Chap. 12 – Antimicrobial Therapy

I. TERMS

1. Chemotherapy – term coined by Paul Ehrlich (father of chemotherapy) - He discovered a drug treatment for syphilis; he also developed the guiding principle of chemotherapy, which is selective toxicity (the drug should be toxic to the infecting microbe, but relatively harmless to the host’s cells). Now the term chemotherapeutic agent describes any chemical substance used in medical practice.

2. Antimicrobial agent – drug used to treat disease caused by microbes.

3. Antibiotic – chemical substance produced by a microorganism that has the capacity to inhibit the growth of bacteria and even destroy them. One of the first antibiotics was penicillin, discovered by Alexander Fleming in while he was carrying out experiments on Staphylococcus (1929). Some of his plates became contaminated with mold spores. As he examined them, Fleming noticed that the Staphylococcus colonies were dissolving as they neared the area where the mold was growing. He reasoned that the mold was secreting something that killed the bacteria. The mold was found to be a member of the genus Penicillium, so he named the bacteria destroying substance “penicillin.” Ten years later Florey and Chain had purified enough penicillin to begin experiments involving the treatment humans. It was enormously useful in the latter part of World War II.

II. GENERAL PROPERTIES OF ANTIBACTERIAL AGENTS

(Antifungal and antiviral agents will be discussed later in the semester as treatment for specific diseases)

A. Selective Toxicity – drug harms the microbe without causing significant damage to the host. When searching for ways to treat disease, scientists look for differences between the human (or animal) host and the pathogen. Ex. Penicillin interferes with cell wall synthesis. Animal cells have no cell walls, so penicillin is not toxic to animals.

B. Spectrum of Activity – the range of different microbes against which an antimicrobial agent acts. Example: Broad spectrum: G(+) and G(-) bacteria vs. Narrow spectrum: G(-) only;

C. Modes of Action (How Do the Drugs Work?)

1. Inhibition of Cell Wall Synthesis - attach to enzymes that cross-link peptidoglycans.

Penicillins – Bactericidal. Natural penicillins are extracted from cultures of the mold Penicillium notatum. Structure: contain beta lactam rings. The discovery of resistant bacterial strains led to the development of semisynthetic penicillins (resistant bacteria have beta lactamase enzymes that can break down beta lactam rings). The first of the semisynthetic penicillins was methicillin (not broken down by beta lactamase enzymes). Other familiar semisynthetics are ampicillin and amoxicillin. Allergy is rare in children but occurs in 1-5% of adults. Also used prophylactically (to prevent infection). Penicillins are not as effective against G(-) due to the presence of the outer membrane.

Cephalosporins – Derived from several species of the fungus Cephalosporium. Have limited antimicrobial action. Their discovery led to the development of a large number of bactericidal, semisynthetic derivatives of cephalosporin. Structure: contain beta lactam rings. Frequently used cephalosporins include cephalexin (Keflex) and ceftriaxone (these 2 are 3^rd generation cephalosporins). Have a fairly wide spectrum of activity, rarely cause serious side effects, and can be used prophylactically. Many people that are allergic to penicillin may also be sensitive to the cephalosporins.

Others – Carbapenems – new group of extremely broad spectrum antibiotics that have a 2-part structure (Ex. Primaxin – consists of a beta lactam antibiotic and cilastatin sodium, a compound that prevents degradation of the drug in the kidneys); Bacitracin – derived from the bacterium Bacillus (highly toxic, so only used topically - used on lesions and wounds of skin and mucous membranes); Vancomycin – produced by Streptomyces (used to treat infections caused by methicillin-resistant staphylococci and enterococci).

2. Disruption of Cell Membrane Function

Antibiotics such as polymyxins act as detergents and distort cell membranes. Polymyxins are obtained from Bacillus polymyxa and are especially effective against G(-) bacteria such as Pseudomonas that have phospholipids in their outer membrane (along with the lipopolysaccharides).

Aminoglycosides – obtained from various species of Streptomyces and Micromnospora. The first was streptomycin (1940’s); many bacteria are now resistant to it. This antibiotic can damage kidneys and the inner ear nerves, so is used only in special situations and usually in combination with other drugs. These drugs have a great ability to act synergistically with other drugs. Gentamycin is a mainstay for the treatment of Pseudomonas (if resistant, then use polymyxins).

