Anti-biotics and anti-fungals Flashcards
What are the two key targets of antibiotics
Bacterial Protein Synthesis
Bacterial Cell wall synthesis.
Describe the key features of bacteria
Bacteria are single cell micro-organisms surrounded by a lipid membrane, which in turn is surrounded by a cell wall. They have no true nucleus for the storage of genetic information and do not
contain any other membrane-bound organelles. For these reason they are known as ‘prokaryotes’.
1/3 of all bacteria are pathogenic
Which phylogenetic domain do bacteria belong to
They constitute an entire phylogenetic domain along with archaea (e.g. Halobacteria, methanogens) and eukaryotes (e.g. protists, fungi, plants & animals) according to the now generally accepted
three domain system originally proposed by Woese, Kandler & Wheelis (1990).
Summarise the different ways of classifying bacteria
There are various systems used to classify bacteria (e.g. DNA sequence, shape, fastidiousness) but for the purpose of this lecture we will focus on their reaction to Gram staining.
Describe the membrane properties of gram positive bacteria
These bacteria have a prominent peptidoglycan cell wall, which means that they take up the Gram stain in a simple biological test. A common Gram positive bacterial species is Staphylococcus Aureus
Cell membrane is surrounded by peptidoglycan cell wall.
Describe the membrane properties of gram negative bacteria
Outer membrane with lipopolysaccharide
E.g. Escherichia Coli
Far less prominent peptidoglycan- sandwiched between LPS and inner membrane
Describe the membrane properties of mycolic bacteria
This genus of bacteria does not fit into the classical Gram negative and positive classification; although technical they are considered to be Gram positive, since they do take up the stain but are acid-fast. These bacteria have an outer layer of mycolic acid and a common mycobacterial species is Mycobacterium tuberculosis.
Peptidoglycan (may be prominent in some mycolic bacteria)- sandwiched between cell membrane and outer mycolic acid layer
Describe the importance of this classification of the membrane properties of antibiotics for pharmacology
This classification is particularly useful for pharmacological purposes since a number of drugs are only effective against one or the other type of bacteria. Drugs that are effective against both Gram positive and Gram negative bacteria are referred to as broad-spectrum antibiotics.
What is meant by acid-fast bacteria
Once stained, acid-fast organisms can resist the acid or ethanol based decolorization common in many staining protocols.
How do E.coli and Mycobacterium Tuberculosis become resistant
By ‘escape’ mechanisms
State the steps involved in the production of THF from PABA
PABA à DHOp (enzyme = dihydropterase synthase)
DHOp à DHF
DHF à THF (enzyme = DHF reductase
Summarise prokaryotic nucleic acid synthesis
Dihydropteroate (DHOp) Produced from paraaminobenzoate (PABA) Converted into dihydrofolate (DHF) Tetrahydrofolate (THF) Produced from DHF by DHF reductase THF Important in DNA synthesis
Describe the importance of THF in prokaryotic nucleic acid synthesis
Nucleic acids are the building blocks of prokaryotic (and eukaryotic) DNA; therefore their synthesis is essential for cell survival and cell division. Tetrahydrofolate (THF) is a critical co-factor involved in the production of numerous amino acids and thymidylate (which is required for uracil synthesis). And there are subtle differences between THF production in prokaryotes that can be exploited by pharmacological reagents.
Describe the enzyme important for prokaryotic DNA replication
DNA replication is the process by which the nucleotide sequence of DNA is copied into an exact complementary sequence, which is required for cell division. The process requires an elaborate interplay between numerous different proteins but for the purpose of this lecture we will focus on the DNA gyrase enzyme. DNA gyrase is a type II topoisomerase, which cuts DNA strands thus releasing the tension from supercoiling and therefore allowing access to all the proteins involved in transcription.
Describe prokaryotic DNA transcription/RNA synthesis
DNA transcription is the ‘process whereby one strand of a DNA molecule is used as a template for synthesis of a complementary RNA by RNA polymerase’. The complementary RNA (known as messenger (m)RNA) is subsequently utilised for the production of proteins.
In bacteria there is only one RNA polymerase responsible for the entire process (as opposed to eukaryotes, who have multiple types of RNA polymerase)
Define protein synthesis
The process of protein synthesis, or translation, can be defined as ‘the ribosome-mediated production of a polypeptide whose amino acid sequence is specified by the nucleotide sequence in an mRNA.’
Describe prokaryotic protein synthesis
Ribosomes Produce protein from RNA templates Differ from eukaryotic ribosomes Eukaryote = 40S + 60S Prokaryote = 30S + 50S
What are the four key steps in prokaryotic protein synthesis that can be exploited pharmacologically
Nucleic Acid Synthesis
DNA replication
RNA synthesis
Protein synthesis
Describe the antibiotics that target prokaryotic nucleic acid synthesis
The enzyme DHOp synthase is not functional in humans and is targeted by the sulphonamide (e.g. sulfadiazine) group of antibiotics. However, these antibiotics are no longer used readily mainly due to the development of bacterial resistance.
Trimethoprim targets the bacterial dihydrofolate (DHF) reductase enzyme with a far greater affinity than the human isoform. It is often used in conjunction with the sulphonamide drug (sulphamethoxazole) in the compound preparation co-trimoxazole but is also prescribed alone mainly for the treatment of urinary tract infections
Describe the antibiotics that target prokaryotic DNA replication
DNA gyrase is not present in humans.
Fluoroquinolones (e.g. Ciprofloxacin) inhibit DNA gyrase & topoisomerase IV
Can also use quinolones
They are generally effective against both Gram negative and Gram positive bacteria (broad spectrum) but they are contraindicated in children and individuals with epilepsy.
Describe the antibiotics that target prokaryotic RNA synthesis/DNA transcription
In bacteria there is only one RNA polymerase responsible for the entire process (as opposed to eukaryotes, who have multiple types of RNA polymerase) and this enzyme is targeted by the Rifamycins (e.g. rifampicin), which are primarily used to treat mycobacterial infections (e.g. tuberculosis).
