3. Molecular Mechanisms That Confer Antibacterial Drug Resistance And Development Of New Antibiotics To Circumvent Resistance Flashcards
What are the three classes of antibiotic resistant pathogens that are a major threat to human health?
- MRSA (methicillin resistant staphylococcus arreus
- Drug resistant gram negative bacteria
- Drug resistant mycobacterium tuberculosis
What is MSRA?
How 80% staph infections are resistant to penicillin , and around 19000 deaths/per year are due to MRSA
Example of drug resistant gram negative bacteria?
Klebsielle pneumoniae
What is drug resistant mycobacterium tuberculosis?
The leading cause of death from infectious disease
What are increased antibiotic resistance correlated with?
Increased antibiotic use (eg 1.4 million kg of antimicrobial drugs annually- 1/2 inappropriately prescribed)
How many antimicrobial drugs are used in animal feeds/year?
14 million kg
What are the two types of resistance to antibiotics?
- Inherent “natural” resistance
2. Acquired resistance
Examples of inherent resistance?
Inaccessibility of antibiotic
Extrusion of antibiotic genes- found in bacterial chromosomes
Describe acquired resistance?
Previously susceptible
Resistance in sub populations or strains of bacterial species
Two types of ways to acquire resistance?
- Vertical gene transfer
2. Horizontal gene transfer
What is vertical gene transfer?
The transfer of spontaneous resistance gene mutations in the bacterial chromosome to bacterial progeny during DNA replication
What is the speed of vertical gene transfer resistance like?
Fast, despite mutation being a rare event, the fast growth of bacteria and absolute number of cells attained means it’s fast- spontaneous mutation frequency for antibiotic resistance is 10^-8 -10^-9
What is the process of vertical gene transfer a matter of?
Darwinian evolution driven by natural selection (in a selective environment of the antibiotic, the wild type are killed while resistant can grow and flourish
What is horizontal gene transfer?
Genetic material contained in small packets of DNA can be transferred between individual bacteria of the same species or even between different species
Three mechanisms of horizontal gene transfer?
- Conjugation
- Transduction
- Transformation
What is the main mechanism of horizontal gene transfer?
Conjugation
What is conjugation?
Transmission of resistance genes following direct contact between two bacteria via pilus (like a bridge)
What are the key players in conjugation and what do they do?
Plasmids, located in the cytoplasm of the donor and recipient cell and exchange through the pilus
What does conjugation resistance transfer allow for?
Resistance to spread among a population of bacterial cells much faster than simple mutation and vertical gene transfer would permit
What is transduction?
Antibiotic resistance genes are transferred between two closely related bacteria by means of bacteria specific viruses (bacteriophages)
What happens in transduction?
Resistance genes are integrated into the chromosome of the recipient cell
What is transformation?
When naked DNA is released into the external environment, normally due to death and lysis of an organism and is taken up by another bacterium
How does transformation work?
The antibiotic resistance gene. And be integrated into the chromosome or plasmid of the recipient cell
What are the three principal resistance mechanisms for bacterial survival?
- Efflux pumps
- Enzymatic degradation of antibiotic
- Enzymatic modification of antibiotic
How do efflux pumps work?
They pump antibiotics back out of bacterial cells through efflux pump proteins to keep intracellular drug concentrations below therapeutic levels
Why does efflux pumps work?
Because for antibiotics to be effective, they must reach their specific bacterial targets and act in some reasonable time frame
What are efflux pumps variants of?
Membrane pumps possessed by all bacteria to move molecules in and out of cells
What is an example of resistance via efflux pumps?
Resistance to tetracyclines- concentrations too low to block protein synthesis
What is enzymatic degradation of antibiotic?
The antibiotic is destroyed by chemical modification by an enzyme that is elaborated by the resistant bacteria
Example of enzymatic degradation of antibiotic?
The hydrolytic deactivation of the B-lactam ring in the penicillin by expression of a hydrolytic enzyme b-lactamase by resistant bacteria
How is B-lactam antibiotics destroyed before it can reach its target?
Lactamase-producing Gram positive bacteria secretes the enzyme into the periplasm
How many penicillin molecules can a single B-lactamase hydrolyse per second?
1000 (so if 10^5 enzymes secreted per resistance cell, then 100 million molecules of penicillin are destroyed every second)
What is enzymatic modification of antibiotic?
Antibiotic is modified by an enzyme so it’s no longer effective
Types of resistance enzymes that modify antibiotics?
Acetyl transferase
Phosphoryl transferase
What can happen to the antibiotic chloramphenicol?
It can be enzymatically inactivated by the addition of an acetyl or GO sphere group
What do modifications to antibiotics do?
Decorate the periphery of the antibiotic and interrupt binding to ribosomes
When was the innovation gap in medicine?
1962 to 2000
When was the golden age of discovery?
1940-1960 (eg sulfa drugs, b-lactams) - natural discovery
When was the golden age of medicinal chemistry?
1990-2010(e.g lipopeptides)
What are 3 approaches to develop new antibiotics to circumvent resistance?
- Modification of common core structures of different antibiotic classes using medicinal chemistry
- Identification of new antibiotic scaffolds through searches of underexplored ecological niches and bacterial taxa
- Bioinformatic analysis of bacterial genomes
What is the modification of common core structures of different antibiotic classes using medicinal chemistry?
Synthetic tailoring is widely used- scaffolds are black, peripheral chemical modifications are red
Examples of modification of core structures?
Penicillin
(Original)
Toglicino (end result)
Two ways tetracycline can be modified?
- Scaffold can be chemically modified (creating a tetracycline derivative like tigecycline that no longer is a substrate for the efflux pump
- A new scaffold like retapumulin can be used instead of tetracycline, as it’s not a substrate for the efflux pump
What’s the identification of new antibiotic scaffolds through searches of underexplored ecological niches like?
More than 2/3 of clinically used antibiotics are natural products or their semisynthetic derivatives
Describe bioinformatic analysis of bacterial genomes?
The genome sequences of large numbers of bacteria and fungi reveal up to two dozen silent clusters for natural product biosynthesis and a variety of approaches have been used to turn on silent biosynthetic gene clusters to evaluate the novelty and activity of the resultant small molecules
