Antibiotic Resistance in Bacteria Flashcards
What are Antibiotics?
= substance produced or derived from certain fungi, bacteria or other organism
= can destroy or inhibit the growth of other microorganisms
= antibiotics widely used in prevention and treatment of infectious diseases
= extremely diverse group of products called secondary metabolites:
= complex organic molecules that are non-essential cell growth or reproduction
What are some antibiotic mechanisms?
Inhibit cell wall synthesis
= B-lactams / SCCMec
= e.g. penciliins
Inhibit protein synthesis
= aminoglycosides
= e.g. streptomycin
Disrupt unique components of the cytoplasmic membrane
= polyenes
= (amphotericin B - fungi only)
Inhibit general metabolic pathway not used by humans
= sulfonamides (e.g. sulfanilamide)
Inhibit nucleic acid synthesis
= (fluoro)quinilones - DNA gyrase
= rifampicin-prokaryotic RNA polymerase
Block pathogen recognitions of or attachment to its host
= attachment antagonists (mainly viral)
= antibiotic more toxic to the pathogens and must be less toxic to the host
How does resistance to beta-lactam antibiotics occur?
Beta lactamases
= degrafe beta lactam ring and inactivates antibiotic
= bypasses antibiotic by providing alternative PBPs
e.g. Penicillin resistance
What is the mode of action for Antibiotics affecting protein synthesis?
Streptomycin
= changes ribosome shape
= can’t read mRNA
Tetracycline
= block docking site of tRNA
Clarithromycin
= blocks proper mRNA movement through ribosome
= synthesis stops
What is the mode of action for Antibiotics affecting metabolic pathways?
e.g. PABA (structural analog - sulfonamide)
= binds to enzyme
= released dihydrofolic acid
Sulfonamide will inhibit folic acid synthesis
= blocks pathway
= no dihydrofolic acid produced
= no DNA synthesis
= no bacterial growth
How does development of resistance in bacteria occur?
Population of microbial cells
Exposure to drug
Sensitive cells inhibited by exposure to drug
Remaining population grows over time
Most cells now resistance
What are the causes of Antimicrobial Drug Resistance?
= incorrect prescribing practices
= non-adherence by patients
= counterfit drugs
= use of anti-infective drugs in animals and plants
= loss of effectiveness
= community-acquired
= hospital-acquired
What are the consequences of Antimicrobial Drug Resistance?
= prolonged hospital admissions
= higher death rates from infection
= requires more expensive, more toxic drugs
= higher health care costs
What are antibiotic resistance reservoirs?
Resistome
= collection of all the antibiotic genes and their precursors in both pathogenic and non-pathogenic bacteria
What are the 5 different types of genes that make up antibiotic resistance genes?
Resistance genes found in pathogenic bacteria
Resistance genes in environmental bacteria
Resistance genes found on antibiotic produces
= e.g. soil-dwelling bacteria and fungi that encode resistances
Cryptic resistance genes
= embedded in bacterial chromosome but do not obviously confer resistance
= cus their level of expression is low or not expressed
Precursor genes
= do not confer antibiotic resistance
= encode protein that confer some basal level activity against antibiotic molecule / have affinity to molecule
= interaction may evolve to a full resistance gene with correct selection pressures
(groups not independent - some overlapping)
What are some global reservoirs for antibiotic reservoirs?
Soil
Freshwater (+aquaculture)
Oceans
Global Resistome
What are the different types of resistance?
Passive
= antibiotic has no target or cannot enter cell
= has no action
Mutation
= target site changes so antibiotic is ineffective
Acquired
= actively acquired resistance
= genetic elements - e.g. plasmids
What is an example of mutation (in Antibiotic Resistance)?
Streptomycin
= binds to 16S rRNA of 30S subunit of bacterial ribosome
= causes inhibition of protein synthesis
= mutation 16S rRNA gene by point mutation, insertion, inversion, deletion or duplication
= changes active site and makes streptomycin ineffective
What is an example of Aquired antibiotic resistance?
Horizontal gene transfer
= acquisition of genes from other microorganisms
= transformation
= transduction
= conjugation
(often plasmid encoded aided by transposons and integrons)
What are modular mobile elements?
= transposable elements
= DNA sequences with ability to move (transpose) themselves to different locations within the genome
2 main types
= DNA transposons = physically cut themselves out of original location and insert into new location in the genome
= Retrotransposons = use copy and paste mechanism, use own reverse transcriptase to create DNA copy of themselves from RNA sequence then insert this copt into new location in genome
What is an example of a multi-resistance plasmid?
= pFBAOT6
= IncU multi-resistance plasmid
= 87,478 bp in length
= 94 predicted coding sequences
(only 12 not assigned a possible function)
= core functions in the first 31 kb
= genetic load region 54 kb
(class I integron, several transposable elements, potential composite transposon of 43 kb)
What are some molecular mechanisms of antibiotic resistance?
