Microbiology: A Systems Approach, 2nd Edition - PDF

Microbiology: A Systems Approach, 2nd ed. 1  Outline and Learning Outcomes 12.1 Principles of Antimicrobial Therapy 1. State the main goal of antimicrobial treatment. 2. Identify the sources for most currently used antimicrobials....

Microbiology: A Systems Approach, 2nd ed. 1  Outline and Learning Outcomes 12.1 Principles of Antimicrobial Therapy 1. State the main goal of antimicrobial treatment. 2. Identify the sources for most currently used antimicrobials. 2  12.1 Principles of Antimicrobial Therapy Goal of antimicrobial chemotherapy: administer a drug to an infected person, which destroys the infective agent without harming the host’s cells Rather difficult to achieve this goal Chemotherapeutic agents described with regard to their origin, range of effectiveness, and whether they are naturally produced or chemically synthesized 3 4 5 The Origins of Antimicrobial Drugs Antibioitics are common metabolic products of aerobic bacteria and fungi – Bacteria: Streptomyces and Bacillus – Molds: Penicillium and Cephalosporium Chemists have created new drugs by altering the structure of naturally occurring antibiotics Also Searching for metabolic compounds with antimicrobial effects in species other than bacteria and fungi 6  Outline and Learning Outcomes 12.2 Interactions Between Drug and Microbe 3. Explain the concept of selective toxicity. 4. List the five major targets of antimicrobial agents. 5. Identify which categories of drugs are most selectively toxic and why. 6. Explain how drugs that inhibit protein synthesis can be selective. 7. Define metabolic analog and discuss its relevance to antimicrobial action. 7  12.2 Interactions Between Drug and Microbe Goal of antimicrobial drugs – Disrupt the cell processes or structures of bacteria, fungi, and protozoa – Or inhibit virus replication Most interfere with the function of enzymes required to synthesize or assemble macromolecules or destroy structures already formed in the cell Drugs should be selectively toxic- they kill or inhibit microbial cells without damaging host tissues 8  Mechanisms of Drug Action Inhibition of cell wall synthesis Inhibition of nucleic acid structure and function Inhibition of protein synthesis Interference with cell membrane structure or function Inhibition of folic acid synthesis 9 10  Antimicrobial Drugs that Affect the Bacterial Cell Wall Active cells must constantly synthesize new peptidoglycan and transport it to the proper place in the cell envelope Penicillins and cephalosporins react with one or more of the enzymes required to complete this process Bactericidal antibiotics 11  Effects of antibiotics on bacterial cell walls. 12 Figure 12.2 Figure 12.3 The mode of action of penicillins and cephalosporins on the bacterial cell wall. (a) Intact peptidoglycan has chains of NAM (N-acetyl muramic acid) and NAG (N- acetyl glucosamine) glycans cross-linked by peptide bridges. (b) These two drugs block the peptidases that link the cross-bridges between NAMs, thereby greatly 13 weakening the cell wall meshwork. Antimicrobial Drugs that Affect Nucleic Acid Synthesis Block synthesis of nucleotides Inhibit replication Stop transcription Inhibit DNA synthesis 14 Antimicrobial Drugs that Block Protein Synthesis Inhibit translation by reacting with the ribosome-mRNA complex Prokaryotic ribosomes are different from eukaryotic ribosomes- selective 15 Sites of inhibition on the prokaryotic ribosome and major antibiotics that act on these sites. All have the general effect of blocking protein synthesis. Blockage actions are indicated by ×. 16  Antimicrobial Drugs that Disrupt Cell Membrane Function Damaged membrane invariably results in death from disruption in metabolism or lysis Specificity for particular microbial groups based on differences in the types of lipids in their cell membranes 17  Antimicrobial Drugs that Inhibit Folic Acid Synthesis Sulfonamides and trimethoprim- competitive inhibition Supplied to cells in high concentrations to make sure enzyme is constantly occupied with the metabolic analog rather than the true substrate of the enzyme. As the enzyme is no longer able to produce a needed product, cellular metabolism slows or stops. 18 The action of sulfa drugs. The metabolic pathway needed to synthesize tetrahydrofolic acid (THFA) contains two enzymes that are chemotherapeutic targe 19 Figure 12.5 para-aminobenzoic acid (PABA)  Outline and Learning Outcomes 12.3 Survey of Major Antimicrobial Drug Groups 8. Distinguish between broad-spectrum and narrow-spectrum antimicrobials and explain the significance of the distinction. 9. Trace the evolution of penicillin antimicrobials, and identify which microbes they are effective against. 10. Explain the significance of beta-lactamases, and where they are found. 11. List other beta-lactam classes of antibiotics and give two examples. 12. List some cell wall antibiotics that are not in the beta- lactam category. 20  Outline and Learning Outcomes 12.3 Survey of Major Antimicrobial Drug Groups 13. Identify two older and two newer antimicrobials that act by inhibiting protein synthesis. 14. Explain how drugs targeting folic acid synthesis work. 15. Identify the cellular target of quinolones and name two examples. 16. Name two drugs that target the cellular membrane. 