Microbial resistance to antimicrobial agents · Bacterial resistance to antimicrobials. Intrinsic...
Transcript of Microbial resistance to antimicrobial agents · Bacterial resistance to antimicrobials. Intrinsic...
Microbial resistance to antimicrobial agents
Prof. Beata M. Sobieszczańska
Department of Microbiology
University of Medicine
What is a drug resistance?
Antimicrobial resistance is the ability of a microorganism to survive and multiply in the
presence of an antimicrobial agent that would normally inhibit or kill this species of
microorganism
Bacterial resistance to antimicrobials
Intrinsic resistance
Bacteria may be resistant because either:
• lack of affinity of the drug for the bacterial target
• inaccessibility of the drug into the bacterial cell
•extrusion of the drug by chromosomally encoded active exporters
• innate production of enzymes that inactivate the drug
Enterococci
Enterococci are inherently resistant to:
cephalosporins
lincozamides
polymyxin B
low concentrations of aminoglycosides
TMP-SMX
ORGANISMS NATURAL RESISTANCE AGAINST: MECHANISM
Anaerobic bacteria AminoglycosidesLack of oxidative metabolism to drive uptake of aminoglycosides
Aerobic bacteria MetronidazoleInability to anaerobically reduce drug to its active form
GP Aztreonam (β-lactam)Lack of penicillin binding proteins (PBPs) that bind and are inhibited by this β-lactam antibiotic
GN VancomycinLack of uptake resulting from inability of vancomycin to penetrate outer membrane
Klebsiella spp. Ampicillin (β-lactam)Production of enzymes (β-lactamases) that destroy ampicillin before the drug can reach the PBP targets
Stenotrophomonas Imipenem (β-lactam)Production of enzymes (β-lactamases) that destroy imipenem before the drug can reach the PBP targets.
P. aeruginosaSulfonamides, trimethoprim, tetracycline, or chloramphenicol
Lack of uptake resulting from inability of antibiotics to achieve effective intracellular concentrations
Enterococci
AminoglycosidesLack of sufficient oxidative metabolism to drive uptake of aminoglycosides
All cephalosporinsLack of PBPs that effectively bind and are inhibited by these β-lactam antibiotics
Acquired resistance
Mutations
Horizontal gene transfer = genes pass from a resistant strain to a non-resistant strain conferring resistance on the latter
The introduction of an antibiotic into the bacterial environment acts as a selective pressure
Acquired resistance - mutation
Horizontal gene transfer
Conjugation
• Transmission of resistance genes via plasmid exchange
• Plasmids that they can pass to other bacteria during conjugation
• (R plasmids carry r-genes)
• This type of acquisition allows resistance to spread among a population of bacterial cells much faster than simple mutation
Transduction
A virus serves as the agent of transfer
between bacterial strains
Transformation
DNA released from a bacterium is picked
up by a new cell
Biochemical mechanisms of bacterial resistance:
1. Production of enzymes which detoxify or modulate the activity of the antibiotic
2. Alteration of the target site to reduce or block binding of the antibiotic
3. Prevention transport of the antimicrobial agent
4. Development of an alternate metabolic pathway to by-pass the metabolic step being blocked by the antimicrobial agent
β-lactamases = bacterial enzymes
•β-lactamase enzymes can destroy the β-lactam ring of penicillins through hydrolysis
•without a β-lactam ring penicillins are ineffective against the bacteria
• Produced by many GP and GN bacteria
bacterial β-lactamases
Penicillinases
Cephalosporinases
ESBL
MBL
AmpC
Extended-Spectrum β-Lactamases (ESBL)
β-lactamases conferring resistance to the penicillins, cephalosporins & monobactams but
not to carbapenems & cephamycins which are inhibited by β-lactamase inhibitors (BLIs)
Produced by GN rods
Cephamycins (2nd gen. of cephalosporins): cefoxitin, cefotetan, cefmetazole
Extended-spectrum β-lactamases (ESBL)
Genes encoding for ESBL are frequently located on plasmids (spread via HGT) which also carry resistance genes for aminoglycosides, tetracycline, TMP-SMX, fluoroquinolones
Clinical implications:
treatment failure
increased morbidity & mortality
outbreaks
Metallo-β-Lactamases (Carbapenemases ) MBL & KPC
• Hydrolyse virtually all β-lactams
• They are resistant to BLIs but sensitive to aztreonam
