lecture 9) antibiotic resistance mechanisms Flashcards
what is antibiotic resistance?
resistance of a microorganism to an antimicrobial drug that was originally effective for treatment of infections caused by that microorganism
how can you measure antibiotic resistance?
antiobiotic resistance can be reflected by an increase in MIC
what is MBC?
minimum bacterial concentration
the minimum concentration of an antimicrobial that kills in vitro
what is the process of determining the MBC?
grow bacteria at different concentrations
plate bacteria on drug free plates
only bacteria that havent been killed will grow back
what is the difference between MIC and MBC?
MIC just looks at growth
MBC is a 2 step process (grow and plate)
how are the values of MIC and MBC correlated with what is used in a clinical setting?
using breakpoints
what is a breakpoint?
the chosen concentration of an antibiotic which defines whether a species of bacteria is resistant to the antibiotic
determines how the antibiotic will be administered
using breakpoints and MIC how is antibiotic resistance determined?
if the MIC is less than the BP the bacteria is susceptible to the antibiotic
if the MIC is greater than the BP the bacteria is resistant to the antibiotic
what are the units for breakpoints?
mg/L
what is a bacteriostatic agent in relation to antibiotic resistance?
an agent that reaches MIC levels in blood or tissues
what is a bacteriocidal agent in relation to antibiotic resistance?
an agent that reaches cidal levels in blood/tissues
describe a broad-spectrum agent in terms of MIC/BP
low MIC for many different bacteria types
describe a susceptible microbe in terms of MIC/BP
inhibited by an agent at a low MIC
describe a resistant microbe in terms of MIC/BP
inhibited by an agent at a high MIC
resistant microbes will only be killed at a dose higher than what a clinician will use
generations are used as an indication for the level of resistance of an antibiotic. describe this method of indication
the higher the generation of antibiotic used to treat the resistant bacteria, the worse the resistance
4 generations: 4th generation is the very very last resort as there is no generation after this and it is likely that the bacteria will become resistant to this one too
name 3 mechanisms of resistance
drug efflux pumps
drug inactivation - enzymes modify drug
drug inactivation - enzymes modify receptor
what mechanism of resistance do beta lactams use?
inactivation of drugs by enzymes - modify drug beta lactamase (enzyme produced by host) breaks down the beta lactam ring in penicillins that binds to a surface protein to prevent the formation of peptide bonds in the peptidoglycan later no beta lactam ring = no inhibiton of infection = bacteria resistant
what makes staphylococcus aureus resistant to penicillin?
presence of penicillinase which is a beta lactamase so will hydrolyse the beta lactam ring
what was used to treat S. aureus infections after they became resistant to penicilin?
methicillin
what infection is induced if S. aureus strains are resistant to methicillin?
MRSA (methicillin resistant staphylococcus aureus)
describe the mechanism of resistance that S. aureus has for methicillin
inactivation of drug using enzymes - modify receptor
resistant strains have an altered pen binding protein that prevents methicillin from binding to the protein. transcription not inhibited so neither is the infection
what genetic components make MRSA resistant?
staphylococcal casette chromosome is mec (SCC-mec)
SCC-mec = mobile genetic elements
each SCC-mec has a casette chromosome recombinase (CCr) and a mec gene complex
mecA encodes for a varient penicillin binding protein (PBP)
PBP has a lower affinity for beta lactams = resistant to virtually all beta lactams
what type of antibiotic is streptomycin?
aminoglycoside
what is streptomycin used to treat?
streptomyces ssp.
how does streptomycin act against streptomyces spp.?
protein synthesis inhibitor
targets 30S subunit that strep ssp. produces
why doesnt streptomycin kill the producer strain?
something attaches to the producer strain making it inactive
what is characteristic about aminoglycoside phosphotransferase (APH) in streptomycin resistance?
chromosomally acquired streptomycin resistance
what is the chromosomally acquired resistance to streptomycin commonly due to?
mutations in gene encoding ribosomal protein s12 and rpsL
what does rspL gene encode for in streptomycin resistance?
encodes for a protein that will be incorporated into a ribosome
strep will want to go there to inhibit the infection
if this gene is modified then the protein wont be there
what is chloraphenicol?
man-made antibiotic
protein synthesis inhibitor targeting 50S subunit
what is the method of resistance bacteria have against chloraphenicol?
modifies acetylation of chloraphenicol by chloraphenicol acetyltransferase
what is the secondary method of resistance bacteria have adapted against chloraphenicol?
chromosomally encoded cmIA produces more OmpA leading to decreased permeability of membrane preventing the action of chloraphenicol
where are drug efflux pumps present?
gram positive and negative bacteria except from the RND family of gram negative bacteria
what is the mechanism of resistance for drug efflux pumps?
