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

2. 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

3. 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.

4. 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

5. 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

6. 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.

7. 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

8. 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

9. 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.

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

11. 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.

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

13. 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.

14. 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)

15. 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!

16. 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)

17. 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

18. 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.

19. 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!

20. 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.

21. 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.

22. 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.

23. 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

24. 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).

25. 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.

26. 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).

27. Controlling Microbial Growth In Vivo

28. 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

29. 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.

30. 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

31. 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

32. 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

33. 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

34. 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.

35. 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)?

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

37. 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.

38. 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.

39. END OF PRESENTATION