A Tough Drug for a Tough Bug
Can colistin help overcome some of the challenges in treating E. coli with antibiotics?
The Limulus Amebocyte Lysate (LAL) test, named for a compound in horseshoe crabs that reacts with exquisite sensitivity to the presence of gram-negative bacteria, is a common quality control tool for pharmaceutical manufacturers. Companies use the LAL kits as an end-product test for endotoxins in human biological products and medical devices. In fact, millions of these tests are performed yearly around the world.
But the LAL test can also be useful in academic research. Such was the case recently when the clinical testing laboratory of Fondazione IRCCS Policlinico S. Matteo, a hospital in Pavia, Italy used our LAL reagent formulation, KTA2, in a study of the drug colistin.
Those who work in infectious diseases or microbiology know that colistin is something of a double-edged sword. The 60-year-old antibiotic was highly effective in treating people, particularly those in intensive care units suffering from acute and chronic gram-negative infections, but it fell out of favor because high doses or long-term use could also be toxic to kidneys.
Today doctors use it sparingly as an antibiotic of last resort against drug-resistant bacteria such as Acinetobacter baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa or carbapenem-resistant enterobacteriaceae (CRE), a superbug and emerging public health threat that is wreaking havoc on hospital wards around the world. The first global case of colistin-resistant CRE, by the way, was just reported last month in the US.
The Italian study set out to assess how well colistin worked in quelling the toxic effects of Enterohaemorrhagic E. coli (EHEC), a strain of the intestinal bug Escherichia coli (E. coli), serotype O157:H7 that is a common source of food-borne illness. EHEC produces toxins, known as verotoxins or Shiga-like toxins (Stx) that resemble those seen in dysentery. Some cases of E. coli O157:H7 give way to hemolytic uremic syndrome (HUS), which occurs when the Shiga toxins released by E. coli bacteria move into the bloodstream. When this happens, the damaged red blood cells start to clog the filtering system in the kidneys, causing the kidneys to fail.
Certain classes of antibiotics, namely quinolones, trimethoprim, sulfamethoxazole or β-lactams, are thought to make the situation in these patients worse, because when the drugs attack E. coli, they split the bacteria open, causing a greater release of Shiga toxins. The drugs are also thought to promote the production of Shiga toxin by the bacteria.
But not all antibiotics necessarily have this effect. Colistin, which works by binding to and neutralizing LPS and phosopholipids in the outer cell membrane of Gram negative bacteria, does not interfere with DNA replication and has been found to have an anti-endotoxin effect. With this in mind, a study led by Elena Percivalle at the Fondazione IRCCS Policlinico S. Matteo, looked at the effects of colistin on Vero cell cytoxicity caused by the E. coli strain and the effects on endotoxin and Stx release to see if the drug might have utility as a treatment against Stx-releasing E. coli infection.
Supernatants of the E. coli strain were grown in Vero cells for 18 hours, some containing colistin and others absent the drug, to see what impact the antibiotic had on both the reduction of Stx and endotoxins, the highly toxic, heat-stable lipopolysaccharides that comprise the outer cell membrane of gram-negative bacteria. Endotoxin concentrations were measured using the LAL kits.
The findings, which were published recently in New Microbiologica, found that the presence of colistin reduced both the bacteria's toxic effects on Vero cells and endotoxin concentrations. The presence of colistin, which markedly reduced endotoxin concentration, protected Vero cells from the lethal injury otherwise caused by the supernatant suggesting that endotoxins released by EC O157:H7 might contribute to the intestinal damage that can occur in patients infected by this bacterium.
Because the study didn't discriminate between Stx1 and Stx2—which is thought to be more virulent—the study couldn't answer whether colistin affected the release of either toxins, or just one. Moreover, the study only tested one E. coli Stx-producing strain.
Still, the ideal antibiotic for the treatment of Stx-producing E. coli infection should reduce the bacterial burden, avoid the release of Stx, and be safe. In this study at least, colistin fit the bill and might be might be worth testing in severely ill patients. Time will tell if such studies occur.
How to cite:
Pauletto, Alessandro. A Tough Drug for a Tough Bug. Eureka blog. June 8, 2016. Available: https://eureka.criver.com/a-tough-drug-for-a-tough-bug/