Prior to the development of effective modern antibacterials, serious systemic bacterial infections were as feared as is AIDS today.
Now we are faced with the major problem of increasing bacterial resistance to antibacterial drugs.
Prior to the discoveries outlined below, bacterial infections were major causes of death. Even relatively insignificant wounds could become sources of "blood poisoning" leading to death. Such infections are now viewed almost casually, except in patients with compromised body defenses.
One of the early pioneers in research on antiinfectives was a German scientist named PAUL EHRLICH. Ehrlich, who was active in the early 20th century, had access to many new compounds from the active German chemical industry. He studied the interactions of these chemicals with infectious protozoans and bacteria. He found that some of the chemicals bound preferentially to some protozoans and were effective as therapeutic agents, but the margin of safety was extremely narrow. He found no chemicals active against bacteria at concentrations that were non-toxic in infected animals, but his general experimental approach and concepts were useful to others who followed. Because the compounds he studied were "chemicals" with known structures, the therapy of infectious diseases became known as CHEMOTHERAPY.
Domagk -- 1930s: In the early 1930s, Domagk, who was also using chemicals from the German chemical industry, made a crucial discovery that led to the development of SULFONAMIDES as therapeutic agents. Domagk found that a chemical named PRONTOSIL was effective in treating experimental infections in vivo, but was not active against the same bacteria in vitro. Nonetheless, prontosil was safe enough to be used therapeutically and constituted the first clinically useful chemotherapeutic agent for bacterial infections. Shortly after its introduction, it was discovered that prontosil was hydrolyzed in vivo to release SULFANILAMIDE, the active ingredient. Thus, prontosil was a PRODRUG, an inactive compound that can be activated by the body.
SULFANILAMIDE, first used in 1936, was the grandparent of the SULFONAMIDE family of drugs that are still extremely useful today. Dramatic proof of the efficacy of this new agent was provided during an outbreak of meningococcal meningitis in the French Foreign Legion in Nigeria. While sulfanilamide was available, there was an 11% mortality rate. After the supply was exhausted, mortality climbed to 75%. Sulfanilamide and its relatives which soon followed were said to have "dethroned the captain of the men of death," such was their effectiveness in treating pneumonias.
PENICILLIN was first discovered by Fleming in 1929 when he found that a Penicillium mold inhibited the growth of bacteria in a petri dish. However, he failed to recognize the therapeutic potential of this and it remained for Florey, an Englishman, to first use Penicillin for therapy in 1940. It was, and is, one of the most active and safe antibacterials available. Because of their effectiveness and large therapeutic index, penicillin and many closely related derivatives, collectively known as the PENICILLINS, and the closely related CEPHALOSPORINS (discovered in the 1960s) are among the most important families of antibacterials available today.
STREPTOMYCIN, the first of the AMINOGLYCOSIDE antimicrobials, was discovered by Waksman in 1944. This was the first of many therapeutically active drugs found to occur naturally in various species of Streptomyces, fungi which are found in soils throughout the world. The aminoglycoside antibiotics are extremely important in clinical medicine, today.
Streptomycin was especially active against many gram negative bacteria which had been resistant to the action of penicillin. With Streptomycin for gram negative bacteria, penicillin for gram positive bacteria, and the sulfonamides for a broader spectrum, it was erroneously thought that bacterial diseases had been conquered for good. In fact, penicillin and streptomycin were so effective when used together that they were combined into a single dose form. Such dose forms are now illegal in human medicine and are frowned on in veterinary medicine for reasons that will become apparent later.
CHLORAMPHENICOL, discovered in 1947, was the first broad spectrum antibiotic (as opposed to antimicrobial). It was isolated from a Streptomyces organism. In addition to a wide spectrum of gram positive and negative bacteria, chloramphenicol is active against such organisms as Rickettsia. Its acute toxicity was so low and efficacy so great that it was used heavily, to the point of abuse. In the mid 1950s, it was discovered that chloramphenicol caused aplastic anemia and other blood dyscrasias in a small proportion of patients. This discovery drastically reduced its use and it is now recommended only for those cases where it is truely needed and other antimicrobials are likely to be ineffective. Chloramphenicol did not lead to the development of a large family of drugs, although a few derivatives are now available or being tested.
CHLORTETRACYCLINE, discovered in 1948 and also isolated from a Streptomyces organism, was the first of an important family of broad spectrum antibiotics. Their spectrum includes many gram positive and negative bacteria as well as Rickettsia and some protozoans. The tetracyclines are very important in current therapy.
Some commonly used terms are defined below. One should pay particular attention to the distinction betweem antimicrobial and antibiotic!
