title: Detection of Escherichia coli colonies on confluent plates of chromogenic media used in membrane filtration.
abstract: MI agar (MI), Chromocult® Coliform agar ES (Chromocult), and DC with BCIG agar (DC) are chromogenic membrane filtration culture-based methods used to assess microbiological water quality. In this study, their ability to detect Escherichia coli colonies on confluent growth plates was determined by testing water samples containing increasing concentrations of a non-E. coli growing bacterium, Citrobacter youngae. Then, their ability to inhibit the growth of non-coliform bacteria was determined by simultaneously testing 603 well water samples. Results were compared with those obtained with mFC and Colilert® methods. Results showed that the E. coli count was difficult to determine on mFC, Chromocult and DC when non-E. coli colonies reached levels of 10(4)colony forming units (CFU)/100 mL. However, the E. coli count did not interfere with Colilert until non-E. coli colonies reached concentrations of 10(7)CFU/100 mL. No inhibition was observed with MI as E. coli colonies could be easily detected in the presence of at least 10(7)CFU/100 mL of C. youngae. Using well water samples, confluent growth plates were observed for 144, 177, and 185 of the 603 well water samples tested with the MI, Chromocult and DC methods, respectively. Among these confluent growth plates, E. coli colonies were not detected for 10, 20, and 31 water samples. However, they were detected by the mFC and/or Colilert methods. Thus, among the three methods tested, the MI method presented the lowest grow rate of atypic colonies and was the only one that presents no interference in the E. coli count.
authors.affiliation: Chaire de recherche en eau potable de l'Université Laval, Québec City, Quebec, Canada; École supérieure d'aménagement du territoire et de développement régional, Université Laval, Québec City, Quebec, Canada. Electronic address: andree.maheux.3@ulaval.ca.
authors.forename: Andrée F
authors.initials: AF
authors.lastname: Maheux
authors.affiliation: Chaire de recherche en eau potable de l'Université Laval, Québec City, Quebec, Canada; Centre de recherche en aménagement et développement de l'Université Laval, Québec City, Quebec, Canada.
authors.forename: Vanessa
authors.initials: V
authors.lastname: Dion-Dupont
authors.affiliation: Chaire de recherche en eau potable de l'Université Laval, Québec City, Quebec, Canada; Centre de recherche en aménagement et développement de l'Université Laval, Québec City, Quebec, Canada.
authors.forename: Marc-Antoine
authors.initials: MA
authors.lastname: Bisson
authors.affiliation: Chaire de recherche en eau potable de l'Université Laval, Québec City, Quebec, Canada; Centre de recherche en aménagement et développement de l'Université Laval, Québec City, Quebec, Canada.
authors.forename: Sébastien
authors.initials: S
authors.lastname: Bouchard
authors.affiliation: Chaire de recherche en eau potable de l'Université Laval, Québec City, Quebec, Canada; École supérieure d'aménagement du territoire et de développement régional, Université Laval, Québec City, Quebec, Canada.
authors.forename: Manuel J
authors.initials: MJ
authors.lastname: Rodriguez
mesh_terms: D002954:Citrobacter / Q000254:growth & development; D015169:Colony Count, Microbial; D003470:Culture Media; D004926:Escherichia coli / Q000235:genetics / Q000502:physiology*; D005374:Filtration; D008828:Microbiological Techniques / Q000379:methods* / Q000592:standards; D014871:Water Microbiology
chemical_list: D003470:Culture Media
keywords: Chromocult coliform ES agar; DC with BCIG agar; E. coli; MI agar; Total coliforms