Haemophilus influenzae is an important cause of respiratory tract infections and certain invasive infections, such as epiglotitis and meningitis, in humans. A MLST scheme based on seven housekeeping gene fragments (adk, atpG, frdB, fucK, mdh, pgi, recA) has been available for 10 years, and more than 1100 Sts have been described. Commensal, non-pathogenic Haemophilus haemolyticus may be difficult to discriminate from H. influenzae, but the absence of the gene encoding fuculokinase (fucK) is a characteristic of H. haemolyticus. However, previous studies have reveled singular isolates of H. influenzae where the fucose operon is partly or entirely deleted; the MLST scheme, that was developed for the unambiguous characterization of H. influenzae, is unable to assign a sequence type to such strains which may erroneously be designated "non-hemolytic H. haemolyticus".
In this study we analyzed a previously described collection of 58 borderline strains [1], of which 34 were negative for fucK. Sequence analysis based on the six other gene fragments of the MLST scheme showed that all fucK-positive and 15 of the 34 fucK-negative strains belonged to H. influenzae, while the remaining 19 fucK-negative strains belonged to H. haemolyticus. MLSA based on the six genes showed that the 15 fucK-negative H. influenzae strains were all located in a distinct cluster in MLST-phylogenetic group II, closely related to H. haemolyticus. This cluster also contained a number of STs with intact fucK genes from the MLST database. The extent of the fucose operon deletion in these strains is presently being characterized. The reason why this operon is deleted in H. haemolyticus and related strains is not clear, but recent data have indicated frequent interspecies recombination between the two species.