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ANTIBIOTICS

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ANTIBIOTICS
ANTIBIOTICS
SOME GENERAL PRINCIPLES
• Antibiotics can be naturally produced,
semi-synthetic, or synthetic substances
• Designed to have as much selective
toxicity on the bacteria as possible
• This is more likely to be achieved
compared to antimicrobials acting against
eukaryotic cells (fungi, protozoa)
EXAMPLES OF SELECTIVE
ACTION
• Penicillin on bacterial cell wall (organisms
without cell wall won’t be inhibited eg
Mycoplasma pneumoniae)
• Sulphonamides prevent bacteria
synthesising folic acid whereas humans
can use preformed folate
• Generally drugs acting on cell membranes
or protein synthesis are more toxic to
humans
ANTIBIOTICS ACTING ON CELL
WALL OF BACTERIA
• Beta lactams:
• Penicillins, cephalosporins, carbapenems,
monobactam
• Glycopeptides:
• Vancomycin, teicoplanin
THE IDEAL
ANTIBIOTIC?:PENICILLIN
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Narrow spectrum
Bactericidal
Very selective mode of action
Low serum protein binding
Widely distributed in body esp. CNS
Excreted by the kidneys
THE DEVELOPMENT OF THE
BETA LACTAMS
• Benzylenicillin and early cephalosporins mainly
active against gram positive bacteria
(strep and staph)
• Then “broad spectrum” penicillins appeared:
ampicillin, ureidopenicillins and cephalosporins:
cefuroxime, cefotaxime
• Carbapenems and latest generation of
cephalosporins, eg ceftazidime more active
against gram negatives
BENZYLPENICILLIN: MAIN
INDICATIONS
 Strep pyogenes sepsis (from sore throat to
fasciitis)
 Pneumococcal pneumonia, meningitis
 Meningococcal meningitis, sepsis
 Infective endocarditis (strep)
 Strep group B sepsis
 Diphtheria
 Syphilis, leptospirosis
Broader spectrum penicillins
• Ampicillin, amoxycillin cover most
organisms hit by penicillin but also Esch
coli, some Proteus (cause UTI’s)
• Augmentin stable to TEM1 beta lactamase
because of the clavulanic acid therefore
more active than ampicillin
• Tazocin: broader coverage than augmentin
against gram negatives including
Pseudomonas
Organisms producing TEM1beta
lactamase
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Haemophilus influenzae
Neisseria gonorrhoeae
Bacteroides fragilis
Staph aureus
Esch coli
Carbapenems
• Imipenem, meropenem: have a very broad
spectrum activity against gram-negative
bacteria, anaerobes, streps
• Now used to treat gram negative infections
due to so called ESBL producing
organisms eg, E coli, Klebsiella
• Ertapenem is a new member of the group
but its not active against Pseudomonas
PENICILLIN IS GENERALLY VERY
SAFE BUT….
• Allergic reactions not uncommon-rashes
• Most severe reaction being anaphylaxis
• A history of anaphylaxis, urticaria, or rash immediately
after penicillin indicates risk of immediate
hypersensitivity after a further dose of any penicillin or
cephalosporin (therefore these must be avoided)
• Allergy is not dependent on the dose given ie, a small
dose could cause anaphylaxis
• Very high doses of penicillin can cause neurotoxicity
• Never give penicillin intrathecally
What antibiotics can be used in
penicillin allergy?
• Macrolides: erythromycin, clarithromycin
• (mainly gram positive cover)
• Quinolones: ciprofloxacin, levofloxacin
(mainly gram positive cover)
• Glycopeptides (serious infections)
• Fusidic acid, rifampicin, clindamycin
(mainly gram positive)
REMEMBER WHAT THE OTHER
BETA LACTAMS ARE:
• All penicillins: ampicillin, augmentin,
piperacillin, cloxacillin
• Cephalosporins: cefuroxime, cefotaxime,
ceftriaxone, ceftazidime (5-10% cross
sensitivity)
• Monobactam: aztreonam (low cross
sensitivity)
• Carbapenems: imipenem, meropenem
CLOXACILLIN
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Narrow spectrum: Staph aureus (MSSA)
Stable to TEM1 beta lactamase
Similar antibiotics are methicillin, nafcillin
Similar safety profile to benzylpenicillin
MRSA emerged in the early 1970’s (MecA
gene encoding additional pbp)
Cephalosporins: main uses
• Cefuroxime: surgical prophylaxis
• Cefotaxime/ceftriaxone: meningitis
nosocomial infections excluding
Pseudomonal,
• Ceftazidime: nosocomial infections
including Pseudomonal
Problems with antibiotic resistance:
how does it happen?
