Detection of public health-relevant genes in estuarine waters of Chubut River

Abstract

Shiga toxin (stx) genes encode pathogenic proteins for humans. Although these genes are found in diverse bacterial species, the most relevant epidemiological information relates to STEC (Escherichia coli producing Shiga toxin), the major cause of human gastrointestinal illness, and Haemolytic Uremic Syndrome (HUS). Water is a relevant vector for STEC infections, and the detection of stx genes in surface waters complements the use of generic E. coli as a microbial indicator of faecal pollution. We investigated pathogen pollution in the Chubut River estuary, analyzing 60 surface water samples collected from seven stations over an 18 km upstream stretch from the river mouth (2022-2023). We analysed the presence of Shiga toxin genes (stx1 and stx2) to estimate STEC and the invA gene for Salmonella presence by culture, DNA extraction, and PCR. The overall prevalence of stx genes was 11.67% (7/60), while invA gene was not detected. Faecal coliforms and E. coli counts ranged from 3 to 7.4×105 MPN/100 ml. STEC genes were detected 2 of 7 times when these counts were below the regulatory limits for recreational waters on two occasions. Stx1, stx2, and stx1-stx2 profiles were detected in 57,14%, 71,43%, and 14,29% of the STEC positive samples, respectively. The prevalence of stx genes was four times higher in the autumn-winter than in the spring-summer seasons. These results demonstrate for the first time the presence of stx genes in the Chubut River estuary. The detection of stx genes was more frequent at the Rawson sewage discharge and the subsequent downstream station, and the identified genetic profiles coincided at both stations in 2 of 12 sampling events. These findings suggest that the stx genes could come from bacteria in sewage that survive in the estuary. Finally, stx genes were not detected in the river mouth station; however, the potential risk cannot be ignored.