Week 4/5 Microbial Growth and Control Flashcards
How to kill microbes: Chemotherapeutic agents
Chemical agents used to treat a disease, it is antibacterial, antifungal, and antiviral.
What is selective toxicity ?
The ability of a drug to only inhibit the pathogen and not damage the host.
What is Therapeutic dose ?
The drug level needed to kill the pathogen in a human being.
What is toxic dose ?
Too high of a drug level that becomes toxic to humans
What is Therapeutic index ?
Ratio of toxic dose to therapeutic dose
Chemotherapeutic agents side effects ?
the undesirable effects of the drug on the host cell.
What is a narrow spectrum drug ?
A drug that attacks only a few pathogens/ specific organisms
What is a broad spectrum drug ?
A drug that attacks a wide range of pathogens/ specific organisms
What is the minimal inhibitory concentration (MIC)
The lowest concentration of the drug needed to kill the pathogen. Used in blood diseases so the cells don’t get lysed and create blood toxins.
What is the minimal lethal concentration ? (MLC)
The lowest concentration of the drug that kills the pathogen. Used in skin bacterial infections to fully lyse the bacteria.
How do you determine the effectiveness of antimicrobials using the dilution susceptibility test ?
Used to determine the minimal concentration of the antibiotic needed to kill the pathogen. (MIC)
Bacteria are inoculated into different concentrations of the antibiotic. The first tube that has the lowest of antibiotic showing no growth is the MIC.
How do you determine the effectiveness of antimicrobials using the disk diffusion test ?
Have a plate with bacteria growing, and disks with antibiotics are added to inhibit growth. The disks will have a halo of clear agar indicating no bacterial growth and that the antibiotic worked.
What do antibiotics target ?
- Inhibit cell wall synthesis
- inhibit protein synthesis
- stop metabolism
- inhibit the synthesis of nucleic acids
Describe Penicillin (B-lactam)
Effective against gram +ve bacteria
acts on growing bacteria
It blocks the enzyme that catalyzes transpeptidation ( cross-links of NAM and NAG) and prevents the synthesis of the cell wall so the cells lyse.
Why is there a range of B-lactam antibiotics ?
Antibiotics like penicillin have been modified so that they can work along with bacteria that mutate and become resistant to certain antibiotics.
Cephalosporins - chemotherapeutic agent
Work similar to penicillin, this is a broad spectrum antibiotic so it works on a lot of bacteria/pathogens. Works on G+ve and G-ve bacteria.
Vancomycin/Teicoplanin
Works on gram +ve bacteria. It stops cell wall synthesis it binds tightly to the peptide chain in peptidoglycan.
Used to treat antibiotic resistant bacteria, as it doesn’t bind to proteins so mutating the protein structure doesn’t affect it.
Aminoglycosides
Have a cyclohexane ring and amino sugars. They bind to the 30s ribosomal subunit and interfere with protein synthesis, by causing misreading of the mRNA.
Works against Gram-ve aerobic bacteria and facultative anaerobic bacteria
Tetracyclines
A 4 ring structure, with a variety of side chains that are able to bind. Wide spectrum antibiotic.
Binds to the 30s ribosomal subunit, and stops the aminoacyl tRNA from binding to the A site in the ribosome.
Bacteriostatic
Macrolides
Bacteriostatic
Contain 12-22 carbon lactone rings that are linked to one or more sugars.
Bind to 23s rRNA of the 50s ribosomal subunit. and inhibits peptide chain elongation.
Sulfa Drugs
Metabolic drugs
Many pathogens make folic acid and this drug mimics a component of folic acid called P-aminobenzoic acid (PABA). They inhibit enzymes that make folic acid.
Bacteriostatic
Trimethoprim
Synthetic antibiotic that interferes with folic acid production. They can be combined with sulfa drugs to be more effective
Quinolones
Broad spectrum
A synthetic drug containing 4 quinolone ring
It inhibits bacterial DNA gyrase and topoisomerase 2.
Bactericidal
Lipopeptides
Only effective against gram +ve organisms
it enters the membrane and aggregates near phosphatidyglycerol, forms holes in the membrane of bacteria.
Antifungal drugs
Disrupt membrane permeability
inhibit sterol synthesis
disrupt spindle formation
inhibit nucleic acid and protein synthesis
Antifungal - Amphotericin B & Clotrimazole
Bind to the fungal steroid ergosterol and cause the membrane to leak.
Antiparasitic drugs
Used to treat malaria and helminths
contain peroxide
can cause antibiotic resistance
Antimicrobial resistance
When you get a bacterial infection, as the bacteria grow they mutate and some are more naturally resistant to antibiotics than others. After selection/ taking antibiotics you are left with a few resistant bacteria that can re-populate. The immune system can take care of the last few bacteria.
If you stop taking your antibiotics before the infection clears and you have Ab resistant bacteria the immune system cant kill them and they will populate. They can transfer their Ab resistance to other bacteria.
