As discussed in Chapter 3 (Cell Structure and Function) and Chapter 6 (Bacteria and Archaea), most bacterial cells contain a rigid cell wall composed of peptidoglycan, which protects the cell from rupturing in hypotonic environments. Young, growing cells must constantly synthesize new peptidoglycan and transport it to the wall (Figure 22.1). Agents such as penicillins and cephalosporins chemically react with the enzymes responsible for completing the peptidoglycan layer, thus preventing the assembly process. This disruption may occur at numerous different sites in the layer and at different points in the process, but in all cases it causes weaknesses at growth points in the layer, which in turn renders the cell vulnerable to lysis. In a hypotonic environment, water then enters the cell and eventually causes it to burst.

FIGURE 22.1 Peptidoglycan layer.
This illustration demonstrates the three-dimensional structure of the peptidoglycan layer.

Inhibition of Protein Synthesis

Most of the agents that inhibit protein synthesis involve disrupting the process of translation (see Chapter 3, Cell Structure and Function) at the ribosome–mRNA complex (Figure 22.2). The ribosomes of eukaryotic cells are structurally different from those of prokaryotic cells, and because of this human cells are usually not affected by the inhibitory action of these agents because they selectively act against bacteria. The specific chemical reactions that disrupt the translation of the mRNA vary and include the following:

FIGURE 22.2 Inhibition of translation.
The inhibition of translation can occur at a number of points, including the following: inhibition of peptide bond formation, interference with the attachment of transfer RNA (tRNA) to mRNA, and changing of the shape of the ribosome, thereby preventing translocation along the mRNA strand.

• Prevention of the initiation of synthesis

• Blocking of the formation of peptide bonds between amino acids on the transfer RNAs (tRNAs)

• Prevention of ribosome translocation along the mRNA strand

Whatever the specific point of attack, the result is the same: inhibition of the synthesis of necessary proteins. Antimicrobial agents that affect bacterial protein synthesis include tetracyclines, chloramphenicol, macrolides, and others.

Inhibition of Nucleic Acid Synthesis

The metabolic process involved in the synthesis of DNA and RNA is a long and complex series of enzyme-catalyzed reactions (see Chapter 3, Cell Structure and Function). The complexity of the process lends itself to disruption at many points along the way and inhibition at any point can block subsequent events in the pathway. As with the other mechanisms the specific target within the process is often dependent on the specific antimicrobial agent being used, but the end result is the same no matter where the point of attack is—disruption of the synthesis of nucleic acids. The interference with nucleic acid synthesis by an antimicrobial drug can be due to:

• Disruption of synthesis of the nucleotide component

• Inhibition of DNA replication

• Interference with transcription of the RNA

An interesting type of mechanism that is employed by sulfonamides and trimethoprim involves mimicking the normal substrate of an enzyme in a process called competitive inhibition. When these agents are supplied to the cell in high concentrations they will substitute for the true substrate of an important enzyme, thus preventing the enzyme from producing a needed product and causing the metabolism of the cell to slow or stop.

Disruption of Plasma Membrane

Damage to the cell membrane is a form of catastrophic damage for any cell. Not only can it affect the transport of substances required for metabolism both in and out of the cell, it is a major factor in the maintenance of the cell’s physical integrity. Most membrane–disrupting agents target the different types of lipids in the cell membrane, which lends to their specificity for a particular microbial group. Because all microorganisms (except for viruses) share the same basic plasma membrane structure, these agents are useful against both bacteria and fungi. Unfortunately, they can also be toxic to human cells.

Antibiotics, specifically polypeptides such as polymyxin B, adversely affect the membrane permeability of microbial cells, resulting in a loss of important metabolites. Antifungal drugs that disrupt fungal plasma membranes include nystatin, miconazole, ketoconazole, and amphotericin B.

