The invention relates to methods of modulating immune cells in a patient by altering microbiota of the patient. The invention also relates to methods of modulating treatments or therapies in a subject organism by altering microbiota of the subject. The invention also relates to cell populations, systems, arrays, cells, RNA, kits and other means for effecting this. In an example, advantageously selective targeting of a particular species in a human gut microbiota using guided nucleic acid modification is carried out to effect the alteration.