I Antimicrobial Chemotherapy
II History of Chemotherapy
III General Properties of Antimicrobial Agents
IV Mechanism of Action
V Antibacterial Agents
VI Antifungal and Antiparasitic Agents
VII Antiviral Agents
VIII Resistance of Microorganisms to Antibiotics
IX Side Effects of Antibiotic Treatment
X Determination of Microbial Sensitivities
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I Antimicrobial Chemotherapy
The first modern drugs to treat disease were discovered in the early 1900s. But it wasn’t until after the Second World War that using antibiotics became routine. It is only recently that heart disease, cancer etc have become significant. Before the 20th century the vast majority of people died of infectious disease and there were no effective cures in most cases. Basically you were on your own. Even if you recovered you might be permanently injured (for example toxins from streptococcal infections may damage the heart irreversibly.)
Despite the modern publicity given to antibiotics, they were not responsible for the abolition of the majority of infectious disease. Modern hygiene (clean water, flush toilets, soap, etc) greatly decreased the spread of infections. Vaccination (discovered by Jenner around 1800) also greatly reduced the frequency of many common infectious diseases such as smallpox and measles. By the time antibiotics were introduced in the mid-20th century, infectious disease had already become a minor cause of death in the industrial nations.
Definitions and Technical Terms:
Chemotherapy is often used to refer to the treatment of cancer by various drugs. However, strictly speaking, a chemotherapeutic agent is any chemical substance used in medical practice &emdash; including antibiotics, aspirin and anti-depressants.
Antibiotic means "against life". Originally it referred to a substance produced by one microorganism which inhibits the growth of other microorganisms. In practice it includes natural products that have been chemically modified (these are sometimes called semi-synthetic) and also compounds which are wholly artificial, such as the sulfonamide antibiotics.
As used today the term "antibiotic" refers to those chemical compounds that inhibit bacteria. Consequently antibiotics are used to treat diseases due to bacterial infection. Antibiotics do not inhibit viruses or fungi or protozoan parasites (e.g. malaria).
So we have the following terms:
Pathogen = microorganism that causes disease
Chemotherapy = treating disease with chemical agents
Chemotherapeutic agent = chemical compound used in medicine
Prophylaxis = giving drug to person at risk, to prevent infection
Antimicrobial agent = chemical compound that inhibits microorganisms
Antibiotic = chemical compound that inhibits bacteria
Antiviral agent = chemical compound that inhibits viruses
Antifungal agent = chemical compound that inhibits fungus
Narrow spectrum = only works on a few microbes
Broad spectrum = having a wide range of action
Note that some antibiotics kill bacteria. Others merely halt their growth.
Bactericidal = kills bacteria
Bacteriostatic = stops growth but does not kill
The endings "-cidal" and &emdash;static" can be added to microbi-, fungi- etc.
Drugs come from three sources:
Naturally occurring = made by some living organism
Molds - penicillin, cephalosporin Bacillus - bacitracin, tyrocidin, polymyxins Streptomyces &emdash; aminoglycosides, tetracyclines, chloramphenicol, erythromycin
Synthetic = made artificially - sulfonamides, quinolones, isoniazid
Semi-synthetic = natural compound that has been chemically modified in the laboratory
II History of Chemotherapy
Rudolf II ruled the Austro-Hungarian Empire from 1552 to 1612. He was a major patron of the arts and sciences and his palace was a center for alchemical research. Here is his physician’s remedy for plague:
"Desiccated toads and pulverized chickens. The menstrual blood of a young maiden. White arsenic, pearls and emeralds from the Orient. This concoction is to be baked into a toad cake and then worn next to the heart in an amulet."
Despite the bizarre superstitions and rituals that often accompany them, some traditional remedies do actually work. E.g.:
Compounds of heavy metals such as mercury or arsenic were used from time to time also. For example, arsenic compounds were often used to treat syphilis. These agents are highly toxic but before the antibiotic age people who were sick didn't have much choice.
The first modern antibacterial drug was salvarsan, which was discovered by Paul Ehrlich in 1910. Salvarsan contains arsenic, whose compounds are usually rather toxic. Ehrlich deliberately set out to find an arsenic derivative that was still highly effective against syphilis but less toxic to humans.
