Lecture 17-18

Question 1

What color will gram positive bacteria stain?

Answer 1

purple

Question 2

What color will gram negative bacteria stain?

Answer 2

pink

Question 3

Name the following types of bacteria in the test tubes below and briefly describe what they mean:

Answer 3

1. obligate aerobe
2. obligate anaerobe
3. facultative anaerobe - uses ATP in the presence of oxygen (aerobe) but switches to fermentation in the absence of oxygen (anaerobic)
4. micraerophilic - prefers lower oxygen and increased CO2 (capnophile)

Question 4

What is the most clinically significant gram -ve bacteria?

Answer 4

E.Coli

Question 5

What types of animal are highly reliant on their GIT flora?

Answer 5

herbivores much more so than carnivors are

Question 6

What are the three main antibacterial targets and name a drug for each mechanism?

Answer 6

1. Inhibitors of cell wall synthesis: Beta Lactams (Penicillins, Cephalosporins)
2. Inhibitors of DNA synthesis and integrity: Sulphonamides and Fluroquinolones
3. Inhibitors or transcription and translation: aminoglycosides, tetracyclins, macrolides

Question 7

How can you find out if a pathogen is sensitive to or resistant to antibiotics that are available?

Answer 7

You can grow the bacterium and then add a disc of antibiotic and observe what happens

Question 8

What is an importantant consideration to consider when looking at disc diffusion tests?

Answer 8

In vitro doesnt always equate to in-vivo efficacy

Question 9

What is a MIC and how are they conducted?

Answer 9

An MIC is a mininum inhibitory concentration. It is the lowest concentration that prevents growth after 24 hours of incubation. This is calculated quantitatively using serial dilution techniques

Question 10

What is the MBC and what does it show?

Answer 10

Minimum bactericidal concentration. It its the lowest concentration where 99.9% of a culture is killed after 24 hours of incubation.

Question 11

What type of antibiotic is it if MBC >>> MIC?

Answer 11

bacteristatic (microorganism arrestor)

Question 12

What type of antibiotic would it be if MBC ~ MIC?

Answer 12

bactericidal (microorganism killer)

Question 13

Are inhibitors of cell wall synthesis bactericidal or bacteristatic?

Answer 13

Inhibitors of cell wall synthesis = bactericidal

Question 14

Are fluroquinolones and aminoglycosides bacteriostatic or bacteriocidal?

Answer 14

bactericidal

Question 15

Are sulphonamides, tetracyclins and macolides bacteristatic or bactericidal?

Answer 15

bacteristatic

Question 16

Differentiate between the terms time dependant and dose dependant killers:

Answer 16

Time dependant killers - duration of time (determines how many bugs are killed)

Dose dependant killers - the difference between the drug concentration and the MIC determines how many bugs are killed

Question 17

What type of killer are inhibitors of cell wall sythesis?

Answer 17

Time dependant killers

Question 18

What classes of antibiotic are dose dependant bactericidals:

Answer 18

Aminoglycosides and flurquinolones

Question 19

What is the post-antibiotic effect and what are the theories on why this effect occurs?

Answer 19

Perisitant suppression of bacterial growth after removal of the antimicrobial agent. The theories on why this occurs include drug persistence and synthesising enzymes prior to replication/slow recovery from non-lethal damage

Question 20

What is the spectrum of activity?

Answer 20

The broad groups of bacteria the antibiotic is effective against. Usually expressed relative to four quadrants.

1. Gram + aerobes
2. Gram + anaerobes
3. Gram - aerobes
4. Gram - anaerobes

Question 21

What are the principle mechanisms of resistance?

Answer 21

1. Intrinsic resistance (e.g aminoglycosides and anaerobes)
2. Decreased permeability to the drug in the cytoplasmic membrane
3. Increased production of a drug destroying enzyme e.g. beta-lactamase
4. The target enzyme/protein is replaced by one with less affinity for the drug e.g. altered penicillin binding protein in MRSA
5. Active extrusion of the drug via MDR P-glycoprotein pump
6. Biofilms provide a barrier to hosts immune system and many antibiotics

Question 22

Briefly explain the mechanism of action of beta-lactams:

Answer 22

1. Penicillin transverses Gram +ve cell wall and reaches cell membrane
2. Binds to penicillin binding protein (PBP)
3. Disrupts cell wall synthesis
4. Bacteria lyse due to impaired osmotic regulation
5. Lysis of bactericidal and beta-lactams can only kill bacteria during their replication phase - time-dependant bactericidal

Question 23

What are the mechanisms of resistance of beta-lactams?

Answer 23

1. Gram -ves: cell wall/membrane is inherently less permeable to beta-lactams
2. Decreased affinity of the PBPs for beta-lactams - e.g. MRSA - this is a dead end resistance mechanism. It cannot be circumvented by a higher dose or higher dose frequency
3. Beta-lactamases, penicillinases and cephalosporinases - hydrolysis of beta-lactamases

Question 24

What is the response of pharamcologists to beta-lactamases?

Answer 24

The pharmacologists response is to use a sacrificial beta-lactam. The second beta-lactam irreversibly binds to beta-lactamase and therefore spares the first (more clinically efficacious) beta-lactam (essentially acts as a suicide inhibitor)

Question 25

What is an example of a beta-lactamase inhibitor?

Answer 25

amoxycillin-clavulanic acid as a result the amoxycillin may still be able to work despite the presence of beta-lactamase

Question 26

What is the spectrum of the natural penicillin and what is an example of one?

Answer 26

Quite narrow spectrum: predominantly gram +ves and anaerobes (G+ & G-) An example: benzylpenicillin

Question 27

What is the spectrum of aminopenicillins?

Answer 27

Broader spectrum: as above but also some gram -ves e.g. E.coli and Proteus Examples include: amoxycillan and ampicillin

Question 28

What is an example of a beta-lactamase stable penicillin?

Answer 28

methicillan (MRSA) and cloxacillin

Question 29

What is an example of an extended spectrum penicillin?

Answer 29

ticarcillin, piperacillin

Question 30

What is an example of a third generation cephalosporin?

Answer 30

Ceftiofur

Question 31

When are carbapenems used and what is an example of this class of drug?

Answer 31

One of the reserve drug classes - these are extremely efficacous - they are often used for severely ill immunocompromised people. Examples include imipenem and meropenem

Question 32

How toxic are the beta-lactams?

Answer 32

these drugs are typically very safe: can be used in geriatrics, juveniles, pregnancy and during lactation

Question 33

What species should oral-beta lactams be avoided in?

Answer 33

Should be avoided in any small animlals like rabbits and guinea pigs

Question 34

What type of drug is cephalexin?

Answer 34

An example of a first generation cephalosporin and is well suited to staph pyoderma

Question 35

What is the duration of action of cefovecin?

Answer 35

Decreased clearance rates in cats and dogs prolongs the duration of action to two weeks. Testing on other species has not shown the same duration of action.

Question 36

What is the most commonly dispensed antibiotics in small animal practice?

Answer 36

Procaine Penicillin (predominantly gram +ve coverage and also has excellent anarobic coverage (G+ and G-)

Question 37

Ceftiofur (a 3rd generation cephalosporin) has a 0wd milk WHP this is often quite misleading why?

Answer 37

The drug does not distribute to the udder very well at all this is why there is a zero day withholding period. Hence this is not a very good drug to give for mastitis.