Around the age when my interest in boys went from beating them at monkey bar tag to stealing smooches behind the high school, my mum had the sex talk with me. 

We talked about consent, respect, not getting pregnant and very importantly: “do not get a sexually transmitted disease.” No glove, no love. 

This was, of course, very good advice. It’s thought by the World Health Organization that worldwide, more than one million people a day acquire a sexually transmitted disease (STD). Some of these infections will affect your fertility, others cause even more worrying health issues. There are many reasons to prevent these unwanted travellers from taking up residence in our body. 

The nasty rep of STDs or STIs (sexually transmitted infections) could be why very little consideration is given to the idea that some of the microbes hitching a ride between individuals on the sexual fluids highway, might be beneficial. What if, in protecting ourselves from the bad bugs we know about, we’re missing out on microbes that could provide all sorts of benefits? 

A growing body of evidence suggests there is good reason to have a closer look.

 

It’s not news that microbes - such as bacteria and viruses - are incredibly important to our health. Within all of us there is a mixture of both beneficial and potentially disease-causing mini critters. If the balance between the two is lost, it can cause problems.

Our bodies have co-evolved with microbes

For example, the Candida genus of yeast is a naturally occurring microbe found in the vagina. Its growth is kept in check by another microbe, the Lactobacillus bacteria. However, if something prevents the bacteria from doing its job, the result is an overgrowth of yeast, causing the uncomfortable symptoms of a yeast infection.

Full of bugs 

Our bodies have co-evolved with microbes. These bacteria, fungi and viruses are on our skin, in our gut and part of our genitalia. While it’s not pleasant to think about bacteria running rampant in our colons, it’s becoming ever clearer that microbes play a key role in our very make up.

The first step to understanding the role microbes play is to identify them. Those that come with sex are called sexually transmitted microbes (STMs). Although we don't yet know much about them, there are some fascinating examples that should motivate researchers to take a closer look, says Chad Smith, an evolutionary biologist from the University of Texas, US.

Take the pea aphid. It’s a juice-sucking insect that makes its way around the globe syphoning the life from legumes. 

Their success could in part be attributed to beneficial STMs that pass between them while mating. The effects are as follows: resistance to parasitoids (that kill or sterilise the host), better tolerance to heat and an improved ability to survive on plants that aren’t legumes, until they find their next meal.

Not to be outdone, some mosquitoes have sexually transmitted bacteria that colonise their gut, testes and the surface of developing eggs. It’s thought this bacterial coating provides nutrition for the larvae, allowing them to develop two to four days faster than those without it. 

Finally, in fungi, beneficial STMs have also been found to increase their tolerance to heat as well as helping their host grow more quickly. 

 

But what about humans? We now know there is one persuasive example that an STM may do some of us some good.  

It's in the form of the GB virus C (GBC-C) formerly known as hepatitis G virus (HGV). It's a sexually transmitted infection that on its own does not appear to cause any major symptoms, though it is often found with other disease-causing viruses such as HIV. 

It can reduce the chance of mothers with HIV passing the infection onto their children

A six-study review found that it was associated with a 59% reduction in the mortality rate of HIV patients. Scientists think GBV-C does this by reducing HIV’s ability to compromise our immune system cells. It may also stimulate other parts of the immune system to actively fight the infection. 

GBV-C can also be passed from mother to child. This is good news as it can reduce the chance of mothers with HIV passing the infection onto their children.   

More recently, GBV-C has also been linked to reduced mortality in those infected with the Ebola virus, somehow reducing the impact of the virus on its host. Understanding how could save lives.

Extraordinary discoveries such as this should make us wonder what else we are missing, says Betsy Foxman, of the University of Michigan, US.

In the past we’ve characterised sexually transmitted microbes as bad, she says. The preventative measures we’ve taken to protect against them may mean that we now lack some that are potentially beneficial.

There are likely a host of beneficial organisms yet to be identified 

Foxman would like to see more work done on the benefits of one pathogen interfering with another. “What could our body have on it that would keep it healthier [for] longer?”

There may even be microbes that help target other kinds of infections, Foxman says. If there are, these could reduce our dependence on medicines such as antibiotics. They often kill a broad spectrum of microbes in order to eradicate the one that is problematic.

