Antibiotics

Question 1

What are antibiotics?

Answer 1

natural products produced by fungi and bacteria in the soil, give the microbes a selective advantage by killing/inhibiting the
growth of other microorganisms

most derived from natural products by fermentation, then modified chemically to give them:

1. pharmacological properties (eg. not secreted with toxic metabolites, doesn’t bind proteins in the blood)
2. an antimicrobial effect

but, some antibiotics can be totally synthetic, e.g. sulphonamides

Question 2

Alexander Fleming’s original culture of Staphylococcus aureus
contaminated with Penicillium notatum

Answer 2

Agar plate with colonies of staph aureus left at the window sill

Mould grew on the agar, and the colonies around the mould had lysed - penicillin had diffused out of the mould and lysed the colonies

Question 3

main principle of antibiotics?

Answer 3

the idea of selective toxicity

select a target that is only in the microbe and not the host, so you don’t harm the host - based on the differences in structure and metabolic pathways between host and pathogen

difficult targeting viruses because they are intracellular, fungi and parasites

variation between microbes
-even within the same species of bacteria, strains can have variation in susceptibility to antibiotics

Question 4

Therapeutic Margin - what is this?

Answer 4

active dose (MIC) versus toxic effect

MIC = minimum inhibitory concentration – the concentration at which you have to give a drug in order for it to a microbiological effect. Need to achieve this MIC without inducing too much toxicity.

Balancing whether an antibiotic has good therapeutic activity vs how toxic it can be (host damage)

If the dose between which the effect switches from effective to toxic is narrow, the drug is said to have a narrow therapeutic margin/index (eg. aminoglycosides, vancomycin)

If a drug is very safe and there is little toxicity, it has a wide therapeutic margin/index.

Question 5

explain the importance of microbial antagonism

Answer 5

one organism can produce something that inhibits another

eg. in the gut, organisms coexisting together, secreting antimicrobial peptides/compounds

Limits growth of competitors and PATHOGENS

all coexist together which MAINTAINS FLORA - loss of means BACTERIAL/PATHOGEN OVERGROWTH

Question 6

give an example of loss of flora and the consequence?

Answer 6

Antibiotic Associated Colitis :
- pseudomembranous colitis

Clostridium difficile - taking broad spectrum antibiotics (broad-spectrum lactams, fluoroquinolones, etc.) messes up balance and allows overgrowth of pathogens including clostridium difficile.

1. Serious hospital cross-infection risks
   - easy to pick up/breathe in the spores
   - if they enter your gut you can then get an infection.
2. Ulcerations causing inflammation
3. Severe acute watery diarrhoea
   - Inflamed colon, lots of mucus being produced
   - loss of colon function (water reabsorption) because c diff produces toxins that affect the colonocytes
   - lack of water absorption from food
   - acute watery diarrhoea - can be life threatening.

Question 7

do antibiotics work alone?

Answer 7

don’t work alone, and its difficult to treat immunosuppressed patients (cancer, babies, elderly, alcoholics, HIV with low CD4) with antibiotics – much more difficult to clear an infection, you need a combination of antibiotics or more toxic antibiotics etc.

Antibiotics in a normal immunocompetent person relies partly on the immune system to help clear the infection.

Question 8

how can antibiotics be classified?

Answer 8

Classified by:-

Type of activity

Structure

Target site for activity

Question 9

Bactericidal v. Bacteriostatic

Type of activity

Answer 9

Bactericidal:

Kill bacteria

Used when the host defense mechanisms are impaired

Required in endocarditis, kidney infection

Bacteriostatic:

Inhibit bacterial growth so immune system can clear it, eg. tetracycline

Used when the host defense mechanisms are intact

Used in many infectious diseases

Question 10

Spectrum of Activity

Type of activity

Answer 10

Broad Spectrum Antibiotics:
Effective against many types
Example: Cefotaxime

Narrow Spectrum Antibiotics:
Effective against very few types
Example: Penicillin G

Some antibiotics only work against gram negative not gram positive

Question 11

Molecular Structure

Answer 11

beta lactams

• beta lactam ring is a chemical structure in some antibiotics
• pencilllins and cephalosporins contain beta-lactam structure

Beta lactams act as natural competitor substrates for enzymes that are involved in making the bacterial cell wall

Question 12

Target site for activity

Answer 12

Bacteria are complex multimeric structures, and have molecules that don’t exist anywhere else - good targets for selective toxicity, eg. pili, capsule, flagellum

1. cell wall synthesis - penicillin
2. folic acid metabolism - sulfonamides
3. DNA and RNA processing - quinolones target DNA gyrase,
4. Protein synthesis - 30S and 50S inhibitors
5. Free radicals such as metronidazole, damage lipids, enzymes
6. bacterial membrane - toxic because affects eukaryotic membranes too, eg. colistin

Question 13

explain how cell wall inhibitors work

Answer 13

GRAM POSITIVE

Big area of cross linking peptidoglycan. Antibiotics that inhibit cell wall synthesis target the enzymes that make peptidoglycan, and the enzymes have to be on the outer side of the inner layer. This is a porous structure, antibiotics can easily penetrate this porous structure to get to and target and enzymes making the peptidoglycan structure. Things like beta lactams are very effective here.