A CHEMOTHERAPEUTIC AGENT is any defined chemical (drug) used to treat disease caused by an invading organism, e.g., bacteria, virus, protozoan, or metazoan. Cancer cells are also considered as living, foreign cells. Chemotherapy is in contrast to PHARMACODYNAMIC therapy. The latter refers to the use of drugs that alter the rate of natural, ongoing reactions and processes in the patient's body.
ANTIBIOTIC refers to compounds isolated from one living organism that kill or inhibit the growth of other organisms. Antibiotics may have e.g., antibacterial, antifungal, antiviral, antiparasitic, or even anticancer activity. The term is loosely used as a synonym for more specific categories such as anticancer, antimicrobial, or antibacterial drug.
ANTISEPTICS are chemicals that are too toxic for systemic use, but that are used as topical antiinfectives DISINFECTANTS are antiinfectives that are so toxic that they are applied only to inanimate objects.
Antimicrobials are derived primarily from three major sources:
Penicillium spp. and Streptomyces spp. are major sources of antibiotics used therapeutically. One should note the preponderance of these as sources when studying the individual antibiotics. Bacillus spp. are the most notable bacterial group from which useful antibiotics have been derived. Synthetic antimicrobials, e.g., the sulfonamides, have always constituted an important source of antimicrobials. Semisynthetic antimicrobials are those derived from chemical modifications of naturally occurring antibiotics. This consitutes an ever more important group of antimicrobials as new drugs, with special properties, are developed.
Familiarity with the variety of schemes used for classifying antimicrobials provides a framework for remembering and using antimicrobials. Three classification schemes will be presented here as follows:
The fundamental and most frequent grouping of antimicrobials is based on their chemical structure. Each of the following groups has a structural component that defines the group. Addition or subtraction of chemical groups from the core structure leads to the various members of the group. The key groups to be studied in this course are the following:
[ Drug Listings ]MECHANISM OF ACTION classification schemes are seldom used in chemotherapeutic drug selection except in cancer therapy. Such schemes are primarily useful in understanding why some drugs have large margins of safety. It also helps one to understand why there may be overlaps in antibacterial action or resistance to a group of drugs. Finally, such schemes help one remember the more detailed mechanisms by which each of the drugs act. Most of the drugs act on one of the following processes:
For clinical purposes, bacteria are said to be resistant to an antimicrobial when they are insignificantly affected by concentrations of the drug that can be achieved at the site of the infection. As might be expected, achievable concentrations vary dramatically from place to place in the body.
Sensitivity of organisms to antimicrobials may be quantified by the minimum concentration required to inhibit their growth (minimum inhibitory concentration, MIC) or by the minimum concentration required to kill them within a specified period of time (minimum bactericidal concentration, MBC). Because they are easier to measure and apply to both bactericidal and bacteriostatic drugs, MICs are more frequently used. Tables of typical MICs for many bacterial species/antimicrobial pairs are widely available. When combined with knowledge of the time course of antimicrobial concentrations at various sites in the body, these MICs can be used to guide rational selection antimicrobials for particular infections. Application of this rational approach to selection is still developing and unexpected results do occur.
The clinical SPECTRUM of organisms is indicated by various terms. BROAD SPECTRUM includes drugs like the tetracyclines that have no strong predilection for gram positive or gram negative organisms and may even act against such organisms as Rickettsia or even protozoans. Few of the broad spectrum antimicrobials affect such a wide range of organisms. Other drugs, such as penicillin G, are primarily active against GRAM POSITIVE orgnisms. Gentamicin is an example of a drug that is especially used to treat infections by GRAM NEGATIVE organisms. Some drugs have such a special clinical niche that they are commonly referred to as a member of that niche despite the fact that they do have other actions. The PENICILLINASE resistant penicillins (e.g., methicillin) and EXTENDED SPECTRUM penicillins (e.g., ticarcillin) are examples. It is important to note that classifications based on spectrum do not imply that a drug will be active against all organisms in the class. Some organisms are notorious for the unpredictability of their response to an antimicrobial whereas others are highly predictable. This will be discussed in more detail below.
CIDAL versus STATIC actions of a drug are most important in the selection of drugs for animals with compromised body defenses and for providing initial guidance in the combination of antimicrobials for treating infections caused by single agents. The designation of a drug as either cidal or static is not absolute. It depends on such things as the drug concentration, bacterial species, phase of growth of the organism, and even the number of bacteria present. For example, low concentrations may be static whereas high concentrations may be cidal. An antimicrobial may be cidal versus one species and static for another. Rapidly growing organisms may be killed by a drug, whereas slowly growing ones may only be inhibited. Despite these caveats, the cidal/static nature of the drug is usually a consideration in drug selection.
