Lecture 8 - Antibiotics Flashcards
Discuss the history of antibiotics.
- Alexander Fleming in 1928 developed the first antibiotic (penicillin)
- Most early antibiotics were naturally occurring bacteria or fungal products
After 1960, chemists and microbiologists began working to synthesise antibiotics
- Most early antibiotics were naturally occurring bacteria or fungal products
What are antibiotics.
Substances that are selectively toxic to microorganisms. They kill or inhibit microorganisms without harming the patient. The basis for this selective toxicity lies in the differences between prokaryotic and eukaryotic cells.
Name some antibiotic mechanisms.
- Inhibition of cell wall synthesis
- Inhibition of protein synthesis
- Injury to plasma membrane
- Inhibition of synthesis of essential metabolites
- Disruption to cytoplasmic membrane
- Inhibition of RNA synthesis
- Inhibition of pathogens attachment to or recognition of host.
Selective toxicity
Penicillins target the peptidoglycan layer found only in bacterial cells wall, this makes penicillin a very safe drug. Other antibiotics which inhibit pathways or target structures that are found in the human cells often cause side effects. E.g. gentamicin causes hearing loss and affects kidney function.
How are antibiotics classified.
Antibiotics can be classified based on, chemical structure, spectrum of activity, mode of action etc.
Classification based on chemical structure:
Beta lactam antibiotics: penicillins, cephalosporins, carbepenems, monobactams
Quinolones: ciprofloxacin
Aminoglycosides: gentamicin, streptomycin
Macrolides: Erythromycin, clarithromycin
Tetracyclines: Tetracycline, doxycycline
Glycopeptides: vancomycin & teicoplanin
Sulpha antibiotics: Cotrimoxazole (Septrin), Trimethoprim
Miscellaneous: Chloramphenicol, nitrofurantoin, rifamycins
What is the difference between bacteriostatic and bacteriosidal antibiotics.
Bacteriocidal refers to agents that kill bacteria
Their action is irreversible. They act by inhibiting the cell wall formation of bacteria, they don’t work with the immune system of the host.
Bacteriostatic refers to agents that prevent the growth of bacteria
Their action is reversible. They act by inhibiting DNA replication and protein synthesis of bacteria, they work with the host immune system to prevent the growth and reproduction of bacteria. When the antimicrobial is removed the bacteria will continue growing.
Bacteriostatic - the total cell count and viable stay count both become stationary when the bacteriostatic is introduced
Bacteriocidal - The total cell count becomes static but the number of culturable cells decreases
Bacteriolytic - Both the number of cells and number of culturable cells decrease
What is meant by the beta-lactam group of antibiotics.
Include penicillins, cephalosporins, monobactams and carbapenems - all have beta lactam group in common.
What are penicillins.
- Examples: Penicillin G, penicillin V, amoxicillin, flucloxacillin.
- Class: Belongs to the beta-lactam group of antibiotics
- First discovered in 1928 by Alexander Fleming.
- First naturally occurring penicillin was penicillin G. Derived from a fungus Penicillium notatum.
- Mechanism of action: They are cidal ( kill bacteria) by binding to and destroying bacteria cell walls thereby resulting in bacterial death.
- Resistance mechanism:
○ Bacteria develop resistance to penicillin by producing an enzyme ( beta-lactamase) which destroys the beta lactam ring of penicillins.
○ Interrupt the binding of the antibiotic and allow cell wall formation to continue - Spectrum of activity/ clinical uses: Penicillins are used to treat a wide spectrum of infections: Skin soft tissues, chest infection, pneumococcal, meningococcal and staphylococcal infections (fluclox) but there are major problems with resistance.
- Side effects: Main side effects are allergic reactions ( rash or anaphylactic reactions)
The cell wall of bacteria contains peptidoglycan which is made up of N-acetylglucosamine and N-acetylmuramic acid polymer chains linked by peptide bridges. New NAG and NAM subunits are inserted into the wall by enzymes allowing the cell to grow. Normally other enzymes link new subunits to old subunits. Penicillin interferes with the linking enzymes an NAM subunits remain unattached to their neighbours. However the cell continues to grow as it adds more NAD and NAM subunits. The cell bursts from osmotic pressure because the integrity of peptidoglycan is not maintained.
What are aminoglycosides?
- Example: Gentamicin.
- Mechanism of action: Binds to ribosomes and prevents protein synthesis. Bactericidal.
- Spectrum of activity: Broad spectrum against mainly Gram negative bacteria such as E coli, pseudomonas, klebsiella , proteus. They have limited activity against Gram positive bacteria, often used in combination with Beta-lactam antibiotics to treat Gram positives such as streptococcal and staphylococcal infections.
