Homology tree of selected AadA proteins involved in combined resistance to spectinomycin and streptomycin based on a multisequence alignment produced with the DNAMAN software (Lynnon-BioSoft, Ontario, Canada). The bacterial source and the database accession number are given for each AadA protein. For a number of AadA proteins, e.g., AadA1 or AadA2, a large number of identical or closely related sequences from different bacterial sources are deposited in the databases. To reduce the complexity of this homology tree, only one representative for each type of AadA protein was chosen. The designations of the different AadA proteins are used as they are deposited in the databases, although these designations do not always reflect the real structural similarities between the different AadA proteins (9). Abbreviations (including reference to the corresponding AadA proteins, if published) are as follows: A. baumannii, Acinetobacter baumannii AadA (20) and AadA4 (32); A. genomosp. 3, Acinetobacter genomospecies 3 AadA1 (40); A. hydrophila, Aeromonas hydrophila; C. jejuni, Campylobacter jejuni AadA2 (22); C. freundii, Citrobacter freundii AadA2 (24); C. glutamicum, Corynebacterium glutamicum AadA2 (34) and AadA9 (33); E. faecalis, Enterococcus faecalis AadA (4); E. coli, Escherichia coli AadA1 (16), AadA2 (28), AadA4 (1), and AadA5 (36); K. oxytoca, Klebsiella oxytoca AadA1 (26); K. pneumoniae, Klebsiella pneumoniae AadA8 (25); P. multocida, Pasteurella multocida AadA1 (38); P. aeruginosa, Pseudomonas aeruginosa AadA1 (15), AadA6 (2), and AadA10 (23); S. Agona, Salmonella enterica serovar Agona AadA23 (17); S. Choleraesuis, Salmonella enterica serovar Choleraesuis; S. Newport, Salmonella enterica serovar Newport AadA7 (6); S. Typhimurium, Salmonella enterica serovar Typhimurium AadA1 (35) and AadA21 (7); S. flexneri, Shigella flexneri; uncult. bacterium, uncultured bacterium; V. cholerae, Vibrio cholerae AadA2 (5).