Antibiotics are substances that can be used to kill or inhibit the growth of bacteria. In the
medical world, they are developed into drugs against pathogenic bacteria. The first antibiotic drug, penicillin, discovered in the late 1920s by Alexander Fleming and first used in the early 1940s was, in its time, a miracle.
The use of penicillin has saved countless lives, but even when discovered, Fleming warned that over time, bacteria would likely evolve to have resistance to it, which has happened in many cases.
Penicillin (and a number of other drugs in the same class) are β-lactams that bind to penicillin-binding proteins (PBP). PBPs are enzymes used during cross-linking of the peptidoglycan cell wall in bacteria, so binding to them and thus blocking them, disrupts the production of the cell wall, causing the bacterium to lyse. Unfortunately, many bacteria have developed β-lactamases, enzymes that break down these β-lactams, so making them inert.
Various other antibiotics have been developed that act in different ways, such as depolarizing the cell membrane, inhibiting protein or nucleic acid synthesis and inhibiting bacterial metabolic pathways. In the past, if one antibiotic failed to work, it was possible to try another type, often with success, but with over-prescription of many of these drugs, more and more bacteria are developing resistance to multiple types of antibiotics. This is clearly a huge problem!
How do bacteria resist?
Other than developing β-lactamases and sharing them around with other bacterial species, bacteria have developed various other mechanisms of resistance to our drugs. These include mechanisms to limit the uptake of the drug, or pump it out with efflux pumps should it enter, inactivation of the drug and modifications to the target of the drug, as seen below.
Gram-positive bacteria, lacking a cell envelope, have limited defenses compared to Gram-negative bacteria, but Gram-negatives can employ all of the above defenses, along with many releasing endotoxins in response to having their cell wall disrupted. Research needs to focus on circumventing these resistance mechanisms to kill and/or limit the growth of our Gram-negative bad boys!
How do we fight resistance?
One such approach has been researched recently which involves just that – rather than attacking the bacteria, maybe it is possible to break down the defenses against antibiotics, so allowing our existing antibiotics to work.
All the defense mechanisms that bacteria deploy are built from proteins that have to be folded in a very specific way to serve their purpose properly.....
Enter DsbA……
DsbA is an enzyme (a protein itself) that is produced by bacteria to help with protein folding. It acts by holding amino acid chains in place and inserting sulfide bonds where needed to create the required shape for the final protein.
The research question considered was whether DsbA proteins were involved in creating the antibiotic resistance proteins and if so, whether inhibiting DsbA in the cell envelope of Gram-negative bacteria would disrupt the building of extracytoplasmic defenses and so allow existing antibiotics to work.
First, experiments were conducted with E. coli. Genes that coded for various antibiotic resistance proteins were collected and inserted into the E. coli genome, resulting in E. coli becoming antibiotic resistant. A chemical inhibitor was then used to block E. coli from producing DsbA and it was found that the antibiotic resistance proteins did not fold properly and became unstable and broke down.
Below is a demonstration with a similar experiment of the effect of a DsbA inhibitor combined with an antibiotic (imipenem) on a strain of K. pneumoniae with multi-drug resistance.
Colistin is an existing drug that targets LPS, disrupting the bacterial cell membrane and also having anti-endotoxin activity. It is an important drug, but unfortunately, many bacteria now have resistance to it. With further tests blocking DsbA on other normally antibiotic resistant Gram-negative bacteria, it was found that the production of both β-lactamases and MCR enzymes that resist colistin was disrupted. DsbA did not appear to disrupt production of efflux pumps, so is not the total answer to disrupting bacterial defense mechanisms. However, allowing some of our existing drugs to work again is still a major step forward.
Further tests were also performed by infecting insect larvae with antibiotic resistant Gram-negative bacteria that could not make DsbA. When then treated with antibiotics, the larvae survival rate increased. DsbA is unique to bacteria, so its inhibition would not affect humans, meaning that this could be a novel approach to fight antibiotic resistance.
The international collaborative team working on this was led by researchers at Imperial college, London and University of Texas at Austin. Other researchers involved were from Spain, France and Switzerland. The team now hopes to develop the research further for safe use in humans as a tool in the war against bacteria and their resistance. Maybe with such techniques and further development, there is hope for the reversal of our currently accelerating trend of increasing antibiotic resistance.
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