The sulfonamides or sulfa drugs were discovered in the mid-1930's by Gerhard Domagk. Sulfonamide compounds were originally invented for use as dyes. It turned out by chance that the sulfonamide part prevented bacterial growth.
Penicillin was discovered by Alexander Fleming in the late 1920s but was not purified until the 1940s. Motivated by World War II, Chain and Florey purified penicillin and developed methods for large scale production. Most of the penicillin produced was used by the military and only after 1945 did much penicillin become available for civilians.
[Some of the work was actually done at the Department of Agriculture's Northern Regional Research Laboratory in Peoria.]
The next antibiotic was streptomycin, made by soil bacteria known as Streptomycetes and discovered by Selman Waksman in the 1940s. It was Waksman who first defined the term "antibiotic".
After this, researchers, especially in pharmaceutical companies, rushed to screen thousands of molds and other microorganisms and found a steady stream of novel antibiotics.
III General Properties of Antimicrobial Agents
1. Ideal Antimicrobial Agents
Bacterial Factors:
1) It inhibits the pathogen effectively at a relatively low concentration.
2) Microorganisms don't become resistant easily.
3) Better if it kills the pathogen rather than just stopping growth
Clinical or Human Factors:
1) Soluble in body fluids - it has to get where the microorganisms are. On the other hand it must not bind tightly to human serum proteins.
2) Selective toxicity &emdash; it must be less toxic to the patient than to the pathogen.
3) Toxicity not easily altered by other factors. Examples are interactions with foods, or other drugs or the effect of clinical conditions such as diabetes.
4) Non-allergenic (i.e. does not trigger an immune reaction).
5. Stable in the body (both degradation and excretion are involved).
7. Stable during storage, preferably at room temperature.
8. Affordable.
2. Selective Toxicity
The basic idea behind antibiotics is to poison the invading microorganism without killing the patient. This is known as selective toxicity. The concept of the "magic bullet" refers to a drug that cures the disease completely and has absolutely no harmful side effects.
Selective toxicity = drug is more toxic to the parasite (infecting microorganism) than to the host (patient).
It is relatively easy to find compounds that will poison plants but not animals (or vice versa). It is more difficult to find chemicals that will poison insects but not people. It is much harder still to find something that kills mice and rats but leaves your cats and dogs unharmed. The more closely related two organisms are, the more difficult it is to selectively poison one of them without damaging the other.
Even in the case of antibacterial agents, many are harmful to humans. There is a vast number of antibiotics but relatively few that are really useful for clinical treatment.
3. Spectrum of Activity
Broad spectrum antibiotics target a wide range of bacteria.
Narrow spectrum antibiotics target a narrow range of bacteria.
Which should you use? If you know what the infecting microorganism is it is better to use a narrow spectrum antibiotic. If the infectious agent is not fully identified then you should use a broad spectrum antibiotic.
Broad spectrum agents will also inhibit the natural bacterial inhabitants ("normal microflora") of your intestines, skin etc.
Using broad spectrum antibiotics also increases the possibility that drug resistance will emerge and spread.
Examples of broad spectrum antibiotics:
Tetracyclines
Sulfonamides
Examples of narrow spectrum antibiotics:
Polymixins &emdash; only against gram-negative bacteria
Erythromycin &emdash; only against gram-positive bacteria
Isoniazid &emdash; only against Mycobacteria (tuberculosis)
There are two important large families of antibiotics. Each contains many individual antibiotics. In both cases, some family members are broad range while others are narrower.
Aminoglycosides - this family includes streptomycin, kanamycin, tobramycin, gentamycin etc.
Beta-lactam antibiotics &emdash; includes both penicillins and cephalosporins.
IV Mechanism of Action
Antiseptics and disinfectants are designed to sterilize non-living objects by killing microorganisms. So it doesn’t really matter if antiseptics and disinfectants are toxic.
Antibiotics must cure infections &emdash; i.e. must inhibit microorganisms that have entered a person or animal. So antibiotics must avoid killing the patient. This means they cannot just denature proteins or disrupt lipids like disinfectants. Therefore antibiotics have a specific target.
Note: Some relatively toxic antibiotics (e.g. polymixin) may be used to treat shallow cuts or surface infections. These can be more toxic than something you would swallow, but cannot be too poisonous.
