Unit 4

 

Control of Microbial Infections

 

Two ways to control microbial infections:

1.     attack the pathogens

chemotherapy -

other microbes -

 

2.     shore up host defenses

immunization

improvements in sanitation, nutrition, and health

 

 

I. Chemotherapy

A. History of Chemotherapy

 

1854-1915 - Paul Ehrlich - staining ΰmagic bullet

A chemical with selective toxicity that would kill the pathogen and not human cells would be an effective treatment

 

1904 – Ehrlich - a dye called Trypan Red was active against trypanosomes

 

1910 – Ehrlich - Salvarsan, an arsenic - against the syphilis spirochete

 

1935 - discovery that Prontosil Red – a leather dye -  was active against staphylococci and streptococci – breaks down in body to a sulfa drug - synthetic drugs - Gerhard Domagk

 

1928 - Fleming - discovery that mold Penicillium would inhibit growth of staphylococci - first antibiotic **

 

1940 – Florey, Chain, Heatley - production and purification of penicillin

 

1944 - streptomycin discovered by Selman Waksman

 

1953 - chloramphenicol, neomycin, tetracycline discovered.

 

Era of antibiotics begins - mainstay of txt against infectious disease for about 50 yrs.


B. Concerns for Antimicrobial Administration, Distribution, and Elimination

 

1.    Routes of administration

 

a.    IV - into a vein -

 

b.   IM - into the muscle -

 

c.    oral (PO - per os, Greek - by mouth) -

 

 

2.    What inhibits antimicrobial distribution in the body?

 

a.    barriers – cell membranes, tight cellular junctions

 

b. Difficult sites to penetrate

o        CNS

o        eye

o        mammary gland

o        prostate

o        testis

o        intracellular

o        inflammatory "capsule" (abscess)

 

3.    How are antimicrobials eliminated from the body?

 

a.    metabolized into a different compound, usually an inactive form, in the liver.

 

b.   removed from blood in the kidneys and excreted in the urine.

 

c.    excreted by liver cells into bile and eliminated in the feces.


C. How Antimicrobial Agents Work

 

Antimicrobials are classified by two different classification systems used simultaneously:

 

1.    microbicidal or microbistatic - whether they kill or inhibit the growth -

 

2.    by target site of the drug

 

 

Antimicrobial agents organized by site of action

 

**5 Sites of Action or Targets of Antimicrobials

 

1.    Cell wall synthesis

2.    Cell membrane function

3.    Nucleic acid synthesis or replication

4.    Protein synthesis

5.    Synthesis of essential metabolites

 

 

1.    Inhibition of cell wall synthesis

Can inhibit peptidoglycan synthesis in two different manners:

a.      inhibit synthesis of the linear strands – Exs. vancomycin and bacitracin

b.     inhibit cross-linking of the strands – Ex. Beta-lactams - penicillins and cephalosporins

 

 

2. Disrupt cell membrane function

a.      phospholipids of gram negative bacteria

b.  sterols of eucaryotic fungi - ergosterol

 

 

3. Inhibit nucleic acid synthesis

a. quinolones inhibit DNA gyrase – prevents relaxation of supercoiled DNA during replication

b. rifampicin inhibits bacterial RNA polymerase ΰ mRNA synth

 

 

4.    Inhibit protein synthesis - review for yourself the steps in protein synthesis

Targets of antimicrobial agents that inhibit protein synthesis:

1.    30S subunit

a.    Aminoglycosides and Streptomycin bind irreversibly to the 30S subunit and prevent formation of the initiation complex - stop protein synthesis -

b.   Tetracyclines also bind to 30S and interfere with attachment of charged tRNA to the “A” site of the ribosome - reversible -

2.    50S subunit

a.    Erythromycin and Chloramphenicol bind reversibly to 50S - inhibit formation of growing polypeptide chain

 

 

5. Inhibit folic acid synthesis

Two antimicrobial agents that inhibit folic acid synthesis:

a.      Sulfonamides - sulfa drugs

b.     Trimethoprim

 

Inhibit folic acid synthesis in 2 different places.

 

Trimethoprim inhibits an enzyme - dihydrofolate reductase

 

Sulfonamides are competitive inhibitors of folic acid synthesis


Antimicrobial agents organized by drug category – we will not have a lecture over the following material but you will need to know it, especially the antibiotics you handle in the lab.