• Mediate broad spectrum β-lactam resistance
• Present on plasmids
• Genes are continuously spreading
• Produced by GN rods: Pseudomonas, Klebsiella pneumoniae, E. coli, Proteus mirabilis, Enterobacter
Metallo-β-Lactamases KPC
• KPC – Klebsiella pneumoniae carbapenemase – isolates resistant to many antimicrobials also aminoglycosides and quinolones
• Genes encoding KPC are on mobile genetic elements e.g. plasmids
• KPC strains – multi-resistant
• First combination of carbapenem with BLI –meropenem+vaborbactam active against KPC-positive Klebsiella
AmpC-type β-Lactamases
• Encoded chromosomally (but may also be on plasmids) among GN rods:
• Serratia, Pseudomonas/Proteus, Acinetobacter, Citrobacter, Enterobacter = SPACE
• Hydrolyse broad spectrum cephalosporins + cephamycins
• They are not inhibited by BLIs
• Often inducible by e.g. cephalosporins
• Hallmark phenotypic pattern: these rods appear to be susceptible to 3rd gen. cephalosporins but resistance can develop upon β-lactam exposure
Aminoglycoside modifying enzymes:
N-Acetyltransferases (AAC) – catalyses acetyl CoA-dependent acetylation of an amino group
O-Adenyltransferases (ANT) – catalyses ATP-dependent adenylation of hydroxyl group
O-Phosphotransferases (APH) – catalyses ATP-dependent phosphorylation of a hydroxyl group
GP and GN bacteria resistance to aminoglycosides (phosphorylation, adenylation, acetylation)
Enterobacteria Resistance to chloramphenicol - acetylation
EnterococciSynergistic combination therapy
cell wall active agent (β-lactam/glycopeptides) + high concentration of aminoglycoside
often provides effective therapy (treatment of endocarditis caused by enterococci)
HLAR (High Level of Aminoglycoside Resistance) – enterococci that acquired genes encoding aminoglycoside inactivating enzymes
The synergism of aminoglycosides with cell wall active agent is lost
Biochemical mechanisms of bacterial resistance:
1. Production of enzymes which detoxify or modulate the activity of the antibiotic
2. Alteration of the target site to reduce or block binding of the antibiotic
3. Prevention transport of the antimicrobial agent
4. Development of an alternate metabolic pathway to by-pass the metabolic step being blocked by the antimicrobial agent
Resistance to β-lactams
PBP (Penicillin Binding Proteins = transpeptidase enzyme) alterations:
Streptococcus pneumoniae resistant to penicillin PRP(Penicillin Resistant Pneumococci)
MRSA (Methicillin Resistant Staphylococcus Aureus)
Listeria monocytogenes, gonococci - resistant to β-lactams
Resistance to glycopeptides & ampicillin
GRE (Glycopeptide Resistant Enterococci)
VRE (vancomycin resistant enterococci)
Ampicillin resistant enterococci (altered PBP)
M. tuberculosis – changes in RNA polymerase
Resistance to rifampin
Resistance to macrolides
Resistance to Macrolide, Lincosamide & Streptogramin B antibiotics (MLSB phenotype) –staphylococci, streptococci
Many GN bacteria - alterations in subunits of DNA gyrase
GP bacteria (pneumococci, staphylococci) - alteration in topoisomerase IV
Resistance to fluoroquinolones
Biochemical mechanisms of bacterial resistance:
1. Production of enzymes which detoxify or modulate the activity of the antibiotic
2. Alteration of the target site to reduce or block binding of the antibiotic
3. Prevention transport of the antimicrobial agent
4. Development of an alternate metabolic pathway to by-pass the metabolic step being blocked by the antimicrobial agent
Strategy 1: Prevention of the antimicrobial from reaching its target by reducing its ability to penetrate into the cell = prevention access
Change in the number or characters (size, selectivity) of porin channels in the outer membrane of a Gram-negative cell wall
Strategy 2: Eliminating antimicrobial agents from the cell by expulsion using efflux pumps
Some bacteria possess membrane proteins that act as an export or efflux pump for antimicrobials – some of them selectively extrude specific antibiotics (macrolides, lincozamides, streptogramins, tetracyclines) whereas others expel a variety of structurally diverse antimicrobials
Examples – prevention access:
1. Pseudomonas aeruginosa – imipenem
2. Enterobacter aerogenes – imipenem
3. Vancomycin resistant S. aureus - thickened cell wall trapping vancomycin