The ideal target is something that exists in a bacterial cell but is absent from an animal cell. Less ideal is a shared structure or pathway that differs significantly in its details.
1) Bacterial Cell Wall - Peptidoglycan.
This structure is totally absent in animal cells. It is the best target in bacteria. Bacterial cell walls are made of peptidoglycan. This polymer is not found in any other type of organism &emdash; including plants or fungi (which also have cell walls but made of different material). The interior of bacteria is at a higher osmotic pressure than animal tissues or most culture media. When bacterial cell walls are disrupted, the bacteria take up water and burst.
Beta-lactam antibiotics (penicillins and cephalosporins) prevent the cross-linking of peptidoglycan. The Beta-lactam ring kills the enzymes that make the cross-links. So the cell wall falls apart.
Notes: Mycoplasmas are bacteria without cell walls so obviously the beta-lactam antibiotics will not work on them.
Gram negative bacteria have an outer membrane that keeps many antibiotics at bay. Many penicillins do not get into gram negative bacteria very well - other penicillins and some cephalosporins do better.
2) Nucleic Acid Synthesis
All organisms have both DNA and RNA. However, the enzymes that work with DNA and RNA are sometimes radically different in different types of organism.
DNA packaging is very different in bacteria and eukaryotes. Bacteria supercoil DNA using DNA gyrase whereas eukaryotes coil DNA around histone proteins. Quinolones (e.g., nalidixic acid, ciprofloxacin) target DNA gyrase.
DNA is transcribed into RNA by RNA polymerase in all cells. However, the RNA polymerase of bacteria is very different from that of higher organisms. Bacterial RNA polymerase is inhibited by rifampin (rifamycin).
3) Metabolic Analogs
Anti-metabolites are compounds that mimic natural metabolites. They may block important pathways (by blocking an enzyme) or be incorporated instead of the natural metabolite.
The best examples of blocking an enzyme are the sulfonamides and trimethoprim, which block two enzymes in the same pathway.
Sulfonamides compete with para-aminobenzoic acid for the active site of the enzyme which makes dihydrofolic acid. Trimethoprim blocks the conversion of dihydrofolic acid to tetrahydrofolic acid (THFA). This acts as a cofactor in the synthesis of purines, thymine and some amino acids. So, overall, sulfonamides and trimethoprim block DNA synthesis.
[Humans need folic acid but do not have the pathway for making it. So mammals must take in folic acid in their diet. Therefore sulfonamides do not prevent human cells getting folic acid. In contrast most bacteria can't take in folic acid; instead they must make it.]
Some anti-metabolites mimic the bases found in DNA or RNA. These may get incorporated and damage the DNA or RNA. In practice these tend to be rather toxic. However they are used against viruses and in cancer therapy because both viruses and cancer cells are multiplying faster than human cells. Because the viruses and cancer cells are making DNA faster they will be hurt most.
4) Protein Synthesis
Bacteria and higher organisms both make protein on ribosomes, but these are different. Bacterial 70S ribosomes are a good target.
Tetracycline, chloramphenicol, aminoglycosides and erythromycin all attack the bacterial ribosome.
Notes: Eukaryotic cells do have 70S ribosomes in their mitochondria. Although these are related, they are not identical to bacterial ribosomes. Also, it takes higher concentrations of antibiotics to get into the mitochondria.
5) Disruption of Cell Membranes
Although all cell membranes are similar there are some differences. For example polymixin damages bacterial membranes, especially the outer membrane of gram negative bacteria. Not surprisingly it is relatively toxic as animal cells also have membranes.
V Antibacterial Agents
1. Inhibitors of Bacterial Cell Wall Synthesis
These all interfere with peptidoglycan synthesis.
Beta-Lactam Antibiotics - This family includes the penicillins, the cephalosporins and some newer groups like the carbapenems.
Penicillins (made by molds Penicillium notatum or Penicillium chrysogenum).
Natural (Penicillin G and V) and semi-synthetic &emdash; in which the original side chain is replaced by a chemically synthesized group.
The original penicillin, PenG, is highly active against G+ bacteria. However, it is sensitive to stomach acid, sensitive to beta-lactamases and does not get into G- bacteria very well due to the outer membrane. [PenV is similar but not acid sensitive so it can be given by mouth.]
