Introduction to antibiotics Flashcards
Describe the trend of yearly antibiotic prescribing over time
- 3.8 million presecriptions in 2012
- As time has progressed we have steasily reduced the amount of prescription
- 3.6 million prescriptions inn 2015
What are antibiotics?
- Greek: anti (against) bios (life)
- ‘Chemical compounds used to treat infections caused primarily by bacteria; they should be sufficiently non-toxic to be given to the infected host’
- Used to supplement the body’s natural defenses to a bacterial infection by either killing bacteria or inhibiting them
- ‘Selective poisons’ for treating bacterial infections not viral
What do we use antibiotics for?
- Treatment of bacterial infections
- Prophylaxis (treatment given or action taken to prevent disease)→Prevention of infection (contacts in outbreaks)
- pre-post surgery or trauma
- Non human uses of antimicrobials:
- Agriculture (as growth promoter-facilitate increase in biomass)
- Aquaculture (e.g. in fish farming)
Classification of antibiotics:
How were they traditionally classified?
Traditionally ‘classified’ on their chemical / biosynthetic origin
Natural antibiotics: (‘true antibiotics’); produced naturally by fungi or bacteria to selectively inhibit the growth of others (Penicillium chrysogenum produces penicillin)
Semi-synthetic antibiotics:chemically modified natural antibiotics (e.g. ampicillin, a modification of penicillin)
Totally synthetic antibiotics: manufactured (e.g. trimethoprim)
Antibiotics are ‘clustered’ within a large group of drugs called ‘Antimicrobials’ or ‘Chemotherapeutic Agents’
Pioneers: The Development of Chemotherapy
- Paul Ehrlich (1854-1915): Proposed the notion of SELECTIVE TOXICITY; exploit differences between structure and metabolism of infecting microorganisms and host cells
- Ehrlich proposed the concept of The Magic Bullet (He envisioned that just like a bullet fired from a gun to hit a specific target, there could be a way to specifically target invading microbes)
- To understand ‘antibiotics’ we need to understand the concept of SELCETIVE TOXICITY
- Primarily achievable in prokaryotes (bacteria) vs eukaryotes
Pioneers: The Discovery of Penicillin
- 1928 Alexander Fleming noticed a secreted antimicrobial produced by the Penicillium fungus (chrysogenum)
- This compound was penicillin
- Between 1928 and 1945 Howard Florey and Ernst Chain devised methods to produce large quantities of penicillin (refined penicllin from the urine of people treated with it)
- The first broad spectrum antibiotic with low toxicity and very good activity.
- Fleming, Chain and Florey shared the Nobel Prize for medicine in 1945
Pioneers: The Discovery of Penicillin
A discovery that changed the world
Penicillin was introduced in the 1940s and was used to treat soldiers during World War II
Previously the infections would have been fatal but now can be treated with penicillin
Many infections including pneumococcal pneumonia (infection in the bloodstream) became survivable!
- Would’ve killed approx 80% infected people within about 20 days
- More people surviving major infectious diseases, fatality rate reduced to about 10-15% uppon implementation of penicillin
The pioneers: The discovery of penicillin
Dorothy Hodgkin (1945)
- Solved the strucure for penicillin
- Biochemist
- Pioneer of X-ray crystallography to determine the alpha helical structure of DNA
- Became a fellow of royal society in 1964
- Nobel prize in chemistry 1964
Pioneers: The Discovery of Streptomycin
- 1944: Incentivised people to find new antibiotics. This era is known as the antibiotic arms race!
- Selman Waksman discovered a ‘new’ antibiotic
- STREPTOMYCIN produced by Streptomyces griseus (Nobel Prize 1952)
- 1st Antibiotic effective against tuberculosis (not kiled by penicillin)
- Albert Schatz made the critical discovery but never recored the evidence of his discovery, Waxsman took all the credit
- 1953: Microorganisms producing CHLORAMPHENICOL, TETRACYCLINE and NEOMYCIN were isolated
How do antibiotics work?
Antibiotics work in one of two ways:
Bactericidal:
Kills the bacteria
Bacteriostatic:
Prevents the bacteria from dividing
Antibiotics can be either:
- Broad Spectrum:
Kill a wide range of bacteria e.g. Penicillin
Streptococci, Staphylococci, Clostridium, and Listeria
- Narrow Spectrum:
Kill a specific type or group of bacteria e.g. Isoniazid
Selective target for mycobacteria
- No effect
- Slowly growing
- Barely Bacteriostatic
- Bacteriostatic
- Bacteriocidal
- Rapidly bactericidal
Antibiotics work on different structures on the bacterium
- Cell wall synthesis inhibitors:
Beta-lactams Glycopeptides Cycloserine, bacitracin
- Cell membrane:
Polymyxin, peptides
- Nucleic acid synthesis:
Sulphonamides, quinolones, rifampicin and Trimethoprim
- Protein synthesis:
Aminoglycosides, tetracyclines, choramphenicol, macrolides, lincosomides, fisuidic acid
Introduction to the β-Lactam Antibiotics
- All have the beta lactam ring in their structure (identified by Dorithy Hodgkin) therefore have the same mechanism of action
- A lactam is a cyclic amide
- Beta lactam ring = 3-carbon and 1-nitrogen ring
The β-Lactam Antibiotics: The Penicillins
Why couldn’t you take penicillin G orally? How is it modified to allow us to do so?
