A taste for human blood may help explain why the MRSA superbug is such a menace, scientists have discovered.

MRSA is a drug-resistant form of Staphylococcus aureus, a microbe that lives harmlessly in the noses of almost a third of the population.

However some people suffer serious or repeated Staphylococcus infections, the worst being caused by MRSA.

The reason could be individual differences in the blood protein haemoglobin, scientists believe.

New research shows that the "vampire" bug favours human haemoglobin over that of other animals.

The bacterium obtains iron by extracting it from haemoglobin. Laboratory tests showed that it binds especially easily to the human version of the blood protein.

Other bacteria that exclusively infect people, including the diphtheria bug, also appeared to latch on to human haemoglobin.

"A big question in staph biology is: why do some people continuously get infected or suffer very serious staph infections, while other people do not?" said lead researcher Dr Eric Skaar, from Vanderbilt University Medical Center in Nashville, Tennessee, US. "Variations in haemoglobin could contribute."

The research is published in the journal Cell Host & Microbe.