Controlling Microbial Growth in Vivo Using Antimicrobial Agents Chapter 9 Instructor: Bertha Escobar-Poni, MD

Controlling Microbial Growth In Vivo Chapter 9 Outline Introduction Characteristics of an Ideal Antimicrobial Agent How Antimicrobial Agents Work Antibacterial Agents Antifungal Agents Antiprotozoal Agents Antiviral Agents Drug Resistance Some Strategies in the War Against Drug Resistance Empiric Therapy Undesirable Effects of Antimicrobial Agents Concluding Remarks

Controlling Microbial Growth In Vivo Introduction Chemotherapy is the use of any chemical (drug) to treat any disease or condition. A chemotherapeutic agent is any drug used to treat any condition or disease. An antimicrobial agent is any chemical (drug) used to treat an infectious disease, either by inhibiting or killing pathogens in vivo. Some antimicrobial agents are antibiotics.

Controlling Microbial Growth In Vivo Introduction antibacterial agents; drugs used to treat bacterial diseases are called antifungal agents; those used to treat fungal diseases antiprotozoal agents; those used to treat protozoal diseases antiviral agents; those used to treat viral diseases

Controlling Microbial Growth In Vivo Introduction An antibiotic is a substance produced by a microorganism that kills or inhibits growth of other microorganisms. Semisynthetic antibiotics: Antibiotics that have been chemically modified to kill a wider variety of pathogens or reduce side effects; examples include semisynthetic penicillins such as ampicillin and carbenicillin

The discovery of penicillin by Alexander Fleming Controlling Microbial Growth In Vivo The discovery of penicillin by Alexander Fleming Colonies of Staphylococcus aureus are growing well in this area of the plate. Colonies are poorly developed in this area of the plate because of an antibiotic (penicillin) being produced by a colony of Penicillium notatum (a mould), shown at C.

Characteristics of an Ideal Antimicrobial Agent Controlling Microbial Growth In Vivo Characteristics of an Ideal Antimicrobial Agent Kill or inhibit the growth of pathogens Cause no damage to the host Cause no allergic reaction in the host Be stable when stored in solid or liquid form Remain in specific tissues in the body long enough to be effective Kill the pathogens before they mutate and become resistant to it

How Antimicrobial Agents Work Controlling Microbial Growth In Vivo How Antimicrobial Agents Work The 5 most common mechanisms of action of antimicrobial agents are: Inhibition of cell wall synthesis Damage to cell membranes Inhibition of nucleic acid synthesis (either DNA or RNA synthesis) Inhibition of protein synthesis Inhibition of enzyme activity

Controlling Microbial Growth In Vivo Antibacterial Agents Bacteriostatic drugs inhibit growth of bacteria, whereas bactericidal drugs kill bacteria. Sulfonamide drugs inhibit production of folic acid (a vitamin) in those bacteria that require p-aminobenzoic acid to synthesize folic acid; without folic acid bacteria cannot produce certain essential proteins and die. Sulfa drugs are competitive inhibitors; they are bacteriostatic.

The effect of sulfonamide drugs. Controlling Microbial Growth In Vivo The effect of sulfonamide drugs.

Controlling Microbial Growth In Vivo Antibacterial Agents In most Gram-positive bacteria, Penicillin interferes with the synthesis and cross-linking of peptidoglycan, a component of cell walls. By inhibiting cell wall synthesis, penicillin destroys the bacteria.

Controlling Microbial Growth In Vivo Antibacterial Agents Colistin and nalidixic acid destroy only Gram-negative bacteria; they are referred to as narrow-spectrum antibiotics.

Controlling Microbial Growth In Vivo Antibacterial Agents Antibiotics that are destructive to both Gram-positive and Gram-negative bacteria are called broad-spectrum antibiotics (examples: ampicillin, chloramphenicol and tetracycline). Multidrug therapy Sometimes one drug is not sufficient; 2 or more drugs may be used simultaneously, as in the treatment of tuberculosis.

