12 Antimicrobial drugs

Question 1

The lectures on antimicrobials will cover

Answer 1

Lecture 1: Antibiotics
Lecture 2: Antifungals
Both 1 & 2 will also discuss mechanisms of resistance against commonly used antimicrobials

Question 2

Learning Outcomes

Answer 2

Know the main classes of antimicrobials with examples of commonly used representatives

Know and understand the mechanisms of action of commonly used antimicrobials

Know the mechanisms leading to resistance to commonly used antimicrobials

Question 3

A bit of history – the discovery of penicillin

Answer 3

Alexander Fleming (1881 - 1955)
Colony of Penicillium notatum
S.  aureus
cells dying
Colonies of
Staphylococcus aureus

The medical application of penicillin required
a lot of additional efforts

Alexander Fleming (1881 - 1955)
Howard Walter Florey (1898–1968)
Ernst Boris Chain (1906–1979)

Collectively were awarded the Nobel prize in 1945

Question 4

Antibiotics’ characteristics

Answer 4

Effect on bacteria:
killing bacteria – bactericidal
inhibiting/arresting growth of bacteria – bacteriostatic

Range of bacterial species affected:
a large number of bacterial species - broad spectrum
a limited number of bacterial species - narrow spectrum
a single species – limited spectrum

Question 5

Characteristics based on target/mode of action

Answer 5

Antibiotics interfere with/inhibit essential cellular structures/processes
Antibiotics target bacteria-specific structures/processes
Such modes of action make them
Toxic to bacteria only
Innocuous to humans (little or no-side effects)

Question 6

The cell wall is a target for many antibiotics

Answer 6

The CW is responsible for cell integrity
The CW protects bacteria from toxic substances
Absence of functional CW = death (by autolysis)

Human cells do not have cell walls
The cell wall is an essential bacteria-specific organelle

Question 7

Bacterial species differ in CW structure

and composition

Answer 7

A simple test (Gram staining test) reveals CW differences

+ve (stained purple)
Staphylococci
Streptococci
Enterococci

ve (coloured pink)
Escherichia coli
Pseudomonas
Salmonella

Question 8

The CWs of Gram +ve bacteria have thick layers of peptidoglycan chains

Answer 8

~40 Peptidoglycan (PG) chains linked together

Question 9

Each peptidoglycan chain is build up of covalently linked sugars

Answer 9

NAM = N-acetylmuramic acid NAG – N-acetylglucosamine
Sugars linked in a chain

In addition short peptide chains
are attached to NAM
Peptide chains (3-5 aa)

Question 10

Peptidoglycan chains cross-link via peptidyl bridges

Answer 10

Cross linking involves a large number of individual chains
It is an essential step in CW biosynthesis (especially in Gram+ve)
It is catalysed by specific enzymes
These enzymes, and the reactions they catalyse, are targets of antibiotics

Question 11

In contrast, the CW of Gram ve bacteria have a thin PG layer

Answer 11

This helps explain why antibiotics targeting PG biosynthesis are not as effective against Gram ve bacteria

Question 12

Antibiotics that act on PG biosynthesis

Answer 12

Beta ()-lactams -

Penicillins

Cephalosporins

Carbapenems

Vancomycin

Question 13

-lactam antibiotics

Answer 13

are effective against growing and dividing cells

kill bacteria by autolysis (bactericidal)

resistant bacterial species produce -lactamase, a secreted enzyme which inactivates antibiotics by breaking down their -lactam ring

Question 14

B-lactam antibiotics

Answer 14

Inhibit the enzymes involved in the transpeptidase cross-linking reaction
Interfere with linking the individual chains together
Disrupt PG synthesis leading to autolysis

Question 15

B-lactam antibiotics

Answer 15

Penicillins
Natural (Penicillin G)
Aminopenicillin
(amoxicillin, ampicillin

Cephalosporins
1st (Cefalexin; cefazolin)
2nd (Cefaclor, cefuroxime)
3rd (Cefixime, cefpodoxime)
4th (Cefepime)

Carbapenems
Meropenem
Ertapenem
Imipenem

Question 16

Key Characteristics

Answer 16

Penicillins
Very effective against Gram +ve
Some allergy reactions
Aminopenicilins better tolerated

