Efficacy of antibiotic treatment verified by FISH. Blochmannia specific fluorescent in situ hybridisation (FISH) of bacteriocytes (Bc) in the midgut epithelium of: (A) and (B) an untreated worker of C. floridanus, and (C) and (D) a worker treated with the antibiotic rifampicin and fed an artificial diet containing all essential amino acids (fluorescence and phase contrast microscopy of the same region, respectively). The midguts of the untreated and the treated worker differ strongly in the number of visible (bacteria containing) bacteriocytes and the number of bacteria per cell. Epithelial gut tissue and other structures visible in phase contrast show no significant autofluorescence. Scale bar 10 μm.