Sensitivity of particular strains of organisms may or may not be highly predictable. This information can be used to predict the need for in vitro sensitivity testing in antimicrobial selection. The following organisms are known to be fairly predictable: penumococci, gonococci, meningococci, and Haemophilus influenzae [human pathogens]; Streptococcus agalactiae, Streptococcus equi, (any beta hemolytic streptococci), Actinomyces pyogenes [animal pathogens]. The following are fairly unpredictable and sensitivity testing is recommended: staphylococci, enterococci, Eschericia coli, Klebsiella, Pseudomonas aeruginosa, Pasteurella haemolytica, and Proteus spp.
Antimicrobials may be ineffective against bacteria because of resistance that is
Natural resistance is chromosomally mediated and is predictable; antimicrobials are not included in sensitivity tests on organisms known to be resistant to them.
Mutational resistance and secondary resistance are similar. Both may be the result of mutations, but secondary resistance occurs after therapy with the antimicrobial in question has begun. The clincally important difference between mutational and secondary resistance is that the use of the drug may cause secondary resistance whereas mutational resistance is pre-existing. Also highly relevant clinically, is the fact that antimicrobial therapy may SELECT for resistant organisms. Normal organisms will be inhibited or killed by the therapy allowing resistant ones to prosper due to lack of competition for food sources and advantageous environmental niches. In the absence of such SELECTION PRESSURE many organisms with mutations will be at competitive disadvantages with their peers and will disappear or be present in extremely low numbers in the population. THAT USE of ANTIMICROBIAL DRUGS CAUSE SELECTION PRESSURE FAVORING RESISTANT STRAINS IS AN EXTREMELY IMPORTANT CONSIDERATION IN THERAPY DECISIONS.
Drug resistance may be transferred from one organism to another. Transferrable drug resistance is plasmid-mediated, i.e., extrachromosomal DNA containing code for the mechanism of resistance is transferred from one organism to another. Key concerns with this type of resistance are that resistance to multiple antimicrobials may be contained in one plasmid, the bacteria acquiring the plasmid are not necessarily compromised as are those with mutational resistance, and the plasmid may be spontaneously lost. Selection pressure is important in the spread of plasmid-mediated resistance.
Conjugation is the most important of the mechanisms by which resistance is transferred, especially with gram negative organisms. A Resistance Transfer Factor (RTF) similar to the codon for the F-pilus is required. The actual information coding for the resistance is the Resistance Factor (RF, plasmid). This extrachromosomal DNA may have one, two, or many genes that code for resistance to one, two, or many antimicrobials. The possibility for transfer of resistance to several drugs at once makes this mechanism extremely important, both in public health and for the effectiveness of drugs in treating specific patients. Plasmids have been found in non-pathogenic organisms which can transfer them to pathogenic species. Thus, low level antimicrobial use can increase the expression of resistance in normal bacterial flora that can then transfer it to invading pathogens rendering many therapeutic agents useless.
Bacteriophage mediated transduction is also possible, e.g., in staphylococci. This is one of the major means by which molecular biologists transfer genetic material in both plant and animal genetic research.
Transposons are minute pieces of DNA that can be part of plasmids. These can then be translocated into other plasmids or into the normal genome of the bacteria leading to more permanent forms of resistance.
Basic biochemical mechanisms of resistance include:
It is important to realize that the means by which organisms become resistant, that is, how they acquire the ability to resist antimicrobials is different from the biochemical mechanism of the resistance itself. BE SURE TO NOTE THIS DIFFERENCE on examination questions. The following could be subclassified according to whether they are transferred by plasmids or are chromosomal mutation-induced. However, the two classes will be combined here.
INACTIVATION OF DRUG:Inactivation of the drug is typified by the enzymatic hydrolysis of the beta-lactam ring of the penicillins and cephalosporins as well as the addition of various substituents to certain sites on the aminoglycoside antibiotics (e.g., gentamicin).
An interesting result of destruction as a means of achieving resistance to antimicrobials is that it may lead to protection of inately sensitive organisms. Gram negative bacteria produce beta-lactamases (enzymes that destroy penicillins and cephalosporins), but these are limited to the periplasmic space of the organism. Thus, they protect only the bacterium that produced them. In contrast, staphylococci produce beta-lactamases that diffuse into the surrounding medium. If there are enough staphylococci present, the concentration of beta-lactamases may reach levels that will protect bacteria that do not produce the destructive enzymes. This could explain why some antibiotics fail when in vitro tests indicate that they should work.
