimicrobial agent. A good example is 4.

Cross-resistance- is resistance to two or more similar antimicrobial agents via a common mechanism. The action of beta-lactamase against penicillin and cephalosporin is an example.

How penicillin and vancomycin inhibits cell wall synthesis- already seen

Above already seen

Fig. 13.7 The effect of lactamase on penicillin

How to Limit drug resistance- 1) high levels of an antibiotic can

be maintained in the bodies of patients long enough to kill all pathogens, including resistant mutants, or to inhibit them sufficiently such that the host defenses can get rid of them

Fig. 13.8 Effects of premature termination of antibiotic treatment

2) Two antibiotics can be administered simultaneously so that they can exert an additive effect termed synergism. For example, when streptomycin and penicillin are combined in therapy, the damage to the cell wall caused by the penicillin allows better penetration by streptomycin.

When clavulanic acid and a penicillin called amoxicillin are given together the clavulanic acid binds tightly to beta-lactamase and prevents the enzyme from inactivating the amoxicillin, i.e., clavulanic acid inhibits penicillinase (beta-lactamase).

3) limiting antibiotics to essential uses only. Restrict the use of antibiotics to cases with bacterial infections. Restrict the use of antibiotics in hospitals. In addition, ban the use of antibiotics in animal feeds.

Why are there so few clinically useful antibiotics?Several hundreds of compounds with antibiotic activity have been isolated from microorganisms over the years, but only a few of them are clinically useful. The reason for this is that only compounds with selective toxicity can be used clinically - they must be highly effective against a microorganism but have minimal toxicity to humans. In practice, this is expressed in terms of the therapeutic index - the ratio of the toxic dose to the therapeutic dose. The larger the index, the better is its therapeutic value.

DETERMINING MICROBIAL SENSITIVITES TO ANTMICROBIAL AGENTS.

The disk diffusion method.

Fig. 13.9 The disk diffusion method of determining microbial sensitivities to various antibiotics

Minimal inhibitory concentration (MIC) microbial susceptibility testing. Organism is sensitive to all but Penicillin R. Most sensitive to methicillin and cefotaxime.

Disc contains a set amount of antibiotic. Circumference of the zone of clearing is compared to standards to determine the effectiveness of the antibiotic against the test organisms. Size of zone of clearance not necessarily indicative of one antibiotic being more effective than another (because the diffusion rates of the antibiotics vary).

Organism showing resistance

Each lollipop is saturated with a different antibiotic or it can be different concentrations of an antibiotic

Fig. 13.10 An E (epsilometer) test.

This test determines antibiotic sensitivity and estimates minimal inhibitor concentration (MIC). A plastic strip containing an increasing gradient of a given antibiotic is placed on the surface of a Petri dish which has been swabbed with the bacterial organism of interest. A zone of inhibition of growth around the strip indicates sensitivity of the organism to that specific antibiotic. The point a which inhibition begins indicates the MIC (minimal inhibitory concentration) for that antibiotic, and can be read off the printed scale.

Antibacterial agents

D-ala

Basic building block of peptidoglycan

Penicillin and cephalosporin block cross-linking of pentaglycine bridge

tetrapeptide starts

as a pentapeptide as shown in the figure to the right

Basic structural unit of the

peptidoglycan

pentaglycine cross-link bridge

This D-ala is cleaved during cross-linking

View the videosIn previous slidesFor the action of theBeta-lactam antibiotics

How Vancomycin inhibits cell wall synthesis

Cephalosporin works essentially the same way as penicillin

http://www.youtube.com/watch?v=UK7TaxmxOQ0

Fig. 13.11 Basic structure of penicillin and cephalosporin

a) A comparison of the penicillin and cephalosporin molecules with beta-lactam rings b)Cephalosporins differs slightly in the attached ring and has two sites for side -chain attachments rather than one, as on the penicillin molecule

Figure 13 bottom

Need not memorize the different R groups of penicillin just wanted to give you some idea of the structural differences among the different types of penicillin and Cephalosporins

You do not have to know the varous types of penicillins.

Additional Beta-lactam antibiotics:Carbapenems represent a relatively new group of bactericidal antibiotics with a two-part structure.Example- Primaxin: Imipenem/cilastatin is a broad spectrum beta-lactam antibiotic containing equal quantities of imipenem and cilastatin.

Imipenem, the active antibacterial agent, is rapidly degraded by the renal enzyme dehydropeptidase if administered alone (making it less effective). It is a cell wall antibiotic

Cilastatin is a dehydropeptidase inhibitor, with no intrinsic antibacterial activity, and must be co-administered with imipenem to ensure its efficacy. Cilastatin blocks the effects of the renal enzyme. Cilastatin has no antibacterial effects and does not affect the antibacterial activity of the imipenim. This drug is given by injection or infusion and is generally used to treat severe infections. It may be less likely to cause an allergic reaction in people who have had an allergic reaction to a penicillin in the past.