• Some bacteria are naturally resistant to
particular antibiotics (Pseudomonas has
permeability barrier to many antibiotics)
• Some typically susceptible species have minority
populations which are resistant by virtue of
mutational resistance (pneumococcus)
• Other species acquire resistance via plasmids
(“infectious resistance”) eg Neisseria
gonorrhoeae, many gram negatives
Current major antibiotic resistance
problems: community infections
• Respiratory tract: penicillin resistance in
pneumococcus (5-10%)
• Gastrointestinal: quinolone resistance in
Campylobacter
• Sexually transmitted: penicillin, quinolone
resistance in gonococcus
• Urinary tract: beta lactam resistance in Esch coli
• MRSA and MDRTB
• Tropical: multidrug resistance in Salmonella
typhi, Shigella spp
Current major resistance problems:
hospital infections
• MRSA: current strains are often multiplyantibiotic resistant
• VISA/GISA: intermediate resistance to
glycopeptides (thickened cell wall)
• VRSA/GRSA: highly resistant (transferable on
plasmids) from enterococci
• VRE: enterococci (multiply resis tant)
• Broad spectrum beta lactam resistant (ESBL)
Esch coli, Klebsiella
• Multiply antibiotic resistant enterobacteria:
Acinetobacter, Stenotrophomonas, Serratia
Other major antibiotic groups:
aminoglycosides
• Gentamicin, amikacin (tobramycin,
streptomycin)
• Mainly active against gram negative
bacteria
• Mainly used to treat nosocomial infections:
pneumonia in ITU, septicaemia
• Limiting factors are nephrotoxicity (and
ototoxicity) and resistance
• Also used in combination
How we give aminoglycosides
• For serious nosocomial infections:
“extended interval” or once daily dosing
• 5 or 7mg/kg for gentamicin (Hartford
nomogram)
• Rationale based on concentrationdependent killing and post-antibiotic effect
• Reduced risk of nephrotoxicity
• In infective endocarditis use lower doses
to give synergy with penicillin
Some indications and limitations
of particular antibiotics
Community acquired pneumonia
• Pneumococcus (and H influenzae) are most
likely: therefore ampicillin, amoxycillin or
augmentin
• Severe pneumonia: cefotaxime
• Severe atypical pneumonia (Legionella):
macrolide or quinolone
• Resistant pneumococcus: vancomycin or
linezolid (new antibiotic!)
• A new quinolone moxifloxacin covers most of
these pathogens (likely to be used more in
community)
Community acquired urinary
infections
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Ampicillin, amoxycillin, augmentin
Oral cephalosporin: cephradine
Trimethroprim
Nalidixic acid
Nitrofurantoin
Ciprofloxacin
Mecillinam
Skin and soft tissue infections
• Cellulitis ? Streptococcal: penicillin or
augmentin
• Infected eczema ? Staphylococccal/mixed:
penicillin+flucloxacillin or augmentin
• Necrotising fasciitis: penicillin+clindamycin
• Septic arthritis: fluclox+fusidic acid
• Gangrene: metronidazole
Where there is deep-seated
infection: bone, abscess
• Need an antibiotic with good tissue and
phagocyte penetration
• Examples are rifampicin, clindamycin,
fusidic acid, ciprofloxacin, metronidazole
• So for treatment of Staph aureus
osteomyelitis: flucloxacillin+ fusidic acid
Why do we use combination
therapy?
• When treating serious infection empirically
we want to cover a broad spectrum
(severe pneumonia:cefotaxime+erythromycin)
• To prevent the emergence of drug
resistance: tuberculosis regimens
• For synergy: infective endocarditis
(aminoglycoside)
• For mixed infections eg, abdominal sepsis
(tazocin+metronidazole)
Factors to consider when
prescribing an antibiotic
• Any history of allergy, toxicity?
• Is it appropriate for the spectrum I want to
cover?
• What route of admin: oral or i.v?
• Any factors affecting absorption ?
• Is it going to reach the site of infection?
• Any drug interactions?
• Any serious toxicity eg, hepatic, renal?
• Does it need monitoring eg aminoglycosides,
vancomycin, streptomycin?
Some other antibiotics occasionally
used
• Co-trimoxazole (Stenotrophomonas)
• Chloramphenicol (typhoid fever,
meningitis)
• Colistin (resistant Pseudomonas) topical
• Neomycin: gut decontamination, topical
Special situations
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Paediatrics
Obesity
Renal failure (haemodialysis/filtration)
Hepatic failure
CNS infections
Epidemiology (contacts of cases
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