ß-Lactams: Mechanisms of Action and Resistance
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Animation Description: This animation starts with the explanation of bacterial cell wall synthesis, the process targeted by ß-Lactams.
Structurally, most bacteria consist of a cell membrane surrounded by a cell wall and, for some bacteria, an additional outer layer. Internal to the cell membrane is the cytoplasm which contains ribosomes, a nuclear region and in some cases granules and/or vesicles. Depending on the bacterial species, a number of different external structures may be found such as a capsule, flagella and pili.
In gram negative bacteria, the gap between the cell membrane and the cell wall is known as the periplasmic space. Most gram positive bacteria do not possess a periplasmic space but have only periplasm where metabolic digestion occurs and new cell peptidoglycan is attached. Peptidoglycan, the most important component of the cell wall, is a polymer made of N-acetyl muramic acid alternating with N-acetyl glucosamine which are cross-linked by chains of four amino acids. The function of the bacterial cell wall is to maintain the characteristic shape of the organism and to prevent the bacterium from bursting when fluid flows into the organism by osmosis.
Synthesis of peptidoglycan and ultimately the bacterial cell wall occurs in a number of stages. One of the first stages is the addition of 5 amino acids to N-acetyl muramic acid. Next, N-acetyl glucosamine is added to the N-acetyl muramic acid to form a precursor of peptidoglycan. This peptidoglycan precursor is then transported across the cell membrane to a cell wall acceptor in the periplasm.
Once in the periplasm, the peptidoglycan precursors bind to cell wall acceptors, and undergo extensive crosslinking. Two major enzymes are involved in crosslinking: transpeptidase and D-alanyl carboxypeptidase. These enzymes are also known as penicillin binding proteins because of their ability to bind penicillins and cephalosporins.
Eventually, several layers of peptidoglycan are formed all of which are crosslinked to create the cell wall. Gram positive bacteria have many more layers than gram negative bacteria and thus have a much thicker cell wall.
Beta-lactam antibiotics include all penicillins and cephalosporins that contain a chemical structure called a beta-lactam ring. This structure is capable of binding to the enzymes that cross-link peptidoglycans.
Beta-lactams interfere with cross-linking by binding to transpeptidase and D-alanyl carboxypeptidase enzymes, thus preventing bacterial cell wall synthesis.
By inhibiting cell wall synthesis, the bacterial cell is damaged. Gram positive bacteria have a high internal osmotic pressure. Without a normal, rigid cell wall, these cells burst when subjected to the low osmotic pressure of their surrounding environment. As well, the antibiotic-penicillin binding protein complex stimulates the release of autolysins that are capable of digesting the existing cell wall. Beta-lactam antibiotics are therefore considered bactericidal agents.
Bacterial resistance to beta-lactam antibiotics may be acquired by several routes. One of the most important mechanisms is through a process known as transformation. During transformation, chromosomal genes are transferred from one bacterium to another.
When a bacterium containing a resistance gene dies, naked DNA is released into the surrounding environment. If a bacterium of sufficient similarity to the dead one is in the vicinity, it will be able to uptake the naked DNA containing the resistance gene.
Once inside the bacterium, the resistance gene may be transferred from the naked DNA to the chromosome of the host bacteria by a process known as homologous transformation. Over time, the bacterium may acquire enough of these resistance genes to result in a remodelling of the segment of the host DNA.
If this remodelled DNA segment codes for cross-linking enzymes (i.e. penicillin binding proteins), the result is the production of altered penicillin binding proteins.
These altered penicillin binding proteins can still cross-link the peptidoglycan layers of the cell wall but have a reduced affinity for beta-lactam antibiotics thus rendering the bacterium resistant to the effects of penicillin and other beta-lactam agents. This transfer process has resulted in penicillin-resistant S. pneumoniae through the acquisition of genes from other naturally occurring penicillin-resistant Streptococcus species.
A second important mechanism by which bacteria become resistant to beta-lactam antibiotics is by the production of enzymes capable of inactivating or modifying the drug before it has a chance to exert its effect on the bacteria.
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