In some embodiments, methods of determining that a subject is likely to have cancer are provided. Such methods may include amplifying a microbial DNA sample in a test sample obtained from the subject to determine an amount of microbial DNA in the test sample, wherein the amount of microbial DNA is determined by an amplification or sequencing technique and determining that the subject is likely to have breast cancer when there is a significantly decreased level of microbial DNA in the test sample when compared to a level of microbial DNA in a control sample. In other embodiments, methods of treating cancer (e.g., breast cancer) are provided. In one aspect, such methods include administering a therapeutically effective dose of a probiotic organism via ductal lavage to a subject suffering from the breast cancer.
