An in vitro method to determine the likelihood that a subject with irritable bowel syndrome will respond to treatment with an IBS intervention diet, wherein said IBS intervention diet is a low FODMAP diet, said method comprising (i) determining for a test sample from the GI tract of a subject with diarrhoea subtype IBS or mixed subtype IBS to be treated with a low FODMAP diet the amount of bacteria from at least one taxonomic group selected from - Acinetobacter junii - Bacteroides stercosis - Bacteroides zoogleoformans - Dorea spp. - Eubacterium siraeum - Clostridium sp. - Eubacterium halli - Bacteroides spp. - Firmicutes - Veillonella spp., Helicobacter spp., and Clostridia; and - Shigella spp. and Escherichia spp.and (ii)(a) comparing the amount of said bacteria from said at least one taxonomic group with a reference value prepared from at least one sample from the GI tract of at least one subject non-symptomatic for IBS and determining if the amount of said bacteria from said at least one taxonomic group in said test sample differs from, or corresponds to, the reference value, wherein an amount of bacteria in the taxonomic groups Acinetobacter junii, Bacteroides stercosis, Bacteroides zoogleoformans, Dorea spp., Eubacterium siraeum, Clostridium sp., or Eubacterium hallii which is greater than the reference values, or an amount of bacteria in the taxonomic groups Bacteroides spp., Firmicutes, Veillonella spp., Helicobacter spp., and Clostridia, or Shigella spp. and Escherichia spp. which is lower than the reference values is indicative that the subject will respond to a low FODMAP diet; and/or (ii)(b) comparing the amount of said bacteria from said at least one taxonomic group with a cut-off value which has been determined as a median amount of said bacteria in at least one sample from the GI tract of at least one IBS subject with said IBS subtype which has been previously shown to be responsive to said treatment and determining if the amount of said bacteria from said at least one taxonomic group in said test sample is greater than or less than said median cut-off value, wherein an amount of bacteria in the taxonomic groups Acinetobacter junii, Bacteroides stercosis, Bacteroides zoogleoformans, Dorea spp., Eubacterium siraeum, Clostridium sp., or Eubacterium hallii which is greater than said median cut-off value for said bacteria, or an amount of bacteria in the taxonomic groups Bacteroides spp., Firmicutes, Veillonella spp., Helicobacter spp., and Clostridia, or Shigella spp. and Escherichia spp. which is less than said median cut-off value for said bacteria is indicative that the subject will respond to a low FODMAP diet. (i) determining for a test sample from the GI tract of a subject with diarrhoea subtype IBS or mixed subtype IBS to be treated with a low FODMAP diet the amount of bacteria from at least one taxonomic group selected from - Acinetobacter junii - Bacteroides stercosis - Bacteroides zoogleoformans - Dorea spp. - Eubacterium siraeum - Clostridium sp. - Eubacterium halli - Bacteroides spp. - Firmicutes - Veillonella spp., Helicobacter spp., and Clostridia; and - Shigella spp. and Escherichia spp.and - Acinetobacter junii - Bacteroides stercosis - Bacteroides zoogleoformans - Dorea spp. - Eubacterium siraeum - Clostridium sp. - Eubacterium halli - Bacteroides spp. - Firmicutes - Veillonella spp., Helicobacter spp., and Clostridia; and - Shigella spp. and Escherichia spp.and (ii)(a) comparing the amount of said bacteria from said at least one taxonomic group with a reference value prepared from at least one sample from the GI tract of at least one subject non-symptomatic for IBS and determining if the amount of said bacteria from said at least one taxonomic group in said test sample differs from, or corresponds to, the reference value, wherein an amount of bacteria in the taxonomic groups Acinetobacter junii, Bacteroides stercosis, Bacteroides zoogleoformans, Dorea spp., Eubacterium siraeum, Clostridium sp., or Eubacterium hallii which is greater than the reference values, or an amount of bacteria in the taxonomic groups Bacteroides spp., Firmicutes, Veillonella spp., Helicobacter spp., and Clostridia, or Shigella spp. and Escherichia spp. which is lower than the reference values is indicative that the subject will respond to a low FODMAP diet; and/or (ii)(b) comparing the amount of said bacteria from said at least one taxonomic group with a cut-off value which has been determined as a median amount of said bacteria in at least one sample from the GI tract of at least one IBS subject with said IBS subtype which has been previously shown to be responsive to said treatment and determining if the amount of said bacteria from said at least one taxonomic group in said test sample is greater than or less than said median cut-off value, wherein an amount of bacteria in the taxonomic groups Acinetobacter junii, Bacteroides stercosis, Bacteroides zoogleoformans, Dorea spp., Eubacterium siraeum, Clostridium sp., or Eubacterium hallii which is greater than said median cut-off value for said bacteria, or an amount of bacteria in the taxonomic groups Bacteroides spp., Firmicutes, Veillonella spp., Helicobacter spp., and Clostridia, or Shigella spp. and Escherichia spp. which is less than said median cut-off value for said bacteria is indicative that the subject will respond to a low FODMAP diet.