Streptococcus pseudopneumoniae (S. pseudopneumoniae) has been described as an emerging pathogen in respiratory tract infections. First identified in Canada in 2004, S. pseudopneumoniae has primarily been found in patients with chronic obstructive pulmonary disease (COPD) and aspiration pneumonia. It is an alpha haemolytic streptococcus belonging to the mitis group of viridans streptococci. Typically appearing as small (<1 mm), smooth, domed colonies with some containing depressed centres which resemble a smaller version of S. pneumoniae, colonies of S. pseudopneumoniae may be overlooked as normal flora of the respiratory tract.
Identification of the mitis group of streptococci can be challenging as members of this group share a high level of genetic homology and are often biochemically inert. The lack of sensitive and specific automated and commercial assays available for identification within this group may pose difficulties for the clinical microbiology laboratory to consider this relatively under-investigated organism as a potential pathogen.
We discuss the identification of four isolates of S. pseudopneumoniae from patient respiratory specimens in our laboratory and methods used to distinguish the organism from S. pneumoniae and S. mitis. We outline a simple algorithm for isolation and identification of S. pseudopneumoniae which should improve detection and lead to a better understanding of the organism’s role in clinical infections.