Some Major Categories of Antibacterial Agents Controlling Microbial Growth In Vivo Some Major Categories of Antibacterial Agents Antibacterial Agent Effect Target/Action Penicillins bactericidal interfere with cell wall synthesis Cephalosporins Tetracyclines bacteriostatic inhibit protein synthesis Aminoglycosides Macrolides bacteriostatic at lower doses; bactericidal at higher doses Fluoroquinolones inhibit DNA synthesis Penicillins (bactericidal; interfere with cell wall synthesis) Cephalosporins (bactericidal; interfere with cell wall synthesis) Tetracyclines (bacteriostatic; inhibit protein synthesis)) Aminoglycosides (bactericidal; inhibit protein synthesis) Macrolides (bacteriostatic at lower doses; bactericidal at higher doses; inhibit protein synthesis) Fluoroquinolones (bactericidal; inhibit DNA synthesis)

Controlling Microbial Growth In Vivo Antibacterial Agents Synergism Antagonism Synergism is when 2 antimicrobial agents are used together to produce a degree of pathogen killing that is greater than that achieved by either drug alone. Synergism is a good thing! Antagonism is when 2 drugs actually work against each other. The extent of pathogen killing is less than that achieved by either drug alone. Antagonism is a bad thing!

Controlling Microbial Growth In Vivo Antifungal Agents Most antifungal agents work in one of 3 ways: By binding with cell membrane sterols (e.g., nystatin and amphotericin B) By interfering with sterol synthesis (e.g., clotrimazole and miconazole) By blocking mitosis or nucleic acid synthesis (e.g., griseofulvin and 5-flucytosine) Antifungal agents and antiprotozoal agents tend to be more toxic to the patient because (like the infected human) they are eucaryotic organisms. (Sterols: cholesterol, ergosterol, phytosterol) (Sterols: cholesterol, ergosterol, phytosterol)

Controlling Microbial Growth In Vivo Antiprotozoal Agents Antiprotozoal agents are usually toxic to the host. Antiprotozoal agents work by: Interfering with DNA and RNA synthesis (e.g., chloroquine, pentamidine, and quinacrine) Interfering with protozoal metabolism (e.g., metronidazole) e.g. Giardia, E. Coli

Controlling Microbial Growth In Vivo Antiviral Agents Antiviral agents are the newest weapons in antimicrobial methodology. Difficult to develop these agents because viruses are produced within host cells. Some drugs have been developed that are effective in certain viral infections, but not others; they work by inhibiting viral replication within cells. Antiviral agent “cocktails” (several drugs that are administered simultaneously) are being used to treat HIV infection.

Drug Resistance “Superbugs” Controlling Microbial Growth In Vivo Drug Resistance “Superbugs” Superbugs are microbes (mainly bacteria) that have become resistant to one or more antimicrobial agent. Infections caused by superbugs are difficult to treat!

Drug Resistance “Superbugs” Controlling Microbial Growth In Vivo Drug Resistance “Superbugs” Bacterial superbugs include: methicillin-resistant Staphylococcus aureus (MRSA); vancomycin-resistant Enterococcus spp. (VRE); multidrug-resistant Mycobacterium tuberculosis (MDRTB); multidrug-resistant strains of Acinetobacter, Burkholderia, E. coli, Klebsiella, Pseudomonas, Stenotrophomonas, Salmonella, Shigella. and N. gonorrhoeae; β–lactamase-producing strains of Streptococcus pneumoniae and Haemophilus influenzae; carbapenemase-producing Klebsiella pneumoniae.

Drug Resistance How Bacteria Become Resistant to Drugs Controlling Microbial Growth In Vivo Drug Resistance How Bacteria Become Resistant to Drugs Some bacteria are naturally resistant because they lack the specific target site for the drug or the drug is unable to cross the organism’s cell wall or cell membrane and thus, … cannot reach its site of action. Resistance of this type is known as intrinsic resistance.

Drug Resistance How Bacteria Become Resistant to Drugs Controlling Microbial Growth In Vivo Drug Resistance How Bacteria Become Resistant to Drugs If bacteria that were once susceptible to a particular drug become resistant, this is called acquired resistance.

Drug Resistance How Bacteria Become Resistant to Drugs Controlling Microbial Growth In Vivo Drug Resistance How Bacteria Become Resistant to Drugs Before a drug enters a bacterial cell it must first bind to proteins on the surface of the cell; these proteins are called drug-binding sites. A chromosomal mutation that affects the structure of a drug-binding site can prevent the drug from binding, resulting in drug resistance

Drug Resistance How Bacteria Become Resistant to Drugs Controlling Microbial Growth In Vivo Drug Resistance How Bacteria Become Resistant to Drugs Bacteria can develop the ability to produce an enzyme that destroys or inactivates a drug. Many bacteria have become resistant to penicillin because they have acquired the gene for penicillinase production during conjugation. A plasmid that contains multiple genes for drug resistance is known as a resistance factor (R-factor).

