The quinolone resistance-determining regions (QRDRs) of topoisomerase II and IV genes from Stenotrophomonas maltophilia ATCC 13637 were sequenced and compared with the corresponding regions of 32 unrelated S. maltophilia clinical strains for which ciprofloxacin MICs ranged from 0.1 to 64 g/ml. GyrA (Leu-55 to Gln-155, Escherichia coli numbering), GyrB (Met-391 to Phe-513), ParC (Ile-34 to Arg-124), and ParE (Leu-396 to Leu-567) fragments from strain ATCC 13637 showed high degrees of identity to the corresponding regions from the phytopathogen Xylella fastidiosa, with the degrees of identity ranging from 85.0 to 93.5%. Lower degrees of identity to the corresponding regions from Pseudomonas aeruginosa (70.9 to 88.6%) and E. coli (73.0 to 88.6%) were observed. Amino acid changes were present in GyrA fragments from 9 of the 32 strains at positions 70, 85, 90, 103, 112, 113, 119, and 124; but there was no consistent relation to higher ciprofloxacin MICs. The absence of changes at positions 83 and 87, commonly involved in quinolone resistance in gram-negative bacteria, was unexpected. The GyrB sequences were identical in all strains, and only one strain (ciprofloxacin MIC, 16 g/ml) showed a ParC amino acid change (Ser-803Arg). In contrast, a high frequency (16 of 32 strains) of amino acid replacements was present in ParE. The frequencies of alterations at positions 437, 465, 477, and 485 were higher (P < 0.05) in strains from cystic fibrosis patients, but these changes were not linked with high ciprofloxacin MICs. An efflux phenotype, screened by the detection of decreases of at least twofold doubling dilutions of the ciprofloxacin MIC in the presence of carbonyl cyanide m-chlorophenylhydrazone (0.5 g/ml) or reserpine (10 g/ml), was suspected in seven strains. These results suggest that topoisomerases II and IV may not be the primary targets involved in quinolone resistance in S. maltophilia.

Given a square matrix A, a Brauer's theorem [Limits for the characteristic roots of matrices IV: Applications to stochastic matrices, Duke Math. J. 19 (1952), [75][76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91] shows how to modify one single eigenvalue of A via a rank-one perturbation without changing any of the remaining eigenvalues. Older and newer results can be considered in the framework of the above theorem. In this paper, we present its application to stabilization of control systems, including the case when the system is noncontrollable. Other applications presented are related to the Jordan form of A and Wielandt's and Hotelling's deflations. An extension of aforementioned Brauer's result, Rado's theorem, shows how to modify r eigenvalues of A at the same time via a rank-r perturbation without changing any of the remaining eigenvalues. The same results considered by blocks can be put into the block version framework of the above theorem.

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