Antibacterial Agents 1 Flashcards
What is meant by the term antibiotic?
Chemical products of microbes which kill or inhibit other organisms
What is meant by antimicrobial agents?
Overarching term including, antibacterial, antifungal, antiviral compounds.
It also includes antibiotics, synthetic compounds and semi synthetic compounds
What is meant by semi-synthetic compounds?
Modified antibiotics - to give them different antimicrobial activity/spectrum, pharmacological properties or toxicity
What is meant by the term bacteristatic?
Inhibit bacterial growth eg. protein synthesis inhibitors
What is meant by bactericidal?
Kill bacteria eg. cell wall-active agents
What is meant by MIC?
Minimum inhibitory concentration
The minimum conc of an Abx at which visible growth is inhibited
Would a more potent antimicrobial agent have a high or a low MIC?
Low
How can mode of bacterial killing change with concentration of an Abx?
Some Abx are bacteristatic at low concentrations and bactericidal at high concentrations
What is meant by synergism?
Activity of 2 antimicrobials together is greater than the sum of their activity seperately
What is meant by antagonism?
One antimicrobial agent diminishes the activity of another
What is meant by indifference?
Activity of an antimicrobial agent is unaffected by the addition of another
Give an example of synergism in clinical use?
beta-lactam and aminoglycaside combination therapy therapy of streptococcal endocarditis
What is meant by antibacterial targets?
The things or processes targeted or inhibited in a bacterial cell by an antibacterial agent, the most common are enzymes, molecules or structures (things bigger than molecules)
What is meant by selective toxicity of antibacterial agents?
They affect bacteria but not humans
Give 2 common ways in which selective toxicity of antibacterial agents is achieved?
1) Antibacterial target is not present in human host
2) Antibacterial target is significantly different in human host
Give 5 common antibiotic targets?
1) Cell wall
2) Protein synthesis
3) DNA synthesis
4) RNA synthesis
5) Plasma membrane
What is the major component of bacterial cell wall?
Peptidoglycan
Is peptidoglycan found in gram positive or gram negative bacteria?
Both
What is peptidoglycan made up of?
Polymer of glucose derivatives and amino acids N-acetyl muramic acid (NAM) and N-acetyl glucosamine (NAG) which form cross links to form a network providing the bacteria with structural rigidity
Why does a cell wall as an antibacterial target provide ideal potential for selective toxicity?
No cell wall found in humans
Give 2 examples of Abx which are cell wall synthesis inhibitors?
1) Beta-lactams
2) Glycopeptides
What is the rough structure of beta-lactam Abx?
All contain the beta-lactam ring which is a 4-membered ring structure (C-C-C-N)
How do beta-lactam Abx prevent cell wall synthesis?
The beta-lactam ring they contain is a structural analogue of D-alanyl-D-alanine (one of the components of peptidoglycan) so beta-lactam Abx can interfere with ‘penicillin binding proteins’ which are transpeptidase enzymes involved in peptidoglycan cross linking and thus synthesis and maintenance of peptidoglycan - a major component of bacterial cell walls
Which was the first true Abx in clinical practise and what type was it?
Benzylpenicillin - a beta-lactam Abx
Name the 4 types of beta-lactam Abx?
1) Penicillins
2) Cephalosporins
3) Carbapenems
4) Monobactams
What is the spectrum of penicillins?
Relatively narrow spectrum
What is the spectrum of cephalosporins?
Broad spectrum
What is the spectrum of carbapenems?
Extremely broad spectrum
What is the spectrum of monobactams?
Gram negative only
Name 3 common penicillins?
1) Benzylpenicillin
2) Amoxicillin
3) Flucloxacillin
Name 2 common cephalosporins?
1) Cefuroxime
2) Ceftazidime
Name 2 common carbapenems?
1) Meropenem
2) Imipenem
Name a common monobactam?
Aztreonam
Which type of bacteria do glycopeptides work against and why?
Only gram-positive bacteria as they are unable to penetrate gram-negative outer membrane porins
Name 2 common glycopeptides Abx?
1) Vancomycin
2) Teicoplanin
What is the structure of glycopeptide Abx and how do they work?
Large molecules, bind directly to terminal D-Alanyl-D-Alanine on NAM pentapeptides. This inhibits binding of transpeptidases and thus peptidoglycan cross linking
How does protein synthesis occur in bacteria?
