Fecal indicator organisms (FIOs) — primarily E. coli and Enterococci — are bacteria that originate in the intestines of warm-blooded animals. Their presence in water signals fecal contamination and the potential presence of pathogens such as Salmonella, Norovirus, or Cryptosporidium. Monitoring FIOs is the standard approach to assessing microbiological water safety under WHO guidelines and the EU Drinking Water Directive.