Tetracyclines – several are obtained from Streptomyces. Semisynthetic tetracyclines have been developed. All are bacteriostatic. They have the widest spectrum of activity of any antibiotic. Unfortunately, because of this they destroy both the pathogenic bacteria and the normal flora. Can cause a variety of mild to severe toxic effects (kidney and liver damage, light sensitivity, interferes with effectiveness of birth control pills, staining of teeth, abnormal bone development in fetus). Used to treat Lyme disease.

Chloramphenicol – originally obtained from Streptomyces, but is now fully synthesized in the lab. It is bacteriostatic and has a broad spectrum of activity. Because of its toxic effects on bone marrow, it is the drug of last choice in the U.S. Be careful - it is sometimes sold without prescription outside the U.S.

Macrolides – Erythromycin, a commonly used macrolide, is produced by Streptomyces. It is bacteriostatic and is valuable in treating infections caused by penicillin-resistant organisms or in patients allergic to penicillin. Considered one of the least toxic of commonly used antibiotics.

Target enzymes involved in nucleic acid synthesis (ex. DNA replication, transcription).

Rifamycin – specifically targets the enzyme involved in the transcription process (mRNA synthesis); produced by Streptomyces and only used in the U.S. for treating tuberculosis; has high drug interaction. Ex. Rifampicin.

Quinolones –new group of broad spectrum antibiotics; targets enzyme that unwinds DNA prior to replication; especially effective against traveler’s diarrhea and UTI’s. Ex. Nalidixic acid used against G(-)’s.

Antimicrobial compounds can function in 2 ways: 1.) by competitively inhibiting enzymes and 2.) by being erroneously incorporated into important molecules such as nucleic acids. The actions of these compounds are sometimes called molecular mimicry because they mimic the normal molecule, preventing a reaction from occurring or causing it to go awry.

a. Competitive Inhibition - Remember our discussion on enzymes, their active sites, and their substrate? In competitive inhibition an antimicrobial compound binds to an enzyme’s active site, so that the enzyme cannot react with its “true” substrate. To bind to the active site, the antimicrobial compound must be similar in structure to the true substrate.

Ex. The drug sulfanilamide is very similar to the compound para-aminobenzoic acid (PABA). Sulfanilamide competitively inhibits an enzyme that acts on PABA. Many bacteria require PABA in order to make folic acid, which they use in synthesizing nucleic acids and other compounds. When sulfanilamide is bound to the enzyme, a bacterium cannot make folic acid. Animals obtain folic acid from their diets (they don’t have the enzymes to make it themselves), so their metabolism is not disturbed by these competitive inhibitors.

b. Nucleic Acid Incorporation – Antimicrobial compounds such as vidarabine and idoxuridine are erroneously incorporated into nucleic acids. These molecules are very similar in structure to the nitrogenous bases. When incorporated into a nucleic acid, they garble the information that it encodes because they cannot form the correct base pairs during replication and transcription. These compounds can harm the host cells as well as the microorganisms (animal cells use the same nitrogenous bases to make nucleotides). These agents are most useful in treating viral infections, because viruses incorporate these “fakes” more rapidly that do cells and are more severely damaged.

III. KINDS OF SIDE EFFECTS

A. Toxicity – Some antimicrobials do exert toxic effects on the patients receiving them. These effects are discussed later in connection with specific drugs.

B. Allergy – An allergy is a condition in which the body’s immune system responds to a foreign substance, usually a protein. For Ex., breakdown products of penicillins combine with proteins in body fluids to form a molecule that the body treats as a foreign substance.

C. Disruption of Normal Microflora – Antimicrobials, especially broad-spectrum antibiotics, mat exert their adverse effects not only on pathogens but also on the normal or indigenous microflora (the microorganisms that normally inhabit the skin and the digestive, respiratory, and urogenital tracts and keep numbers of unwanted “nonnative” microorganisms in check). When these native populations are reduced, other organisms not susceptible to the antimicrobial agent invade and multiply rapidly (called superinfections) Ex. Oral ampicillin and long-term use of penicillin can often leads to destruction of normal microflora in the gut and in turn, growth of the yeast Candida. Cephalosporins, tetracyclines, and chloramphenicol often lead to oral and vaginal yeast infections. Live-culture yogurts or acidophilus in the form of a pill (both contain lactobacilli) can be given to counteract this effect (basically you are reestablishing the normal microflora).