Semi-synthetic penicillins involve a choice:
Resistant to beta lactamases (and acid) e.g. methicillin, cloxacillin and nafcillin. These are almost inactive against G- as they cannot cross the outer membrane.
Get into G- bacteria much better e.g. ampicillin, carbenicillin and amoxicillin. However these are broken down very well by beta lactamases.
Cephalosporins (made by mold Cephalosporium)
Similar structure to penicillins and therefore also destroyed by beta lactamases.
All of the clinically useful cephalosporins are semi-synthetic. Cephalosporins have two side chains so the number of possible alternative structures is much higher than for penicillins. An enormous number have been made and they are divided into "generations". Some are both moderately resistant to beta lactamases and are effective against both G+ and G- bacteria. Often used to treat infections resistant to other antibiotics. They are rather expensive.
Other Cell Wall Antibiotics:
Bacitracin - A small circular polypeptide made by Bacillus licheniformis. Too toxic to be taken internally but can be used topically.
Vancomycin - Made by Streptomyces (filamentous soil bacteria). Too large to cross the outer membrane of G- and so only works against G+. Used against penicillin resistant Staphylococcus. Moderate toxicity, must be injected due to poor intestinal absorption.
2. Disrupters of Cell Membranes
Few are important as antibacterials. They tend to be toxic since animal cells have membranes too. Some are important as antifungal agents.
Polymyxins (from Bacillus polymyxa)
Used against G- and mostly used topically because of their toxicity.
Tyrocidins (from Bacillus brevis) polypeptides
Used against G+, only used topically because of their toxicity.
3. Inhibitors of Protein Synthesis
All are targeted against some part of the 70S ribosome. Some prevent the start of protein synthesis (on 30S subunit) but don't affect the synthesis of a chain that is already started (streptomycin) others inhibit the addition of amino acids to a growing chain (tetracycline 50S, chloramphenicol & erythromycin 30S).
Streptomycin (from Streptomyces)
The next antibiotic to be discovered after penicillin. Penicillin worked well with G+ but poorly against many G- bacteria. Streptomycin and other members of the aminoglycoside family (neomycin, kanamycin, gentamycin, amikacin, tobramycin etc) work well against G- and Mycobacteria.
Aminoglycosides bind to 30S subunit of ribosome and prevent initiation step in protein synthesis. In addition aminoglycosides (including streptomycin) distort the shape of the 30S subunit and cause misreading.
Streptomycin is not used much any more. Partly toxicity &emdash; it may damage the kidneys and the auditory nerve (resulting in ringing in the ears and dizziness). Also, many microorganisms are now resistant. Streptomycin is mostly used in combination with other drugs, for example for tuberculosis with isoniazid, or for tularemia or plague with tetracycline. Other inhibitors of protein synthesis that are used more often now:
Erythromycin (from Streptomyces)
Fewer toxicity problems than most others. Effective against G+ but does not get into G- very well. Particularly important in treating Legionella, mycoplasmas and some chlamydias. Erythromycin binds to the 23S rRNA; resistance occurs when 23S rRNA is methylated. Resistance genes are carried on plasmids.
Tetracycline (from Streptomyces)
Broad spectrum, acts on both G+ and G- as well as mycoplasmas, chlamydias, and rickettsias. Kills most of normal intestinal bacteria and may cause gastrointestinal problems. Moderate toxicity. Has the strange problem of permanently staining the teeth of young children and this can even happen to a fetus if tetracycline is taken by a pregnant woman.
Chloramphenicol (from Streptomyces)
Originally isolated from Streptomyces, but nowadays chemically synthesized. Broad spectrum like tetracyclines, but more toxic. Damages bone marrow in occasional individuals who develop serious anemia which may be fatal. Rarely the drug of first choice. Used principally in very serious infections for which there isn't a better choice: typhoid fever, brain abscesses, some kinds of meningitis, severe rickettsial infections. Unusual as it is also effective on yeasts.
4. Inhibitors of Nucleic Acid Synthesis
Rifampin (from Streptomyces)
Rifampin is a chemically modified member of rifamycin family. Binds to RNA polymerase and inhibits transcription. Doesn't enter G- very well.
Used for meningococci, for tuberculosis in combination with isonizaid and for leprosy with dapsone (a sulfone). It interacts with some other medications and sometimes causes liver damage.