Drug Resistance How Bacteria Become Resistant to Drugs Controlling Microbial Growth In Vivo Drug Resistance How Bacteria Become Resistant to Drugs Bacteria can also become resistant to drugs by developing the ability to produce multidrug-resistance (MDR) pumps (also known as MDR transporters or efflux pumps). An MDR pump enables the cell to pump out drugs before they can damage or kill the cell.

Drug Resistance How Bacteria Become Resistant to Drugs Controlling Microbial Growth In Vivo Drug Resistance How Bacteria Become Resistant to Drugs Summary: Bacteria can acquire resistance to antimicrobial agents by chromosomal mutation or by the acquisition of new genes by Transduction [ bacteriophage carry DNA from on bacteria to another] Transformation [naked DNA from environment] most commonly, by Conjugation (plasmid containing such gene).

Controlling Microbial Growth In Vivo

Drug Resistance β-Lactamases Controlling Microbial Growth In Vivo Drug Resistance β-Lactamases Every penicillin and cephalosporin molecule contains a double-ringed structure (referred to as a “house and garage”). The “garage” is known as the β-lactam ring. Penicillin core structure

Drug Resistance β-Lactamases Controlling Microbial Growth In Vivo Drug Resistance β-Lactamases Some bacteria produce enzymes, β-lactamases, that destroy this ring; when the β–lactam ring is destroyed, the drug no longer works. 2 types of β-lactamases - penicillinases and cephalosporinases; some bacteria produce both types.

Sites of β-lactamase Attack on Penicillin and Cephalosporin Molecules. Controlling Microbial Growth In Vivo Sites of β-lactamase Attack on Penicillin and Cephalosporin Molecules. Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins

Drug Resistance β-Lactamases Controlling Microbial Growth In Vivo Drug Resistance β-Lactamases Drug companies have developed special drugs that combine a β–lactam antibiotic with a β-lactamase inhibitor. Augmentin (brand name): clavulanic acid combined with amoxicillin) Timentin: Clavulanic acid combined with ticarcillin Unasyn: Sulbactam combined with ampicillin Zosyn: Tazobactam combined with piperacillin

Some Strategies in the War Against Drug Resistance Controlling Microbial Growth In Vivo Some Strategies in the War Against Drug Resistance Education of healthcare professionals and patients Patients should stop demanding antibiotics every time they are, or their child is, sick Physicians should not be pressured by patients and should prescribe drugs only when warranted Clinicians should prescribe a narrow-spectrum drug if lab results indicate that it kills the pathogen

Some Strategies in the War Against Drug Resistance Controlling Microbial Growth In Vivo Some Strategies in the War Against Drug Resistance Patients should destroy any excess or out-dated medications Antibiotics should not be used in a prophylactic manner Healthcare professionals should practice good infection control Patients should take drugs in manner prescribed

Controlling Microbial Growth In Vivo Empiric Therapy Empiric therapy is when drug therapy is initiated before laboratory results are available (i.e., before the pathogen is identified and/or before susceptibility test results are available). Empiric therapy is sometimes necessary to save a patient’s life. Clinicians make an “educated guess” based on past experience with the type of infectious disease and the most effective drugs.

Controlling Microbial Growth In Vivo Empiric Therapy Clinicians must take a number of factors into consideration before prescribing antimicrobial agents If pathogen identity is known, use the “pocket chart” of antimicrobial susceptibility test data from past year. Is the patient allergic to any antimicrobial agents? What is the age of the patient? Is the patient pregnant? Inpatient or outpatient? In the hospital formulary? Site of the infection? What other medication(s) is the patient taking? What other medical problems does the patient have? Is the patient leukopenic or immunocompromised? What is the cost of the drug(s)?

Controlling Microbial Growth In Vivo Pocket chart for aerobic Gram-negative bacteria. The chart is a quick reference whenever empiric therapy is necessary.

Undesirable Effects of Antimicrobial Agents Controlling Microbial Growth In Vivo Undesirable Effects of Antimicrobial Agents Reasons why antimicrobial agents should not be used indiscriminately Organisms susceptible to the agent will die, but resistant ones will survive; this is known as selecting for resistant organisms. The patient may become allergic to the agent. Many agents are toxic to humans and some are very toxic. With prolonged use, a broad-spectrum antibiotic may destroy the normal flora, resulting in an overgrowth of bacteria known as a superinfection.

Selecting for drug-resistant organisms Controlling Microbial Growth In Vivo Selecting for drug-resistant organisms Indigenous microflora of patient before antibiotic therapy. (S = susceptible; R = resistant) After antibiotic therapy has been initiated Resistant organisms multiply and become the predominant organisms.

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