Bacterial Mediated Disease

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Antibiotics - Classification

Selective Toxicity = the aim of antibiotics = kill or inhibit growth of pathogen without casing serious harm to host cells

Antibiotics can be classified by

Bacteriostatic or Bactericidal

Broad or Narrow Spectrum e.g. Penicillin G is naarrow spectrum = minimum effect on intestinal flora BUT Cefotaxime a Cephalosporin = Broad Spectrum - Polymicrobial although it has been reported that its seems to have a higher effect on Gram negatives!

Mechanism of Action

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Inhibitors of Nucleic Acid Synthesis

There are 5 ways in which Antimicrobial agents interfere with nucleic acid synthesis - will mention 2

1. Altering the base pairing in DNA template strand e.g. Acridines - double the distance between adjacent base pairs = deleterious as affects reading frame of amino acid = distortion of DNA helical structure = bacteriostatic! = Antiseptic use only as too toxic for clinical use

2. Inhibiting DNA gyrase (Topo 2) e.g. Fluoroquinolones - Inhibition of this gyrase = affects supercoiling + gene regulation e.g. Salmonella enterica requires it when in macrophage environment for survival genes + invasion genes to be switchef on - if not = dies! = RESISTANCE exists = mutant Topo (less affinity of inhibitor OR pumping out drug!

3. Inhibitors of RNA Polymerase - interrupts DNA transcription! e.g. Rifampicin

4. Interfering with Nucleotide Synthesis = Folate antimetabolites = target Class 2 Reactions

Trimethoprims are folate antagonists that mimic folate enough to fool the bacterial enzyme dihydrofolate reductase

This enzyme = converts dihydrofolate to tetrahydrofolate - form of folate used in purine synthesis and thymidylate synthesis

The human equivalent enzyme is not as sensitive to this bacteriostatic drug = Selective Toxicity

They resemble the pteridine moiety in folate

Sulfonamides are PABA structural analogues (p-aminobenzoic acid) and PABA is an important precursor to folic acid synthesis.

Sulfonamides compete with PABA for the bacterial enzyme dihydropteroate synthetase

Active part = Sulphanilamide

Using both Sulfonamides + Anti-folates like Trimethoprims in combination therapy = synergistic effect = block 2 steps in the same pathway + less chance resistance occurring = Co-trimoxazole

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Inhibitors of Protein Synthesis

Steps in Protein Synthesis

1. Initiation

- Stage 1

2. Ribosome

binds to m-RNA at start codon -recognized by initiator tRNA = methionine start

codon

Elongation

- Stage 2

1. New

tRNA arrive at A site, old one move to P site and transpeptidation occurs

-elongating polypeptide chain moved to A site only for transpeptidation -moves

to P site to receive another t-RNA molecule -exiting = E site

2. The

tRNA binds by complementary base pairing with m-RNA via its anticodon

3. The

tRNA with amino acid = aminoacyl t-RNA

4. The

tRNA with growing peptide chain = peptidyl t-RNA

Termination

- Stage 3

1. Stop

codon reached

2. Translation

terminated

3. Polypeptide

chain released

Those that inhibit 30S

subunit = Aminoglycosides and Tetracyclines e.g. Oxytetracycline

1. Interrupt with

anticodon reading of tRNA by the mRNA and compete with A site with aminoacyl

tRNA

Those that inhibit 50S subunit e.g Choramphenicols

1. Inhibit Transpeptidation = no peptide bond formation = no polypeptide elongation

Broad spectrum -affects both Gram positives and Gram negatives

2. Bacteriostatic to most bacteria

3. Bactericidal to H. influenzae and N. meningitidis

4. Resistance to it = bacterial production of acetyl-transferases which can inactivate drug

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Inhibitors of Cell Wall Synthesis

All B-Lactams interfere with the synthesis of bacterial cell wall peptidoglycan (Made up of N-acetyl-muramic acid and N-acetyl-glucosamine plus peptide chains)

They are Bactericidal - kill bacteria

They bind to Penicillin Binding proteins such as Transpeptidase

Binding to transpeptidase = inhibits enzyme = no cross-linkages can be formed with pentapeptide chains in backbone of peptidoglycan between Pentaglycine and D-alanyl in pentapeptide chain

Inactivate inhibitor of autolytic enzymes = lysis in bacterium

B-Lactam ring and thiazolidine ring in their structure = make up penicillin nucleus

B-lactamases target B-lactam ring that is important for penicillin antibacterial activity

1st, 2nd and 3rd gen - extended spectrum ones such as 3rd gen Piperacillin = effective against most bacteria

Examples = Penicillins and Cephalsporines

Vancomysin = Glycoprotein = binds pentapeptide + prevents Transpeptidase cross linking

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Bacterial Resistance

Some are born great (Gram negative's cell membrane), some achieve greatness (spontaneous mutation) and others have greatness thrust upon them! (Mobile Genetic elements -plasmid etc.)

Since 1940s, development of safe and effective drugs for bacterial and other infections = revolutionized medical treatment

Mortality and Morbidity associated with the diseases = dramatically reduced

Counterbalanced by emerging drug resistant organisms

Bacteria possess resistomes = pool of genes that favour development of resistance

Restriction of medical treatment against infections = worrying! - MRSA (nosocomial infections) - resistant to many antibiotics - now Vancomycin also emerging VISA, VRSA

Plasmids = extrachromosomal elements - self-replicate + circular DNA

1. B-lactamases - Augmentin = clavulanic acid shields ampicillin from the enzyme

2. Acetyl-transferases

3. Chromosomal and Ribosomal subunit resistance - rare and clinically not important

4. Plasma-mediated genetic exchange - carry R genes = R plasmids = Conjugation transport between bacteria

5. Genetic Duplication and Amplification of pre-existing R genes

6. Transfer within bacteria = Transposons

7. Others = integrons + Gene Cassettes

Example mecA gene = Methicillin resistance = encodes different PBP2a = new transpeptidase = mutant = evades target! - not recognized

Biofilms making stuff harder = antibiotic neuralization, reduced penetration, altered physiology of microbes in Biofilm

Biofilm = stress response e.g. Ethanol with S. aureus = PIA/PNAG ica operon expression!

Bacterial Resistance was always present = more prevalent = selective pressure = High adaptability of Bacteria!

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Types of Bacteria

Types of Bacteria/Pathogens

that exist = Primary Pathogens = cause disease in healthy organism e.g. M.

leprae, Opportunistic = cause disease in immunocompromised patients e.g.

P. aeruginosa and Harmless Bacteria!

Harmless Bacteria - can be

beneficial to Humans = Various commensal bacteria and often in harmony in

healthy humans - Microflora in Skin, Mucous Tissues, Oral Cavity and most

importantly = Microflora in GIT

Microflora in GIT =

Bifidobacterium, Lactobacillus and Streptococcus among others = Involved

in several metabolic reactions e.g. Vitamin synthesis like Vitamin K, Gas

Production like CO2 and Steroid Metabolism!

Microflora = carry reactions and compete with Pathogenic bacteria = less risk of infections and disease

Changing normal microflora = via Antibiotics = important to use Probiotics = to restore!

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Pathogenesis of Bacteria Part 1

Microbial Pathogenesis = the process by which microorganisms causes disease

Virulence = ability of Bacteria to cause disease

Virulence factor = certain bacterial characteristics which contribute to virulence (its ability to cause disease)

Measuring of Virulence = LD50 test = how many cells injected into animals that is lethal to 50% of the animals - if high virulence = only small difference (narrow therapeutic index) between the amount of cells to kill 50% to the amount needed to kill 100%