Molecular basis of virulence in clinical isolates of Escherichia coli and Salmonella species from a tertiary hospital in the Eastern Cape, South Africa

Background

Apart from localized gastrointestinal infections, Escherichia coli and Salmonella species are major causes of systemic disease in both humans and animals. Salmonella spp. cause invasive infections such as enteric fever, septicemia, osteomyelitis and meningitis while certain types of E. coli can cause systemic infections, including pyelonephritis, meningitis and septicemia. These characteristic requires the involvement of a myriad of virulence factors.

Methods

This study investigated the virulence factors of Escherichia coli and Salmonella species in clinical specimens from patients with diarrhoea presenting to health care centres in Oliver R. Tambo District Municipality, Eastern Cape Province, Republic of South Africa. Microbiology analysis involved the use of cultural and molecular techniques.

Results

Out of a total of 315 samples screened, Salmonella isolates were obtained in 119 (37.8%) of cases and these comprised: S. choleraesuis (6%), S. enteritidis (4%), S. eppendorf (1%), S. hadar (1%), S. isangi (8%), S. panama (1%), S. typhi (52%), S. typhimurium (25%) and untyped Salmonella spp. (2%). Among the Salmonella species 87 (73.1%) were invasive. Using molecular diagnostic methods, diarrheagenic E. coli were detected in 90 cases (28.6%): the greater proportion of this were enteroaggregative E. coli (EAEC) 37 (41.1%), enteropathogenic E. coli (EPEC) 21 (23.3%) and enterohemorrhagic E. coli (EHEC) 21 (23.3%). The predominant virulence gene among the diarrheagenic E. coli was EAEC heat-stable enterotoxin astA genes while the virulence genes identified in the Salmonella strains were 15 (12.6%) flic and 105 (88.2%) inv genes. The amino acid identity of the representative genes showed 95-100% similarity to corresponding blast searched sequence.

Conclusions

This study showed the diversity of virulence gene expression in two major enteric pathogens. S. typhi and enteroaggregative E. coli were the predominant enteropathogens in our study area with an indication that EAEC is endemic within our study population. It was observed among other things that some diarrheagenic E. coli isolated from apparently asymptomatic subjects expressed some virulence genes at frequency as high as seen in diarrheagenic cases. This study underlines the importance of understanding the virulence composition and diversity of pathogens for enhanced clinico-epidemiological monitoring and health care delivery.