GRAM NEGATIVE

Peptidoglycan sits in the periplasmic space. Outer membrane is an impermeability barrier, the only way things can get across is by porins. If the membrane doesn’t have a transport mechanisms to transport an antibiotic, the antibiotic won’t get through.

Question 14

How does a bacteria make peptidoglycan structures?

Answer 14

FIRST PRECURSOR MOLECULE = terminal (last 2 peptides) on the monomer are isomers for alanine, labelled d-ala, d-ala - specific and needed for peptidoglycan synthesis

then it travels across the cytoplasm by linking to a lipid transport molecule – this transport can be inhibited

cross linking of 5 amino acids, then polymerisation in the cell wall via transcarboxypeptidases - these enzymes recognise the D-ala D-ala, cleaves off the last D-ala and links the other D-ala to another pentapeptide

-these enzymes are in bacterial cell walls and inhibited by beta lactams

vancomycin binds to D-ala D-ala, and the enzymes then can’t get to the structure as the big vancomycin structure blocks them from having their effect.

Question 15

give an example of an antibiotic acting as a structural mimic for bacterial enzymes?

Answer 15

beta lactams

act as substrate mimicks and competitive inhibitors for the cross linking enzymes

but, some bacteria have enzymes have destroy the beta lactam ring, breaking up the 3D structure - means it no longer acts as a competitive inhibitor for the enzyme

PBP, penicillin binding proteins, synthesise the peptidoglycan and beta-lactams bind here. This means bacteria cannot make peptidglycan, so it lyses itself and dies – this is why most beta lactans are bactericidal

also sulphonamides are structural mimics for p-benzoic acid

Question 16

Folic Acid Synthesis Inhibitors

Answer 16

structure of sulfonamide is almost identical to structure of p–aminobenzoic acid – competes against it and therefore blocks it from binding

Question 17

Protein synthesis inhibitors

Answer 17

erythromycin blocks translocation of peptidyl t-RNA

eryhtromycin is an example of a macrolide, used to treat Gram-positive infections esp. in those allergic to beta-lactams

Question 18

When do we use antibiotics ?

Answer 18

treat bacterial infections

use them propholactically to prevent someone from getting an infection

use them peri-operatively before an operation, particularly gut operation where there are lots of bacteria

people with increased subceptiblity to infection, eg. COPD patient, people with sickle cell disease or patients that lack a spleen

Inappropriate use - viral sore throats - patient pressure

Question 19

Route of Administration

Answer 19

often orally by GP

systemic infections resulting in hospitalisation often mean rapid delivery of antibiotics via an i/v - often unable to take oral due to vomiting, unconscious, poor gut absorption due to trauma

Topical - conjunctivitis, superficial skin infections, burns, antiseptic creams, heavy metal ointments

Question 20

what is MIC and what factors affect MIC dose?

Answer 20

minimum inhibitory concentration
-the concentration at which the antibiotic will kill off or inhibit the growth of the organism

depends on:

1. age, weight, renal and liver function of the patient
2. severity of infection
3. the susceptibility of the organism
4. depend upon properties of the antibiotic i.e. enough to give a concentration higher than the MIC (minimum inhibitory concentration)

Question 21

why might antibiotic combinations be favourable?

Answer 21

• with polymicrobial infections
• to achieve less toxic doses of individual drugs
• reduce antibiotic resistance

Question 22

penicillins

Answer 22

Basic eg. penicillin V

• Active against streptococci, pneumococci, meningococci, treopnemes.
• Most strains of Staphylococcus aureus are resistant

Anti-staphylococcal penicillins e.g. flucloxacillin

• narrow spectrum, G+ves, beta-lactamase resistant, less potent that PenG
• Not MRSA

Question 23

Beta-lactams: Cephalosporins

Answer 23

cefalexin - oral agent primarily used to treat UTIs

• bactericidal activity
• binds to and inactivates PBP’s located on the inner membrane of the bacterial cell wall

Question 24

Aminoglycosides

Answer 24

This group includes gentamicin and streptomycin

• cannot be absorbed from the gut and must be given parenterally
• active predominantly against Gram-ve bacteria including Pseudomonas aeruginosa
• nephrotoxic (kidney) and ototoxic (ear), so serum levels must be monitored

Question 25

Glycopeptides

Answer 25

vancomycin

Active only against Gram-positive organisms

Parenteral only (administered or occurring elsewhere in the body than the mouth and alimentary canal)

Usually reserved for situation when other agents cannot be used e.g. against MRSA

Question 26

Tetracyclines

Answer 26

Includes oxytetracycline, doxycycline

Broad spectrum
Used mainly for treating:
Chlamydia
Mycoplasma pneumoniae
Acne

Question 27

Quinolones

Answer 27

Includes ciprofloxaxin, moxifloxacin

Older drugs such as ciprofloxacin active mostly against Gram-negatives
-useful for complicated UTIs and gastrointestinal infections

Newer agents have better anti-Gram-positive activity
-useful for some respiratory tract infections

Question 28

Fusidic acid

Answer 28

narrow spectrum, used in combination to treat Staphylococcal infections only.