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Antibiotics Part 1
Dr P Gayo Munthali
Consultant Microbiologist UHCW
Honorary Associate Clinical Professor
University of Warwick
Objectives
•
By the end of this lecture you should be able
to:
1) Explain the mode of action of beta lactams,
aminoglycosides, fluoroquinolones, macrolides,
tetracyclines and glycopeptides
2) Mention the major side effects of the antibiotic
groups in (1)
3) Appreciate different types of resistance and in
simple terms, explain the mechanisms of resistance
to beta lactams
4) Explain some limitations in the use of antibiotics in
(1)
5) Understand the general spectrum of activity of
antibiotics in (1)
Antibiotics, Point of Action
Folic acid Metabolism
Trimethoprim, Sulphonamides
Cell wall Synthesis
Beta-lactams,
Glycopeptides
Daptomycin
Fosfomycin
Cell membrane
Polymixin, bacitracin,colistin
50S
30S
DNA Replication
Quinolones
Protein Synthesis
DNA Dependent RNA 30S Tetracyclines,Aminoglycosides
50S Chloramphenicol, Clindamycin,
Pol. Rifampicin
Erythromycin, Linezolid,Streptogramin
Important mechanisms of
resistance to antibiotics
Mechanism Typical example
Antibiotics affected
Inactivation
ß-Lactamases
ß-Lactams
Aminoglycoside
modifying enzymes
Aminoglycosides
Changes in
Changes in
ß-Lactams/
target binding PBPs/Peptide Terminal
Glycopeptides
Ribosomal methylation Macrolides
Efflux and
permeability
changes
DNA gyrase mutation
Fluoroquinolones
Efflux pumps
Pump specific
Porin protein loss
Most except polymyxins
and aminoglycoside
ß-Lactams
Β-Lactam
Ring
Thiazolidine Ring
Penicillins and Cephalosporins
s
R-CONH
Penicillins 1940-
N
o
R-CONH
s
COOH
R
N
o
COOH
Cephalosporins 1948-
Carbapenems and Others
CHз
Carbapenems 1976-
HO
S
R
HO
N
o
o
COOH
N
o
Clavulanic acid 1976
COOH
Mobactams
R
Monobactam 1981-
R-CONH
N
o
R
Mechanisms of Action
• Inhibit bacterial enzymes involved in cell wall
synthesis
– Penicillin binding proteins (PBPs) essential for
peptidoglycan synthesis
• Trigger membrane associated autolytic enzymes that
destroy cell wall
• Inhibit bacterial endopeptidase and glycosidase
enzymes which are involved in cell wall growth
• Time dependent activity
Beta Lactams Against Bacterial Cell
Wall
Cell wall
Osmotic
Pressure
Cell Membrane
Antibiotic against cell wall
Osmotic
Pressure
Cell membrane
Rapture
Spectrum of Activity
•
•
•
•
Very wide
Gram positive and negative bacteria
Anaerobes
Spectrum of activity depends on the agent
and/or its group
• Aztreonam only active against gram
negatives
Pharmacokinetics
• Absorption
– PO forms have variable absorption
– Food can delay rate and extent of absorption
• Distribution
– Widely to tissues & fluids
– CSF penetration:
IV – limited unless inflamed meninges
IV 3rd & 4th generation cephalosporins, meropenem, &
Aztreonam – penetrate well
• Metabolism & Excretion
– Primarily renal elimination
– Some have a proportion of drug eliminated via the liver
– ALL -lactams have short elimination half-lives
Adverse Effects
Penicillin hypersensitivity – 0.4% to 10 %
– Mild: rash
– Severe: anaphylaxis & death
• There is cross-reactivity among all Penicillins
• Penicillins and cephalosporins ~5-15%
• Penicillins and carbapenems~1% (may be higher)
– Desensitization is possible for mild hypersensitivity
• Aztreonam does not display cross-reactivity with
Penicillins and can be used in penicillin-allergic
patients
Resistance to ß-Lactams
•Penicillin-Binding Protein (PBP)
mediated Resistance
•ß-Lactamase
•Efflux pumps/loss of porins
Penicillin-Binding Protein (PBP)
mediated Resistance
•
•
•
•
PBP over expression
Acquisition of Foreign PBPs genes
Mutation by recombination with foreign DNA
Point mutation
PBP over expression
• Rare
– The more PBPs are expressed, the more an
organism becomes resistant
• S.aureus increased resistance to methicillin by
over expression of PBP4
• E.faecium strains that over express PBP5 have
increased resistance to penicillin.
