Pharmaceutical giant Merck and Co Inc says it has successfully redesigned a drug to block the mechanism used by certain bacteria to fight off penicillin-type antibiotics.

It has not yet been used on humans, but has been tested on monkeys without causing dangerous side effects.

Antibiotics have lost their power over many bugs

Bacteria are constantly developing new tricks to defeat the drugs used against them. And they reproduce so quickly that they can evolve new forms in just a few years.

Doctors say 70% of bacteria involved in infections that people get in hospitals are now resistant to at least one antibiotic.

Last year the Institute of Medicine noted that 90% of all strains of staphylococcus aureus, the most common cause of infections, some of them deadly, are now resistant to penicillin.

The breakthrough may make it possible to bring some penicillin-type antibiotics back into use against bugs that have built up defences against them.

Side effects

The new drug, called by its experimental name L-786,392, targets a bacterial protein known as PBP2a, short for penicillin-binding protein.

It is found in drug-resistant staphylococci and enterococci, and blocks the chemical doorways that penicillin usually uses to get into and kill bacteria.

The scientists first developed the technique of neutralising the resistance mechanism a few years ago, but they had to go back to the drawing board when the drugs caused harmful side effects.