Bacterial Hitchhikers: Who’s Really in the Driver’s Seat?

Bacteria can influence the sexual behavior of their Nasonia hosts.
Bacteria can influence the sexual behavior of their Nasonia hosts.
Like all species of animals and plants, we humans are unwitting hosts to our own set of bacterial travelers. We carry thousands of different species of microbes, which scientists refer to as “the human microbiome.” In fact, only one among every 10 cells in our bodies is human while the other nine
are bacterial.

Known as commensal bacteria, these bacterial cells greatly outnumber our own, but they go largely unnoticed because they are much smaller than human cells. Bacteria inhabit the gut, skin and mucous membranes, where they perform a number of crucial functions such as aiding digestion and strengthening the immune system.

Since the early 20th century, scientists have debated what role these colonies of bacteria might also play in promoting or inhibiting formation of new species. Now a team of Vanderbilt biologists is attempting to answer this long-standing question, aided by a $1.3 million award from the National Science Foundation.

The team will use a tiny genus of wasp, called Nasonia. These wasps carry bacteria that have demonstrated the ability to influence their host’s sexual behavior.

“By receiving this award,” says Assistant Professor of Biological Sciences Seth Bordenstein, “we have been challenged by the National Science Foundation to answer one of life’s most fundamental questions.”

Seth Bordenstein, right, and Robert Brucker examine a bottle filled with Nasonia wasps.
Seth Bordenstein, right, and Robert Brucker examine a bottle filled with Nasonia wasps.

Two opposing theories exist regarding the role that commensal bacteria have played in furthering environmental diversity, the process that has filled our planet with more than 1.8 million different species. One theory says they don’t play any role. Instead, speciation is driven exclusively by the host’s genes. The other theory holds that the bacteria are in the driver’s seat, encouraging the formation of new species when they stand to benefit and discouraging formation when they do not.

Bordenstein and his team are looking for evidence to support the second contention by studying the characteristics of the microbiomes of several closely related species of Nasonia and the hybrids produced when they mate. By analyzing the genomes and microbial communities of this wasp genus, which recently underwent the process of speciation, graduate student Robert Brucker and his colleagues in the Bordenstein lab are attempting to identify specific microbes that increase the rate of mortality in the hybrid offspring.

The team will identify the host bacteria that colonize the different hybrids; assess the hybrids’ ability to regulate their own immune systems and defend against certain pathogens; and determine whether any increased lethality is due to decreases in the numbers of beneficial bacteria or to increases in the numbers of pathogenic bacteria.

Bordenstein and his team hope their observations will provide new insights into the role that host bacteria play in this vital evolutionary process.

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