Microbiology Chapter 10

Chemotherapy

Treatment of disease(not limited to cancer) with chemical substances.

Antibiotic

An antimicrobial agent, usually produced by a bacterium or fungus. Used to help inhibit other competitors in their environment. Not effective against viruses. 

Greatest number of antibiotics derived

The greatest number of antibiotics is derived from bacteria in the genera Streptomyces and Bacillus or molds in the genera Penicillium and Cephalosporium. 

Selective toxicity

The property of some antimicrobial agents to be toxic to a microorganism and less/non-toxic to the host. You want antibiotic drugs to be like this

Broad spectrum

An antibiotic that is effective against a wide variety of organisms (both Gram+ and Gram-). EX. Norfloxacin. Drawback is that it is effective against many types of bacteria, which means it will kill the good bacteria as well as the bad. 

Narrow spectrum

An antibiotic that is effective against only specific types of organisms. EX. Penicillin - it targets peptidoglycan and is effective against Gram+. The drawback is if you don’t know what is infecting you, and you use this, it may not work. 

Superinfection

An infection following a previous infection. This is usually caused by microorganisms that have become resistant to the antibiotics used earlier.

 Why is it so difficult target a pathogenic virus?

They live within host cells so they are hard to target because we don’t want to damage the host while trying to kill the virus. They are also non-living, which means you can’t target cell walls or protein synthesis.

 Why is it so difficult target a pathogenic protozoan?

They are eukaryotic and animal-like, since they have more in common with us. This means we cannot target peptidoglycan, 70s ribosomes, or metabolism. 

 Why is it so difficult target a pathogenic fungi?

They are eukaryotic, which means there are fewer differences between fungal cells and our cells, making them difficult to kill.

 Why is it so difficult target a pathogenic helminth?

They are eukaryotic and animal cells. These organisms have a lot in common with us, which makes them difficult to kill. 

Goal of antibiotic drugs

1. Disrupt cell processes or structures of bacteria, fungi, or protozoa

2. Inhibit virus replication

3. Interfere with the function of enzymes required to synthesize or assemble macromolecules

4. Destroy structures already formed in the cell

Modes of antimicrobial inhibition

1. Protein synthesis inhibitors: Inhibit 70s ribosomes. Bacteriostatic. EX. Selectively toxic because bacteria use 70S ribosomes and humans use 80S (Drawback is the mitochondria, which use 70S).

2. Inhibition of cell walls: They disrupt the cell wall by blocking synthesis and repair. Bactericidal. Selectively toxic because bacteria have a cell wall, and human cells do not.

3. Damage to the cell membrane: Causes loss of selective permeability. Bactericidal. Not selectively toxic because bacterial cell membranes are similar to ours (both phospholipid bilayers).

4. Inhibition of nucleic acids: Targets DNA replication or transcription. May inhibit RNA polymerase or gyrase (unwinding enzyme). Bacteriostatic. Selectively toxic because bacteria have circular chromosomes, and we have linear chromosomes.

5. Inhibition of metabolism: Blocks pathways or inhibits metabolism. Bacteriostatic. Selectively toxic because bacteria synthesize folic acid, but we get it from our diet.

Protein synthesis inhibitors

Inhibit 70s ribosomes. Bacteriostatic. EX. Selectively toxic because bacteria use 70S ribosomes and humans use 80S (Drawback is the mitochondria, which use 70S).

Inhibition of cell walls

They disrupt the cell wall by blocking synthesis and repair. Bactericidal. Selectively toxic because bacteria have a cell wall, and human cells do not.

Damage to the cell membrane

Causes loss of selective permeability. Bactericidal. Not selectively toxic because bacterial cell membranes are similar to ours (both phospholipid bilayers).

Inhibition of nucleic acids

Targets DNA replication or transcription. May inhibit RNA polymerase or gyrase (unwinding enzyme). Bacteriostatic. Selectively toxic because bacteria have circular chromosomes, and we have linear chromosomes.

Inhibition of metabolism

Blocks pathways or inhibits metabolism. Bacteriostatic. Selectively toxic because bacteria synthesize folic acid, but we get it from our diet.

If you have an infection with a Gram-negative organism, should you choose a drug that is bactericidal or bacteriostatic? Why?

You should give a Gram - organism a bacteriostatic drug because bactericidal antibiotics can cause Gram - bacteria to rupture and release lipid A, which is an endotoxin that can cause the patient to go into shock. 

Penicillin

Inhibits cell wall synthesis. It contains a ꞵ-lactam ring, which is responsible for its action. Need it to make peptidoglycan. The types are differentiated by the chemical side chains attached to the ring. It prevents the cross-linking of peptidoglycans, interfering with cell wall construction. This targets Gram+ more than Gram -. 

