Antibiotics: Antibacterial Agents
Humans, and our domestic animals, can serve as hosts to a wide variety of disease-causing organisms (pathogens):
- bacteria
- viruses
- fungi
- protozoans
- helminths (worms).
This page will examine only those chemical agents that are used to combat bacterial pathogens.
The Problem
There are many chemicals that are lethal to bacteria - cyanide does a good job - but they cannot be used to cure infections because they are lethal to the host as well. The problem, then, is to find substances that attack a metabolic pathway found in the bacterium but not in the host. This is not an insurmountable problem for bacterial pathogens because prokaryotes differ in many respects from eukaryotes.
The Solution
- Natural products.
A number of natural products, penicillin for example, have been discovered that are antibiotics suitable for therapy.
They were originally discovered as secretions of fungi or soil bacteria. Soils are complex ecosystems, and it is not surprising that its inhabitants have evolved chemical defenses against each other.
The photo (courtesy of Merck & Co., Inc.) shows how the growth of bacteria on the agar in a culture dish has been inhibited by the three circular colonies of the fungus Penicillium notatum. The antibiotic penicillin, diffusing outward from the colonies, is responsible for this effect. Today, penicillin is made from cultures of Penicillium chrysogenum that has been specially adapted for high yields. |
- Semi-synthetic products.
These are natural products that have been chemically modified in the laboratory (and pharmaceutical facility) to
- improve the efficacy of the natural product
- reduce its side effects
- circumvent developing resistance by the targeted bacteria
- expand the range of bacteria that can be treated with it
- Completely synthetic products.
The sulfa drugs are examples.
Sulfa Drugs
Sulfanilamide was the first antibacterial agent. Many other sulfa drugs (such as sulfamethoxazole) have since come into use.
The Chink in the Armor
Both bacteria and their human hosts require folic acid for
However,
- bacteria synthesize their folic acid starting with para-aminobenzoic acid (PABA), while
- we must ingest our folic acid already formed; that is, for us it is a vitamin.
Sulfanilamide, and the other sulfa drugs, are analogs of PABA; they compete with PABA and, when chosen, block the synthesis of folic acid. Mammals ignore PABA and its analogs and thus can tolerate sulfa drugs.
Folic Acid Analogs
These synthetic molecules block the final step in the conversion of PABA to folic acid so they, too, block nucleotide and protein synthesis in bacteria but not in mammals.
Trimethoprim is one of several in current use. These folic acid analogs are often used in combination with a sulfa drug.
The beta-lactams get their name from the characteristic ring structure - shown here in blue - that they all share. (The green arrow shows the bond that is broken by the beta-lactamases that are synthesized by many penicillin-resistant bacteria.)
They include the
- penicillins such as
- penicillin G (a natural product) produced by the fungus Penicillium chrysogenum
- ampicillin (a semi-synthetic)
- amoxicillin (semi-synthetic)
- cephalosporins
There are over two dozen of them in current use. Most are semi-synthetics derived from the secretion of the mold Cephalosporium.
Some examples:
- cephamycin
- cefonicid
- cefotetan
- cephalothin
The beta-lactams all work by interfering with the synthesis of the bacterial cell wall - a structure that is not found in eukaryotes.
The walls of bacteria are made of a complex polymeric material called peptidoglycan. As its name suggests, it contains both amino acids and sugars.
The sugars are of two kinds
- a nitrogen-containing hexose called N-acetylglucosamine (NAG) and its close relative
- N-acetylmuramic acid (NAM).
These two form a linear polymer of NAG alternating with NAM. The links are between the #1 and #4 carbons and are oriented in the same way they are in cellulose. (This is the linkage attacked by lysozyme.)
Side chains containing 4 or 5 amino acids are attached to each NAM. These form covalent bonds with amino acids in adjacent chains. The bonds may
- be direct to the next chain or
- include additional peptide cross bridges (e.g., 5 glycine residues) which
- extend to chains in the same plane (shown here) as well as to chains above and below.
This elaborate, covalently cross-linked structure provides the great strength of the cell wall. It also leads to the remarkable conclusion that the bacterial cell wall meets the definition of a single molecule!