AAC 39:2415-2422, AAC 38:1980-1983, AAC 45: 1480-1486
Acquisition of Foreign PBPs
• Represented best by Methicillin Resistant
S.aureus (MRSA)
• S.aureus acquires foreign PBP2a encoded by
mecA gene
• PBP2a has low affinity for all ß-lactams
• PBP2a can perform all the combined functions
of all the S. aureus PBPs
• Almost all MRSAs express ß-Lactamase
Clin. Microbiol. Rev.10:781-791, J.Infect.Dis.162:705-710
Result
• All PBPs in S.aureus become redundant
–MRSA is resistant to all ß-lactams
Mutation by Recombination with
Foreign DNA
• Streptococcus pneumoniae and Neisseria are
capable of picking up foreign DNA and
integrating it with their own DNA
– Form mosaic gene
• Pneumococcus picks up resistant genes from
alpha haemolytic streps
– Reduced affinity to beta lactams
• Seen as penicillin resistant Pneumococci
9
8
7
6
MICs
5
Isolate
Series1
Series2
BSAC
Series3
MIC for
meningitis
4
3
2
1
im
e
ro
x
ef
u
C
Pi
pe
ra
c
illi
n
e
ia
x
ef
tr
C
ta
x
ef
o
C
JAC 1992,30(3);279-288
on
im
e
m
M
er
op
e
ne
ne
ip
e
Im
ni
c
illi
n
m
0
Pe
MIC 90 Against 100 Isolates/BSAC MIC90
Resistance Breakpoints
Beta Lactam Activity Against 100 Penicillin
Resistant Pneumococci from Spain
Efflux pumps/Loss of Porins
• Important type of resistance in Pseudomonas
– A combination of ß-Lactamase production and
porin loss can lead to complex resistance pattern
• Can lead to carbapenem resistance without
carbapenemase production
Porins and Pumps
Porins
Overexpressed Efflux pumps
Adapted from Journal of Bacteriology, April 2006, p. 2297-2299, Vol. 188, No. 7
Resistance due to ß-Lactamases
• Mode of action
– Classification
ß-Lactamase
ß -pleated
sheet-5
ά-helices
AAC 39:2593-2601
Bound ß-lactam by ß-Lactamase
ß-Lactamases-action
R-CONH
s
C
N
o
s
R-CONH
HOH
Enzyme
CH3
COOH
Enzyme-Ser-OH
o
CH3
C
O
Ser
N
H
CH3
CH3
COOH
Annu.Rev.Microbiol.45:37-67
Beta Lactam Classification
• You do not need to know the classification
or detailed information on ß-Lactamases
• However you need to appreciate the following
concepts;
– Simple betalactamases
– Extended spectrum betalactamases (ESBL)
– Betalactamases against the Carbapenems
Simple ß-Lactamases
• Many Based on genes called TEM-1 and SHV-1
found on mobile DNA elements
– TEM-1 and SHV-1 are simple penicillinases in
Enterobacteriaceae
– Inactive against cephalosporins
– Confer resistance to Penicillins such as Benzylpenicillin and
amoxicillin
– On mobile elements and therefore transmissible
• Staphylococci also produce simple beta lactamases
not based on TEM-1 and SHV-1
– Flucloxacillin designed to resist betalactamases in
Staphylococcus aureus
AAC 33:1131-1136
Extended Spectrum ß-lactamases
• Based on TEM-1 and SHV-1
• Amino acid mutations in active site
progressively increase their activity against
cephalosporins
– When they hydrolyze extended-spectrum
cephalosporins
• They are then called ESBLs
– Also attack a monobactam Aztreonam
-On mobile elements thus transmissible
– Carry other resistance genes, Gentamicin,
Ciprofloxacin
ESBLs
• Hydrolyze extended-spectrum cephalosporins
with an oxyimino side chain
• These include;
– Cefotaxime
– Ceftazidime
– Ceftriaxone
• Loose term
• Among the beta-lactam, only the
Carbapenems are stable against ESBLs
– Imipenem, Meropenem, Ertapenem and
Doripenem are in clinical use
Characteristics of ESBLs
• May appear sensitive to some cephalosporins
and combinations of piperacillin and
tazobactam as well as amoxicillin and
clavulanic acid
– However, use of these ß-lactam agents will lead to
microbiological and clinical failure
– Only carbapenems among the ß-lactams can be
used successfully
AmpC ß-Lactamases
• Produced by almost all gram-negative bacteria
• Chrosomally encoded versions important in
Citrobacter freundii, Enterobacter aerogenes,
Enterobacter cloacae, Morganella morganii,
Pseudomonas aeruginosa and Serratia
marcescens (not found in Salmonella and
Klebsiella)
• AmpC ß-Lactamase genes have been found
on transferable plasmids
Class C ß-Lactamases
• All ß- lactams induce AmpC ß-lactamase
production
– Only carbapenems are resistant to AmpC ßlactamases
• If there is loss of porins as well, this will lead to
carbapenem resistance
– Other ß- lactams will be hydrolysed
Metallo-ß-Lactamases
• Require Zinc or other heavy metal for activity
• Hydrolyse all ß-Lactams including
carbapenems
• Most will be associated with resistance to many
antibiotic classes
• Currently New Delhi Metallo-ß-Lactamase
(NDM-1) is a new flavour in the UK
– Associated with India
– Resistant to almost all antibiotics in use in the UK
Aminoglycosides
•Highly positively charged compounds, concentration dependent
activity
•Inhibit bacterial protein synthesis by irreversibly binding to
30S ribosomal unit
•Naturally occurring:
•Streptomycin
•Neomycin
•Kanamycin
•Tobramycin
•Gentamicin
•Semisynthetic derivatives:
•Amikacin (from Kanamycin)
•Netilmicin (from Sisomicin)
30S Ribosomal Unit Blockage by
Aminoglycosides
•Causes mRNA decoding errors
•Block mRNA and transfer RNA translocation
•Inhibit ribosome recycling
Ribosome recycling follows the termination of protein synthesis
Spectrum of Activity
• Gram-Negative Aerobes
– Enterobacteriaceae;
