It’s a little while since we talked about what you might call our hidden self — the vast army of bugs that colonises our nooks and crannies, especially our intestines, and that is essential to our survival.
In Our Inner Self we noted that these little guys outnumber the human cells that make up the body by about ten to one. Actually that estimate has recently been revised — downwards you might be relieved to hear — to about 1.3 bacterial cells per human cell but it doesn’t really matter. They are a major part of what’s called the microbiome — a vast army of microorganisms that call our bodies home but on which we also depend for our very survival.
In our personal army there’s something like 700 different species of bacteria, with thirty or forty making up the majority. We upset them at our peril. Artificial sweeteners, widely used as food additives, can change the proportions of types of gut bacteria. Some antibiotics that kill off bacteria can make mice obese — and they probably do the same to us. Obese humans do indeed have reduced numbers of bugs and obesity itself is associated with increased cancer risk.
In it’s a small world we met two major bacterial sub-families, Bacteroidetes and Firmicutes, and noted that their levels appear to affect the development of liver and bowel cancers. Well, the Bs & Fs are still around you’ll be glad to know but in a recent piece of work the limelight has been taken by another bunch of Fs — a sub-group (i.e. related to the Bs & Fs) called Fusobacterium.
It’s been known for a few years that human colon cancers carry enriched levels of these bugs compared to non-cancerous colon tissues — suggesting, though not proving, that Fusobacteria may be pro-tumorigenic. In the latest, pretty amazing, installment Susan Bullman and colleagues from Harvard, Yale and Barcelona have shown that not merely is Fusobacterium part of the microbiome that colonises human colon cancers but that when these growths spread to distant sites (i.e. metastasise) the little Fs tag along for the ride!
Bacteria in a liver metastasis of the same bowel tumour. Though more difficult to see, the red dot (arrow) marks the presence of bacteria from the original tumour. From Bullman et al., 2017.
In other words, when metastasis kicks in it’s not just the tumour cells that escape from the primary site but a whole community of host cells and bugs that sets sail on the high seas of the circulatory system.
But doesn’t that suggest that these bugs might be doing something to help the growth and spread of these tumours? And if so might that suggest that … of course it does and Bullman & Co did the experiment. They tried an antibiotic that kills Fusobacteria (metronidazole) to see if it had any effect on F–carrying tumours. Sure enough it reduced the number of bugs and slowed the growth of human tumour cells in mice.
Growth of human tumour cells in mice. The antibiotic metronidazole slows the growth of these tumour by about 30%. From Bullman et al., 2017.
We’re still a long way from a human therapy but it is quite a startling thought that antibiotics might one day find a place in the cancer drug cabinet.
Bullman, S. et al. (2017). Analysis of Fusobacterium persistence and antibiotic response in colorectal cancer. Science 358, 1443-1448. DOI: 10.1126/science.aal5240