Decreased permeability (in biofilm)
Low expression of target proteins
Persister cells
Movement of AMR genes
Cell survival
What is MRSA?
= Methicillin Resistant Staphylococcus aureus
= any strain of S. aureus that has developed (through process of natural selection) resistance to beta-lactam antibiotics
= inlcuding: penicillins (methicillin, dicloxacillin, nafcillin, oxacillin) and the cephalosporins
= gram-positive bacteria acquire resistance to beta-lactam antibiotics through production of protein called: PBP2a
= only known way for Staphylococcus strains to spread the gene in the wild is:
horizontal gene transfer
= main treatment is Vancomycin
(BUT vancomycin strains have been found in hospitals)
= 25-30% of population have S. aureus in their system without signs of illness
= global issue
= though to originate from S. aureus found on skin of hedghogs that survived next to fungus: Trochophyton erinacei (which produces own antibiotics)
How does MRSA Virulence occur?
Sccmec
(staphylococcal cassette chromosome mec)
= mobile genetic element
= includes mecA gene coding for resistance to the antibiotic methicillin
mecA gene encodes protein PBP2a
(penicillin binding protein 2A)
PBP2a
= has low affinity for beta-lactam antibiotics
(e.g. penicillin and methicillin)
= enables transpeptidase activity in the presence of beta-lactams
= preventing them from inhibiting cell wall synthesis
Now variants on SccmecA
= I,II,III,IV
ACME
(arginine catabolic mobile element)
= a virulence factor present in many MRSA strains
(BUT not prevalent in MSSA)
= facilitates stable skin colonisation, wound infection , person-to-person transmission
What is the model for MecA induction?
- Antibiotic exposure
= S. areus exposed to beta-lactam antibiotics - Activation of sensor kinase
= antibiotic triggers activation of sensor kinase
= (membrane bound protein that detects presence of antibiotic and transmits signal to cytoplasm) - Phosphorylation of response regulator
= signal received by response regulator
= becomes phosphorylated and activates the transcription of downstream genes - Activation of mecA expression
= one of the downstream genes activated
= encodes regulatory protein that controls expression of antibiotic resistance genes - Expression of antibiotic resistance genes
= mecA binds to and inhibits the activity of another regulatory protein: Blal
= Blal normally repressed antibiotic resistance genes
= inhibition leads to expression of these genes
(EXTRA READING)
What is the action of penicillin?
Penicillin binds to and inhibits activity of penicillin binding proteins (PBPs)
= bacteria are then unable to build new cell wall material
= their existing cell wall becomes weakened and susceptible to damage
= these bacteria are unable to grow and divide properly
= eventually killed by the antibiotic
Resistance through PBP2A
= PBP2A structurally different, not inhibited by beta-lactam antibiotics
= carry out own cell wall synthesis activity
= bacteria maintain structural integrity and survive
What is NDM-1?
= New Dheli Metallo-beta-lactamase-1
Carbapenems
= class of beta-lactam antibiotics
= developed to overcome antibiotic resistance mediated by beta-lactamase enzymes
blaNDM-1 gene
= produces NDM-1 = carbapendemase bet-lactamase
= enzyme that hydrolyses and inactivates to a broad range of beta-lactam antibiotics
(inclusing carbapenem antibiotics)
NDM-1
= first isolated in Klebsiella pneumoniae isolate
= most commonly found in gram negatives
(e.g. E. coli, K. pneumoniae)
What is the spread of NDM-5?
NDM-1 has variants 1-17
NDM-5
= new variant of NDM carbapenemase
= identified in a MDR E. coli ST648 isolate
Resistant to:
= all cephalosporins
= carbapenems
= aminoglycosides
= quinolones
Suceptible to:
= colistin
= tigecycline
NDM1-8
= have different amino acid alignments
= the leucine variant = makes it more easy to spread = hydrolyses carbamazepine
What are the 2 approaches to manage antibiotic resistance?
2 approaches:
- Managing existing resistant pathogens
= improving hygiene in hospitals
= screening hospital visitors
= isolate patients
(for MRSA) - Avoid future evolution of more resistance
= changing selection on bacteria
What management practices are there fore antibiotic resistance?
Reduce inappropriate prescription of antibiotics
= increase public awareness that many diseases cannot be cured with antibiotics
= educate doctors
Reduce us of agricultural antibiotics
Combat use of low-quality antimicrobial products
= risks poor health outcomes + spread of resistance
Increase number of patients who finish full course of antibiotics
= make more affordable to low income regions
Restrict use of new antibiotcs
Where possible use other treatments
= vaccines
= ?phage treatment
Get drug companies to develop new antibiotics
= last new class was in 1980s
= e.g. Teizobactin
= active against: Clostridium difficile, Bacillus anthracius, MRSA, M. tuberculosis
= ineffective against most gram-negative bacteria
= effective at low dose
= published in 2015 but still not in commercial use
