Use of API NH system for identification of Moraxella catarrhalis.

Barbe and coworkers tested 318 isolates of Haemophilus and Neisseria species as well as Moraxella catarrhalis in an evaluation of the API NH kit in a study primarily addressing the ability of that kit to accurately identify organisms in its intended area of application (1). They found that only 1 of the 305 strains that were included in the kit database was identified incorrectly, and of the 13 isolates tested that were not in the database, 3 were not identified and the rest were incorrectly identified. It is unfortunate that strains of Moraxella species were not included in the study, because a problem that we have encountered may have come to light (8). We have previously described the identification of an isolate of Moraxella osloensis as M. catarrhalis with this kit. Since then, we have seen two further genital tract isolates of Moraxella species also misidentified as M. catarrhalis. The tributyrin test is important for identification of M. catarrhalis (5), and the lipase cupule in the API NH kit may detect the enzyme responsible for tributyrin hydrolysis (7). The authors recognize that a positive lipase test does allow M. catarrhalis to be differentiated from Neisseria and Haemophilus species. Unfortunately, it is not widely recognized that many other Moraxella species also produce an enzyme capable of hydrolyzing the triglyceride tributyrin (3). Although any laboratory test has the potential to give misleading results, the dangers that this poses to laboratory workers is often underemphasized. One of us has previously worked with a blood culture isolate of Brucella melitensis which was designated Moraxella phenylpyruvica by a similar gallery strip test kit (the API 20E system) before its final identity was recognized (6). The authors emphasize the importance of using a heavy standardized suspension to inoculate the cupules. Potential users of the kit should be aware of the risks that this may pose to operators, especially as the kit is intended for use with organisms capable of causing infections by the airborne route. Reports of staff acquiring meningococcal infections (and brucellosis), probably by exposure in this manner, have already been documented (2, 4). In the past, identification tests and kits deemed suitable for M. catarrhalis have been evaluated against isolates of this and Neisseria species. It would seem appropriate to include strains of Moraxella species when tests aimed at identifying M. catarrhalis are evaluated.