The human body harbors many harmless bacterial residents, known as commensals. One such bacterium, called Neisseria meningitidis, makes its home in the upper respiratory tract, usually without consequence. But certain strains of N. meningitidis -- known as 'hyperinvasive' strains -- occasionally find their way into the blood stream and then into the brain, where they trigger a potentially fatal immune response (meningitis).
Tinsley and colleagues have now discovered that many hyperinvasive stains of N. meningitidis contain a bacteriophage (a virus-like organism that infects bacteria) inserted into their genetic material. The researchers studied strains of bacteria that had been isolated from humans and found that the strains that carried this virus-like stowaway were more likely to have caused disease. How this bacteriophage causes N. meningitidis to become more dangerous is unknown. It may help the bacteria get into the bloodstream, suggests Tinsley, either by altering the bacteria or by disrupting the immune response that normally protects against it.