Component 3A - Antibiotics Flashcards
What are bactericidal antibiotics?
They kill bacteria, who destroy the bacterial cell wall
What are bacteriostatic antibiotics?
They prevent growth and multiplication. The bacteria resume their normal metabolism when the antibiotic is no longer present
What are broad-spectrum antibiotics?
They affect many different gram-positive and gram-negative species
What are narrow-spectrum antibiotics?
They are much more selective. For example they kill gram-positive or gram-negative bacteria only
Why are antibiotics safe to use in humans to kill bacteria?
Antibiotics used medically affect bacterial metabolism but do not interfere with the host cell metabolism.
How does the structure of gram-negative cell walls protect against many antibiotics and immune defences?
• It contains peptidoglycan (murein) consisting of molecules of
polysaccharide cross linked by amino acid side chains. The cross
linking provides strength and the wall protects against osmotic lysis.
• It is surrounded by an outer layer of lipoprotein and
lipopolysaccharide.
• The presence of the extra layers protects the cells from the action of
some antibacterial agents such as lysozyme and penicillin.
What reflects the more complex structure of gram-negative bacteria?
The Gram reaction: lipid-containing molecules are disrupted by decolorisation stage causing violet to leak out and then stained red by Safranin.
How does penicillin affect the structure of bacteria and how does it do this?
Penicillin affects the formation of cross linkages in the cell wall during the growth and division of bacterial cells. It does this by binding to and inhibiting the enzyme responsible for the formation of cross-links between molecules of peptidoglycan.
What does the change in structure caused by penicillin cause to happen to the bacteria?
The wall is weakened so water can enter the cell by osmosis and this causes cell lysis
Which type of bacteria is penicillin more effective against?
Penicillin is more effective against Gram positive organisms than Gram negative due to the difference in the structure of the cell wall.
Describe how tetracycline affects bacteria?
It acts by acting as a competitive inhibitor of the second anticodon-binding site on the 30S subunit of bacterial ribosomes and prevents the binding of a tRNA molecule to its complementary codon. In this way tetracycline inhibits the translation stage of protein synthesis.
Why is tetracycline a good antibiotic?
Affects protein synthesis, a metabolic process common to all bacteria, and is effective against a broader range of bacteria.
Why are viruses not affected by antibiotics?
Because they don’t have cell walls or metabolic pathways
What are two sources of antibiotic resistance
• Bacteria divide rapidly under optimum conditions and have a high
mutation rate;
• Naturally occurring mutations that confer resistance to antibiotics can be acquired in plasmids and this gives these bacteria a selective advantage (resistance gets passed on)
Why is it the continuous use of high levels of antibiotics a problem?
1) It has resulted in the selection of bacterial strains that are completely unaffected by some antibiotics
2) In the absence of antibiotics the antibiotic-resistant bacteria no longer have an advantage over non-mutated forms, so it is the use of antibiotics that causes the problem
