Lecture 9 - Antibiotic resistance Flashcards
What is antibiotic resistance?
- This is the ability of an organism to resist the antimicrobial drug
- Penicillin introduced 1940, by 1959 95% of all staphylococcus aureus strains were resistant to penicillin.
- It can occur naturally or can be acquired from other bacteria.
Resistance is now a global threat to public health.
Why is antibiotic resistance increasing.
- Inappropriate use of antibiotics
○ Animal husbandry
○ Agriculture
○ Doctors - Difficult as don’t want to miss meningitis- Weak or absent antimicrobial resistance surveillance and monitoring systems
- Poor infection prevention and control practices - prevention is better than cure
- Insufficient diagnostic, prevention and therapeutic tools
- Overuse of antibacterial products
- Disruption of commensal microflora, natural protection from pathogens. - leads to increased use of antibiotics
Antibiotics in water supplies.
Why is antimicrobial resistance a problem?
- Leads to treatment failure
- Increased mortality
- Increased healthcare cost
- Resistance spread in the community
Spread of resistance from acute to primary care and from county to country
How does resistance happen.
Intrinsic resistance (naturally occurring): Resistance occurring due to normal genetic,, structural or physiological state of the organism - defines ‘spectrum’ of antibiotic sensitivity.
Acquired resistance: Resistance that results from altered genetic, structural or physiological state of the organism.
Induced genetic mutation (after pressure from antibiotics)
Acquired: By transfer of genetic material from one bacteria to another.
Describe the way in which microorganisms can be resistant to antibiotics.
- Lack of target binding site or altered binding site
○ Bacteria without cell wall are resistant to penicillin- Efflux pump:
○ Some bacteria possess pumps that exports the antibiotic outside the cell - Production of an enzyme that inactivates antibiotics.
○ E.g. beta lactamases, cephalosporinases, carbapenemase - Alternative metabolic pathways
○ This is when an antibiotic acts by inhibiting a metabolic pathway in the microorganism the cell reverts to using an alternative pathway to evade the action of the antibiotic - Decreased permeability or loss of porin channels
○ Lack of porin channels prevents antibiotic entry - Formation of biofilm
○ In a biofilm the bacteria adhere to a surface, stick to each other and are covered with a polymeric substance (slime). This prevents penetration of the antibiotics. The bacteria in a biofilm do not actively replicate, this reduces the activity of antibiotics.
- Efflux pump:
Describe some of the resistance mechanisms for common antibiotics.
- Beta-lactam antibiotics
e.g. penicillins & cephalosporins - Resistance mechanism - Enzymatic destruction of the beta lactam ring by beta lactamase.- Aminoglycosides
E.g. gentamicin - Resistance mechanism - Modifying enzymes alter the aminoglycosides molecule and make it difficult to bind to ribosomes. Reduces uptake due to decreased porin channels. Altered ribosomal biding site. - Tetracyclines
Resistance mechanism - Reduces uptake & efflux pump - Quinolones
Resistance mechanisms - Decreased uptake & altered DNA gyrase binding site.
- Aminoglycosides
How is antibiotic resistance aquired by genetic transfer.
- Genes that confer resistance can be carried on chromosomes or plasmids
- Plasmids are extra-chromosomal genetic material that moves easily from one bacteria to the other by
○ Transduction
○ Conjugation
Transformation
- Plasmids are extra-chromosomal genetic material that moves easily from one bacteria to the other by
Describe transduction.
- A phage infects the donor bacterial cell
- Phage DNA and proteins are made and the bacterial chromosome is broken down into pieces
- Occasionally during phage assembly, pieces of bacterial DNA are packaged in a phage capsid. Then the donor cell lyses and releases phage particles containing bacterial DNA.
- A phage carrying bacterial DNA infects a new host cell, the recipient cell.
Temperate phage can adopt different lifestyles
* Lytic cycle - Cell lyses
Lysogenic cycle
Describe conjugation.
- The F+ donor cell containing an F plasmid is capable of synthesising a sex pilus
- The sex pilus contracts the recipient F+ cell.
- The plasmid is activated for transfer when an endonuclease cleaves one strand of DNA at the origin of transfer.
- The sex pilus retracts and pulls the donor and recipient cells together. The F- plasmid is transferred as a single- stranded DNA molecule.
The complementary strands to both the F plasmid strands are synthesised in the doner and recipient cells both cells are F+ and synthesise sex pilus.
Describe transformation.
Free DNA is picked up
1. Recipient cell takes up donor DNA
2. Recombination occurs between donor DNA and recipient DNA
Double stranded DNA binds to the plasma membrane. Enzymes cut the DNA. Other enzymes break down one DNA strand. A protein bind the DNA. DNA replaces a portion of the bacterium’s DNA.
* New imagining method shows how bacteria use their long and mobile appendages - called pili to bind to or ‘harpoon’ DNA in the environment.
Describe the mechanism of beta-lactams-1
Mechanism of beta-lactams-1
* B-lactams are a group of antibiotics acting on the cell wall of a bacterial cell
* These include - penicillins, cephalosporins, carbapenems and monobactams
* These bind to, and inhibit, carboxypeptidases and transpeptidases.
* These are the cell wall synthesising enzymes, also called penicillin-binding proteins, PBPs, that catalyse the D-ala-D-ala cross linkages of the peptidoglycan wall that surrounds the bacterium
* Penicillin binds to and inactivates the PBP
* As a result there is a weakening of the cell wall structure, leading to cell lysis.
Resistance to mechanism of B-lactams
In Gram -ve bacteria the resistance is mostly due to a combination of endogenous acquired B-lactamases, along with a natural up-regulated impermeability and efflux.
What are Extended spectrum B-lactamases (ESBLs)
Extended spectrum B-lactamases (ESBL)
* A group of enzymes produced by Gram negatives such as E. coli responsible for extensive resistance to a range of β- lactam antibiotics.
* The gene for this resistance is carried on a plasmid and transferable from one bacteria to the other.
* Previously seen in hospital isolates but now found in lots of community isolates.
* Causes hospital outbreaks and can only be treated with a limited number of antibiotics.
Treatment for ESBLs
Commonly used medications include:
* carbapenems (imipenem, meropenem, and doripenem)
* cephamycins (cefoxitin and cefotetan)
* fosfomycin
* nitrofurantoin
* beta-lactamase inhibitors (clavulanic acid, tazobactam, or sulbactam)
* non-beta-lactamases
colistin, if all other medications have failed
What laboratory tests are used to test resistance.
Laboratory detection for resistance
* Disc diffusion testing: has standard zone measurement for interpretation
No zone -resistant
* E-test method with antibiotic strips, Minimum inhibitory concentration
* Molecular detection - PCR
○ Normal - presence or absence
Real time quantitative PCR
Name some solutions to the problem of resistance.
- Synergistic action of multiple antimicrobials
- Look after the antimicrobial agents that we have
–Antimicrobial stewardship globally
–Better diagnostics and surveillance - Find alternative antimicrobial agents and technologies
–New Antimicrobial drug classes
–Vaccine development and immunotherapy
- Look after the antimicrobial agents that we have
UK strategy
* Optimising prescribing practice
* Improving infection prevention and control
* Raising awareness and changing behaviour
* Improving the evidence base through research
* Developing new drugs, vaccines and other diagnostics and treatments
* Improving the evidence base through surveillance
Strengthening UK and international collaboration
