antibiotic resistance Flashcards
how do you identify resistance?
antibiotic sensitivity and susceptibility testing (growing the organisms in the presence of antibiotics, if it grows in the presence of a high concentration it is resistance and if lower is sensitive
low MIC = sensitive
detection of antibiotics resistance genes - see if the genes are present in the organism
how do you carry out antibiotics sensitivity testing?
on a liquid media microtitre plate - add antibiotics to wells in doubling dilutions and add microorganisms. Ensure that there is one row without AB and one without microorganisms. Incubate to make microorganisms grow and look at the colour change. see the concentration of the AB that is sufficiently high to kill bacteria and find MIC and compare to find breakpoint
what is the breakpoint?
it is the MIC value whereby if the MIC is lower than or the same then the organism if sensitive to the antibiotics
why are there different breakpoints?
concentration of different antibodies to human body is different
what is disc sensitivity testing?
it is on a solid media where you add organisms to an agar plate and use a swab dipped into culture to spread evenly. You add antibiotics of known concentrations and incubate overnight, Compare the zones against the published breakpoint sizes and interpret and report
how do you detect antibiotic resistance genes?
nucleic acid amplification tests such as PCR - can be applies to any sample type
what are the advantages an disadvantages of the antibiotic resistance genes tests?
they are sensitive and fast
they are expensive and the presence of the gene does not necessarily mean resistance
what are the resistance mechanisms?
innate or acquired
what is innate resistance?
it is a fundamental property of a bacteria or antibiotics or combination that is usually to do with the entry into a cell or permeability to ABs
what are examples of innate immunity?
gram positives - glycopeptides, daptomycin
gram positives - aztreonam or colistin
anaerobes/streptococci - aminoglycosides
why wont aminoglycosides work in an anerobic bacteria?
they are taken into the cell to work which is aerobic meaning that they do not work in anaerobic
what is meant by acquired resistance?
acquisition of a gene that encodes a resistance mechanism
what are examples of acquired resistance?
horizontal transfer, new mutation - usually an antibiotic modifying enzyme or a target alteration
why might antibiotics not work?
wrong type, enzymatic degradation, target alteration so cannot bind or decreased permeability
what are examples of decreased permeability?
vancomycin - gram negative bacilli have an outer membrane that is impermeable to it
gentamicin - anaerobes - uptake of aminoglycosides is O2 dependent
what are examples of the target modification?
flucloxacillin - MRSA has an altered penicillin binding protein meaning that beta lactams cannot bind
trimethoprim - gram negative bacilli have mutations in the dihydrofolate reductase genes
vancomycin - VRE has altered peptide sequence in gram negative peptidoglycan meaning that they binding of vancomycin is reduced 1000 fold
what are examples of enzymatic degradation?
penicillins and cephalosporins - there are extended spectrum beta lactamases, penicillinases and carbapenamases
other enzymes will inactivate aminoglycosides and chloramphenicol
what are examples of drug efflux?
multiple antibiotics, especially in gram negatives, antifungal triazoles and candida spp
what are the main reasons for development and spread of resistace?
horizontal and vertical transfer, single genes encoding resistance and resistance genes encoded in plasmids
what are examples of single gene resistance?
antibiotic modifying or altered targets
what are plasmids for in resistance?
these are circular DNA sequences that are transmitted within and between species mainly bhy conjugation
what role does horizontal transfer play in resistance?
enabled by transponons and integrons, DNA sequences are designed to be transferred from plasmid to plasmid or to chromosome/genome between or within species - often contain cassettes with multiple resistance genes
what role does vertical transmission play in resistance?
chromosomal or plasmid borne resistance genes transferred to daughter cells on bacterial cell division - binary fission
what are the causes of resistance?
bacteria are subject to constant spontaneous mutations that may confer a survival advantage that favours the growth and propogation of the mutant strain. If the organisms is growing in the presence of a sub-inhibitory concentration of ABs then the development of the resistance gene will confer an advantage, out compete and produce a new strain therefore making it more likely to reinfect