Of course, at times antibiotics are necessary to save lives, but it would be “nice to have something a little more judicious and targeted”, Foxman adds.

We're not sure which beneficial STMs are passing between people, but Foxman suggests Lactobacillus, the bacteria found in yogurt and naturally present in humans, is one of them. There are likely a host of other beneficial organisms yet to be identified, she says.

Cost benefit

This all sounds like great news, right? There could be a whole bunch of little known STMs floating around that could be beneficial to our health.

Microbes that cause sexual diseases need to ensure they can hop from human to human

There’s just one problem. If we get them by having sex that leaves us open to getting other harmful infections as well.

There may be other ways to acquire them in the future. Once beneficial STMs are identified, scientists working in public health can hopefully develop safe ways to inoculate people, or figure out how to replicate their effects, without risking unprotected sex.

Most sexually transmitted infections such as chlamydia and gonorrhoea don’t tend to kill their hosts. They are also often asymptomatic. “They’re just trying to make a living,” says Foxman.   

There could even be certain STMs that encourage humans to have sex more often

Microbes that cause sexual diseases need to ensure they can hop from human to human. This transaction is more likely to happen if the person hosting the microbes appears healthy, she adds. “If someone looks sick, from an evolutionary standpoint, they’re a bad bet,” she says. 

It could be that as humans evolved, well before condoms existed, the risk of getting an STD was, at times, outweighed by acquiring an important microbe. It’s an evolutionary cost-benefit analysis, says Smith. 

Those who carry beneficial STMs could even be unknowingly signaling that they do so, says the evolutionary biologist Michael Lombardo. They might then be favoured as mates. 

It gets more complicated. It may not just be the physiological benefits, such as disease prevention or tolerance to heat, that make certain STMs advantageous. These microbes could also affect their host's behavior in a good way. 

That microbes can do so is a relatively new and promising area of research. For example, certain gut bacteria are thought to affect the brains of young mice, reducing their anxiety levels. 

Individuals that mate with more than one partner are more likely to acquire a beneficial STM

Microbes may also modify the chemical signals that animals produce, says Smith. This was discovered in research on fruit flies as they acquired bacteria from different diets. The result? Fruit flies preferred to mate with others that shared those same microbial communities.

Getting around

These are examples of gut bacteria altering behaviour and therefore mate choice, but can sexually transmitted microbes do the same?

There is little evidence for this so far, says Smith, but it would make sense. If a male has a beneficial microbe and it has a positive affect on him, females that choose to mate with him would acquire it too, he adds.

“Individuals that mate with more than one partner are more likely to acquire a beneficial STM,” says Smith. If that’s the case, it could be one of the reasons a species might favour mating with more than one partner.

In the common lizard, for example, the females that mate with more than one partner have been found to have more diverse microbial communities than those that don’t. Scientists who study them speculate that this could mean a difference in health between sexually active females and asexual reproducers.

Lombardo believes that some female birds that mate repeatedly with the same partner, or with more than one partner, may receive benefits from STMs by acquiring viruses that kill harmful bacteria, or by acquiring less virulent strains of pathogens.

These could limit the impact of more virulent strains. Beneficial microbes that produce bacteria-killing chemicals could also help fight existing infections. This is one of many theories that attempts to explain why birds have extramarital sex outside their main partnership.

There could even be certain STMs that encourage humans to have sex more often, says Foxman.“What if it just felt better to have sex when you had a particular bug?

 “That bug could increase mucus flow. It could be as simple as that.”

Foxman says there are conditions in both men and women that create chronic pain feedback loops related to sexual activity, yet their cause is unknown. To identify a beneficial STM that would alleviate these conditions could mean all the difference to a person’s sex life. It’s worth looking into, she says.

“We are sexual beings. Things that make people feel better sexually, tend to make them feel better overall.”

It appears that whether you’re an aphid, bird, lizard or human, odds are that you are never quite alone with a mate. There are perhaps thousands of other organisms waiting for the magic to happen so that they can make their way back and forth between you, and perhaps out into the population. 

We may know more of them soon. Smith says that with the exciting and explosive growth of interest and research into the human microbiome, there are now “new techniques to survey and quantify the function of microbes that we didn’t have ten years ago.” 

In the mean time, stay tuned and play safe.