The document reviews the history and mechanisms of β-lactam antibiotics, detailing their structure and classes, including penicillins, cephalosporins, carbapenems, and monobactams. It explains the discovery of penicillin by Alexander Fleming, subsequent developments by Howard Florey and Ernst Boris Chain, and advancements in antibiotic production and resistance. Moreover, it highlights the improvements in antibiotic formulations to enhance stability, broaden their spectrum of activity, and combat bacterial resistance.

1. Antibiotics

2. -lactam antibiotics
N
S
RCONH
CH3
CH3
O CO2
H
penicillins
N
S
NHCO
O
CO2
H
CH3
R
cephalosporins
N
O CO2
H
S
NH2
H
C
H3
H H
OH
carbapenems
N
O
N
S O 3H
R
O
monobactams
The most important class of antibiotics affecting cell wall biosynthesis are -lactams.
-lactam group (a four-atom cyclic amide) is the pharmacophore of all -lactam
antibiotics. -lactam rings were unknown before the discovery of penicillin and it took
big effort to determine the structure of the drug.
The most important classes of -lactam antibiotics are penicillins, cephalosporins,
carbapenems and monobactams.

3. The mechanism of action of -lactam antibiotics is based on the similarity of
structures of the C - N bond in the -lactam ring and the peptide bond
connecting two D-alanine residues of the peptidoglycan precursor. TPase
recognizes the -lactam as its substrate and forms a covalent bond with the
antibiotic molecule. This adduct is very stable and because of that TPase is
irreversibly inactivated.
N
H
C
CH 3
C COOH
CH 3
N
H
C
O
NH C C
H S
R C
O
H
C
O N
C
CH
COOH
CH3
O
Ser
TPase
CH3
D-Ala-D-Ala penicillin
Covalent complex of penicillin with TPase
Mechanism of -lactam action
N
S
RCONH
CH3
CH3
O CO2
H

4. Penicillin Discovery . . .
• In 1928 after returning to his lab following a two
week vacation Fleming encountered the place in its
usual disarray
• Fleming had a inoculated a number of petri dishes
with staphylococci prior to leaving on vacation
• He hadn’t placed them in an incubator because he
knew that the staph would sufficiently multiply over
the long vacation
• Little did he know that penicillium mold grows well
at room temperature

5. Fleming’s observation
• Fleming returned to his lab
to find many of his culture
plates contaminated with
fungus
• He immediately started
preparing to clean all his
plates but it happened that a
former member of his lab
was visiting that day
• Fleming took some of the
contaminated cultures to
show his visitor and that’s
when he noticed the
inhibition zone around the
fungus

6. Fleming’s Observation cont.
• Fleming was not very
knowledgeable about
fungi but knew that the
mold in his dish was a
species of penicillin
• Eventually determined
to be Penicillium
notatum

7. 1929 Paper
• In 1929 Fleming published a paper detailing
his discovery
• This was also a crucial moment because his
ideas reached a large audience
• But it wasn’t until ten years later that other
scientists began trying to use penicillin to treat
clinical disease

8. 1929-1931
• Fleming continued to work on and off with penicillin
during this time but was never able to produce it in
quantities necessary for practical testing or
applications
• Fleming found that many of his cultures were
unstable and stopped producing mold after eight
days
• Interestingly, Fleming initially conceived of penicillin
as a topical agent and did not think of using it as an
injectable or ingestible medication

9. Florey and Chain

10. Florey, Chain continued
• Soon after beginning his
research, Chain discovered
that penicillin was not an
enzyme but a molecule
• He was intrigued by the fact
that penicillin was a very
unstable molecule
• Chain was able to freeze- dry
the penicillin and produce a
stable brown powder
• Tested on mice, a huge dose
proved safe

11. Florey, Chain continued
• Another important observation was that the
penicillin powder turned the mice’s urine brown – it
passed unaltered and without loss of effects into the
urine
• This meant that PCN could pass through the body
and fight infections wherever they were
• The Oxford team was ecstatic about their discovery
and began work immediately to prove their findings
were correct

12. Experimental and Human Testing
• Florey next experimented with mice and lethal doses of
streptococci
• Eight mice were injected with the bacteria and only four
mice received penicillin prior to the bacterial injection:
the four “PCN mice” survived and the others all died
• The first landmark paper detailing the mice experiments
were published in August 1940
• After the researchers were confident that PCN was safe
in mice they began human testing

13. Large Scale Production of Penicillin
• Started with a mold ‘Penicillium chrysogenum’
isolated from a cantaloupe
• Strain improvement by production of mutants
• Submerged cultures
• Medium modifications using corn steep liquor
• Fed batch cultures
• Semisynthetic penicillins using penicillin
nucleus (6-aminopenicillanic acid)

14. Penicillins : penicillin G
In penicillins, the -lactam ring is fused to thiazolidine ring.
Originally, penicillin was produced in the form of Penicillin G (benzylpenicillin) by
fermenting Penicillium mold in the presence of phenyl acetic acid
N
S
HN
CH3
CH3
O CO2
H
O
Good activity, but only against Gram-positive bacteria
Acid- and alkali-labile
Sensitive to the action of inactivating penicillinases

15. N
S
H2N
O
6-APA
Presently, many penicillins are produced semisynthetically starting from
6-aminopenicillanic acid (6-APA) as a precursor.
6-APA can be generated from penicillin G by cleaving off the benzyl moiety of
penicillin G.
Various new side chains can be then attached to the penicillin molecule through
the amino group of 6-APA
N
S
R C O NH
CH3
O
CO2H
New penicillins
6-APA
N
S
HN
CH3
CH3
O CO2
H
O
Benzyl-penicillin
CH3
CH3
CH3
CO2H