17. Discuss two possible ways that microbes acquire antimicrobial resistance. 18. List five cellular or structural mechanisms that microbes use to resist antimicrobials. 21  12.3 Survey of Major Antimicrobial Drug Groups About 260 different antimicrobial drugs Classified in 20 drug families Largest number of antimicrobial drugs are for bacterial infections 22 23  Antibacterial Drugs Targeting the Cell Wall Penicillin group – Most end in the suffix –cillin – Can obtain natural penicillin through microbial fermentation – All consist of three parts: a thiazolidine ring, a beta-lactam ring, and a variable side chain 24 Figure 12.6 Chemical structure of penicillins. All penicillins contain a thiazolidine ring (yellow) and a beta-lactam ring (red), but each differs in the nature of the side chain (R 25 group), which is also responsible for differences in biological activity. Subgroups and Uses of Penicillins 26 The Cephalosporin Group of Drugs Newer group Currently account for a majority of all antibiotics administered 27 Figure 12.7 The structure of cephalosporins. Like penicillin, they have a beta-lactam ring (red), but they have a different main ring (yellow). However, unlike penicillins, they have two sites for placement of R groups (at positions 3 and 7). This makes possible several 28 generations of molecules with greater versatility in function and complexity in structure Subgroups and Uses of Cephalosporins Broad-spectrum Resistant to most penicillinases Cause fewer allergic reactions than penicillins Four generations of cephalosporins exist based on their antibacterial activity 29  Other Beta-Lactam Antibiotics Imipenem Aztreonam 30  Other Drugs Targeting the Cell Wall Bacitracin Isoniazid Vancomycin Fosfomycin tromethamine 31  Antibacterial Drugs Targeting Protein Synthesis Aminoglycoside Drugs – Products of various species of soil actinomycetes in the genera Streptomyces and Micromonospora – Relatively broad spectrum because they inhibit protein synthesis – Subgroups and uses Treating of aerobic gram-negative rods and certain gram- positive bacteria Streptomycin: Bubonic plague and tularemia and good antituberculosis agent Gentamicin: Less toxic and used for gram-negative rods 32 The structure of streptomycin. Colored portions of the molecule show the general arrangement of an aminoglycoside. 33  A colony of Streptomyces, one of nature’s most prolific antibiotic producers. 34 Figure 12.9  Tetracycline Antibiotics Bind to ribosomes and block protein synthesis Broad-spectrum Subgroups and uses – Gram –positive and gram-negative rods and cocci – Aerobic and anerobic bacteria – Mycoplasmas, rickettsias, and spirochetes 35 Broad-spectrum antibiotics. Tetracyclines. These are named for their regular group of four rings. The several types vary in structure and activity by substitution at the four R groups. 36 Figure 12.10  Erythromycin and Clindamycin Erythromycin – Large lactone rinig with sugars attached – Relatively broad-spectrum – Fairly low toxicity – Blocks protein synthesis by attaching to the ribosome – Mycoplasma pneumonia, Chlamydia infections, diphtheria Clindamycin – Broad-spectrum – Derived from lincomycin – Causes adverse reactions in the gastrointestinal tract, so applications are limited 37 Broad-spectrum antibiotics. Erythromycin, an example of a macrolide drug. Its central feature is a large lactone ring to which two hexose sugars are attached. 38 Figure 12.10 Antibacterial Drugs Targeting Folic Acid Synthesis Sulfonamides, Trimethoprim, and Sulfones – Sulfonamides Sulfa drugs Very first modern antimicrobial drug Synthetic Shigellosis, acute urinary tract infections, certain protozoan infections – Trimethoprim Inhibits the enzymatic step immediately following the step inhibited by solfonamides in the synthesis of folic acid Often given in combination with sulfamethoxazole One of the primary treatments for Pneumocystis (carinii) jiroveci pneumonia (PCP) in AIDS patients 39  40 Figure 12.5  Antibacterial Drugs Targeting DNA or RNA Fluoroquinolones High potency Broad spectrum Inhibit a wide variety of gram-positive and gram-negative bacterial species even in minimal concentrations 41  Antibacterial Drugs Targeting Cell Membranes Polymyxins: narrow-spectrum peptide antibiotics – From Bacillus polymyxa – Limited by their toxicity to the kidney – B and E can be used to treat drug-resistant Pseudomonas aeruginosa Daptomycin – Lipopeptide made by Streptomyces – Most active against gram-positive bacteria 42  Interactions Between Microbes and Drugs: The Acquisition of Drug Resistance Drug resistance: an adaptive response in which microorganisms begin to tolerate an amount of drug that would ordinarily be inhibitory Can be intrinsic or acquired Microbes become newly resistant to a drug after – Spontaneous mutations in critical chromosomal genes – Acquisition of entire new genes or sets of genes via transfer from another species (plasmids called resistance (R) factors) Specific Mechanisms of Drug Resistance 43 44  Natural Selection and Drug Resistance The events in natural selection for drug resistance. (a) Populations of microbes can harbor some members with a prior mutation that confers drug resistance. (b) Environmental pressure (here, the presence of the drug) selects for survival of these mutants. (c) They eventually become the dominant members of the population 45 Figure 12.14  Outline and Learning Outcomes 12.4 Interactions Between Drug and Host 24. Distinguish between drug toxicity and allergic reactions to drugs. 