Other antibacterial agents that act on cell walls

Bacitracin a small bactericidal polypeptide derived from Bacillus licheniformis, is used only on lesions and wounds of the skin or mucous membranes because it is poorly absorbed and toxic to the kidneys. This antibiotic prevents the transfer of the peptidoglycan building block synthesized within the cell from going through the plasma membrane to the outside of the cell.

Vancomycin- Can be used to treat infections caused by methicillin-resistant staphylococci and enterococci. Because it is poorly absorbed through the GI tract, it must be administered intravenously. Vancomycin prevents incorporation of N-acetylmuramic acid (NAM)- and N-acetylglucosamine (NAG)-peptide subunits into the peptidoglycan matrix; which forms the major structural component of Gram-positive cell walls. Vancomycin binds to the D-alanyl-D-alanine moieties of NAM/NAG peptides and blocks cross-linking.

Polymyxin mode of action:

1.Alters cytoplasmic membrane permeability by binding to a negatively charged site in the lipopolysaccharide layer which has an electrostatic attraction for the positively charged amino groups in the cyclic peptide portion2.Fatty acid portion dissolves in hydrophobic region of membrane and disrupts membrane integrity 3.Leakage of cellular molecules, inhibition of cellular respiration 4.Binds and inactivates endotoxin (LPS)5.Relative absence of selective toxicity: nonspecific for cell membranes of any type, highly toxic

Active ttuberculosis will kill about two of every three people affected if left untreated. Treated tuberculosis has a mortality rate of less than 5%.The standard "short" course treatment for tuberculosis (TB), is isoniazid (inhibits mycolic acid-major cell wall component), rifampicin (inhibits RNA polymerase), pyrazinamide (thought to disrupt membrane structure and interfere with energy metabolism), and ethambutol (blocks the formation of the cell wall of the tuberculosis organism), for two months, then isoniazid and rifampicin alone for a further four months. The patient is considered cured at six months (although there is still a relapse rate of 2 to 3%). For latent tuberculosis, , the standard treatment is six to nine months of iisoniazidalone.If the organism is known to be fully sensitive, then treatment is with isoniazid, rifampicin, and pyrazinamide for two months, followed by isoniazid and rifampicin for four months. Ethambutol need not be used.

Fig. 4.6c-Acid fast cell wall

Mycobacterium tuberculosis (TB organism is an acid fast organism )

Lipid bilayer

peptidoglycan

arabinogalactan

Mycolic acid

Acyl lipids

lipoarabinomannan (LAM)

The complex structure of the acid fast cell wall makes it a good barrier against many physical agents, such as phagocytic cell digestion, penetration by antibacterial agents. However, the treatment in the slide above, combination of drugs described in the previous slide, seems to be very effective against TB.

Inhibitors of protein synthesis

You do not need to memorize any of the antibiotic producing organisms.

Aminoglycosides (irreversibly binds to the 16S subunit of the 30S ribosome)- An important property of this class of antibiotics is its ability to act synergistically with other antibiotics.

For example, gentamicin (an aminoglycoside) and penicillin or ampicillin are effective against penicillin-resistant streptococci.

Some examples: gentamicin, kanamycin, neomycin, and streptomycin.

binds here30s sub-unit

Tetracyclines (Also binds to the 16S subunit of the 30S ribosome) Some examples: i) tetracycline ii) chlortetracycline (aureomycin) and oxytetracycline (terramycin) Newer semisynthetic teteracyclines include minocycline and deoxycycline.

All are bacterostatic (reversibly bind to the 30S ribosome) at normal doses, are readily absorbed from the digestive tract, and become widely distributed in tissues and body fluids. The fact that tetracyclines have the widest spectrum of activity of any antibiotics (Gram positives, Gram negative, rickettsia, mycoplasma and some fungal infections) both beneficial and a problem:

i) produce severe gastrointestinal disorders because they destroy so much of the normal flora

ii) recalcitrant superinfections of tetracycline-resistant Proteus, Pseudomonas and Staphylococcus as well as yeast infections, also can result. Also Clostridium difficile infection of the GI tract

30S subunit

Chloramphenicol- Chloramphenicol binds to the 50S-ribosomal subunit and blocks translation. Chloramphenicol is now

fully synthesized in the laboratory. Like tetracyclines it is bacteriostatic, is rapidly absorbed from the digestive tract, is widely distributed in tissues, and has a broad spectrum of activity.

It is used to treat typhoid fever, infections due to penicillin-resistant strains of meningococci and Haemophilus influenzae, brain abscesses and severe rickettsial infections. However, it is used sparingly because it can cause aplastic anemia (body stops producing red and white blood cells in sufficient quantity)-in a dose-dependent manner- which is often a fatal disease. Aplastic anemia appears days to months after treatment is discontinued and is most common in newborns.

