Human innards are teeming with viruses that infect bacteria. What are they up to?

You've probably heard of the microbiome – the hordes of bacteria and other tiny life forms that live in our guts. Well, it turns out those bacteria have viruses that exist in and around them – with important consequences for both them and us.
Meet the phageome.
There are billions, perhaps even trillions of these viruses, known as bacteriophages ("bacteria eaters" in Greek) or just "phages" to their friends, inside the human digestive system. Phageome science has skyrocketed recently, says Breck Duerkop, a bacteriologist at the University of Colorado Anschutz School of Medicine, and researchers are struggling to come to grips with their enormous diversity. Researchers suspect that if physicians could harness or target the right phages, they might be able to improve human health.
"There will turn out to be good phages as well as bad phages," says Paul Bollyky, an infectious disease physician and researcher at Stanford Medicine. But for now, it's still not clear how many phages occupy the gut – perhaps one for each bacterial cell, or even fewer. There are also bacteria that contain phage genes but aren't actively producing viruses – the bacteria are just living their lives with phage DNA tagging along in their genomes.

And there are lots of phages still unidentified. Scientists call these the "dark matter" of the phageome. A big part of current phage research is to identify these viruses and their host bacteria. The Gut Phage Database contains more than 140,000 phages, but that's surely an underestimate. "Their variety is just extraordinary," says Colin Hill, a microbiologist at University College Cork in Ireland.
Scientists find phages by sifting through genetic sequences culled from human faecal samples. That's where researchers found the most common gut phage group, called crAssphage. (named for the "cross-assembly" technique that plucked their genes out of the genetic mishmash.) In a recent study, Hill and colleagues detailed a light-bulb shape for crAssphages, with a 20-sided body and a stalk to inject DNA into host bacteria.

腸道是一個生態系統，就像樹林一樣，噬菌體是細菌捕食者，就像狼是鹿捕食者。腸道需要噬菌體，就像森林需要狼一樣。
T9娛樂城目前尚不清楚 crAssphages 是否會對人類健康產生影響，但考慮到它們會感染最常見的腸道細菌之一——擬桿菌，如果它們確實如此，希爾也不會感到驚訝。其他也感染擬桿菌的常見族群包括 Gubaphage（腸道擬桿菌噬菌體）和 LoVEphage（許多病毒遺傳元件）。
噬菌體因人而異。正如 Hill 及其同事在《2023 年微生物學年度回顧》中所描述的那樣，它們還會根據年齡、性別、飲食和生活方式而改變。

儘管噬菌體會感染細菌，有時會殺死細菌，但其中的關係比這更複雜。希爾說：“我們曾經認為噬菌體和細菌在戰鬥，但現在我們知道它們實際上是在跳舞；它們是夥伴。”
噬菌體可以透過引入新基因來使細菌受益。當噬菌體顆粒在受感染的細菌內部組裝時，它有時會將細菌基因連同其自身的遺傳物質一起塞入其蛋白質外殼中。杜爾科普說，隨後，它將這些基因注入新的宿主，而那些意外轉移的基因可能會有所幫助。它們可能會產生抗生素抗藥性或消化新物質的能力。
希爾說，噬菌體透過不斷地咬住細菌的腳跟來保持細菌種群的健康。擬桿菌的外表面可以呈現多達十幾種類型的糖衣。不同的皮毛有不同的優點：例如逃避免疫系統，或是佔據消化系統的不同角落。但希爾說，當噬菌體存在時，擬桿菌必須不斷更換外殼，以躲避識別一種或另一種外殼的噬菌體。結果：在任何給定時間，都存在具有不同皮毛類型的擬桿菌，使整個族群能夠佔據各種生態位或應對新的挑戰。

噬菌體還可以防止細菌族群失控。腸道是一個生態系統，就像森林一樣，噬菌體是細菌捕食者，就像狼是鹿捕食者。腸道需要噬菌體，就像森林需要狼一樣。當這些掠食者與獵物的關係改變時，就會導致疾病：研究人員觀察到發炎性腸道症候群（IBS）、腸躁症和大腸直腸癌中噬菌體的變化——例如，患有IBS的人的病毒生態系通常多樣性較低。

人們試圖透過飲食來重新平衡腸道微生物群，或者在極端的醫療情況下，透過糞便移植來重新平衡腸道微生物群。希爾說，對付噬菌體可能提供更精細的方法。舉一個恰當的例子，科學家正在尋找可用於治療感染引起胃潰瘍的細菌的噬菌體。
也許我們應該感謝管理腸道生態系統的數兆噬菌體。希爾表示，如果沒有它們，幾種細菌可能很快就會佔據主導地位——可能會讓你無法消化某些食物，並容易產生脹氣和腹脹。
野生而奇妙的噬菌體是細菌和人類的舞伴。