Penicillinases

An enzyme produced by bacteria, which allow bacteria to become resistant to penicillin. This is because it breaks the β-lactam ring and turns penicillin into penicilloic acid (not active to inhibit peptidoglycan). These enzymes are produced by bacteria, but most notably by Staphylococcus. 

Types of penicillin

1. Natural penicillin

2. Semisynthetic penicillin

3. Penicillinase-resistant penicillins

4. Penicillins plus β-lactamase inhibitors

Natural penicillin

Extracted from penicillium (fungi) cultures. Penicillin G must be injected, while Penicillin V must be taken orally. These have a narrow spectrum of activity (target Gram+). The issue with these is that they are susceptible to enzymes called penicillinases (β-lactamases), which make bacteria resistant to penicillin. 

Semisynthetic penicillin

It is partially derived from mold and partially synthesized. Contains chemically added side chains, making it resistant to penicillinase or extending the spectrum of activity. EX. Oxacillin or Ampicillin. 

Penicillinase-resistant penicillins

Methicillin and oxacillin

Extended-spectrum penicillins

Effective against gram-negatives as well as gram-positives. EX. Aminopenicillins: ampicillin, amoxicillin

Penicillins plus β-lactamase inhibitors

Contain clavulanic acid, a noncompetitive inhibitor of penicillinase.

Polypeptide antibiotics

Inhibit cell wall synthesis. Types: Bacitracin and Vancomycin

Bacitracin

A narrow-spectrum antibiotic. It is a topical antibiotic used for skin infections post-surgical (wipes). Used to kill Gram positive bacteria on our skin, like Staph. aureus or Streptococcus species . Found in Neosporin (triple antibiotic ointment)

Vancomycin

A narrow-spectrum antibiotic. It used to be a last-line drug for MRSA, but now it is a first-line drug. Important against MRSA.

What disease is isoniazid and ethambutol used to treat? How do they work?

Used against tuberculosis because it has mycolic acid in its cell wall. They work by inhibiting mycolic acid synthesis in actively growing cells and inhibiting the incorporation of mycolic acid.

Antimycobacterial antibiotics

Inhibit cell wall synthesis. Narrow-spectrum drugs. Types: Isoniazid and Ethambutol

Isoniazid (INH)

Inhibits mycolic acid synthesis in actively growing cells. It is used in combination with other anti-TB drugs

Ethambutol

Inhibits the incorporation of mycolic acid. It’s a relatively weak anti-TB drug and is usually used as a secondary drug to avoid resistance problems.

Drugs used to inhibit protein synthesis (considered selectively toxic)

1. Chloramphenicol

2. Clindamycin

3. Aminoglycosides

4. Tetracyclines

5. Macrolides

Chloramphenicol

Broad spectrum and inexpensive, but it is toxic and affects the formation of blood cells. It binds to the 50S portion of the ribosome, and it inhibits the formation of the peptide bond.

Clindamycin

Used for acne. Narrow spectrum. 

Aminoglycosides

Streptomycin is toxic and can cause auditory damage; rarely used

Tetracyclines

Broad spectrum; penetrate tissues, making them valuable against rickettsias and chlamydias, but can suppress normal intestinal microbiota. It will interfere with the attachment of tRNA to the mRNA complex.

Macrolides

Erythromycin, azithromycin, clarithromycin: used as an alternative to penicillin to treat streptococcal and staphylococcal infections (to treat ear, skin, and respiratory infections)

Drugs used to inhibit nucleic acid synthesis

1. Rifamycin

2. Quinolone and fluoroquinolones

Rifamycin

Inhibits mRNA synthesis. It is able to penetrate tissues; making it useful for antitubercular activity. Limited spectrum because it can’t pass through the cell envelope of many Gram negative bacteria. Side effects include orange/red color in urine, feces, and various body fluids.

Quinolone and fluoroquinolones

Nalidixic acid is synthetic and can inhibit DNA gyrase. DNA gyrase is enzyme used to unwind DNA for replication (only bacteria have, so it is selective). Norfloxacin and ciprofloxacin (Cipro) are broad spectrum; relatively nontoxic. Used for urinary tract infections (UTI) and for anthrax (Cipro). 

Drugs used to disrupt the plasma membrane

1. Daptomycin

2. Polymyxin B

Daptomycin

Produced by streptomycetes and used for skin infections. It attacks the bacterial cell membrane. Active against Gram +. 

Polymyxin B

Topical; bacteriocidal; effective against gram-negatives. Combined with bacitracin and neomycin in nonprescription ointments. Also used to treat drug-resistant Pseudomonas aeruginosa and severe UTI

Drug used to inhibit the metabolic pathway

Sulfonamides (sulfa drugs) inhibit the folic acid synthesis needed for nucleic acid and protein synthesis. They competitively bind to the enzyme for PABA production, a folic acid precursor. 