The beta-lactam antibiotics bind to and inhibit enzymes needed for the synthesis of the peptidoglycan wall. While they have little effect on resting bacteria, they are lethal to dividing bacteria as defective walls cannot protect the organism form bursting in hypotonic surroundings.
These are products of actinomycetes (soil bacteria) or semi-synthetic derivatives of the natural products.
Examples are:
The Chink in the Armor = the bacterial ribosome
The 70S bacterial ribosome differs in several ways from the 80S eukaryotic ribosome. The aminoglycosides bind to the 30S subunit of the bacterial ribosome and
- interfere with the formation of the initiation complex
- cause misreading of the mRNA.
Although the eukaryotic ribosome in the cytosol is unaffected by these drugs, ribosomes in the mitochondria are 70S and sensitive to their effects.
These are natural products derived from soil bacteria called actinomycetes or their semi-synthetic derivatives.
Examples:
- chlortetracycline (trade name = "aureomycin")
- oxytetracycline (trade name = "terramycin")
- doxycycline
The Chink in the Armor = the bacterial ribosome
Tetracyclines bind to the 30S subunit of the bacterial ribosome. They prevent the transfer of activated amino acids to the ribosome so protein synthesis is halted.
Macrolides are products of actinomycetes (soil bacteria) or semi-synthetic derivatives of them. They bind the 50S subunit of the bacterial ribosome thus inhibiting protein synthesis.
Erythromycin is a widely-prescribed macrolide.
These relatives of the macrolides also inhibit bacterial protein synthesis by binding to the 50S ribosomal subunit.
Quinupristin and dalfopristin are examples. As of 1 October 1999, they will be sold as a mixture under the tradename Synercid. Combined, they show great promise in treating certain infections resistant to vancomycin - currently the antibiotic of last resort for some hospital-acquired infections.
Ciprofloxacin, norfloxacin and ofloxacin are examples.
The Chink in the Armor = DNA gyrase
The quinolones block the action of the bacterial enzyme that relieves the coils that form in DNA when the helix is being opened in preparation for
Eukaryotes have no DNA gyrase.
The most common of these are the polymixins.
They behave as detergents, increasing the permeability of the membranes that encase bacteria and causing the contents of the bacterial cell to leak out.
This semi-synthetic antibiotic binds to the bacterial RNA polymerase and prevents it from carrying out its role in transcription. Its affinity for the equivalent eukaryotic enzyme is much lower.
This antibiotic blocks the action of the bacterial isoleucine tRNA synthetase, the enzyme responsible for attaching the amino acid isoleucine (Ile) to its tRNA in preparation for protein synthesis, so protein synthesis is inhibited. It spares the equivalent eukaryotic enzyme.
Cycloserine inhibits synthesis of the bacterial cell wall but by a different mechanism than the beta-lactam antibiotics discussed above. Cycloserine is an analog of D-alanine and blocks the incorporation of D-alanine into the peptide bridges in the bacterial cell wall (look back). It is derived from an actinomycete.
This product of a streptomyces achieves its effect by interfering with the 30S subunit of the bacterial ribosome. It is particularly effective against the gonococcus, the bacterium that causes the sexually-transmitted disease (STD) gonorrhea.
Glycopeptides also interfere with the synthesis of the bacterial cell wall but by a different mechanism than the beta-lactams.
Vancomycin is a widely-used glycopeptide in the U.S. It binds to the D-alanines on the precursors of the peptidoglycan cross bridges preventing their cross-linking (look back). It has become the antibiotic of last resort as resistance to the other antibiotics has become more and more common.
These new antibiotics, e.g., linezolid (tradename = Zyvox), are still in clinical trials. Early results indicate that they will be effective against many bacteria that have developed resistance to the older antibiotics.
These antibiotics attack a previously-unexploited chink in the bacterium's armor: the proper assembly of the complex of fMet-tRNA with the messenger RNA (mRNA) and the 30S ribosomal subunit. Eukaryotes do not begin translation with fMET.
Resistance to Antibiotics
None of the antibiotics discussed above is effective against all bacterial pathogens.
Intrinsic resistance
Some bacteria are intrinsically resistant to certain of the antibiotics. Example: gram-positive bacteria are much less susceptible to polymixins than gram-negative bacteria. [The "gram" designations refer to the behavior of the bacteria when stained with the Gram stain; this behavior is a reflection of the very different organization of their cell walls.]