25. Explain what a superinfection is and how it occurs. 46 12.4 Interaction Between Drug and Host 47  Toxicity to Organs Drugs can adversely affect the following organs: Liver, kidneys, gastrointestinal tract, cardiovascular system and blood-forming tissue, nervous system, respiratory tract, skin, bones, and teeth 48  Allergic Responses to Drugs Allergy: heightened sensitivity The drug acts as an antigen and stimulates an allergic response Reactions such as skin rash, respiratory inflammation, and rarely anaphylaxis 49  Suppression and Alteration of the Microbiota by Antimicrobials Biota: normal colonists or residents of healthy body surfaces – Usually harmless or beneficial bacteria – Small number can be pathogens If a broad-spectrum antimicrobial is used, it will destroy both infectious agents but also some beneficial species 50  Superinfection When beneficial species are destroyed, microbes that were once kept in small numbers can begin to overgrow and cause disease- a superinfection – Using a broad-spectrum cephalosporin for a urinary tract infection; destroys lactobacilli in the vagina; without the lactobacilli Candida albicans can proliferate and cause a yeast infection 51 The role of antimicrobials in disrupting microbial biota and causing superinfections. (a) A primary infection in the throat is treated with an oral antibiotic. (b) The drug is carried to the intestine and is absorbed into the circulation. (c) The primary infection is cured, but drug-resistant pathogens have survived and create an intestinal superinfection. 52  Outline and Learning Outcomes 12.5 Considerations in Selecting an Antimicrobial Drug 26. Describe two methods for testing antimicrobial susceptibility. 27. Define therapeutic index and identify whether a high or a low index is preferable. 53  12.5 Considerations in Selecting an Antimicrobial Drug Three factors must be known – The nature of the microorganism causing the infection – The degree of the microorganism’s susceptibility to various drugs – The overall medical condition of the patient Identifying the Agent – Direct examination of body fluids, sputum, or stool is a rapid initial method – The choice of drug will be based on experience with drugs that are known to be effective against the microbe: the “informed best guess” Testing for the Drug Susceptibility of Microorganisms Figure 12.17: Figure 12.18 Technique for preparation and interpretation of disc diffusion tests. (a) Standardized methods are used to seed a lawn of bacteria over the medium. A dispenser delivers several drugs onto a plate, followed by incubation. Interpretation of results: During incubation, antimicrobials become increasingly diluted as they diffuse out of the disc into the medium. If the test bacterium is sensitive to a drug, a zone of inhibition develops around its disc. Roughly speaking, the larger the size of this zone, the greater is the bacterium’s sensitivity to the drug. The diameter of each zone is measured in millimeters and evaluated for susceptibility or resistance by means of a comparative standard (see table 12.7). Figure 12.18: Alternative to the Kirby-Bauer procedure. Another diffusion test is the E-test, which uses a strip to produce the zone of inhibition. The advantage of the E-test is that the strip contains a gradient of drug calibrated in micrograms. This way, the MIC can be measured by observing the mark on the strip that corresponds to the edge of the zone of inhibition. (IP = imipenem and TZ = tazobactam) Figure 12.19: Tube dilution test for determining the minimum inhibitory concentration (MIC). (a) The antibiotic is diluted serially through tubes of liquid nutrient from right to left. All tubes are inoculated with an identical amount of a test bacterium and then incubated. The first tube on the left is a control that lacks the drug and shows maximum growth. The dilution of the first tube in the series that shows no growth (no turbidity) is the MIC. (b) Microbroth dilution in a multiwell plate adapted for eukaryotic pathogens. Here, amphotericin B, flucytosine, and several azole drugs are tested on a pathogenic yeast. Pink indicates growth and blue, no growth. Numbers indicate the dilution of the MIC, and Xs show the first well without growth.  The MIC and Therapeutic Index MIC- minimum inhibitory concentration: the smallest concentration (highest dilution) of drug that visibly inhibits growth Once therapy has begun, it is important to observe the patient’s clinical response  If Antimicrobial Treatment Fails If antimicrobial treatment fails, the failure is due to – The inability of the drug to diffuse into that body compartment – A few resistant cells in the culture that did not appear in the sensitivity test – An infection caused by more than one pathogen, some of which are resistant to the drug  Best Choice of Drug Best to choose the drug with high selective toxicity for the infectious agent and low human toxicity – Therapeutic index (TI): the ratio of the dose of the drug that is toxic to humans as compared to its minimum effective dose – The smaller the ratio, the greater the potential for toxic drug reactions