1) Translation of RNA into a protein takes place on a ribosome
2) Ribonucleoprotein complexes (2/3 RNA, 1/3 protein)
3) 50s and 30s subunits come together to form 70s initiation complex
Aminoglycosides kill bacteria how?
Protein synthesis inhibitors
Bind to the 30s ribosomal subunit
Mechanism of action not fully understood
In terms of Abx what does MLS stand for?
Macrolides, Lincosamides, Streptogramins
Give an example of 2 macrolides?
Erythromycin, Clarithromycin
Give an example of a lincosamide?
Clindamycin
How do the MLS category of Abx kill bacteria?
Protein synthesis inhibitors
How do lincosamides kill bacteria?
Protein synthesis inhibitors
Bind to 50s ribosomal subunit - blockage of exit tunnel for newly synthesised protein
Inhibit protein elongation
Name 2 tetracycline Abx?
1) Tetracycline
2) Doxytetracycline
How do tetracylcines kill bacteria?
Protein synthesis inhibitors
Bind to 30s ribosomal subunit
Inhibit RNA translation - interfere with tRNA binding to rRNA
Give an example of an oxazolidnone?
Linezolid
How does the oxazolidinone linezolid kill bacteria?
Protein synthesis inhibitor Inhibits initiation of protein synthesis Binds to 50s ribosomal subunit Inhibits assembly of initiation complex May also bind to 70s subunit
Name 6 types of Abx which act by inhibiting protein synthesis in various ways?
1) Aminoglycosides
2) MLS (macrolides, Lincosamides, Streptogramins)
3) Tetracyclines
4) Oxazolidinones
5) Mupirocin
6) Fusidic acid
How do Trimethoprim and Sulfonamides kill bacteria?
Inhibit DNA Synthesis
Inhibit folate synthesis which is a purine synthesis precursor
Trimethoprim inhibits Dihydrofolate reductase
Sulfonamides inhibit Dihydropteroate synthetase
Which enzyme is inhibited by trimethoprim?
Dihydrofolate reductase
Which enzyme is inhibited by sulfonamides?
Dihydropteroate synthetase
What is co-trimoxazole?
Trimethoprim and Sulfonamides combined
How do quinolones and fluoroquinolones kill bacteria?
Inhibit DNA synthesis
Inhibit one or more of two related enzymes, DNA gyrase and topoisomerase IV, involved in remodelling of DNA during DNA replication
Give 3 examples of quinolones or fluoroquinolones?
1) Nalidixic acid
2) Ciprofloxacin
3) Levofloxacin
How does Rifampicin kill bacteria?
RNA synthesis inhibitor
RNA polymerase inhibitor
Prevents synthesis of mRNA
Give an Abx which targets the cell membranes of gram-negative bacteria?
Colistin
Give an abx which targets the cell membranes of gram positive bacteria?
Daptomycin
Dpatomycin has what kind of structure?
Cyclic lipopeptide which destroy plasma membranes
What are the 4 main possible adverse effects of use of Abx?
1) Adverse effects of all drugs
2) Generation of Abx resistance
3) Fungal infection
4) C.Diff infection
What are the possible adverse effects of all drugs? 3
1) General - nausea, vomiting, headache, skin rashes etc.
2) Infusion reactions (when drugs given IV)
3) Allergic reactions
What are the 2 main mechanisms of generation of Abx resistance through Abx treatment?
1) Selection of resistant strains in patients (resistant bacteria colonies no longer reduced by the presence of other bacteria as that is destroyed by abx)
2) Preferential colonisation on exposure to resistant strains (if pt on abx touches another pt, the resistant strains are most likely and able to colonise)
What is a common fungal infection caused by the use of Abx?
Superficial and invasive candidiasis (vaginal and oral thrush)
What are the specific adverse effects of aminoglycosides? 2
1) Reversible renal impairment on accumulation
2) Irreversible ototoxicity (toxic to the ear)
(therapeutic drug monitoring indicated)
What are the specific adverse effects of beta-lactams?
Main problem is allergic reactions
Generalised rash in 1-10%
Anaphylaxis in approx. 0.01%
What is the specific adverse effect of linezolid?
Bone marrow depression
What symptoms would indicate an intolerance rather than an allergy to beta-lactams?
Nausea, diarrhoea, headaches etc
What symptoms would indicate a minor allergy to beta-lactams?
Non-severe skin rash
What symptoms would indicate a severe allergy to beta-lactams?