Name an antibiotic derived from the chemical modification of penicillin V
Penicillin G is the natural penicillin discovered by Fleming, Chain and Florey, it is acid labile so stomach acid would break it down and reder it ineffective (hydrolyse beta lactam ring)
Fleming, Chain and Florey identified that if you grow penicillium chrysogenum in a phenyoxy acetic acid containing medium rather than a phenyl acetic acid containing medium it would add extra oxygen atom into the background structure of penicillin and the resulting chemical structuer is acid stable (allows us to take it orally)
Amoxicillin derived through chemical modification of penicillin V + Methicillin
The β-Lactam Antibiotics: The Penicillins
Describe their conserved structure
- They each contain a β-lactam ring fused with a thiazolidine ring
Penicillin V→Phenoxymethylpenicillin
Penicillin G→ Benzylpenicillin
Amoxicillin
Methicillin
How does Penicillin work?
- Penicillin kills bacteria by weakening their cell walls (BACTERICIDAL) causing osmotic rupture
- An essential part of the bacterial cell wall is a chain-link fence structure (peptidoglycan or PG)
- PG is a mesh around the bacteria that is springy but strong made up of individual links called NAG and NAM
- The links (NAG & NAM) joined together in repeating filaments by β-(1,4)-glycosidic bonds by TRANSGLYCOSYLASE but between fibres (attached to NAM) are pentapeptide crossbridges that are joined together by an enzyme (TRANSPEPTIDASE)
- Transpeptidase enzyme is blocked by ß-lactam antibiotics (Penicillin)
- Holes open up in the chain-link fence
- Causes bacteria to rupture under their own pressure (lysis)
TRANSPEPTIDASE (Penicillin Binding Proteins) catalyse the final stages of peptidoglycan formation
Transpeptidase (PBPs-penicillin binding proteins- target of penicillin); membrane bound
Transpeptidase facilitates the pentapeptide CROSS-LINKING
Penicillin binds to transpeptidase and stops it building crossbridges between peptidoglycan fibres so chainlink fence can’t form
Inhibition of Transpeptidase by β-Lactam Antibiotics
β-lactam antibiotics inhibit the transpeptidase (similar structure to D-alanyl D-alanine in the peptide chain)
competitive inhibition; irreversible binding
- Lysine in position 3 of stem forms an interpeptide pentaglycine crossbridge with terminal D-alanyl-D-alanine residues that are attached to NAM
- Transpeptidase binds to terminal D-alanine D-alanine to form the peptide bond with the crossbridge (the free amino end of an l-Lys residue)
- Beta lactam and thiazolidine are an analog (chemically analogous) to the terminal D-alanine D-alanine structure so that this molecule will bind to transpeptidase irreversibly (penicillin is a competitive inhibitor) and block the enzyme from working
- Hydrolysis of Beta lactam ring (and amide bond) and a covalent bond (irreversible) formation with the transpeptidase enzymes catalytic serine residue in the active site.
- As a result the cell wall falls apart
β-Lactam antibiotics: clinical usage
They are broad spectrum thefore have a variety of uses such as:
- URTI- upper respiritory tract infection (eg. tonsillitis)
- LRTI- lower respiratory tract infection (eg. pneumonia)
- STI (eg. gonorrhoea, syphilis)
- Skin and tissue infections
NB. (allergy) Hypersensitivity and anaphylactic shock in some patients; alternative antibiotics warranted
Why do antibiotics stop working?
What doesn’t kill them makes them makes them stronger
Persister population (a few isolates metabolising at different rates or have advantageous mutations/selective advantage) They have been exposed to a sub lethal amount of antibiotics
How do bacteria become resistant to antibiotics?
- Make enzymes which alter or destroy the antibiotic
- Alteration of target site
- Prevent antibiotics getting into the cell
- Pump (efflux) the antibiotic out of the cell
- Temporarily change their metabolism (dormancy)
Antibiotic sensitivity testing in the clinical microbiology laboratory:
- Determine whether bacteria are ‘sensitive’ or ‘resistant’ to different antibiotics
(a) Right antibiotic is chosen to treat the infection
(c) Prevention of bacterial resistance - Laboratory tests fall into two main categories
(a) Diffusion: Disk Diffusion Assay (Pseudo-quantitatve/qualitative)
In this test, wafers containing antibiotics are placed on an agar plate where bacteria have been placed, and the plate is left to incubate. If an antibiotic stops the bacteria from growing or kills the bacteria, there will be an area around the wafer where the bacteria have not grown enough to be visible- this is called the zone of inhibition
(b) Dilution: Minimum inhibitory / bactericidal concentrations (MIC / MBC) (Quantitative)
Diffusion: Disk Diffusion Assay
What are the advantages?
What are the disadvantages?
- Advantages: simple; screen lots of antibiotics; inexpensive
- Disadvantages: qualitative; no information on antibiotic concentration
Dilution: Minimum Inhibitory / Bactericidal concentration (MIC / MBC)
What is MIC?
What is MBC?
What are the advantages and disadvantages?
How is this process carried out?
MIC: Lowest concentration of an antibiotic needed to inhibit visible growth in broth
MBC: Lowest concentration of an antibiotic needed to kill 99.9% of the original bioload
- Advantages: greater accuracy than agar disk diffusion; Quantitative provides information that CAN be used to modify treatment regimes
- Disadvantages: more expensive and labour intensive
- Relevant controls: No antibiotic (positive control, no inoculum (negative control), with neat antibiotic and with a series of serial dilutions of the antibiotic
- You can identify any positive bacterial growth within the culture
- Measure the optical density (cloudy=bacterial growth,clear= bacterial inhibition) that gives you MIC
- To get MBC you need to take a sample from each culture, plate it out and do a CFU count