16. Penicillin improvements:
better acid stability
The amide bond in the β-lactam ring is highly strained and relatively unstable in
acidic solutions. The rate of acid hydrolysis depends on the chemical nature of
the side chain. Electron-withdrawing side chains decrease the rate of acid
hydrolysis. Because of that, amoxicillin or cloxacillin are more acid-stable: they
can withstand the acidic pH of the stomach and can be used orally.
amide bond
N
S
C O N H
C H
3
C H
3
O C O
2
H
C H
N H
2
O
H
Amoxicillin
N
S
C ONH
C H3
C H3
O C O2H
CH3
O
H
N
Cl
Cloxacillin

17. Penicillin improvements: broader spectrum
Penicillins enter the periplasmic space of Gram-negative bacteria through the ‘holes’ in
the outer membrane (porins). Hydrophobic side chains (e.g. benzyl group in penicillin
G) interfere with passage through porins.
More polar groups, such as -NH2 or -COOH facilitate crossing the outer membrane and
increase access of β-lactams to the periplasmic space of Gram-negative bacteria.
N
S
C H3
C H3
O C O2H
CH
NH2
ampicillin
CONH
N
S
CONH
CH3
CH3
O CO2H
CH
NH2
O
H
amoxicillin
N
S
C H3
C H3
O C O2H
CH
COOH
carbenicillin
CONH

18. Penicillin improvements:
resistance to -lactamases
The main mechanism of resistance to penicillin is based on the secretion by bacteria
of enzymes, beta-lactamases that can hydrolyze amide bond in -lactam ring. The
presence of a bulky side chain in the drug may hinder access of a beta-lactamase to
the amide bond. Therefore, penicillin derivatives containing bulky side chains are
fairly resistant to the beta-lactamase action.
N
S
C ONH
C H3
C H3
O C O2
H
OC H3
OCH3
methicillin
N
S
C ONH
C H3
C H3
O C O2
H
CH3
O
H
N
Cl
cloxacillin

19. -lactamase inhibitors
Clavulanic acid
N
O
O
COOH
CH2OH
Sulbactam
N
S
O
COOH
O O
A useful way to overcome -lactamase-based resistance is to administer a -lactam drug in
combination with -lactamase inhibitors. Such inhibitors (clavulanic acid, sulbactam,
tazobactam) possess a -lactam ring and generally resemble -lactam antibiotics. They
function by binding to the -lactamase enzymes and inactivating the enzyme without being
degraded.
Such inhibitors, which look like -lactam antibiotics, have only weak antimicrobial activity.
Popular combinations are amoxicillin with clavulanate (augmentin) or ampicillin with
sulbactam (unasyn).

20. Cephalosporins have been first obtained from a fungus Cephalosporium
acremonium. Similar to penicillins, many cephalosporins are produced semi-
synthetically either starting from 7-aminocephalosporanic acid (7-ACA) or by
converting relevant penicillins into cephalosporins.
N
S
H2N
O
CO2H
O CH3
O
7-aminocephalosporanic acid (7-ACA)
2
3
7
Cephalosporins

21. Active against Gram-positive cocci and streptococci.
cephalothin cafazolin cephalexin
Cephalosporins are classified by generations
N
S
N HC O
O
C O
2
H
O C H
3
O
S S
N N
N
S
NHC O
O
C O
2
H
S CH
3
N
N
N
N
N
S
NHC O
O
C O
2
H
C H
3
NH
2
I
II
cephaclor
cefamandole nofate
N
S
O
C O
2
H
Cl
N HC O
N H
2
N
S
NHC O
O
C O
2
H
CH
2
S
OC HO
N N
N
N
C
H
3
cefotaxime cefixime
N
S
N HC O
O
C O
2
H
C H
2
OC O C H
3
N O C H
3
N
S
N
H
2
N
S
N HC O
O
C O
2
H
C H
2
N O C H
2
C O
2
H
N
S
N
H
2
Improved activity against some Gram-negatives, for example, H. influenzae.
Better activity for Gram-negatives though, somewhat reduced activity against Gram-positive
pathogens.
III

22. Carbapenems combine chemical features of penicillins and cephalosporins. Prototype
carbapenem thienamycin, has been isolated from Streptomyces cattleva. It exhibits
excellent activity against a broad spectrum of Gram-positive and Gram-negative
organisms.
thienamycin
Thienamycin penetrates very easily through the outer membrane of Gram-negative
bacteria (through porin "holes") . It is resistant to the action of extended spectrum -
lactamases (ESBL) which can inactivate penicillins and cephalosporins. Carbapenems can
target another enzyme of the cell wall biosynthesis, Ld transferase (LdT) which sometimes
can help cell to bypass the need for TPase. Therefore, carbapenems shows excellent
activity against some Gram-positive strains which developed resistance to penicillins and
cephalosporins.
Carbapenems
N
O CO2H
S
NH2
H
H3C
H H
OH

23. Monobactams
Aztreonam (Azactam)
Monobactams were developed as narrow-spectrum antibiotics specifically
targeting aerobic Gram-negative bacteria. Monobactams are particularly useful for
the treatment of individuals allergic to penicillin. Such patients can still be treated
with the monobactams, which are sufficiently structurally different to not induce
allergic reactions.
SO3H
N
O
C
H3
C
H3
COOH
N
H2
N
O
CH3
N
O
S
N

24. The Waksman era
• Discovered many antibiotics such as actinomycin-
D, neomycin, streptomycin from soil bacteria in
1940s
• Streptomycin (an aminoglycoside) was used
against M.tb and gram-negative bacteria.
• Neomycin used as a topical antibacterial agent.
• Many more drugs including chloramphenicol and
tetracyclins, macrolides (erythromycin and
nystatin) have been discovered since then.

25. Mechanism of Action of Antibiotics