A French military surgeon in 1850 was the first to describe an infection with Pseudomonas aeruginosa when he discovered blue pus in the dressings of wounded soldiers.1 The colour resulted from the secretion by the bacterium of its characteristic pigment pyocyanin. By 1984 more than 102 different species were included in the family Pseudomonadaceae , most of which were plant pathogens or soil saprophytes. All are straight or curved Gram negative rods that are motile by polar flagella. Most are strictly aerobic, can grow in temperatures from 4–43°C and are usually oxidase positive. Application of DNA technologies to microbial taxonomy has led to further divisions within the Pseudomonadaceae . This has resulted in a proliferation of “new” genera and species much to the consternation of clinicians and clinical microbiologists alike. The taxonomic tool used most frequently is analysis of 16S and 23S ribosomal RNA (rRNA) cistrons. These regions are relatively well conserved. For example, it is estimated that the average substitution (mutation) rate for 16S rRNA within a particular eubacterium is 1% per 50 million years.2 However, there is sufficient variability to permit delineation of genera and species. This analysis can be done by DNA/DNA hybridisation but is currently most often achieved by polymerase chain reaction (PCR) amplification of a large portion of the 16S rRNA gene and sequencing the amplicon; Pseudomonadaceae have thus been split into five rRNA groups (table 1). View this table: Table 1 Medically important pseudomonads The most important human pathogen in this group is P aeruginosa . It has a ubiquitous distribution in the environment and can adapt to living and multiplying in such unpromising habitats as distilled water and disinfectants. Sinks, taps, and drains in hospitals are always colonised but these isolates seem rarely to infect patients. It is not normally part of the skin, pharyngeal …