TolC and AcrA are genes involved in forming the transporter part of the efflux pump that removes the antibiotic from cell
why is the TolC and AcrA channel needed in gram negative bacteria?
dont have periplasmic intermediate therefore if the antibiotic was pumped out of the cell it would come straight back in again as there would be no control mechanism
what makes efflux pumps multifunctional tools?
they can make secretion systems
in E.coli, AcrA and TolC have been shown to extrude what antibiotics?
chlorampenicol fluoroquinolone tetracycline novobiolin beta lactams
AcrA and TolC in S. typhimurium can expel what antibiotics?
quinolons
chloramphenicol
tetracycline
nalidixic acid
describe the role of TolC
outer membrane bound protein
attaches and hooks any protein
fuse components in inner and outer membrane
how many rings are there in tetracycline?
4
what does tetracycline specifically target?
30S subunit in protein synthesis
what is a plasmid?
extra chormosomal material
can replicate at the same or different time as chromosom
contains all the genes needed to make an efflux pump
what is an R plasmid?
resistant plasmid
how would you detect anomalies in plasmids?
each plasmid has different GC content
plot GC content to detect abnormalities
what is GC content?
guanine-cytosine content of a chromosome
what are transponons?
mobile genetic elements that regenerate into a chromosome
why would transponons be considered dangerous?
mediator of resistance
excise from chromosome and integrate elsewhere
can also excise from chromosomes and integrate into plasmids
where can antibiotic genes be found?
chromosomes
plasmids
how can antibiotic resistant genes be transferred?
horizontal and vertical transmission
explain vertical transmission of antibiotic resistant genes
selective advantage means a spontaneous strain arise
antibiotic is added
susceptible cells will die
resistant cells will remain alive, grow, replicate and propagate their selective advantage onto subsequent generations
explain horizontal transmission of antibiotic resistant genes
genetic material is transferred between 2 different bacterial cells
in the presence of antibiotic the suscpetible cells will die (majority are susceptible)
resistant cells will remain alive
resistance genes transfer from cell to cell
what type of gene transfer is the main reason for antibiotic resistance?
horizontal gene transfer
what is the main difference between horizontal and vertical gene transfer of antibiotic resistance?
vertical transmission is genes being propagated on to subsequent generations therefore there is an increase in number of cells
horizontal transmission is when genes are transferred from cell to cell so there is no increase in overall cell number but there is an increase in the number of cells that are resistant to antibiotic
what is the main driver of vertical and horizontal gene transmission?
selective advantage
name the mechanisms of horizontal gene transfer
natural transformation
conjugation and transduction
what happens to the bacteria during natural transformation of horizontal gene transfer?
manipulation of bacteria = can take up DNA
what species is natural transformation of horizontal gene transfer found in?
neisserria
bacillus
streptococcus
haemophillus
what do the bacteria develop during natural transformation of horizontal gene transfer and when does this happen?
develop T6SS when in competition for food, water etc
what do certain species in vibrio use T6SS for in regards to natural transformation of horizontal gene transfer?
use it to kill cells around it and then take up their DNA
use it as a survival mechanism
what mediates natural transformation in horizontal gene transfer?
competence proteins
what is a competent bacteria?
bacteria that can take up DNA
what does the secretin do in competence proteins in natrual transformation of horizontal gene transfer?
forms a ring in outer membrane that is part of protein secretion system
pairs with something involved in protein synthesis that will pull in DNA
what is the ring in the outer membrane formed by secretin normally involved in?
extrusion of material
what is conjugation with regards to antibiotic resistant gene transfer?
mechanisms of transferring antibiotic resistant genes from one cell to another via horizontal gene transfer
what type of replicate are involved in conjugation of horizontal gene transfer?
chromosome replicates only even though plasmids can integrate into the chromosome and replicate
what does transduction use during horizontal gene transfer?
bacteriophage
how are bacteriophages used in gene transfer?
go into cell and attack it
inject their DNA into host’s DNA to hijack the host
phage DNA integrates into host chromosome/plasmid
phage particles released from bacteria
package DNA from host
name the gene for conjugation and efflux pumps
F factor
what is the F factor?
gene for conjugation and efflux pumps
fertility factor: origin of transfer
what type of enzymes are antibiotic degrading enzymes?
beta lactamases
where can you find efflux pumps?
inner and outer membrane
what can beta lactamases modify?
their own target, different target or can break target down
what is the consequence of altering the permeability of the membrane?
prevents things entering the cell