Anaphylaxis, Urticaria, angio-oedema, bronchospasm, severe skin reaction (Steven-johnsons syndrome)
What type of beta-lactam abx are safe to use in patients with a non severe penicillin allergy? 2
1) Cephalosporins
2) Carbapenems
Which beta-lactam Abx is safe to use in patients with a severe penicillin allergy?
Aztreonam
What is the most commonly diagnosed cause of Abx associated diarrhoea?
C.Difficile infection
How does C.Diff cause infection due to Abx use?
Occasional constituent of the normal intestinal flora (in 20% of elderly people)
Produces toxins A&B
Assumed cause by abolition of colonisation resistance
Combination of enterotoxin and spore production accounts for clinical features and transmissibility
What is significant about the hypervirulent C.Diff strain 027?
Causes more severe disease
What are the 4 common precipitating abx for C.Diff infection (the 4 C’s)?
1) Co-amoxiclav
2) Cephalosporins
3) Ciprofloxacin
4) Clindamycin
What are the 4 less common precipitating abx for c.diff infection?
1) Benzylpenicillin
2) Aminoglycasides
3) Glycopeptides
4) Piperacillin-Tazobactam
What is the strategy for abx use?
Following a history taking and examination a clinical diagnosis is made, following this:
1) Empiric therapy - based on predicted susceptibility of likely pathogens and local antimicrobial policies
Following a lab investigation you get a microbiological diagnosis, following this:
2) Targeted therapy - Based on predicted susceptibility of infecting organism and local antimicrobial policies
Following antimicrobial sensitivity results:
3) Susceptibility-guided therapy - based on susceptibility testing results
What happens to the level of knowledge of infecting organism and antimicrobial spectrum of the agent used as you move along the abx prescribing strategy?
The level of knowledge of infecting organism increases
The spectrum of the antimicrobial agent used narrows
What is the policy for Abx prescribing from the Dept of Health?
Start Smart then Focus (at 48 hours)
What is meant by ‘start smart’ in the dept of health policy for abx prescribing?
1) Only start using abx if there is clinical evidence of infection
2) Use local guidelines
3) Document indication, duration or review date
4) Obtain cultures first
5) Use single dose abx for surgical prophylaxis
What is meant by ‘Focus’ in the dept of health policy for abx prescribing?
1) Stop abx if their is no evidence of infection
2) Switch abx from IV to oral
3) Change abx - ideally to a narrower spectrum - or broader if required
4) Continue and review again at 72 hours
5) Outpatient parenteral abx therapy (OPAT)
Which key abx is used to treat staph aureus (not MRSA)?
Flucloxacillin
Which key abx is used to treat streptococcus pyogenes?
Benzylpenicillin
Which key abx is used to treat gram negative bacilli?
Cephalosporins (avoid in the elderly)
Which key abx is used to treat anaerobes?
Metronizadole
Which key abx is used to treat gram positives (including MRSA)?
Vancomycin
Which key abx is used to treat most clinically relevant bacteria?
Meropenem
Which key abx is the last option for multi-resistant gram negatives?
Colistin
An important determinant of in vivo efficacy is concentration at site of action, which abx has good and which has poor availability in CSF?
Good availability in CSF in presence of inflammation = beta-lactams
Poor availability in CSF = Aminoglycosides and vancomycin
An important determinant of in vivo efficacy is concentration at site of action which abx has good and which has poor availability in urine?
Good availability in urine = trimethoprim and beta-lactams
Poor availability in urine = MLS abx
Combination therapy should only be carried out with good reason, what are the 3 reasons for combining abx?
1) To increase efficacy - synergistic combination may improve outcome
2) To provide adequately broad spectrum - single agent may not cover all organisms eg. polymicrobial infection, emperic treatment of sepsis
3) To reduce resistance - Organism would need to develop resistance to multiple agents simultaneously eg. antituberculous chemotherapy
When considering pharmacodynamics what is meant by conc. dependent abx?
Main determinant of bacterial killing is the factor by which concentration exceeds MIC
How would a conc dependent abx be administered differently to a time dependent abx?
1) Conc dependent - administered intermittently at high conc to achieve high peaks
2) Time dependent - administered frequently to maintain a high level
Give an example of a conc dependent and a time dependent abx?
Conc = Aminoglycosides Time = Beta-lactams
When considering pharmacodynamics what is meant by time dependent abx?
Main determinant of killing is the amount of time for which the abx conc exceeds MIC