Inhibition of Metabolic Pathways

Antimicrobial agents not only are able to disrupt the metabolic pathways involved in the synthesis of proteins and nucleic acids, but can affect numerous cellular metabolic processes that can lead to cell death. Disruption of crucial enzyme activity/production as well as blocking the synthesis of other essential metabolic compounds such as NAD or folic acid can be catastrophic to the overall metabolism of any cell, thus allowing use of these agents against a wide variety of microorganisms. The sulfonamides (sulfa drugs) and trimethoprim are broad-spectrum antimicrobial drugs that disrupt the synthesis of tetrahydrofolic acid, a precursor to nucleic acids, necessary for microbial metabolism.

Characteristics of Antimicrobial Agents

Almost all antimicrobial agents share a set of common characteristics that are important to remember when selecting a drug for treatment. These characteristics provide a great deal of information regarding how the agents work, and in some cases why they don’t work, in the treatment of an infection. These characteristics of an antimicrobial agent include the following:

• Its selective toxicity

• Whether it is microbicidal rather than microbiostatic

• How readily and easily the agent may be delivered to the site of infection

• Whether it is readily soluble in body fluids and will maintain its potency long enough to be effective

• How long the agent remains active in the body

• The degree of antimicrobial resistance to the agent

• Whether it complements or aids in host body defenses

• Is it nonallergenic?

• Does it have a long shelf life?

• Is it affordable/available to all patients who might need it?

Spectrum of Action

Most antimicrobial drugs do not inhibit the growth of all pathogens. Antimicrobial drugs have a spectrum of activity, which is the range of pathogen types against which a given drug is effective (Table 22.1). On the basis of the spectrum of activity, drugs can be classified as antimicrobials with a broad spectrum of activity and those that have a narrow spectrum of activity.

TABLE 22.1

Spectrum of Activity of Some Antimicrobial Drugs

Drug	Activity against:
Penicillin G	Gram-positive bacteria
Streptomycin	Gram-negative bacteria, mycobacteria
Tetracycline	Gram-negative bacteria, gram-positive bacteria, chlamydias, rickettsias
Isoniazid	Mycobacteria
Acyclovir	Viruses
Ketoconazole	Fungi
Mefloquine (malaria)	Protozoans
Niclosamide (tapeworms)	Helminths
Biltricide (flukes)	Helminths

• Drugs with a broad spectrum of activity are effective against a large variety of microorganisms. An example would be the sulfonamides, which are effective against both gram-positive and gram-negative bacteria, actinomycetes, chlamydias, and some protozoans. These drugs exert their effects on all cell components of the microbes. An advantage of using broad-spectrum antibiotics is the high possibility of efficacy against an unidentified pathogen. The disadvantage of using such a drug is the likelihood of also destroying the normal flora (see Chapter 9, Infection and Disease). Destruction of the normal flora can lead to disease if one organism of the normal flora is replaced by a pathogen. This then leads to what is referred to as a superinfection.

• Drugs with a narrow spectrum of activity, such as isoniazid and penicillin G, are effective only against a relatively small number of microbes, and generally avoid destruction of the normal flora. They target a specific component that is present only in certain bacteria.

• Drugs with a medium spectrum of activity are effective against some gram-positive and gram-negative bacteria, but not all of them.

Selective Toxicity

An ideal antimicrobial drug will kill harmful microorganisms without significant damage to the host. This principle is referred to as selective toxicity. The differences in structure and/or metabolism between the microbe and the host make selective toxicity possible. With greater differences between the pathogen and the host it is easier to develop an effective antimicrobial drug. Regarding toxicity, it is preferable to have a wide range between the toxic dosage level, which can cause damage to the host, and the therapeutic dosage level, which will eliminate the infectious pathogen over a prescribed period of time (see Chapter 21, Pharmacology).

