treatment results in changes to the structure of the gut microbiome both during and after treatment in mice and humans. It is currently unknown what the long term effects of this dysbiosis are on systemic immunity. Deviations from the standard regimen There is evidence supporting some deviations from the standard regimen when treating pulmonary TB. Sputum culture-positive patients who are smear-negative at the start of treatment do well with only 4 months of treatment (this has not been validated for HIV-positive patients); sputum culture-negative patients do well on only 3 months of treatment (possibly because some of these patients never had TB at all). It is unwise to treat patients for only three or four months, but all TB physicians will have patients who stop their treatment early (for whatever reason), and it can be reassuring to know that sometimes retreatment is unnecessary. Elderly patients who are already taking a large number of tablets may be offered 9HR, omitting PZA which is the bulkiest part of the regimen. It may not always be necessary to treat with four drugs from the beginning. An example might be a close contact of a patient known to have a fully sensitive strain of tuberculosis: in this case, it is acceptable to use 2HRZ/4HR (omitting EMB and STM) in the expectation that their strain will be INH susceptible also. Indeed, this was previously the recommended standard regimen in many countries until the early 1990s, when isoniazid-resistance rates increased. TB involving the brain or spinal cord (meningitis, encephalitis, etc.) is currently treated with 2HREZ/10HR (12 months of treatment in total), but there is no evidence to say that this is superior to 2HREZ/4HR. There is no difference in relapse rates amongst those who are treated with 6 months or longer period of time. However, more well-designed studies are needed to answer this question. Regimens omitting isoniazid Isoniazid resistance accounts 6.9% of isolates in the UK (2010). Worldwide, it is the most common type of resistance encountered, hence the current recommendation of using HREZ at the beginning of treatment until sensitivities are known. It is useful to know of current reported outbreaks (like the current outbreak of INH-resistant TB in London). If patients are discovered to be infected with an isoniazid-resistant strain of TB having completed 2 months of HREZ, then they should be changed to RE for a further 10 months, and the same thing