IV. GENERAL PROPERTIES OF ANTIBACTERIAL AGENTS

A. How Resistance is Acquired

1. Spontaneous Mutations - Most bacteria acquire antibiotic resistance by spontaneous mutations in their genetic material. Bacteria reproduce so rapidly that billions of cells can be produced in a short time; among them there will always be a few mutants. If a mutant happens to be resistant to a drug, that mutant and its progeny will survive, whereas the nonresistant cells will die. After a few generations, most of the survivors will be resistant to the drug. Understand that antibiotics do not induce mutations, but they do create environments that that favor the survival of mutant resistant organisms (see section E below).

2. R Plasmids – Resistant genes usually found on R plasmids can transferred from one bacterium to another by conjugation through pili, transduction (using a viral vector), or transformation (Remember Chapter 8?)

3. L forms - Some species of bacteria can lose their cell wall and swell into irregularly shaped cells called L forms. L forms can arise spontaneously and can persist and divide repeatedly. They can spontaneously revert to normal-walled cells. In the L form, bacteria are resistant to antibiotics that effect cell wall formation (ex. penicillin).

B. Mechanisms of Resistance – include the following: development of an enzyme that destroys or inactivates the drug; alteration of an enzyme that allows a formerly inhibited reaction to occur (the drug no longer works on the target enzyme); alteration of a metabolic pathway – a new chemical reaction bypasses the chemical reaction effected by the drug; alteration of membrane permeability – drugs can no longer cross the membrane, so they have no effect.

C. First Line, Second-Line, and Third-Line Drugs – as a strain of bacteria acquires resistance to a drug, another drug must be found, and so on.

D. Cross-resistance – resistance to 2 or more similar antimicrobials via a common mechanism. Ex. Penicillin contains a structure called a beta lactam ring. Some bacteria have an enzyme called beta lactamase that will break the beta lactam ring in penicillins. Bacteria that have this enzyme are also resistant to some cephalosporins that also contain beta lactam rings.

E. Limiting Drug Resistance

1. High levels of antibiotic can be maintained in the bodies of patients long enough to kill all pathogens, including resistant mutants or to inhibit them so that body defenses can kill them.

2. Two antibiotics may be administered simultaneously so they can exert an additive effect (synergism). Ex. penicillin is often added to another antibiotic (the penicillin damages the cell wall, allowing better penetration by the other antibiotic). In the following example, another antibiotic is added to penicillin to increase its effectiveness. Augmentin = penicillin (amoxicillin) and clavulanic acid. Clavulanic acid binds to beta lactamases and prevents them from inactivating the amoxicillin.

3. Antibiotics can be restricted to essential uses only (ex. not for colds, etc.). In addition, the use of antibiotics in animal feeds could be banned.

F. Special Problems with Drug-Resistant Hospital Infections – Resistant organisms are found more often in hospitalized patients than among outpatients. Why?

2. Sick people live in close proximity.

3. Hospitalized patients tend to be more severely ill and many have lowered resistance to infections because of their illness or because they are taking immunosuppressant drugs.

4. Hospitals typically make intensive use of a variety of antibiotics (resistant strains readily emerge and spread among patients).

V. DETERMINING MICROBIAL SENSITIVITIES TO ANTIMICROBIAL AGENTS

A. The Disk Diffusion Method (Kirby-Bauer method) – A bacteria is uniformly spread over an agar plate. Filter paper disks are saturated with the drug and placed on the agar surface. Clear areas called zones of inhibition appear on the agar as round disks where the drugs inhibit the bacteria. Important to realize the results obtained in vitro (in the lab) often differ from those obtained in vivo (in a living organism). Metabolic processes in a living organism may inactivate or inhibit a drug. We’ll do this procedure in lab.

B. The Dilution Method – A constant quantity of microbial inoculum is introduced into a series of broth cultures containing decreasing concentrations of a drug. After incubation, the tubes are examined and the lowest concentration of the drug that prevents visible growth (indicated by turbidity) is noted. Advantage to using this method over the disk method: Samples from tubes that show no growth can be used to inoculate broth that contains no drug to see if the drug was bactericidal or bacteriostatic.

C. Serum Killing Power – Obtain patient’s blood sample while the patient is receiving an antibiotic. Bacteria are added to the patient’s serum (blood plasma minus clotting proteins). Growth (turbidity) after incubation indicates that the antibiotic is ineffective.

D. Automated Methods – Bacteria are added to wells in trays to which a variety of antimicrobial agents have been added. The trays are inserted into a machine that measures microbial growth. Advantages: efficient, fast, inexpensive, and allows physicians to prescribe an appropriate antibiotic early in an infection rather than prescribing a broad-spectrum antibiotic while awaiting lab results.