Microbicidal versus Microbiostatic

Antimicrobial drugs that kill microorganisms are referred to as bactericidal; if they prevent microbes from growing they are called bacteriostatic. Ideally the drug being used would eliminate the pathogens by destroying them. Inhibition of microbial growth does not necessarily eliminate the pathogens already present and may require prolonged use of the drug to ensure that high-enough levels are maintained to keep populations from recovering and increasing. Because of the potential negative effects the drugs may have on the host, the use of a killing agent instead of an inhibitory one is not always a simple decision. Microbicidal agents can also kill natural flora in areas such as the healthy intestine, creating a potential digestion problem because the normal resident flora are vital to proper digestion. The destruction of the normal flora population anywhere in or on the body may also create an advantage for competing pathogens that would normally be kept in check by the resident population (superinfection).

Delivery to the Site of Infection

To be effective, drugs need to be easily and effectively delivered to the site of infection. Therefore, an important criterion for the effectiveness (efficacy) of a drug is its ability to cross biological barriers (see Chapter 21, Pharmacology). Potential barriers may be physiological, such as the lining of the body compartments and the blood–brain barrier, or those induced by inflammation and disease. Abscess walls, exudates, and necrotic material are classic examples of host reactions to pathogens and can impede the transport of drugs to the invading microorganisms.

Furthermore, if the antimicrobial drug is not sufficiently soluble or becomes too dilute once in solution, the effectiveness of the drug is greatly diminished. In addition, if the means of delivery and/or transport of the agent does not efficiently and rapidly deliver the antibiotic to the site of infection, the drug will not have the opportunity to fight the pathogen even if it is in solution and in the correct concentration. For example, some agents are effective against a particular infection but are not lipid soluble. Therefore, these compounds are not able to cross the blood–brain barrier, rendering them ineffective against an infection in the central nervous system, such as meningitis. Whereas most antibiotics cannot cross the blood–brain barrier, some third-generation cephalosporins (cefotaxime, ceftizoxime, and ceftriaxone) and metronidazole can.

Time of Activity

It is desirable for an antimicrobial drug to be active over a long period of time for an optimal effect. As covered in Chapter 21 (Pharmacology), the duration of action time is a major consideration when evaluating the overall effect of a drug on an infection/pathogen. If the destruction of a population of pathogens requires a sustained concentration of agent over a specified period of time, any shortening of this time may significantly decrease the effectiveness of the treatment. By subjecting the organisms to the agent for a time that proves to be nonlethal to many of them, the risk of producing a resistant population is increased.

Not Subject to Antimicrobial Resistance

The ideal antimicrobial drug is an agent that is not subject to the development of antimicrobial resistance. If the nature of the agent and its mechanism is such that populations of pathogens are less likely to develop resistance to the agent, its effectiveness and scope of use would be dramatically increased. Because of microbial genetic mutations as well as the acquisition of resistance genes from other cells, viruses, and the environment; it is difficult to develop an antibiotic against which microbes will not eventually develop some resistance (see Resistance to Antimicrobial Drugs).

Complements/Aids in Host’s Own Defenses

The natural defenses of a host, including barriers at portals of entry, protective cells and environments within the host, and an acquired immune system (both active and passive), do an excellent job in fighting off infections in the healthy host (see Chapter 20, The Immune System). When these layers of defense are compromised, antimicrobial drugs need to be administered in order to aid the immune system. Any time a foreign substance is introduced into a host there is a risk of upsetting this complex balance of defenses and actually inhibiting the body’s natural response to pathogens. For example, if an antibiotic harms an organ or system it may adversely affect the natural defenses of the host, causing such things as excessive damage to the kidneys.

Nonallergenic

Certain antimicrobial agents are capable of inducing a hypersensitivity reaction (Chapter 20, The Immune System) in certain individuals. Patients with allergies to specific agents such as penicillin must be careful to inform medical personnel about their allergy, or the shock that may result from receiving penicillin could be fatal.

