Xanthomonas maltophilia misidentified as Pseudomonas cepacia in cultures of sputum from patients with cystic fibrosis: a diagnostic pitfall with major clinical implications.

Pseudomonas cepacia infection in patients with cystic fibrosis (CF) has major significance in terms of infection control, psychosocial issues, and medical treatment. We describe three instances in which the diagnostic laboratory misidentified Xanthomonas maltophilia as P. cepacia in cultures of sputum from patients with CF. These errors were recognized when 3 (9%) of 32 isolates, which had all been identified as P. cepacia and had been submitted to the Canadian Pseudomonas Repository Laboratory (Vancouver, BC), were correctly identified there as X. maltophilia. Each of the three isolates grew well on P. cepacia media, turned a characteristic vivid pink color, were polymyxin-resistant, and were lysine-positive. All three were initially characterized incorrectly as oxidase-positive and DNase-negative. The diagnostic laboratory then reexamined 24 other isolates that had been identified as P. cepacia; complete biochemical testing confirmed that all were indeed P. cepacia. Because infection due to P. cepacia has major implications for patients with CF, when a possible strain of P. cepacia is isolated, careful and complete characterization should be performed.