Understanding penicillin resistance
The action of β-lactams, though extremely effective, is not entirely foolproof. Bacteria begin to develop resistance to antibiotics over time in a number of ways. Resistance can be limited to one specific antibiotic, or encompass multiple medicines. As the prevalence of antibiotic resistant bacteria increases it leads to the longer hospital stays for infected patients, requires stronger courses of treatment with greater side effects and, in some less developed countries, can result in premature deaths, which would have been treatable, had the bacterial species not developed a resistance to the antibiotic prescribed. Antibiotic resistance is a growing threat, which we must endeavour to understand in order to construct more effective treatments for bacterial infections.
So, how do bacteria develop resistance against β-lactam penicillins? The mode of action of penicillins depends greatly on the presence of the β-lactam ring. This structure produces a molecular ‘key’ that is able to interact with the active site ‘lock’ of the transpeptidase enzyme; disrupting its ability to create peptidoglycan crosslinks in the bacterial cell wall.
Bacteria can use β-lactamases to break open the β-lactam ring
Just as β-lactam rings can interfere with the activity of enzymes, so they can also be cleaved through enzymatic activity. Enzymes called β-lactamases can catalyse opening of the β-lactam ring through reaction with water, in a hydrolysis reaction. In earlier sections we have seen that hydrolysis gives a penicilloic acid, which is not antibacterially active; is readily excreted from the body once produced.
Designing penicillins that resist being inactivated by β-lactamases Many bacterial species can synthesise their own β-lactamases which will target and disable penicillins. Penicillins themselves can be modified to provide some protection against these enzymes. For example, in methicillin, a bulky substituted benzene (at the variable R site of the penicillin molecule) provides a steric hindrance to any enzymes that try to interact with the β-lactam ring of the penicillin. Despite this there have been classified strains of methicillin resistant Staphylococcus aureus or MRSA since the 1960s, which have developed through the over-prescription of methicillin and related penicillins. As such bacterial resistance to penicillins still remains a serious concern to the medical world.
Bacteria can become resistant to penicillin by modifying enzymes that make the cell wall
Some bacteria, including Streptococcus phenominae, have developed resistance to β-lactams through modification of their penicillin binding proteins (or PBPs), which make up the active site of transpeptidase enzymes. Where normally β-lactams react with PBPs to form a relatively stable ring-opened product, mutations in the genetic coding for proteins making up the transpeptidase enzyme can result in multiple PBPs that have a low binding affinity for β-lactams. As such our β-lactam ‘key’ no longer fits into the transpeptidase active site ‘lock’. Penicillins do not bind as effectively to their target PBPs and the transpeptidase enzymes continue to function, binding to their original substrate rather than the antibiotic.
Alternative options are available to help tackle the surge in antibiotic resistance, which will be discussed when we investigate other β-lactam antibiotics and non-β-lactam antibiotics.
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