An apparent outbreak of infection with Acinetobacter calcoaceticus reconsidered after investigation by pyrolysis mass spectrometry.

In a previously reported apparent outbreak of infection due to Acinetobacter calcoaceticus on a Regional Burns Unit in which both clinical and environmental isolates were available for study, investigations of the organisms by electrophoretic typing of surface proteins, plasmid profiling and antibiograms were inconclusive and unable to identify a single epidemic strain of the organism. The same collection of isolates has been analysed for inter-strain comparison by pyrolysis mass spectrometry (PyMS). PyMS analysis suggested that a few isolates were very different from the majority but that most of the collection comprised a group of closely similar but nonetheless distinct isolates. These isolates may well be representatives of a limited variety of strains occupying an ecological niche but not yet a single emergent strain. The ability of PyMS to detect a single epidemic strain of A. calcoaceticus when present in such a collection of isolates was demonstrated by analysis of a "constructed outbreak collection", using some of the original isolates. This study illustrates the versatility of PyMS for inter-strain comparison studies of species not yet amenable to conventional typing methods. The application of rapid, highly discriminatory, high-volume inter-strain comparison methods such as PyMS to apparent outbreaks of nosocomial infection is likely to reveal patterns of organism acquisition other than point-source transmission.