Their analysis of stool samples in a study of more than 3,600 twins found evidence that some of this bacteria is inherited.
What is contained in faeces bacteria could therefore partly explain why obesity passes down through families.
The study is published in Genome Biology.
The research team extracted information from study participants about the human faecal microbiome - the bacteria present in faeces samples - and compared these to six different measures of obesity, including body mass index (BMI) and different types of body fat.
iWonder - What does your poo say about you?
The researchers found the strongest links with visceral fat, where participants with a high diversity of bacteria in their faeces had lower levels of visceral fat.
This type of body fat is bad news because it is stored in the stomach area around important organs such as the liver, pancreas and intestines and is linked with higher risks of cardiovascular disease and diabetes.
Dr Michelle Beaumont, lead study author from the department of twin research and genetic epidemiology at King's College London, said although the study showed a clear link, it was not yet possible to explain why it existed.
One theory is that a lack of variety in faecal bacteria could lead to the domination of high levels of gut microbes which are good at turning carbohydrates into fat.
Dr Beaumont said: "As this was an observational study we cannot say precisely how communities of bacteria in the gut might influence the storage of fat in the body, or whether a different mechanism is involved in weight gain."
And she indicated more research was needed to investigate how microbes in our guts and in our faeces can influence our health.
But there is a growing body of evidence to suggest that gut bacteria may play a role in obesity.
It is known that at least 50% of human faeces is made up of bacteria shed from the gut.
Dr Beaumont said that eating a broad diet including a variety of different types of food - much like that of the early hunter-gatherers - could increase the diversity of microbes in our faeces.
If the theory that microbes are passed on down the generations is correct, she said they may play an important role in how fat develops around the body and the health risk it presents.

We have a mass of information for you to digest on our election hub at Election 2015 - Northern Ireland - BBC News
We have profiles of all 18 constituencies and the main political parties and, over the coming weeks, will take an in-depth look at the party leaders.
There will, of course, be analysis from our top team of political experts.
All our stories are tagged, enabling you to personalise your news feed on your mobile; whether you want to follow a particular political party or you want to keep an eye on a constituency, we will keep you up to date.
We also have a clickable guide to the issues at the heart of this election.
And there will be a full list of candidates by the close of nominations on 9 April.
We will have all the stories as the campaign builds momentum, manifestos are launched and political broadsides are exchanged between candidates and parties.
On election night, the constituency streams will really come into play as you can keep across how the vote is looking in your part of Northern Ireland.
We will also have an overnight live page bringing you constant updates on how the parties are faring, who is out who is in, the big stories of the election and latest from the count centres.
We'll bring you up-to-date content from our reporters and correspondents in the count centres,  and will be across social media with the very latest commentary.
In this election, more than ever before, the focus could be on the Northern Ireland parties if there is a hung parliament.
So even after the results are in, we'll bring you all the wheeling and dealing that could result in another coalition government or a minority one.
And we'll bring you analysis of what this will mean for Northern Ireland.
What are the top issues for each political party at the 2015 general election?
Policy guide: Where the parties stand

The authority said traffic often tried to switch lanes at the "dangerous junction" on Berkeley Place and Triangle South.
The scheme was financed by a local office development as part of its planning agreement.
Mayor George Ferguson said the changes would be "welcomed by all".
"I don't believe there are many people who drive, walk or cycle in the area who would contest how dangerous that corner is," he added.
The work will take eight weeks to complete.
The make-up of the bacteria found in human faeces may influence levels of dangerous fat in our bodies, say researchers from King's College London.
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The general election campaign is now in full swing and you can follow every step of the way on the BBC News NI website.
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A Â£95,000 scheme to add a signalled pedestrian crossing to a busy section of road in Bristol will make it safer, the city council has said.