DECREASED ACCUMULATION:Decreased accumulation of the drug either as a result of decreased penetration to the site of action or to increased removal of drug from the organism. Decreased penetration is characteristic of gonococci and penicillin G. In some cases, organisms develop the ability to transport drugs out of their cytoplasm leading to insufficient concentration inside the cell to be effective. The tetracyclines are an example of this (as are some anticancer drugs).
DECREASED AFFINITY / BINDING:Decreased binding of the drug to the active site because of changes in the enzyme or receptor that lead to decreased affinity for the drug. The is characteristic of some penicillin derivatives (penicillinase resitant antistaphylococcal derivatives) and staphylococci. Streptomycin resistance can result from a single amino acid change in a ribosomal subunit leading to decreased streptomycin binding.
METABOLIC BY-PASSMetabolic by-pass of the target reaction is typified by trimethoprim. Some organisms can synthesize new dihydrofolate reductase (DHF reductase), thus overproducing the affected enzyme. In other cases, the activity of entirely different pathways may be enhanced. In either case, the impact of the block is lessened.
Adverse effects of antimicrobials may be divided into three broad categories:
Allergic reactions are especially likely and may represent the major toxicity of the penicillins and cephalosporins. They are more likely to occur with parenteral than oral drug administration. If there is doubt about how a patient will react to a drug, i.e., the patient has a history of drug allergies, then a sensitivity test should be done or a drug from a different family should be used.
In addition to direct effects on the patient, such as toxicoses and allergies, antimicrobials may also cause adverse efects indirectly through their effect on the patient's microflora. This category of adverse effects refers only to the antibacterials because there are no normal flora from other types of microorganisms. Superinfection (sometimes called suprainfection) is the superimposition of an infection on an existing infection. This results because an antimicrobial may depress normal flora and/or a predominant pathogen, thus allowing another pathogen to flourish. Such infections are especially likely to occur on mucous membranes, e.g., the gut, mouth, and vagina. The pathogens are commonly yeasts, like Candida albicans. Broad spectrum antimicrobials like the tetracyclines are especilly prone to produce these infections.
A special case of a biological adverse effect is antibiotic-induced enterocolitis. This condition, which occurs in a wide variety of species including humans, is cause by overgrowth of Clostridium diffcile, among others. Antimicrobials which achieve high concentrations in the gut and which are especially active against the normal anaerobes of the gut are most associated with this condition including: ampicillin, tetracyclines, LINCOMYCIN, and other broad spectrum antibacterials. For therapy of Clostridium difficile induced antibiotic-induced enterocolitis (antibiotic-induced pseudomembranous enterocolitis) vancomycin is the drug ofchoice.
Important toxic effects are those that are life threatening or that are so distasteful to the patient/client that they cause problems in compliance. Examples of distasteful effects are vomition, abdominal pain, diarrhea, and dizziness, all on a short term basis. Note that these so-called "distasteful" effects can also become serious. Minocycline is an example of a drug that causes reversible dizziness.
Example of more severe toxicoses include the following:
Nephrotoxicoses, e.g., proximal tubular degeneration with aminoglycosides like gentamicin or tubular crystal formation with older sulfonamides.
Ototoxicities, including permanent loss of hearing (cochlea) and/or disturbances to vestibular function resulting in dizziness and disorientation. Aminoglycosides, e.g., gentamicin, may harm both structures.
Bones and teeth, including deformation and discoloration. This is espcially likely in fetus, neonates, and young patients. Example: tetracyclines.
Curare-like respiratory paralysis may be caused, e.g., by aminoglycosides at doses that may be achieved with improper therapy and excessive doses.
Adverse effects on the immune system are not easily detected in patients being treated for infections, thus they have not been emphasized in the past. However, they are obviously extremely important because the body defenses enhance the chance for cure. Hauser & Remington 1984 (Ristuccia & Cunha, pp 55+) have assembled a list of measurable effects of antimicrobials on various immune system parameters. Some of these are presented here as examples of the kinds of process that may be affected.
Lymphocyte transformation is inhibited by some antimicrobials as are delayed-type hypersensitivity (cell-mediated) types of responses. Antibody production is inhibited by the following drugs which are all main-line drugs: amphotericin B, cefoxitin, doxycycline, rifampin, cefotaxime, chloramphenicol, moxalactam, and trimethoprim-sulfamethoxazole.
Various aspects of PMN leukocyte function are inhibited by some drugs. Chemotaxis is inhibited by the following: amikacin, chloramphenicol, gentamicin, tetracycline, amphotericin B, doxycycline, rifampin, and tobramycin. Phagocytosis is inhibited by the following: amphotericin B, doxycycline, and tetracycline. Microbiocidal activity is inhibited by the following: amikacin, cephalothin, gentamicin, sulfonamides, tetracycline, and tobramycin.