Week 12 - Antimicrobial Resistance Flashcards
Innate Antimicrobial Resistance
Already encoded in genome
Natural and inherited
Predictable once know particular bacterial group
Mechanisms of Drug Resistance
Alter the target site Bypass mechanisms of resistance Destroy the antimicrobial agent Modify the antibacterial agent Reduce intracellular levels of antibiotics
Microbial Biofilms
Can stop penetration of antimicrobials
Biofilms responsible for numerous infections that are difficult to manage
Can be mixed or pure
What are biofilms made up of?
Polysaccharide matrix (75-95%) Bacterial cells (5-25%)
Acquired Antimicrobial Resistance
Results from altered cellular physiology and structure caused by changes in microorganisms usual genetic makeup
- successful genetic mutation
- acquisition of genes from other organism
- combination of mutation and gene transfer
Origins and Evolution of Antibiotic Resistance Genes
Bacterium can become resistant by 2 methods: - Mutation - Acquisition of a new gene Emergence of new genes Spread of old genes to new hosts
Spread of Antibiotic Resistance Genes
Bacteria transsfer resistance genes among themselves by 3 mechanisms:
- Transformation
- Transduction
- Conjugation
Transformation
Incorporation of naked DNA into recipient cell
Recipient cell capable of taking up naked DNA said to be competent
Only occurs naturally in about 40 species
Transduction
DNA transferred as part of a virus
Two ways:
- generalised
- specialised
Conjugation
Promotes horizontal transfer of genetic material from a donor cell to a recipient cell via cell-to-cell contact
Genes from conjugation usually borne on conjugative plasmids and occasionally on conjugative transposons
IS elements
Way microorganisms get information
Elements of DNA that can move around and regulate themselves
Don’t care about homology (can line up in genome wherever they like)
Smallest one made up of transposase bounded by inverted repeats (IR)
Transposase - binds to transposon and catalyses movement by cut and paste mechanism
IR 15-25 bp at each end of element and unique to the IS
Transposase makes staggered cut in target DNA molecule
IS inserts into cut and is replicated
Transposons
Similar to IS elements but include gene not required for transposition
Encode additional proteins that offer selective advantage to their host
How is antibacterial resistance emerging?
Selective pressures
Over prescription
Incorrect following of directions
How can microorganisms alter the target site?
Mutate 30S ribosomal unit so streptomycin can’t bind
Mutations in B unit of RNA pol reduces affinity of rifampicin
Producing altered transpeptidases reduces affinity for B-lactam antibiotics
Producing altered 50S ribosomal subunit that still functions but antibiotic can no longer bind
How can microorganisms bypass mechanisms of resistance?
Instead of using traditional pathway to make folic acid use alternate enzymes so that it can bypass sulphonamides
How can a microorganism destroy the antibacterial agent?
Penicillinases or B-lactamases destroy the B-lactam ring
- Klebsiella sp. produce B-lactamases that destroy ampicillin before drug can reach target
Amidases remove the R-group from penicillins
How can a microorganism modify the antibacterial agent?
Chloramphenicol Acetyltransferase
- attaches an acyl group chloramphenicol preventing it from binding to ribosomes
- lock and key doesn’t work
IMPORTANT!!!
How can a microorganism reduce intracellular levels of antibiotics?
Pumping the drug out of the cell as soon as it comes in
Some cells naturally have resistance to some antimicrobial agents
- penicillin can’t pass through outer membrane of GN cells
Some cells can alter permeability of cell membrane to antibiotic
- Pseudomonas aeruginosa alters size or conc of porins in outer layer
- providing resistance to B-lactamases
Efflux Pumps
Allows cell to remove drugs
Multidrug Resistance Transporters
Get drugs and pump out other harmful components such as heavy metals and compounds
2 classes:
- secondary transporters driven by proton (H+ ions) or sodium motive force
- ATP-binding cassette primary transporters which use hydrolysis of ATP to fuel transport
Generalised Transduction
Phage infects bacterial cell
Hijacks replication machinery making lots of copies of the phage
During packaging, accidentally picks up bacterial DNA instead
This makes a transducing phage
This phage doesnt have genetic code for phage, has code for bacteria
Puts bacterial DNA into new cell
Now doesnt code for hijacking replication machinery, might code for antibacterial resistance gene
Pseudomonas aeruginosa Resistance Characteristics
Produces B-lactamases
Impermeable outer membrane - alters size or concentration of porins in the outer membrane
Active efflux pumps
Forms biofilms
