What are antimicrobials?
Antiseptics
Disinfectants
Antibiotics
What are antibiotics?
Produced by fungi and act on bacteria but not viruses and not on eukaryotic cells. They treat bacterial infection without harm to the patient. Broad spectrum antibiotics affect many different Gpos and Gneg species but narrow spectrum affects specific types.
What are different types of antibiotics?
Bactericidal antibiotics kill bacteria
Bacteriostatic antibiotics prevent bacterial multiplication but don’t cause death.
What is the structure of the bacterial cell wall?
Petidoglycan or murein forms part of the wall. It contains polysaccharide and short amino acid chains. Transpeptidase enzymes cross link the polysaccharide molecules by attaching them to side chains of amino acids. The cross linking makes the cell wall strong, gives it shape and allows it resist bursting due to osmosis.
Describe the structure of gram positive bacteria.
Cell walls are made of a thick layer of murein. Pores in the murein close during decolorisation of gram stain protocol and so crystal violet is retained within the cell so it stains violet. Safranin is a counter stain and turns violet GPos cells purple. The murein is accessible to molecules outside the cell so is susceptible to penicillin and lysozyme.
Describe the structure of gram negative bacteria.
Thin layer of murein and it is surrounded by a layer of lipoprotein and lipopolysaccharide. These lipid containing molecules are disrupted by the decolorisation stage of gram staining and crystal violet leaks
s out of the cell, leaving it unstained. Safranin turns the cells red. The lipid containing layer protects the murein from agents like penicillin.
How does penicillin work?
- Readily diffuses through the cell wall of Gpos bacteria and enters some Gneg through surface molecules called porins
- Bacteria continually make and break down parts of their cell wall
- DD-transpeptidase catalyses the condensation reactions that make cross links. Penicillin binds.
- Breakdown by hydrolysis continues and more cell wall is lost
- As no peptide cross links are made precursor molecules build up which are hydrolysed too
- Cell wall weakened, water enters by osmosis, wall is too weak to withstand the pressure and cell lyses.
How does tetracycline work?
It acts against Gpos and Gneg bacteria and so it has wide medical use against the bacteria that cause acne and common infections. It inhibits protein synthesis. It both diffuses and is pumped into bacteria cells. It binds to the small subunit of ribosomes and blocks TRNA attachment in the second position so no Amino acids can be added to the polypeptide chain. It binds reversibly
Why has resistance occurred?
Antibiotics have been used frequently, specifically by farms to prevent infection rather than treat it.
How does resistance occur?
Antibiotics in the environment kill susceptible individuals but any that have mutation will resist and survive. They have a selective advantage and reproduce, passing on the resistant alleles and builds a resistant population.
What are the 2 sources of antibiotic resistant alleles?
- every time bacterial cell DNA replicates a mutation conferring resistance may arise. Bacteria divide rapidly when conditions are suitable so have high mutation rate
- bacteria may acquire plasmids carrying an allele conferring resistance from their environment. The plasmids replicate inside the bacterium and are passed on.
Why has overuse causes resistance?
The mutated genes code for proteins that prevent antibiotics working. If there were no antibiotics the energy used to synthesise these proteins would put bacteria at a selective disadvantage and they would die. They only have a selective advantage when antibiotics are present. High levels of antibiotics therefore causes resistance
