| Abstract: | Pseudomonas aeruginosa is an important cause of pulmonary infection in cystic fibrosis (CF). Its correct identification ensures effective patient management and infection control strategies. However, little is known about how often CF sputum isolates are falsely identified as P. aeruginosa. We used P. aeruginosa-specific duplex real-time PCR assays to determine if 2,267 P. aeruginosa sputum isolates from 561 CF patients were correctly identified by 17 Australian clinical microbiology laboratories. Misidentified isolates underwent further phenotypic tests, amplified rRNA gene restriction analysis, and partial 16S rRNA gene sequence analysis. Participating laboratories were surveyed on how they identified P. aeruginosa from CF sputum. Overall, 2,214 (97.7%) isolates from 531 (94.7%) CF patients were correctly identified as P. aeruginosa. Further testing with the API 20NE kit correctly identified only 34 (59%) of the misidentified isolates. Twelve (40%) patients had previously grown the misidentified species in their sputum. Achromobacter xylosoxidans (n = 21), Stenotrophomonas maltophilia (n = 15), and Inquilinus limosus (n = 4) were the species most commonly misidentified as P. aeruginosa. Overall, there were very low rates of P. aeruginosa misidentification among isolates from a broad cross section of Australian CF patients. Additional improvements are possible by undertaking a culture history review, noting colonial morphology, and performing stringent oxidase, DNase, and colistin susceptibility testing for all presumptive P. aeruginosa isolates. Isolates exhibiting atypical phenotypic features should be evaluated further by additional phenotypic or genotypic identification techniques.The accurate identification of Pseudomonas aeruginosa is a critical component of cystic fibrosis (CF) patient management. Once established within CF lungs, P. aeruginosa is rarely eradicated, leading to increased treatment requirements and an accelerated decline in pulmonary function, quality of life, and life expectancy (10, 13, 27). Emerging evidence indicates that aggressive antipseudomonal therapy at the time of initial acquisition may eliminate P. aeruginosa, preventing the development of chronic infection for months or even years (37). Similarly, separating patients with P. aeruginosa from other CF patients may reduce the spread of multiple-antibiotic-resistant strains capable of person-to-person transmission (16). Such strategies are contingent upon the early and correct identification of these organisms (30).While there is much emphasis on misidentifying P. aeruginosa as another species (39), less attention is paid to falsely identifying other species as P. aeruginosa. Nevertheless, accurate identification of P. aeruginosa is important, as this may avoid prolonged and sometimes unnecessary antibiotic treatments, which could select for other antibiotic-resistant pathogens (6). Similarly, in CF clinics where cohort isolation is practiced as an infection control measure, false identification could mean exposure of the CF patient to potentially transmissible bacteria (2, 17, 28, 33).While most clinical strains of P. aeruginosa are easily identified, respiratory isolates from patients with CF can present a taxonomic challenge (15, 24). Phenotypic identification of P. aeruginosa from patients with CF is often complicated by slow growth, auxotrophic metabolic activity, loss of pigment production, multiple antibiotic resistance, atypical colonial morphology, and development of mucoid exopolysaccharide (14, 25). Commercial identification platforms are also considered unreliable (18, 21, 39). Moreover, CF respiratory secretions may contain other nonfermenting gram-negative bacilli, such as Achromobacter, Stenotrophomonas, and Burkholderia species, which can further impede the identification of P. aeruginosa (29, 32, 35, 39).Although several molecular strategies have been developed recently (1, 35, 39), most clinical microbiology laboratories still identify P. aeruginosa by traditional phenotypic techniques. However, there are few published data describing the frequency at which bacterial species in CF sputum are falsely identified as P. aeruginosa by phenotypic methods. In this study, we used P. aeruginosa-specific duplex real-time (PAduplex) PCR assays, phenotypic analysis, amplified rRNA gene restriction analysis (ARDRA), and partial 16S rRNA gene sequence analysis to assess the rate and extent of misidentification of P. aeruginosa isolates in CF sputum by Australian clinical microbiology laboratories. |