Quinolones (artificially synthesized)
Inhibit an enzyme needed for bacterial DNA replication - DNA gyrase. This enzyme is needed for unwinding DNA before replication. Eukaryotes do not have DNA gyrase as they compress their DNA by coiling it around histones instead of supercoiling like bacteria. Broad spectrum.
Nalidixic acid was the first quinolone to be synthesized. Norfloxacin, ciprofloxacin etc are more recent quinolones.
Problem: interferes with the development of cartilage and therefore limited to adults (excluding pregnant women).
5. Antimetabolites
Sulfonamides (artificially synthesized)
Mimics para-aminobenzoic acid and inhibits folic acid synthesis. Still used sometimes for urinary tract infections and also if there is an allergy to penicillin. Sometimes used along with trimethoprim which is also an anti-metabolite in the same pathway (synthesis of folic acid) but at the next step. Cotrimioxazole = trimethoprim and sulfamethoxazole mixture. When used together only about 1/10 concentration is needed.
Problems: allergies, toxicity; because they have been used for such a long time resistance is common.
Sulfa drugs are listed in textbook under inhibitors of nucleic acid synthesis because they inhibit the production of folic acid, which is a coenzyme necessary for the synthesis of the nitrogen bases needed for DNA and RNA. Folic acid is also needed for other reactions (1 carbon transfers) but some of the other end products may be supplied to invading bacteria by mammalian cells (methionine and some vitamins). So in infections the main effect is inability to make nitrogen bases.
Isoniazid (artificially synthesized)
Used only against tuberculosis in combination with other antibiotics. Interferes with action of two cofactors/vitamins &emdash; nicotinamide and pyridoxal. As a result it interferes with the synthesis of mycolic acids which part of the unusual cell wall of mycobacteria.
It enters mammalian cells readily. This is important because M. tuberculosis is mostly intracellular. Resistance develops easily and so it is usually given in combination with another drug.
VI Antifungal and Antiparasitic Agents
Fungi and parasites are eukaryotes and so they are more closely related to animal cells than bacteria are. Therefore they are more difficult to treat. Relatively few choices compared to antibacterial agents and many are toxic, especially if taken internally. Toxicity ranges from uncomfortable skin irritations to serious liver and kidney damage and anemia. An agent that works on one kind of fungus may not work on another. In particular one that works on a fungus causing a skin infection may not work well on fungi causing systemic infections.
In addition to "natural" infections there is the problem of superinfection &emdash; when fungi colonize the intestines after the normal flora has been wiped out with antibacterial drugs.
1. Antifungal Agents
Imidazoles and Triazoles (artificially synthesized)
Clotrimazole, fluconazole and others. Many available without prescription. Used for fungal skin infections and vaginal yeast infection e.g. Monistat. Disrupt synthesis of membrane sterols. Therefore relatively toxic, mostly limited to surface infections.
Amphotericin B and Nystatin(from Streptomyces)
Bind to a sterol (ergosterol) that is only found in fungi plus a few algae and protozoans but is not present in humans.
Amphotericin B is used for systemic infections, although it is toxic &emdash; little alternative in some cases. Nystatin is used topically for Candida and for intestinal infections (not absorbed from intestine).
2. Antiparasitic Agents
Quinine (from cinchona bark) and artificial quinolines (chloroquine, mefloquine etc) are used to treat malaria.
Metronidazole for amebic dysentery and some other anaerobic protozoans.
VII Antiviral Agents
Even less choice with antivirals. Since most viral diseases are self limited and not fatal the most effective health care for viruses is immunization and good nursing.
Base Analogs (artificially synthesized)
Base analogs are synthetic analogs of DNA and RNA bases and therefore interfere with nucleic acid synthesis. Therefore most are toxic and some are too toxic for anything but topical use. Acyclovir (acycloguanosine) has been used for herpesviruses; probably the antiviral in most general use.
Problems: none work with all kinds of viruses and most are only effective with a very few or maybe only one kind of virus, such as herpesviruses, or influenza or smallpox. Effectiveness is often limited - that is they decrease replication and reduce numbers of the virus but they never completely eradicate the virus.
Interferons (human proteins)
Interferons are proteins produced by animal cells when they are infected with viruses &emdash; they are part of the natural defense against viruses. Still experimental. Interferons are being made and modified by genetic engineering techniques. Whether or not they will be used for viral infections remains to be seen.
