The cause of bacterial meningitis is, by definition, a bacterial infection - but it may not be quite that simple. The true culprit may be a virus that infects the bacteria.

Bacteria and viruses are very different organisms. Basically, bacteria are self-sufficient when it comes to replication, but viruses need to infect a host cell and highjack its genetic machinery in order to reproduce.

Some viruses, called bacteriophages, specialize in infecting bacterial cells.

Now European researchers report that a bacteriophage may be the trigger that makes the common and generally harmless bacterium, Neisseria meningitidis, invade the blood stream and cross the blood-brain barrier, leading to meningitis.

“We have shown that a bacteriophage increases the ability of N. meningitidis, a (harmless) bacterium, to become pathogenic,” investigator Dr. Xavier Nassif told.

Most cases of illness due to N. meningitidis are caused by a small number of “hyperinvasive” types of the microbe, Nassif of the National Institute of Health and Medical Research, Paris, and colleagues note in the Journal of Experimental Medicine.

To investigate further, the researchers analyzed genetic material from hyperinvasive organisms and from organisms not associated with disease. They found that a “prophage” - the early stage of a bacteriophage - was present in disease-casing bacteria.

The team points out that “the carriage of virulence determinants by phages is not an uncommon situation in bacterial pathogens.” This is the case in both cholera and diphtheria.

The findings “may help to design a better strategy to fight cerebrospinal meningitis by pinpointing strains at higher risk of being pathogenic,” Nassif concluded.

SOURCE: Journal of Experimental Medicine, June 20, 2005.