Control of microbes with antimicrobials

Question 1

Viable count:

Answer 1

only living cells included

Question 2

Total

Answer 2

living and dead cells

however lysed cells do not contribute to total count

Question 3

Antimicrobials

Answer 3

• chemicals that are selectively toxic to microbes
• they are made by a microbe and attacks another microbe
• more selective toxicity than disinfectants (least selectively toxic)

Question 4

Therapeutic dose:

Answer 4

minimum conc. of agent required for effective disease treatment

Question 5

Toxic Dose:

Answer 5

max. conc. of agent tolerated without unacceptable side-effects

Question 6

Therapeutic index:

Answer 6

window between therapeutic dose and toxic dose (Toxic dose/ therapeutic dose)

Question 7

Bacteriostatic

Answer 7

stops growth, does not kill, thus reversible (if take agent away, population will come back and grow)

Question 8

Bacteriocidal

Answer 8

stops growth, kills cells, irreversible

Question 9

Bacteriolytic

Answer 9

stops growth, lyses cells, irreversible.

However if bacteria is lysed could release an endotoxin which is toxic thus could essentially kill the patient. E.g. Thyphoid fever - thus why we use bacteriostatic

Question 10

Antibiotic modes of Action

Answer 10

• Inhibit cell wall synthesis
• Inhibit protein synthesis
• Inhibit nucleic acid synthesis
• Injure cytoplasmic membrane
• Block metabolic pathways

Question 11

Inhibit cell wall synthesis

Answer 11

Targeting the peptidoglycan layer will destroy the rigidity fo the bacteria.

Agents targeting the cell wall are bacteriolytic (e.g. Penicillin, Vancomycin)

Question 12

Pencilin

Answer 12

• High therapeutic window, target the prokaryote-specific e.g. cell wall PG
• Effective for Gram +ve
• Bind to transpeptidase enzyme. Substrate analogs of D-alanly-Dalanine, goes to active site and stops it. Thus can’t make cross-links
• Can’t have natural ones as will be destroyed by stomach acid if taken orally

Question 13

Aminoglycosides

Answer 13

• synthesised by actinomycetes (e.g. Streptomyces)

- target the small subunit fo ribosome –> bacteriocidal

Question 14

Resistance:

Answer 14

• Spontaneous chromosomal mutations of rRNA
• Single base change
• Problem with TB, one base change can’t allow drug to work
• Vertically inherited / horizontal
• mobile genes
• resistance can be broad spectrum, if resistance to one mycin, resistant to others.

Question 15

MIC

Answer 15

minimum inhibitory concentration: lowest amount of antimicrobial that prevents growth

Question 16

MLC

Answer 16

Minimum lethal conc.: lowest conc. of antimicrobials that kills the organisms

Question 17

Advantages and Disadvantage of Agar diffusion plate

Answer 17

Adv:

• Rapid: can assay multiple antibiotics on plate
• cheap

Disadvantages:

• less accurate, MIC calculated from tables
• can’t directly antibiotics
• Can’t be used for slow-growing bacteria i.e. TB (by the time TB would develop, the antibiotic would have diffused throughout whole plate)

Question 18

Kirby-Bauer Assay

Answer 18

Standard method of sensitivity testing –> aimed to limit number of variable –> more reliable
- KB assay inhibition zone can be used to calculate MIC

Question 19

Antifungal agents

Answer 19

• eukaryotes which are similar to humans
• most antifungal drugs target sterols in the cell wall either directly or inhibiting biosynthesis
• Difficult to find useful antifungal due to host toxicity
  E.g. Candida infection of mouth

Question 20

Antiviral agents

Answer 20

• obligate parasites, use host system
• Do not have their own metabolites instead use ours, thus we can’t target our metabolism
• these agents mainly target DNA polymerase - viral polymerase has unique which is different to humans.