The types of selective pressure operating on the outer membrane protein C (ompC) of Enterobacter aerogenes strains, the causative agent for nosocomial infections, and Salmonella sp., the hazardous pathogen are investigated using the maximum likelihood-based codon substitution models. Although the rate of amino acid replacement to the silent substitution (ω) across the entire codon sites of ompC of E. aerogenes (ω=0.3194) and Salmonella sp. (ω=0.2047) indicate that the gene is subjected to purifying selection (i.e. ω<1), approximately 3.7% of ompC codon sites in E. aerogenes (ω=21.52) are under the influence of positive Darwinian selection (i.e. ω>1). Such contrast in the intensity of selective pressures in both pathogens could be associated with the differential response to the adverse environmental changes. In E. aerogenes, majority of the positively selected sites are located in the hypervariable cell-surface-exposed domains whereas the trans-membrane domains are functionally highly constrained.