E. coli, K. pneumoniae, Proteus sp.
Citrobacter, Enterobacter sp.
Morganella, Providencia, Serratia
– Pseudomonas aeruginosa
– Acinetobacter
• Gram-Positive Aerobes (Usually in
combination with ß-lactams)
S. aureus and coagulase-negative staphylococci
Viridans streptococci
Enterococcus sp. (gentamicin)
Mechanisms of Resistance
• Ribosome changes
– Prevents binding
• Loss of cell permeability
• Expulsion by efflux pumps
• Enzyme inactivation by Aminoglycoside
modifying enzymes
– This is the most important mechanism
Pharmacokinetics
• All have similar pharmacologic properties
• Gastrointestinal absorption: unpredictable but always
negligible
• Distribution
– Hydrophilic: widely distributes into body fluids but very poorly into;
• CSF
• Vitreous fluid of the eye
• Biliary tract
• Prostate
• Tracheobronchial secretions
• Adipose tissue
• Elimination
– 85-95% eliminated unchanged via kidney
– t1/2 dependent on renal function
– In normal renal function t1/2 is 2-3 hours
Adverse Effects
• Nephrotoxicity
– Direct proximal tubular damage - reversible if caught early
– Risk factors: High troughs, prolonged duration of therapy,
underlying renal dysfunction, concomitant nephrotoxins
• Ototoxicity
– 8th cranial nerve damage – irreversible vestibular and
auditory toxicity
• Vestibular: dizziness, vertigo, ataxia
• Auditory: tinnitus, decreased hearing
– Risk factors: as for nephrotoxicity
• Neuromuscular paralysis
– Can occur after rapid IV infusion especially with;
• Myasthenia gravis
• Concurrent use of succinylcholine during anaesthesia
Macrolides
• Erythromycin is the prototype antibiotic for
this group
• Bacteriostatic- usually
• Inhibit bacterial RNA-dependent protein
synthesis
• Bind reversibly to the 23S ribosomal RNA
of the 50S ribosomal subunits
– Block translocation reaction of the polypeptide
chain elongation
Macrolides
Lactone Ring
14
14
Erythromycin
15
Azithromycin
Telithromycin
14
Clarithromycin
Mechanisms of Resistance
• Altered target sites
– Methylation of ribosomes preventing antibiotic binding
• Resistance to macrolides , lincosamides (Clindamycin) and
streptogramin B
• Can be induced by macrolides
• Efflux pumps
– Resistance to macrolides only
• Cross-resistance occurs between all macrolides
Spectrum of Activity
• Gram-Positive Aerobes:
– Activity: Clarithromycin>Erythromycin>Azithromycin
• MSSA
• S. pneumoniae
• Beta haemolytic streptococci and viridans streptococci
• Gram-Negative Aerobes:
– Activity: Azithromycin>Clarithromycin>Erythromycin
• H. influenzae, M. catarrhalis, Neisseria sp.