Dilution/Minimal Inhibitory Concentration Method

In the dilution/minimal inhibitory concentration method a given quantity of the pathogen culture is added to a series of tubes containing decreasing concentrations of a specific antibiotic. The tubes are then incubated and examined for growth. Turbidity (cloudiness) indicates bacterial growth; lack of turbidity signifies that bacterial growth is either inhibited or the organisms have been killed by the antimicrobial agent (Figure 22.4). The goal is to identify the tube with the lowest concentration of drug that inhibits visible growth. This concentration is referred to as the minimal inhibitory concentration (MIC) for a specific agent acting on a selected microorganism. In the past this test was performed in individual test tubes. However, kits are now available wherein the test is performed in shallow wells on standard testing plates. This test is widely used in hospitals and clinics today; however, there are also shortcomings with this method. As with the Kirby-Bauer method, the basic test procedure will not indicate whether an antibiotic is microbicidal or microbiostatic. A second test using this method can be added to determine whether the organisms are being inhibited or killed. In this test a sample from the tube with the lowest concentration showing no growth is subcultured in order to see whether there are any organisms still surviving in the tube. This lowest concentration containing no living organisms is called the minimal bactericidal concentration (MBC).

FIGURE 22.4 Minimal inhibitory concentration.
The minimal inhibitory concentration (MIC) for a specific agent is indicated by the absence of growth in the tube containing the lowest drug concentration.

Serum Killing Power

In the serum killing power method a sample of a patient’s blood is obtained while they are receiving an antibiotic. The blood is then processed to extract the serum, which will contain the drugs that are in the bloodstream. A bacterial suspension is then added to a measured quantity of the patient’s serum. If the bacteria grow in the tube of serum this indicates that the antibiotic is ineffective, whereas no growth indicates it is effective. Using serial dilutions, the lowest concentration that still provides effective serum killing power can be easily determined.

Side Effects

All drugs are potential poisons (see Chapter 21, Pharmacology) and the administration of therapeutic drugs can possibly lead to side effects that can be potentially serious. Drugs can adversely affect the liver (hepatotoxic), kidneys (nephrotoxic), gastrointestinal tract, cardiovascular system, red bone marrow (hemotoxic), nervous system (neurotoxic), respiratory system, integument, bones, and teeth. The administration of drugs involves a previous assessment of the risks and benefits. This type of assessment is referred to as the therapeutic index (see Chapter 21).

The liver is responsible for the metabolism and detoxification of chemical substances in the blood. Liver damage can occur due to a drug or its metabolic by-products. The kidneys are in charge of the excretion of drugs and drug metabolites. Both drugs and their metabolic by-products have the potential to damage the structures of the nephron and therefore are capable of interfering with the filtration and reabsorption ability of the nephron.

One of the most common side effects to antimicrobial therapy is diarrhea, which may progress to severe intestinal irritation and colitis. Most often these irritations occur because of disruption of the normal intestinal flora. The organisms in the normal flora of the skin, intestine, urogenital tract, and oral cavity are with a few exceptions harmless and also prevent the colonization of potentially harmful organisms. However, with the use of broad-spectrum antibiotics these harmless microbes can be destroyed, allowing resistant organisms to overgrow, resulting in a superinfection.

For example, the use of a broad-spectrum cephalosporin for the treatment of Escherichia coli–induced urinary tract infections can also destroy lactobacilli in the vagina. Lactobacilli and Candida albicans are both part of the normal vaginal flora. Lactobacillus will be killed by the broad-spectrum antibiotic, allowing Candida albicans to flourish and cause a yeast infection. Similar superinfections by Candida can also occur in the oral cavity (thrush) and the large intestine.

Antibiotic-associated colitis, a serious and potentially fatal condition, can be caused by oral administration of:

• Tetracyclines

• Clindamycin

• Broad-spectrum penicillins

• Cephalosporins

The condition is a result of the overgrowth of Clostridium difficile, an antibiotic-resistant and endospore-forming bacterium. The organism invades the intestinal mucosa, where it releases toxins (enterotoxins) that result in diarrhea, fever, and abdominal pain. Other drug-induced side effects may involve the skin, either because of an allergic reaction or by interaction of the drugs with sunlight, causing photodermatitis.

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