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Carbapenems synthesis
Penicillin synthesis
Cephalosporin synthesis

b-lactams synthesis and antibacterial activity

Penicillins, cephalosporins, and certain other antibiotics belong to a family of compounds, collectively known as b-lactam antibiotics, which share the structural feature of a b-lactam ring. In the penicillins the b-lactam ring is fused to a five-membered thiazolidine ring, whereas the cephalosporins display a fused b-lactam/dihydrothiazine ring structure (Fig. 1). other structures include oxygen or carbon atoms in place of the sulfur atom and virtually every possible permutation has been attempted. The b-lactam ring is the Achilles' heel of this group of antibiotics because many bacteria possess enzymes (b-lactamases) that are capable of opening the ring and rendering the molecule inactive.

Figure1. general structure of main classes of b-lactams

Mechanism of Action

Penicillins are dipeptide analogs of D-ala-D-ala. Radiolabelled Penicillin G binds to several different proteins in the cell wall of bacteria (Table 1). Bactericidal action is primarily due to inhibition of PBPs 1a & 1b and PBP3 that function as transpeptidases in the biosynthesis of the peptidoglycan. Inhibition is result of acylation of the enzyme by attack of serine on the b-lactam ring. In Figure 2 is shown the different binding pattern of some b-lactam antibiotics  to the PBPs.

Table 1. Penicillin Binding Proteins (PBPs) of E. coli

Penicillin Binding
Protein

% of total
3H-penicillin G Bound

Protein Function

1a
1b

8

transpeptidases
(inhibition is lethal)

2

0.7

maintenance of rod shape

3

2

septum formation

4

5

6

4

65

2

D-alanine carboxypeptidase
(non-lethal inhibition)

 

Figure 2. Binding of b-lactams to the PBPs of E. coli W7. Ampicillin (1 mg/ml), mecillinam (10 mg/ml), cefsulodine (150 mg/ml), and
cefmetazole (20 mg/ml), Numbers indicate the time in hours at which samples were taken.

Penicillins and other b-lactam antibiotics are categorized as bactericidal agents, but this is true only when bacteria are actively dividing. The response of bacteria to b-lactam antibiotics is affected by subtle differences in the mode of action. Several other features of the response that may sometimes have therapeutic implications have also been discovered.

b-lactam antibiotics interfere with the final cross-linking reaction that provide strength to the cell wall. Several forms of the enzyme that performs this reaction are needed to maintain the complex molecular architecture of the cell and are inhibited at different extent by various b-lactam agents. These target enzymes belong to a group of proteins to which penicillin and other b-lactam antibiotics bind (penicillin-binding proteins; PBPs). E. coli, the best-studied species, has seven of these proteins, numbered 1a, 1b, 2, 3, 4, 5, and 6 in order of decreasing molecular weight. PBPs 1a and 1b are jointly concerned in cell elongation, PBP2 in shape determination, and PBP3 in cell division [1]. Pen G and other beta-lactam antibiotics mainly bind to PBP 1a, 1b, 2 and 3 of E. coli. PBPs 4-6 are thought to be unconnected with the antibacterial effect of b-lactam agents, since mutants lacking these proteins do not seem to be disabled in any way. Binding to the remainder has been correlated with the various morphological effects of b-lactam antibiotics on Gram-negative bacilli. Thus, cefalexin and its close analogs, as well as aztreonam, bind almost exclusively to PBP 3 inhibiting the cell division process and resulting in the gowth of filamentous bacteria. The amidinopenicillins, bind preferentially to PBP 2 and cause a generalized effect on the cell wall so that the bacteria gradually assume a spherical shape. Other b-lactam antibiotics bind to PBPs 1-3 and, in sufficient concentration, induce the formation of osmotically fragile, wall-deficient forms (called spheroplasts), which typically emerge at the cell wall growth site as the cell starts to divide. The morphological events are illustrated in Figures 3-7 (from Greenwood D, O'Grady F, Journal of Medical Microbiology, 1969; 2: 435-41).

                      

Figure 3. Strain of swarming Pr. niirabilis after an exposure             Figure 4. The same strain of Pr. mirabilis,

of 60 minutes  to ampicillin, showing emergent spheroplasts,          showing emergent spheroplast.

abnormal bulges in the central portion of some cells, cell ghosts      ESM. x 24,000.

 and at least one apparently unaffected cell.

Electroscan micrograph (ESM). x 6700.

Figure 5. The same strain of Proteus mirabilis after 60 minutes’ exposure showing the “ Hasenform ” of Liebernieister and Kellenberger.

 ESM. x 3200.

                       

Figure 6. The same strain of P. mirabilis after an exposure of            Figure 7. A strain of E. coli after an exposure of 60 minutes to

120 minutes to ampicillin in the dilution system. Spheroplast            ampicillin. There is a large amount of cellular debris. Central

formation is well advanced and some cells show multiple cell-wall     and terminal cell-wall lesions can be seen in the centre of the

lesions. ESM. x 7400.                                                                   picture. and in the lower part cells that may be unaffected.

                                                                                                ESM. x 10,700.

An important consequence of differences in binding is that compounds  which bind only to PBP 3 or PBP 2 (e.g. cefalexin, aztreonam, and mecillinam), are much more slowly bactericidal to Gram-negative bacilli than those that bind PBPs 1, 2, and 3.
In Gram-negative bacilli, rupture of spheroplasts can be quantitatively prevented by raising the osmolality of the growth medium, so cell death appears to be an osmotic phenomenon. In contrast, the lethal event in Gram-positive organisms, which have much thicker cell walls, appears to be autolysis triggered by the release of lipoteichoic acid as a consequence of exposure to b-lactam antibiotics.

Increasing the concentration of b-lactam antibiotics in Gram positives often results in a reduced bactericidal effect. The mechanism of this effect (known as the Eagle phenomenon after its discoverer) is not clear, but may be related to the inhibition of PBPs that have lower affinity for b-lactams that may rapidly block cell growth thereby preventing the lethal events that normally follow inhibition of another PBP by lower drug levels.

Persisters and penicillin tolerance

In both Gram-positive and Gram-negative bacteria, a proportion of the population, called persisters, survive exposure to concentrations of b-lactam antibiotics lethal to the rest of the culture.  As long as the antibiotic is present, they remain dormant and resume growth when it is removed. Moreover, some strains of staphylococci and streptococci display "tolerance" to b-lactam antibiotics and are killed much more slowly than usual. The therapeutic significance, if any, of persisters is unknown, but penicillin tolerance has been implicated in therapeutic failures in bacterial endocarditis where bactericidal activity is crucial to the success of treatment.

 


1. Curtis N.A.C. "Penicillin-binding proteins in theory and practice". J. Antimicrob. Chemother. 8: 85, (1981).



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