 

A. Antibacterial Agents– fill the blanks for antibacterial in for yourself as you read the chapter.

 

1. Beta-lactams – antibiotics containing a beta-lactam ring

 

a. Penicillins – end in “illin”

b. Cephalosporins – begin with “ceph” or “cef”

 

·        Mode of action -

 

·        Administration route -

 

·        Distribution in the body -

 

·        Mechanism of elimination -

 

·        Special uses -

 

·        Adverse side effects -

 

·        Examples - penicillin, ampicillin, amoxicillin, methicillin

      cephalothin, cephalexin, cefaclor

 

 

 

 

 

 

 

 

 

 


2. Aminoglycosides – end in “mycin” or “micin”

 

·        Mode of action -

 

·        Administration -

 

·        Distribution -

 

·        Elimination -

 

·        Uses -

 

·        Side effects -

 

·        Examples - streptomycin, gentamicin, amikacin, neomycin

 

 

3. Tetracyclines – end in “cline”

 

·        Mode of action -

 

·        Administration -

 

·        Uses -

 

·        Side effects:

 

GI

 

children

 

liver

 

v    Added to animal feed - resulting in widespread resistance

 

·        Examples – oxytetracycline, doxycycline

 

 

 

4. Chloramphenicol

 

·        Mode of action -

 

·        Administration -

 

·        Distribution -

 

·        Elimination -

 

·        Uses -

 

·        Side effects -

 

 

5. Macrolides, lincosamides - mycin

 

·        Mode of action -

 

·        Administration -

 

·        Distribution -

 

·        Elimination -

 

·        Uses -

 

·        Examples:

Macrolide - Erythromycin

 

Lincosamide - Clindamycin

 

 

 

 

 

 

 

 

6. Sulfonamides - synthetic

 

·        Mode of action -

 

·        Administration -

 

·        Distribution -

 

·        Elimination -

 

·        Uses -

 

·        Side effects -

 

·        Example – Sulfamethoxazole (GantanoleR)

 

 

7. Quinolones / Fluoroquinolones- synthetic

 

·        Mode of Action -

 

·        Administration -

 

·        Distribution -

 

·        Elimination -

 

·        Uses -

 

·        Side effects –

 

·        Examples - (nalidixic acid), norfloxacin, ciprofloxacin


B. Anti-Fungals

 

Fewer in number

Selective toxicity more difficult – superficial infections respond well to topical antifungals but systemic fungal infections are challenging to cure.

 

Examples:

1. Polyenes – produced by Streptomyces

 

·    Ex.

Nystatin

Amphotericin B

·    Mode of action -

Both bind to ergosterol in the fungal membrane ΰ leakage ΰ cell death

 

·    Administered -

Topical

IV

 

·    Uses -

 

Poor penetration into fluids. Used for serious systemic infections such as cryptococcal meningitis, histoplasmosis.

 

·    Side effects -

 

Nephrotoxicity ΰ permanent kidney damage in 80% of treated patients.

 

 

2.    Azoles – lots of them

·        Mode of action – inhibit the synthesis of ergosterol

·        Uses – skin and deep systemic mycoses

·        Exs. miconazole, ketoconazole, fluconazole

IV               PO              PO or IV

 

 

3. Griseofulvin – from Penicillium

·        Mode of action – impairs mitotic spindle ΰ inhibits fungal cell division

·        Uses – dermatophytes – a 1st line txt but being replaced by newer antifungals like the azoles.


C. Anti Virals – Development of antiviral chemotherapy has lagged behind the others but has been spurred by HIV/AIDS. Since viruses use host structures and enzymes for replication, inhibiting viral replication without toxicity to the host is difficult.

 

Targets for antivirals – in theory, any step from attachment & entry ΰ exit.

1. Inhibit early viral processes

·        Ex. Amantadine

·        Mode of action – blocks channels in viral envelope ΰ prevents viral uncoating.

·        Administration – PO – 2-4 hrs to peak blood levels.

·        Uses – Protection from and txt of influenza. Prophylaxis for high risk patients. There is fear now that Americans will stockpile antivirals against pandemic flu, use them incorrectly and cause development of antiviral resistance in Influenza virus.

 

2. Inhibit viral nucleic acid synthesis

·        Ex. Acyclovir (ZoviraxR)

·        Mode of action – interferes with viral polymerase (herpesviruses bring their own polymerase)

·        Uses – herpes, varicella infections – to prevent reactivation & encephalitis – can not resolve latent infections – not a cure.

·        Ex. Ribavarin (VirazoleR)

·        Uses – broad range anti-viral. Used for children w/ severe RSV & adults w/ severe influenza or measles.

 

3. Inhibit proteases ΰ formation of defective HIV

·        Ex. saquinavir (Hoffman LaRoche, FDA approved ‘95)

·        Never use in monotherapy due to risk of developing resistance.

 

4. Inhibit reverse transcriptase - retroviruses including HIV (RNA ΰ DNA)

·        Ex. AZT = Azidothymidine (Zidovudine, RetrovirR)

·        Mode of action – analogue of thymidine, interferes with reverse transcription.

·        Administration – PO

·        Uses – slows the progression of immune failure. Given to pregnant women so they won’t pass HIV to their fetus.

·        Side effects – nausea ΰ bone marrow toxicity

·        Never use in monotherapy.