• NO activity against any Enterobacteriaceae
• Anaerobes: upper airway anaerobes
• Atypical Bacteria
• Other Bacteria: Mycobacterium avium complex
Pharmacokinetics 1
• Erythromycin ( Oral: absorption 15% - 45%)
• Short t1/2 (1.4 hr)
• Acid labile
• Absorption (Oral)
– Erythromycin: variable absorption of 15% - 45%
– Clarithromycin: 55%
– Azithromycin: 38%
• Half Life (T1/2)
– Erythromycin 1.4 Hours
– Clarithromycin (250mg and 500mg 12hrly) 3-4 & 5-7 hours respectively
– Azithromycin 68hours
– Improved tolerability
•
Excellent tissue and intracellular concentrations
–
•
•
Tissue levels can be 10-100 times higher than those in serum
Poor penetration into brain and CSF
Cross the placenta and excreted in breast milk
Pharmacokinetics 2
• Metabolism & Elimination
– Clarithromycin partially eliminated by the
kidney
– ALL hepatic elimination
Adverse Effects
• Gastrointestinal (up to 33 %) (especially
Erythromycin)
•
•
•
•
Nausea
Vomiting
Diarrhoea
Dyspepsia
• Thrombophlebitis: IV Erythromycin &
Azithromycin
• QTc prolongation, ventricular arrhythmias
• Other: ototoxicity with high dose erythromycin in
renal impairment
Fluoroquinolones
Quinolone pharmacore
Fluoroquinolones
• Synthetic antibiotics
• Concentration-dependent bactericidal activity
• Broad spectrum of activity
• Excellent pharmacokinetics
• bioavailability, tissue penetration, prolonged half-lives
• In common use
• Ciprofloxacin
• Levofloxacin
• Moxifloxacin
Mechanism of Action
• Inhibit bacterial topoisomerases which is used
by bacteria to;
• Relax supercoiled DNA before replication
• DNA recombination
• DNA repair
• DNA gyrase – Primary target for gramnegatives
• Topoisomerase IV – Primary target for
gram-positives
Resistance
• Altered target sites due to point mutations.
• The more mutations, the higher the resistance
to Fluoroquinolones
• Most important and most common
• Altered cell wall permeability
• Efflux pumps
• Cross-resistance occurs between
fluoroquinolones
Spectrum of Activity
• Gram-positive (MSSA Streptococcus
pneumoniae )
– Moxifloxacin is most active
• Gram-Negative (Enterobacteriaceae H.
influenzae, M. catarrhalis, Neisseria sp.
Pseudomonas aeruginosa)
– Ciprofloxacin is most active
• Atypical bacteria: all have excellent activity
Pharmacokinetics
• Absorption
• Good bioavailability
• Oral bioavailability 60-95%
• Divalent and trivalent cations (Zinc, Iron, Calcium,
Aluminum, Magnesium) and antacids reduce GI absorption
• Distribution
• Extensive tissue distribution but poor CSF penetration
• Metabolism and Elimination
• Combination of renal and hepatic routes
Adverse Effects
• Cardiac
• Prolongation QTc interval
• Assumed to be class effect
• Articular Damage
• Cartilage damage
• Induced in animals with large doses
Tetracyclines
•Hydronaphthacene nucleus containing four fused rings
•Tetracycline
•Short acting
•Doxycycline
•Long acting
Mechanism of Action
• Inhibit protein synthesis
• Bind reversibly to bacterial 30S ribosomal subunits
• Prevents polypeptide synthesis
• Bacteriostatic
Resistance
• Efflux
• Alteration of ribosomal target site
• Production of drug modifying enzymes
Spectrum of Activity
• All have similar activities
• Gram positives aerobic cocci and rods
– Staphylococci
– Streptococci
• Gram negative aerobic bacteria
• Atypical organisms
–
–
–
–
Mycoplasmas
Chlamydiae
Rickettsiae
Protozoa
Pharmacokinetics
• Incompletely absorbed from GI, improved by
fasting
• Metabolised by the liver and concentrated in bile
(3-5X higher than serum levels)
• Excretion primarily in the urine except
doxycycline ( 60% biliary tract into faeces,40% in
urine)
• Tissue penetration is excellent but poor CSF
penetration
– Incorporate into foetal and children bone and teeth
Avoid in pregnancy and children
Adverse Effects
• Oesophageal ulceration
• Photosensitivity reaction
Glycopeptides
• Vancomycin
• Teicoplanin
Vancomycin
Mechanism of Action
• Inhibit peptidoglycan synthesis in the bacterial
cell wall
• Complex with D-alanyl-D-alanine portion of the cell
wall precursor
Resistance
• Modification of D-alanyl-D-alanine binding site
of peptidoglycan
• D-alanyl-D-alanine terminal then ends in D-alanylD-lactate
• Leads to lower glycopeptide affinity
• Complex reactions to achieve this
Spectrum of Activity
• Gram positive bacteria only including
MRSA
Pharmacokinetics
• Absorption
• oral is negligible
• IV required therapy for systemic infections
• Distribution
– Distributes widely into body tissues and fluids,
including adipose tissue
– Variable penetration into CSF, even with inflamed
meninges
• Elimination
– Primarily eliminated unchanged by the kidney
Adverse Effects
• Red-Man Syndrome
– Erythema multiforme-like reaction with intense
pruritus, tachycardia, hypotension, rash involving
face, neck, upper trunk, back and upper arms
• Related to infusion rate
• Resolves spontaneously after discontinuation
• Lengthen infusion (over 2 - 3 hr)
• Hematological
– Neutropaenia
– Eosinophilia