Both studies were on patients receiving immunotherapy, which boosts the body's own defences to fight tumours.
One study, at the Gustave Roussy Cancer Campus in Paris, looked at 249 patients with lung or kidney cancer.
They showed those who had taken antibiotics, such as for dental infection, damaged their microbiome and were more likely to see tumours grow while on immunotherapy.
One species of bacteria in particular, Akkermansia muciniphila, was in 69% of patients that did respond compared with just a third of those who did not.
Boosting levels of A. muciniphila in mice seemed to also boost their response to immunotherapy.
Meanwhile, at the University of Texas MD Anderson Cancer Center, 112 patients with advanced melanoma had their microbiome analysed.
Those that responded to therapy tended to have a richer, more diverse microbiome than those that did not.
Tissues samples showed there were more cancer-killing immune cells in the tumour of people with the beneficial bacteria.
The team then performed a trans-poo-sion, a transplant of faecal matter, from people to mice with melanoma.
Mice given bacteria from patients with the "good" mix of bacteria had slower-growing tumours than mice given "bad" bacteria.
Dr Jennifer Wargo, from Texas, told the BBC: "If you disrupt a patient's microbiome you may impair their ability to respond to cancer treatment."
She is planning clinical trials aimed at altering the microbiome in tandem with cancer treatment.
She said: "Our hypothesis is if we change to a more favourable microbiome, you just may be able to make patients respond better.