Cephalosporins
Different primary 
coverage:
1st – Gram +ve
2nd – Anaerobes
3rd – Gram –ve
4th - Pseudomonas

Carbapenems
Broad spectrum
Generally effective 
against all BUT:
MRSA and VRE
Only available IV

All are bactericidal

Question 17

Vancomycin

Answer 17

An inhibitor of CW biosynthesis
different mode of action to beta-lactams
different chemical structure

Glycopeptide antibiotic
(more allergic reactions)

Effective against MRSA
Administered intravenously

However emerging cases of resistance
(VRSA; VRE)

Question 18

Protein synthesis requires rRNA-Protein

complexes known as ribosomes

Answer 18

The ribosome has 2 subunits
There is no synthesis of proteins in the absence of
functional ribosomes

Bacterial ribosomes differ from those in humans
In bacteria = 30S + 50S
In humans = 40S + 60S

Question 19

Examples of antibiotics which inhibit protein synthesis

Answer 19

Some bind to, and inhibit protein components of the 30S subunit
Tetracycline
Aminoglycosides (Gentamycin, Streptomycin)

Others bind to, and inhibit protein components of the 50S subunit
Macrolides (Erythromycin)
Chloramphenicol

Question 20

Antibiotics inhibiting PS

Answer 20

Broad spectrum (effective against both Gram +ve and Gram –ve species)
Most are bacteriostatic
Associated with greater toxicity (human mitochondrial ribosomes are inhibited too)

Question 21

Antibiotics inhibiting DNA biosynthesis

Answer 21

Fluoroquinolones

Question 22

Common representatives

Answer 22

Fluoroquinolones
ciprofloxacin
norfloxacin
levofloxacin
moxifloxacin

Question 23

Fluoroquinolones

Answer 23

Broad-spectrum, synthetic
Inhibit bacterial enzymes (DNA gyrase) with essential roles in DNA replication
Effective against Gram –ve bacteria and intracellular pathogens (Legionella, Mycoplasma)
Higher levels of toxicity associated with them

Question 24

Antibiotics inhibiting RNA biosynthesis

Answer 24

RNA biosynthesis requires specialised enzymes known as DNA-dependent RNA polymerases

Question 25

Rifampicin

Answer 25

Inhibits bacterial but not human RNA polymerases

Used predominantly for treating tuberculosis

Question 26

Biosynthesis of nucleotides requires folic acid

Answer 26

DHPdiP (dihydropteroate diphosphate + PABA (para-aminobenzoic acid)
Dihydropteroic acid -
Bacteria but NOT humans can make folic acid
Folic acid (Vit B9)
dihydrofolic acid
Tetrahydrofolic acid
Synthesis of nucleotides
 DNA and RNA

Question 27

Folic acid synthesis as a target of man-made antibiotics (antimetabolites)

Answer 27

Sulfonamides –
analogues of PABA, act
by substrate competition

Trimethoprim inhibits
dihydrofolate reductase

Co-trimoxazole – a mixture
of both (5:1), inhibits both
enzymes

Question 28

Sources of commonly used antibiotics

Answer 28

Natural – produced by:

fungi: penicillin, cephalosporin
bacteria: erythromycin, rifampicin, streptomycin, tetracycline

Semi-synthetic – ampicillin

Synthetic – sulfonamides, trimethoprim

Question 29

Choice of antibiotic treatment depends on:

Answer 29

bacterial species
susceptibility to drug
site of infection
safety of drug
cost of therapy
patient factors

Question 30

Resistance to antibiotics arises by

Answer 30

Mutations in bacterial chromosomal genes encoding targets of common antibiotics (VGT)
Transfer between organisms of resistance genes carried by plasmids (HGT)
Biofilm formation

Biochemically resistance is manifested by:

    (i) decreased accumulation of the drug – increased efflux 
    or reduced permeability of the drug 
    (ii) enzymatic inactivation of the drug - secretion of - lactamase 
    (destroys beta-lactam) or chloramphenicol acetyl 
    transferase  (inactivates chloramphenicol)