Lecture 6 - Antibiotic resistance Flashcards
Delivery of antibiotics
Humans - Pills, injection (wounds), intravenous delivery (larger amounts and more sustainable)
Animals/farming - Used to increase animal yield e.g. tetracycline, mass production
Humans (again) - raw meats to prevent contamination
What are antibiotics
- Substance produced by fungi/bacteria/other organism that destorys/inhibits microorganism growth
- Widely used in prevention and treatment of infectious disease
- Extremely diverse group called secondary metabolites: complex organic molecules that are non-essential cell growth or reproduction
Antibiotic mechanism
Antibiotic mode of action: Cell wall
Beta-lactam antibiotic
Inhibit cell wall synthesis
Penicillin inteferes linking enzymes, NAM subunits remain unattached to neighbours
Resistance to beta-lactam antibiotics
Beta lactamases - degrades beta-lactam ring and inactivates antibiotic
Bypasses antibiotic by providing alternative PBPs
Penicillin resistance
Penicillin -> Penicilloic acid by beta lactamases
beta lactamase breaks bond in beta lactam ring of penicillin to disable molecule. Bacteria with this enzyme resist effects of penicillin and other B-lactam antibiotics
Antibiotics: metabolic pathway
PABA pathway
PABA -> Dihydrofolic acid by enzymes
Enzymes:
PABA and other substrates enter enzyme, dihydrofolic acid released
Can be inhibited by sulfonamide
Dihydrofolic acid -> Tetrahydrofolic acid by enzymes
Tetrahydrofolic acid -> Purine and pyramidine nucleotides
Purine and pyramidine nucleotides -> DNA and RNA
Antibiotic consumption in England
2007 - 35 million prescriptions (19M for penicillin)
Aim: 15% reduction by 2024
WHO priority pathogens
Critical:
Acinetobacter baumannii - Carbapenem-resistant
Pseudomonas aeruginosa - Carbapenem-resistant
Enterobacteriaceae - Carbpenem-resistant, ESBL-producing
High:
Enterococcus faecium - Vanomycin resistant
Helicobacter pylori - Clarithromycin resistant
Medium:
Streptococcus pneumoniae - penicillin-non-susceptible
Haemophilus influenzae - ampicillin-resistant
Development of resistance in bacteria
Drug-sensitive cell and drug resistant mutant exposed to drug in population of microbial cells
Sensitive cells inhibited by exposure to drug
Remaining population grows, and most cells now drug resistant
What causes drug resistance
Causes:
Incorrect prescribing practices
non-adherence by patients
Counterfeit drugs
Use of anti-infective drugs in animals & plants
Loss of effectiveness
Community-acquired (TB, Pneumococcal)
Hospital-acquired (Enterococcal, Staphylococcal infections- MRSA)
Consequences of drug resistance
Consequences
Prolonged hospital admissions
Higher death rates from infection
Requires more expensive, more toxic drugs
Higher health care costs
New Research 2017
Air pollution (black carbon) increases resistance S. aureus and streptococci (and increases transfer from nose to lung in the latter)
(Hussey et al 2017 Env. Microbiol doi:10.1111/1462-2920.13686)
Antibiotic resistance reservoirs
Resistome is a collection of antibiotic resistance genes and their precursors in both pathogenic and non-pathogenic bacteria
Soil, air, water — global
Types of resistance
Passive:
- Antibiotic has no target or cannot enter cell
- Antibiotic has no action
Mutation:
- Target site changes so antibiotic is ineffective
Acquired:
- Actively acquired resistance (genetic elements - e.g. plasmids)
Horizontal gene transfer
Transformation:
Free DNA in the environment and a competent recipient - state of donor is dead or alive - state of recipient is living
Transduction:
Bacteriophage - donor is bacteriophage - state of recipient is living
Conjugation:
Cell-to-cell contact. Plasmid (fre in cytosol or on chromosome of donor) - donor and recipient living
Modular mobile elements
Mobile gene cassette - Can carry single or multiple resistances
-> Integron - Promotes dissemination of GCass
Drives expression Amr genes
-> Transposon
Composite Tn: flanked by IS or IR
Complex Tn: has own transposase/rec’ase
Conjugative Tn: promotes own transfer
pFBAOT6
- Isolated September 1997 from Westmorland General Hospital effluent
- IncU multi-resistance plasmid
- 87,478bp in length
- 94 predicted coding sequences, only 12 not assigned a possible function
- Core functions in first 31kb
- Genetic load region 54kb:
- Class I integron
- Several transposable elements
-A potential composite transposon of 43kb
Methicillin resistant Staphylococcus aureus
Opportunist pathogen-
25-30% of population already have S. aureus present in their system without any signs of illness.
1959: Methicillin introduced
1961: MRSA discovered in England
1968: MRSA found in the US (Boston).
1974: 2% of hospital Staphylococcus aureus infections.
1981: MRSA acquired in the community.
2002: Community & hospital acquired infections are
found to be genetically different.
2007: In the US:
94,000 severe infections/yr
19,000 deaths/yr
What is MRSA resistant to?
Ampicillin and other penicillins and cephalosporins
Spread by horizontal gene transfer
PBP2a protein production cause SA to become resistant to Beta-lactam antibiotics
Vancomycin used to treat MRSA, but resistant strains have been found and are beginning to establish in hospitals
MRSA virulence
SCCmec, or staphylococcal cassette chromosome mec, is a mobile genetic element which includes the mecA gene coding for resistance to the antibiotic methicillin.
mecA gene encodes the protein PBP2A (Penicillin binding protein 2A).
PBP2A has a low affinity for beta-lactam antibiotics such as methicillin and penicillin. This enables transpeptidase activity in the presence of beta-lactams, preventing them from inhibiting cell wall synthesis.
Now variants on SCC mecA (I, II, III, IV)
New Delhi carbapenemase
NDM-1 was first detected in a Klebsiella pneumoniae isolate from a Swedish patient of Indian origin in 2008.
Most commonly found in: Gram-negative such as Escherichia coli and Klebsiella pneumoniae,
Gene for NDM-1 can spread from one strain of bacteria to another by horizontal gene transfer and disseminated by travel.
Metallo-beta-lactamase-1 (NDM-1)
Carbapenems are a class of beta-lactam antibiotics
Developed to overcome antibiotic resistance mediated by bacterial beta-lactamase enzymes.
blaNDM-1 gene produces NDM-1, which is a carbapenemase beta-lactamase - an enzyme that hydrolyzes and inactivates to a broad range of beta-lactam antibiotics including carbapenem antibiotics.
NDM1 in 2019
Located in arctic, carried in migrating bird faeces
Spread of NDM-5
NDM-1 variants 1-17
NDM-5 new variant of the New Delhi metallo-enzyme (NDM) carbapenemase
identified in a multidrug-resistant Escherichia coli ST648 isolate recovered from the perineum and throat of a patient in the UK with a recent history of hospitalization in India.
Resistant to :
all cephalosporins carbapenems (ertapenem, imipenem, meropenem,
aminoglycosides (gentamicin, tobramycin, amikacin),
quinolones (ciprofloxacin)
susceptible to colistin and tigecycline