VIII Resistance of Microorganisms to Antibiotics
1. Types of Resistance
Many bacteria can develop resistance to antibiotics. This is becoming a serious problem nowadays as many pathogens are becoming resistant to several different antibiotics. Resistance may be genetic (i.e. inherited) or non-genetic.
Non-genetic "resistance" is not due to a genuine change in resistance of the pathogen but more in the nature of temporary protection. Examples:
a) Non-growing cells. Some antibiotics only work on growing and dividing bacteria. Sometimes bacteria may stay alive but stop dividing. They will then be resistant to antibiotics that inhibit cell wall synthesis, like penicillin.
b) Some bacteria hide themselves deep in tissues and/or inside human cells where they are difficult to reach. Others may secrete layers of protective material. Mycobacterium tuberculosis hides away like this and in addition, some cells, called persisters, become inactive.
c) Some bacteria lose their cell walls and become L-forms. Consequently they become resistant to antibiotics that target cell walls. This is only possible when the bacteria are osmotically protected inside human tissues. L-forms quite often revert to normal cells and become sensitive again.
Genetic resistance &emdash; two subclasses:
I) Due to mutation of a gene on the bacterial chromosome. Will be inherited by the direct descendents of the cell that mutated.
II) May be acquired from another cell by movement of DNA between bacteria. Most often the resistance genes are carried on a plasmid. These may be transferred by conjugation, transduction or transformation. In nature plasmids move from cell to cell by conjugation. They may also be carried inside virus particles &emdash; transduction.
First-line, Second-line and Third-line Drugs
These terms refer to "lines of defense". If a pathogenic microorganism gains resistance to the drug of first choice (first line of defense) then another antibiotic must be used. If the pathogen becomes resistant to the second-line drug then a third-line drug must be found. And so on. Some pathogens have become resistant to so many antibiotics that it is getting difficult or impossible to treat them effectively.
2. Mechanisms of Resistance
(a1) Target Alteration. The target is altered so that the drug no longer affects it. Example: Adding a methyl group to 23S ribosomal RNA prevents erythromycin binding to 23S rRNA and so the cell gains resistance.
(a2) Target Replacement. The sensitive target remains in the cell, but a new component is made that can perform the same role and that is resistant to the antibiotic. Example: Sulfonamide resistance is usually due to a new resistant enzyme made from a gene carried on a plasmid.
(b) Changes in Transport. The cells may change so that the drug cannot get in as well. In other cases the drug may be actively expelled.
Example: Tetracycline is actively expelled by the tetracycline resistance protein. The gene for this protein is carried on many plasmids, which therefore make cells resistant to tetracycline.
(c) Inactivation of the Antibiotic. A cell acquires the gene necessary to make an enzyme that destroys the antibiotic.
Examples: Beta lactamases destroy penicillins and cephalosporins by splitting them. Some other antibiotics are inactivated by adding phosphate groups or acetyl groups, which block their binding. Chloramphenicol acetyl-transferase inactivates chloramphenicol. Many aminoglycosides are inactivated by adding phosphate or acetyl groups.
3. Interactions Between Drugs
Cross resistance is when a change that makes a cell resistant to one antibiotic also makes it resistant to another.
Example: Some beta-lactamases can split both penicillins and cephalosporins. Some changes in cell permeability reduce the entry of both chloramphenicol and penicillin.
Synergy is when two antibiotics help each other, e.g. a mixture of penicillin and streptomycin. The damaged cell wall (due to penicillin) allows the other antibiotic to enter the cell more easily.
Antagonism is when two antibiotics hinder each other. A bacteriostatic drug, which stops growth (e.g. tetracycline), may actually protect a cell against a drug like penicillin that only works on dividing cells.
4. Limiting Drug Resistance
Some Major Problems:
a) Assorted surveys suggest that one third to one half of drug prescriptions are not really needed.
b) Antibiotics are often included in animal feeds to increase growth yield. The amounts added are relatively low and provide an ideal situation for bacteria to acquire resistance. Many European countries have banned this.
c) Many patients stop taking antibiotics when they begin to fell a bit better. This allows surviving bacteria to make a come-back. In particular, those that have gained low level resistance may divide and then develop higher resistance. It is important to give enough antibiotics to wipe out all the pathogens and to complete the course of treatment. This is especially a problem in poor countries where they skimp on antibiotics to save money. It is also especially bad for anti-viral therapy where many of the drugs have unpleasant side effects.