Acquired resistance
Many bacteria acquire resistance to one or more of the antibiotics to which they were formerly susceptible.
Example: In the U.S. in the decade from 1985-1995, resistance of Shigella (which causes gastrointestinal illness) to ampicillin grew from 32% to 67%. And, while only 7% of these isolates were resistant to the combination of sulfamethoxazole and trimethoprim at the start of the decade, that figure had grown to 35% by the end of the decade.
Bacteria develop resistance by acquiring genes encoding proteins that protect them from the effects of the antibiotic. In some cases the genes arise by mutation; in others, they are acquired from other bacteria that are already resistant to the antibiotic. The genes are often found on plasmids which spread easily from one bacterium to another - even from one species of bacterium to another.
Examples:
- Synthesis of the enzyme penicillinase - or other beta-lactamases - provides protection from the beta-lactam antibiotics. These enzyme break the beta-lactam ring at the position shown with the green arrow in the diagram of penicillin G.
- Likewise synthesis of cephalosporinases defeats the cephalosporins.
- Some bacteria modify their DNA gyrase thus defeating quinolones.
- Some bacteria synthesize "pumps" in their plasma membrane through which they remove antibiotics like tetracyclines from the interior of the cell.
- Bacteria may methylate their ribosomes obscuring the target of antibiotics that ordinarily bind to and inactivate the ribosome.
- Bacteria may modify the structure of their peptidoglycan wall and thus avoid the inhibitory effects of antibiotics like cycloserine.
An alarming number of human pathogens have acquired genes to combat all the presently-used antibiotics except vancomycin and recently vancomycin-resistant bacteria have appeared. These multidrug-resistant strains are particularly common in hospitals where antibiotic use is heavy, and the patients often have weakened immune systems.
Measuring Antibiotic Resistance
The figure illustrates the simplest method of the several available for measuring antibiotic resistance.
- A suspension of the bacteria to be tested (e.g. cultured from the infected patient) is spread over the surface of a petri dish containing a solid culture medium.
- Disks of several different antibiotics are pressed on the surface of the agar. The concentration of antibiotic in each type of disk is standardized.
- Incubate overnight.
- The bacteria will grown into a "lawn" except where an antibiotic to which they are sensitive has diffused out from its disk.
- Measure the diameter of any zones of inhibition that are formed.
What can you do to delay the spread of antibiotic resistance?
- Don't ask your doctor for an antibiotic to treat a viral disease (e.g., a cold) for which antibiotics are useless. (However, your doctor may prescribe an antibiotic if you are infected by an influenza virus - not to fight the virus but to protect you against a secondary bacterial infection of your damaged lungs.)
- Stay the course. Use all doses prescribed even though you are feeling better. This will minimize the opportunity to select for resistance among the bacteria that remain late in the infection.
- Don't save unused antibiotics for later self-medication.
Farmers can help as well by avoiding the use of antibiotics in their livestock that are similar to those used in humans. Antibiotics are widely used in healthy livestock to improve their growth rate (by an unknown mechanism).
An article in the 20 May 1999 issue of The New England Journal of Medicine documents the recent development of quinolone resistance in Campylobacter jejuni, the most frequent bacterial cause of gastroenteritis in humans. The rise coincides with the approval in 1995 of the use of quinolones by U. S. poultry farmers (chickens also become infected by C. jejuni).
Similar recent increases in quinolone-resistant C. jejuni have been reported in Holland and also in Spain (where as many as 50% of human infections are now caused by bacteria resistant to the antibiotic). In each country, the appearance of resistant strains followed the widespread introduction of quinolone treatment for animals.
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Drug companies - after many years of complacency - are now responding to the threat of antibiotic-resistant bacteria. Over a dozen new antibiotics are being developed and some have already reached clinical trials.
Many of these are semi-synthetic modifications of already-existing antibiotics, including new
- beta-lactams
- macrolides
- glycopeptides
- quinolones
- modifications of vancomycin
Others (e.g., linezolid) are entirely new, attacking previously-unexploited chinks in the bacterial armor.
28 September 1999