Different O and K serotype distributions among clinical and environmental strains of Serratia marcescens

Recent revision of the O serotyping scheme for Serratia marcescens has allowed the definitive serological identification of a collection of 511 epidemiologically distinct strains in terms of both lipopolysaccharide (O) antigens and capsular (K) antigens. High levels of typability were achieved, 88% and 91% respectively, with only 2% failing to ape with either method. In most cases, non-typability was due to a lack of antigen, i.e., the strains produced only rough LPS or were acapsular, suggesting that typability would be little improved by the discovery of additional serotypes. The distribution of the 58 O:K serotypes was very uneven, with O14:K14 accounting for 30% of the 423 clinical strains in the collection, but only 5% of the 88 non-clinical, environmental strains. Thus, the prevalence of O14:K14 strains in hospitals is not reflected in the environment. Similar conclusions were valid for O27:K14, O21:K3 and O21:K14 strains, as well as those with rough lipopolysaccharide. Conversely, the proportions of O6:K3, O6:K14, O8:K14 or O28:K28 strains were significantly lower among the clinical collection than among their environmental counterparts (12% in total rather than 65%). This suggests that O14:K14 may have a selective advantage in colonising or infecting hospitalised patients and, therefore, that the O14 and K14 polysaccharides themselves may contribute towards the apparent pathogenicity of these serotypes.