Some Solutions:
Treat the infection with two antibiotics simultaneously. The idea is that it is extremely unlikely for resistance to develop to two different antibiotics at once. This is done frequently with tuberculosis. Mycobacterium tuberculosis is difficult to eradicate and treatment takes months.
Combine an antibiotic with an agent to overcome resistance. Clavulanic acid binds to beta-lactamases and blocks their action. So giving penicillin together with clavulanic acid overcomes resistance.
Special Problems with Resistant Hospital Infections
The problem of resistant organisms is especially serious in hospitals. They provide ideal situations that select for resistant organisms. This is because there are large numbers of sick people concentrated close together. Many of them carry infectious microorganisms and are taking antibiotics. Others are seriously weakened and so are highly susceptible to new infections. Especially in danger are patients with immune suppression &emdash; whether due to AIDS or to therapy.
It is very difficult to keep microorganisms from spreading from a patient into the environment, although good aseptic procedures will keep this to a minimum. But when they do spread, other patients are good candidates to pick up whatever is around. What can be done? Use antibiotics only when necessary, take the whole amount prescribed, and use a narrow spectrum drug whenever possible. And of course, good aseptic technique.
IX Side Effects of Antibiotic Treatment
1) Toxicity
Some antibiotics damage host cells as well as the infectious agent.
2) Allergy
Not the same as toxicity. May be mild or serious. Allergy is a reaction mounted by the immune system against a foreign substance. Allergic reactions do not depend on the action of an antibiotic, but occur merely because it is regarded as foreign. Allergies will be covered later.
3) Disruption of Natural Microflora
The intestines and body surfaces are normally inhabited by a variety of bacteria. Some are merely harmless, others are positively useful. The natural inhabitants tend to help keep out harmful invaders (both other bacteria and fungi). Some bacteria also make vitamins and/or help digest certain foods.
Extensive use of broad spectrum antibiotics may kill off a lot of the normal flora and leave only a limited normal flora and antibiotic resistant strains. Since fungi are not affected by antibacterial agents, this provides a good opportunity for pathogenic fungus (e.g. Candida) to get established.
X Determination of Microbial Sensitivities
1. Disk Diffusion Method
Spread test organism on agar so as to give a lawn of growth after incubation. Before incubating, place a filter paper circle soaked in antibiotic solution onto the agar. The drug will diffuse out and, if it is effective, will give a clear zone where growth is prevented. The size of the inhibition zone indicates the level of sensitivity. Used routinely in hospital labs.
2. Dilution Methods
A more complicated method but it gives more information.
A series of tubes are set up with dilutions of an antibiotic or other drug. A standard inoculum is added and the tubes incubated, then examined for visible growth. The lowest concentration of the drug that inhibits growth is the minimum inhibitory concentration (MIC).
If you want to know whether the organism was killed or just inhibited, take a sample from each of the no-growth tubes. Use it to inoculate fresh tubes that contain media only, no drug. Samples from tubes where the organisms were killed will show no growth in the new tubes. If the organisms were inhibited but not killed, they will grow when taken out of the drug.
3. Serum Killing Power
A clinical check rather than a microbiological sensitivity test. Blood is taken from a patient receiving an antibiotic and allowed to clot; the liquid remaining is the serum. It is inoculated with the infecting organism and incubated. If no growth occurs, that is evidence that the concentration of antibiotic in the blood is adequate.
4. Automated Methods
Several different automated methods are available both to identify microorganisms and to measure growth in the presence of antibiotics. Some measure turbidity, some measure release of CO2, and some measure DNA hybridization.
They are faster than conventional methods, which is an advantage in several ways. Perhaps most important, a faster answer about the organism’s identity and antibiotic sensitivities allows the physician to prescribe the best choice antibiotic.
5. Choice of Drugs
Toxic dose = dose that harms the host (patient).
Therapeutic dose = dose that cures the disease.
Chemotherapeutic index = maximum tolerable dose divided by the minimum dose that will cure the disease. The higher the chemotherapeutic index the better.