Exposure to antibiotics to which bacteria are resistant worsens the damage they unleash on the body, new research has shown.
Antibiotics trigger the evolution of resistance in bacteria. One notable example is MRSA - methicillin resistant Staphylococcus aureus - which has earned notoriety as a hospital superbug.
MRSA routinely tolerates exposure to drugs like penicillin by making a chemical change to its outer coat so that the drug can no longer bind on.
Microbiologists had reasonably assumed that dosing MRSA with penicillins would amount to little more than just a waste of antibiotics.
However, University of California Los Angeles scientist Sabrina Muller and her colleagues, writing in the journal Cell Host and Microbe, have found that exposing MRSA to drugs to which it is resistant in fact fires up the infection and results in a potentially more serious clinical presentation.
The team grew cultures of MRSA in the presence of penicillin-like drugs. These treated bugs, they found, provoked a far fiercer response from immune cells called macrophages compared with MRSA grown in normal medium.
Injecting fragments of these penicillin-exposed bacteria and even heat-killed whole bacteria into the skin of mice also produced a more severe tissue reaction when the bugs had been grown beforehand in pencillin-dosed medium.
Surprisingly, the reason is not down to enhanced virulence on the part of the bacteria. Instead, exposure to penicillin switches on the defence processes in the bug, which, despite the fact that the bacteria are still able to grow, nonetheless leads to a cell wall that is more haphazard and ragged in its construction.
This modified surface provokes a stronger immune response than the normal bacterial coat. So, paradoxically, the damage done by MRSA is often down not to the bug so much as the individual's own immune system.
The discovery is pertinent, on the eve of World Antibiotic Awareness Week, because clinicians are increasingly finding themselves under considerable pressure to avoid using so-called "big guns" antimicrobial drugs. But risk treatment with agents to which the bug may be resistant may be stoking - rather than extinguishing - the fire...