GISA (Glycopeptide Intermediate Resistant Staphylococcus Aureus)
VISA (Vancomycin Intermediate Resistant Staphylococcus Aureus)
VRSA (Vancomycin Resistant Staphylococcus Aureus)
4. Many GN bacteria – aminoglycosides, quinolones
Examples – efflux pumps:
1. enterobacteria – tetracyclines, chloramphenicol
2. staphylococci – macrolides, streptogramins
3. staphylococci, pneumococci - quinolones
Biochemical mechanisms of bacterial resistance:
1. Production of enzymes which detoxify or modulate the activity of the antibiotic
2. Alteration the target site to reduce or block binding of the antibiotic
3. Prevention of transport of the antimicrobial agent
4. Development of an alternate metabolic pathway to by-pass the metabolic step being blocked by the antimicrobial agent
Overcoming drugs that
resemble substrates and
tie-up bacterial enzymes
Production of greater amounts of the limited enzyme that is being tied up or inactivated by the
antibiotic
Mode of action
Prevent cross-linking
Resistance
• Enzymatic destruction of β-lactam ring
• Target (PBP) modification
• Reduced intracellular accumulation
• Target modification - productionof false targets
Target & bind PBP
Glycopeptides
β-lactams
Quinolones
Target DNA gyrase and topoisomerase IV = inhibit DNA supercoiling
• Target modification
• Reduced intracellular accumulation
Mode of action Resistance
• Enzymatic modification
• Target modification
• Reduced uptake
• Target modification
• Reduced intracellular uptake
Target & bind 30S ribosomal subunit = protein inhibition
Macrolides
Aminoglycosides
Tetracyclines • Target modification
• Reduced intracellular accumulation
Target & bind 50S ribosomal subunit = protein inhibition
Target & bind 30S ribosomal subunit = protein inhibition
Mode of action Resistance
• Target modification
• Target modification
Target RNA polymerase = block RNA synthesis
Sulphonamides
Rifamycins
Target dihydropteroate synthase = inhibition folic acid synthesis
Examples
Altered target (PBP)
Mechanism of resistance
Resistance of staphylococci to penicillin
Resistance of enterobacteria to penicillins, cephalosporins, monobactams ESBL
Resistance of staphylococci to methicillin and oxacillin MRSA
Enzymatic destruction
(β-lactamases)
β-lactams
Resistance of Enterobacter, Klebsiella and Pseudomonas to imipenem
Decreased uptake
(porin channel formation is decreased)
Examples
Altered target
alteration in the molecular structure of cell wall precursor
Mechanism of resistance
Resistance of enterococci to vancomycin, teicoplanin
VRE, GRE
Glycopeptides
Examples
Altered targetmodification of ribosomal proteins or of 16S rRNA
Mechanism of resistance
Resistance of many GN and GP bacteria to aminoglycosides
e.g. enterobacteria, enterococci
Resistance of many GN bacteria to aminoglycosides
Enzymatic modification
Aminoglycosides
Resistance of Mycobacterium to streptomycin
Decreased uptake
(change in number and character of porin channels)
Examples
Altered targetchanges in DNA gyrase subunits
Mechanism of resistance
Resistance of GN bacteria and staphylococci (efflux only) to various quinolones
Fluoroquinolones
Resistance of GN and GP to various quinolones
Decreased uptake
(alterations in the outer membrane – diminished uptake or efflux pump)
• Bacteria are able to resist antimicrobials by: preventing intracellular access, immediately removing antimicrobial substances through efflux pumps, modifying the antimicrobial through enzymatic breakdown, or modifying the antimicrobial targets within the bacterial cell. Successful development of resistance often results from a combination of two or more of these strategies
• Antimicrobial resistance traits are genetically coded and can either be intrinsic or acquired
• Intrinsic resistance is due to innately coded genes which create natural resistance to a particular antibiotic. Innate resistance is normally expressed by virtually all strains of a particular bacterial species
• Acquired resistance is gained by previously susceptible bacteria either through mutation or horizontally obtained (HGT) from other bacteria possessing such resistance via transformation, transduction, or conjugation. Acquired resistance is limited to subpopulations of a particular bacterial species and may result from selective pressure exerted by antibiotic usage