- Clinical uses: Used for UTIs, intra-abdominal infections, gynaecological infections, used in combination with penicillins to treat endocarditis.
- Side effects: Their use is limited by serious side effects such as hearing loss (Ototoxicity) & renal impairment (nephrotoxicity).
- They are only available as IV formulations. Levels have to be monitored strictly to prevent side effects.
- Resistance: results from altered binding site and efflux pump effect.
The mechanism of action - inhibition of protein synthesis
* Some aminoglycosides cause change in 30S ribosome shape; mRNA is misread
* Tetracycline and some aminoglycosides block docking site of tRNA
Lincosamides or macrolides bind to 50S ribosome subunit, blocking proper mRNA movement through ribosomes so synthesis stops
Mechanism of resistance - streptomycin (an aminoglycoside)
Mutation means that streptomycin can no longer fit into its active site on the ribosome therefore synthesis isn’t stopped.
What are tetracyclins.
Tetracyclines
* Examples are tetracycline ad doxycycline.
* Mechanism of action: They prevent bacterial protein synthesis and bacteriostatic rather than cidal.
Bind to A site of 30S ribosomal subunit preventing the binding of tRNA to A site preventing protein synthesis.
* Spectrum of activity & clinical uses: Very broad spectrum against Gram positives, Some Gram negatives ( except pseudomonas), atypical bacteria such as mycoplasma and rickettsial infections. Used to treat skin soft tissue infections, chest infections, Lyme disease and rickettsial infections.
* Metabolism: Tetracyclines are well absorbed after oral administration. Penetration is good into bone, skin, lung and kidneys. It is inactivated by the liver and excreted into faeces and urine. They bind to calcium and magnesium to form an insoluble complex. Their absorption is therefore inhibited by dairy produce.
* Contraindication/ side effects: Tetracycline’s should not be given to pregnant women and children because it affects skeletal development and causes permanent teeth discolouration. They also cause photosensitivity reaction on exposure to sunlight.
Describe the mechaisms of tetracycline resistance.
Mechanism of tetracycline resistance
Tetracycline Efflux: A way to limit access of tetracycline to ribosomes is to reduce intracellular concentrations of tetracycline by pumping it out of the cell at a rate equal to or greater than its uptake. The efflux proteins exchange a proton for a tetracycline-cation complex against a concentration gradient. This resistance mechanism is the best-studied and most familiar mechanism of tetracycline resistance. Efflux resistance genes are generally found on plasmids. The resistance gene product is a cytoplasmic membrane protein that is an energy-dependent tetracycline transporter. Efflux genes are found on the two types of bacteria gram-positive and gram-negative.
Ribosomal Protection: There are nine ribosomal protection proteins namely tet(M), tet(O), tet(S), tet(W) tet(Q), tet(T) otr(A), tetP(B) and tetc. These are cytoplasmic proteins that protect the ribosomes from the action of tetracycline. They interact or associate with the ribosome, making it insensitive to tetracycline inhibition. Ribosomal protection proteins bind to the ribosome causing an alteration in ribosomal conformation that prevents tetracycline from binding to the ribosome, without altering or stopping protein synthesis. The genes for ribosomal protection have been found on plasmids and self-transmissible chromosomal elements.
Tetracycline Inactivation: The tet(X) gene is the only example of tetracycline resistance due to enzymatic alteration of tetracycline. The tet(X) gene was found because it is linked to erm(F), which codes for a rRNA methylase gene. The erm(F) gene was cloned into E. coli, and the clones were found to confer tetracycline resistance in E. coli when grown aerobically. The tet(X) gene product is a 44-kDa cytoplasmic protein that chemically modifies tetracycline in the presence of oxygen and NADPH.
* Enzyme alters antibiotic so no longer functional
What should be considered before someone is prescribed antibotics
- Have they got an infection?
- What type of infection do they have?
○ Viral : varicella (chicken pox), rubella (German measles), common cold, hepatitis, mumps – do not need antibiotics for primary infection
○ Fungal – only a few available
○ Bacterial – main target of antibiotics - Can I wait for culture results or do they need empiric antibiotics?
- Aim to give targeted antibiotics whenever possible. Cultures must be sent!
- Not all positive microbiology needs antimicrobial treatment.
- What type of infection do they have?
Factors to consider - choice of an empiric antibiotic
* Organism related factors - Site of infection (likely pathogens), Organisms & pattern sensitivity in the local area
* Patient related factors - Immune status, renal and hepatic function, side effects, pregnancy and breast feeding, severity of illness.
* Antibiotic related factors - Absorption, half-life, protein binding, metabolism, drug interactions.
Targeted antibiotics aren’t always possible
Antibiotic resistance is a real threat
* Acinetobacter
* Staphylococcus aureus
Neisseria Gonorrhoeae