Other antibacterial agents that affect protein synthesismacrolides- Erythromycin Binds to the 50S ribosomal

subunit and blocks translation. A commonly used macrolide (large ring compound) is produced by several strains of Streptomyces erythreus and exerts a bacteriostatic effect.

It is recommended for infections caused by streptococci, pneumococci, and corynebacteria but is also effective against Mycoplasma, Chlamydia, and Campylobacter.

Several antibiotics are effective against pneumonias,

however, erythromycin is the only common antibiotic that will combat Legionnaires’ disease.

Inhibitors of Nucleic Acid SynthesisRifampin- produced by Streptomyces mediterranei, only the

semisynthetic rifampin is currently used. Easily absorbed from the digestive tract except when taken directly after a meal, it reaches all tissues and body fluids. Although it is bactericidal and has a wide spectrum of activity, it is approved in the US only for treating tuberculosis and eliminating meningococci from the nasopharynx of carriers. Rifampin blocks RNA transcription.

Quinolones- inhibit bacterial DNA synthesis by blocking DNA gyrase a new group of synthetic bactericidal analogues of nalidixic acid, are effective against many Gram-positive and Gram-negative bacteria. Quinolones’ mode of action is to inhibit the enzyme that unwinds the DNA double helix preparatory to its replication. Norfloxacin, ciporofloxacin and enoxacin are examples of this group of antibiotics.

Antifungal Agents are being used with greater frequency because of the emergence of resistant strains and an increase in the number of immunosuppressed patients, especially those with AIDS.

Imidazoles and Traizoles They appear to affect fungal plasma membranes by disrupting membrane sterols.

Several agents in this family of drugs are currently in use: clotrimazole, ketoconazole, miconazole, and fluconazole. All these agents are used topically.

Polyenes the polyene family of antibiotics consists of antifungal agents that contain at least two

double bonds. Amphotericin B (and Ampho E) and nystatin are two of the most

common polyene antibiotics. Amphotericin B increases membrane permeability such that glucose, potassium and other substances leak from the cell. This drug binds to membrane

ergosterol found in fungi and some algae and protozoa but not in humans The drug is poorly absorbed by the GI tract and is given intravenously.

Amphotericin B (and E) is the drug of choice in treating most systemic fungal infections, especially cryptococcosis, coccidioidomnycosis, and aspergillosis.

Nystatin has the same mode of action as amphotericin B but is also effective topically in the treatment of Candida yeast infections.

Griseofulvin-originally from Penicillium griseofulvum, is used primary for superficial fugal infections. The drug binds to tubulin, interfering with microtubule function, thus inhibiting mitosis.

General mechanism of action of amphotericin B

Antifungal antibiotics

Polyene antibiotics (ene= double bonds)superficial fungal infectionse.g., athletes foot

Amphotericin B and Nyastatin are membrane disrupters but unlike what your book shows they have different structures (as shown above).

Do not have to know these structures. Only that that Amphotericin B and Nystatin are polyene antiobiotics

Antiviral Agents:Purine and pyrimidine analogues-

Idoxuridine and trifluridine, both analogues of thymine, are administered in eye drops to treat

inflammation of the cornea caused by a herpesvirus. They block DNA replication.Vidarabine (ARA-A)- an analogue of adenine, has been used effectively to treat viral encephalitis, an

inflammation of the brain caused by herpesviruses and cytomegaloviruses. Also blocks DNA synthesis.Ribavirin (Virazole), a synthetic nucleotide analogue of guanine, blocks replication of certain RNA and DNA viruses. Ribavirin blocks RNA replication in RNA viruses, how it blocks DNA virus replication is less clear. In an aerosol spray, it can combat influenza viruses; in an ointment, it can help to heal herpes lesions. In the U.S. the oral (capsule or tablet) form of ribavirin is used in the treatment of hepatitis C, in combination with interferon drugs

Acyclovir- (Zovirax) analogue of guanine (Blocks HSV DNA synthesis by competing with HSV thymidine kinase), is much more repeatedly incorporated into virus infected cells than into normal cells. Thus, it is less toxic than other analogues. It can be applied topically or given orally or intravenously. It is especially effective in reducing pain and promoting healing of primary lesions n a new case of genital herpes.

Ganiciclovir is an analogue of guanine similar to acycovir. The drug is active against several kinds of herpesvirus infections and in particular cytomegalovirus eye infections.

Amantadine- The tricyclic amine amantadine prevents influenza A viruses from penetrating cells.. In terms of the mechanism of its antiviral properties, amantadine interferes with a viral ion channel which is required for the viral particle to become "uncoated" once it is taken inside the cell by endocytosis.

Chloramphenicol can cause aplastic anemia, a deadly form of cancer

A. TrueB. False

True

False

50%50%

Mode of Action of Tamiflu- neuraminidase inhibitor

Neuraminidase required for release of influenza virus from the cell surface

Fig. 13.17 the use of double antibiotic therapy to eradicate resistant-strain infections