Beta-lactam antibiotics like penicillins and cephalosporins inhibit bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs). This prevents the final cross-linking stage of peptidoglycan assembly, leading to cell lysis as precursor units accumulate. Resistance can occur via beta-lactamase production or mutations that alter PBP binding or outer membrane permeability. A variety of mechanisms allow different bacteria to develop resistance to beta-lactam antibiotics.

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MODE OF ACTION OF BETA-LACTAM
ANTIBIOTICS

2. β- Lactams
 Beta-lactams comprise a large
family of different groups of
bactericidal compounds, all
containing the beta-lactam ring.
All β- lactam drugs are selective
inhibitors of bacterial cell wall
synthesis and therefore active
against growing bacteria.
 The different groups within the
family are distinguished by the
structure of the ring attached to
the beta-lactam ring – in
penicillins this is a five-
membered ring, in
cephalosporins a six- membered
ring – and by the side chains
attached to these rings.

3.  Beta-lactams contain a beta-lactam ring and inhibit cell wall
synthesis by binding to cell receptors, penicillin-binding
proteins(PBPs).
 PBPs are membrane proteins (e.g. Carboxypeptidases,
transglycosylases and transpeptidases) capable of binding to
penicillin (hence the name PBP) and are responsible for the
final stages of cross-linking of the bacterial cell wall structure.
 Inhibition of one or more of these essential enzymes results in
an accumulation of precursor cell wall units, leading to
activation of the cell’s autolytic system and cell lysis.

4.  PBPs are under chromosomal control, and mutations may alter
their number or their affinity for β-lactam drugs.
 After β -lactam drug has attached to one or more receptors,
the transpeptidation reaction is inhibited and peptidoglycan
synthesis is blocked.
 The next step probably involves removal or inactivation of an
inhibitor of autolytic enzymes in the cell wall. This activates
the lytic enzyme and results in lysis if the environment is
isotonic.
 In a hypertonic environment, the microbes change to
protoplasts or spheroplasts, covered only by the fragile cell
membrane. In such cells, synthesis of proteins and nucleic
acids may continue for some time.

5.  Within the periplasmic space of gram-negative beta-
lactamases can inactive beta-lactams before they reach
their target PBPs, thereby protecting the cell from
antibiotic action. Alternatively, mutant PBPs fail to bind
beta-lactase, thus allowing peptidoglycan synthesis to
occur.
 In gram-positive bacteria beta-lactams may be
extracellularly destroyed by beta-lactamases or rendered
ineffective, as in gram-negatives, by the mutant PBPs.

6.  The difference in susceptibility of gram-positive and gram
negative bacteria to various penicillins or cephalosporins
probably depends on structural differences in their cell walls
(e.g., amount of peptidoglycan, presence of receptors and
lipids, nature of cross-linking, activity of autolytic enzymes)
that determine penetration, binding, and activity of the drugs.
 Resistance to penicillins may be determined by the organism's
production of penicillin-destroying enzymes (beta-lactamases).
 Beta-lactamases open the beta-lactam ring of penicillins and
cephalosporins and abolish their antimicrobial activity.

8.  Different beta-lactams have different clinical uses, but are
not active against species that lack a cell wall
 Some, such as penicillin, are active mainly against gram-
positive organisms, whereas others (e.g., semi-synthetic
penicillins, carboxypenems, monobactams, second-, third-,
fourth- generation cephalosporins) have been developed for
their activity against gram-negative rods.
 It is important to note that beta-lactams are not active against
species that lack a cell wall (e.g., Mycoplasma) or those with
very impenetrable walls such as mycobacteria, or intracellular
pathogens such as brucella, legionella and chlamydia.

9.  There is one group of beta-lactamases that is occasionally found in certain
species of gram-negative bacilli, usually Klebsiella pneumoniae and
Escherichia coli. These enzymes are termed extended-spectrum beta-
lactamases(ESBLs) because they confer upon the bacteria the additional
ability to hydrolyze the beta-lactam rings of cefotaxime, ceftazidime, or
aztreonam.
 The classification of beta-lactamases is complex, based upon the genetics,
biochemical properties, and substrate affinity for a beta-lactamase inhibitor
(clavulanic acid).
 Clavulanic acid, sulbactam, and tazobactam are beta–lactamase inhibitors
that have a high affinity for and irreversibly bind some beta-lactamases
(e.g., penicillinase of Staphylococcus aureus ) but are not hydrolyzed by
the beta-lactamase.
 These inhibitors protect simultaneously present hydrolyzable penicillins
(e.g., ampicillin, amoxicillin, and ticarcillin) from destruction. Certain
penicillins (e.g., cloxacillin) also have a high affinity for beta-lactamases.

10. Resistance to beta-lactams
1.Resistance by alteration in target site
 Methicillin-resistant staphylococci synthesize an additional PBP which has a
much lower affinity for beta lactams than the normal PBPs and is therefore able
to continue cell wall synthesis when the other PBPs are inhibited.
2. Resistance by alteration in access to the target site
 This mechanism is found in gram-negative cells where betalactams gain access
to their target PBPs by diffusion through protein channels(porins) in the outer
membrane. Mutations in porin genes result in a decrease in permeability of the
outer membrane and hence resistance.
3. Resistance by production of beta lactamases
 Beta-lactamases are enzymes that catalyse the hydrolysis of the beta-lactam ring
to yeild microbiologically inactive products.
 The beta-lactamases of gram-positive bacteria are released into the extracellular
environment and resistance will only be manifest when a large population of
cells is present. The beta-lactamases of gram-negative cells, however, remain
within the periplasm.

11.  There are two other types of resistance mechanisms.
 One is due to the absence of some penicillin receptors
(penicillin-binding proteins; PBPs) and occurs as a result of
chromosomal mutation; the other results from failure of the
beta-lactam drug to activate the autolytic enzymes in the cell
wall.
 As a result, the organism is inhibited but not killed. Such
tolerance has been observed especially with staphylococci and
certain streptococci.
 Examples of agents acting by inhibition of cell wall synthesis
are penicillins, the cephalosporins, vancomycin, and
cycloserine.