Pathogen Factors
Pathogenesis and epidemiology of NSTIs caused by E. coli are not well studied in comparison to uropathogenic E. coli . There are two methods to genetically classify E. coli . The conventional method for classification of E. coli is by phylogenetic group. Recently, however, another classification method for E. coli has been developed. The MLST method characterises isolates of bacterial species using the sequences of internal fragments of 7 house-keeping genes. In this case, the causative E. coli strain was ST127 in MLST and virulent phylogenetic group B2. Recently, ST127 was recognised as a pathogen in urinary tract infection (9,10). This strain was also reported as a pathogen in necrotising enterocolitis in preterm infants (11) and in bloodstream infections (12). What is notable about this strain is that, despite having high antibiotic sensitivity, it possesses many pathogenic genes. However, the pathogenesis of the ST127 strain in the setting of skin infection has yet to be studied.
In this case, it was also possible to identify the genes of the pathogen. Virulence factors such as adhesion, toxins, iron acquisition and invasion were analysed. The virulence patterns of the strains from the skin and soft tissue infection in this patient were similar on whole-genome analysis to strains from uropathogenic infection and sepsis (13); however, the available epidemiologic data with a large number ofE. coli strains was insufficient. The virulence factors suspected of having an association with skin and soft tissue infection were cytotoxic necrotising factor 1 and hemolysin (4,14). These toxins were both positive in this patient, and were reported in other cases to trigger inflammation during infection (15,16) as a result of interactions with neutrophils and monocyte-derived dendritic cells. The patient in this case had a weakened immune system caused by liver cirrhosis, which may have altered the appropriate immune response to interaction with these toxins.