What two drugs are synergistic and target folic acid synthesis? How do they work?

Combination of trimethoprim and sulfamethoxazole (TMP-SMZ) is an example of drug synergism. They target two different steps in the pathway to block folic acid. 

Bacteria in biofilms

Behave differently then when they are free-living. They are often unaffected by antimicrobials, antibiotics often cannot penatrate the sticky extracellular material surrounding the biofilm, and the bacteria in biofilms express a different phenotype. 

Biofilm treatment strategies

1. Interrupting quorum sensing pathways to inhibit the formation of the biofilm. EX. Daptomycin: Shown success

2. Adding DNAse to antibiotics aids penetration through extracellular debris. This helps breakdown the biofilm to get drugs where they need to be.

3. Impregnating devices with antibiotics prior to implantation.

Factors to know before antimicrobial therapy

1. The identity of the microorganism causing the infection

2. The degree of the microorganism’s susceptibility (or sensitivity) to various drugs

3. The overall medical condition of the patient

Kirby Bauer Test

Used to determine the appropriate antibiotic for treatment. Advantages: Common, easy, cheap, little skill required, and standardized. It measures the zone of inhibition to see if a bacterium is sensitive, resistant, or intermediate. 

E test

A gradient diffusion method. A plastic strip (epsilometer) is coated with an increasing concentration gradient of the drug. This is used to determine the minimal inhibitory concentration (MIC). This is the lowest concentration that will inhibit bacterial growth.

Failure of antimicrobial treatment is due to

1. The inability of the drug to diffuse into that body compartment (brain, joints, skin)

2. Resistant microbes in the infection that did not make it into the sample collected for testing

3. An infection caused by more than one pathogen (mixed), some of which are resistant to the drug

4. The patient did not take the antimicrobials correctly

Therapeutic Index

The ratio of the dose of the drug that is toxic to humans as compared to its minimum effective (therapeutic) dose. The smaller the ratio, the greater the potential for toxic drug reactions. The drug with the highest therapeutic index has the widest margin of safety

Mechanisms of antibiotic resistance

1. Restricted permeability

2. Restricts access of antibiotics

3. Altered target of the antibiotic

4. Antibiotic inactivation

Restricted permeability

Decreased expression of porins, physical change in the porin protein to reduce permeability, or a change to the cell wall structure (such as being able to produce capsules or slimes. EX. Antibiotics affected by this are the beta-lactam antibiotics or aminoglycosides.

Restricts access of antibiotics

By producing an efflux pump (Even if the drugs make it in, there are drug pumps that kick the antibiotic right back out, like a bouncer. EX. Seen for tetracyclines, quinolones, and macrolides.

Altered target of the antibiotic

Prevents the antibiotic from binding to the target molecule caused by a mutation (Seen for almost every antibiotic class). EX. The enzyme that converts PABA to make folic acid changes so that sulfanilamide can no longer bind and inhibit folic acid synthesis.

Antibiotic inactivation

Either through degradation of the antibiotic or by modifying the antibiotic, which prevents the antibiotic from binding to its target. EX. Beta-lactamases (penicillinase) can inactivate beta-lactam antibiotics such as penicillin and cephalosporins.

Vertical gene transfer

Occurs during reproduction between generations of cells. During replication, DNA polymerase can cause a mutation, which can lead to antibiotic resistance. 

Horizontal gene transfer

The transfer of genes between cells of the same generation. This can lead to antibiotic resistance. The main types are transformation, transduction, and conjugation. 

Transformation

Naked DNA is transferred from a dead donor into a competent recipient. 

Transduction

A virus (Bacteriophage) acts as a genetic vector, passing DNA from the donor to the recipient.

Conjugation

The transfer of genetic material from one cell to another involving cell-to-cell contact.

Alternatives to antibiotics

1. Bacteriophage therapy

2. Anti-quorum-sensing drugs

3. Fecal microbiota transplants

4. Antibody therapy

5. Targeting biofilm and adherence

6. Bdellovibrio and like organisms (BALOs)

Bacteriophage therapy

Viruses that target bacteria specifically. They are so specific in targeting bacteria that they won’t harm the host cell. 

Anti-quorum-sensing drugs

Quorum sensing is used by bacteria to recruit other bacteria to the biofilm. These drugs would disrupt that.

Fecal microbiota transplants

A fecal transplant that will be put up the colon to restore the good bacteria. 

Antibody therapy

Giving patients antibodies against the pathogen.

Targeting biofilm and adherence

Prevents biofilm from forming.

Bdellovibrio and like organisms (BALOs)

Small predatory bacteria that target Gram (-) bacteria. It will be used to fight Gram - bacterial infections.