An evaluation of fluorogenic and chromogenic assays for the direct enumeration of Escherichia coli

Microbial source tracking of private well water samples across at-risk regions in southern Ontario and analysis of traditional fecal indicator bacteria assays including culture and qPCR

Many people living in rural areas rely on privately owned wells as their primary source of drinking water. These water sources are at risk for fecal contamination of human, wildlife, and livestock origin. While traditional bacteriological testing involves culture-based methods, microbial source tracking (MST) assays present an opportunity to additionally determine the source of fecal contamination. This study investigated the main host sources of contamination in private well water samples with high levels of Escherichia coli (E. coli), using MST with human and multi-species specific markers. Fecal contamination of human origin was detected in approximately 50% of samples, indicating that current contamination prevention strategies require reconsideration. The relationship between cattle density and fecal contamination of bovine origin was investigated using a Bovine Bacteroidales specific MST assay. Regional variations of microbial sources were examined, and may inform local primary prevention strategies. Additionally, in order to assess MST and E. coli quantitative real time polymerase chain reaction (qPCR) assays as indicators of fecal contamination, these were compared to E. coli culture methods. Variation in results was observed across all assay methods investigated, suggesting the most appropriate routine bacteriological testing methodology cannot be determined without comparison to a method that directly detects the presence of fecal contamination.

Use of 8-hydroxyquinoline-beta-D-glucuronide for presumptive identification of Shiga toxin-producing Escherichia coli O157

8-hydroxyquinoline-beta-D-glucuronide (HQG) was used to improve the presumptive identification of Shiga toxin-producing Escherichia coli O157 (STEC O157) on sorbitol MacConkey agars (SMAC). Advantages of HQG are (i) that it is less expensive than 5-bromo-4-chloro-3-indoxyl-glucuronide; (ii) that it is visible in normal daylight and (iii) that it does not diffuse into the agar like 4-methylumbelliferryl-beta-D-glucuronide (MUG). Sixteen STEC O157 isolates, 91 bovine mastitis-associated E. coli isolates and 222 faecal E. coli isolates from apparently healthy cattle were used in this study. 4-methylumbelliferryl-beta-D-glucuronide detected beta-glucuronidase activity in more isolates than HQG (P < 0.05). On SMAC with HQG, cefixime and tellurite all STEC O157 isolates grew as cream-coloured colonies (100% sensitivity), whereas all non-STEC O157 E. coli except one grew either not at all or as purple or black colonies (99.7% specificity). No difference was found between faecal and mastitis isolates for the proportion of isolates that hydrolysed HQG or MUG or fermented sorbitol. However, significantly more mastitis isolates were able to grow in the presence of the cefixime-tellurite supplement. 8-Hydroxyquinoline-beta-D-glucuronide is a useful substrate for the identification of STEC O157 on SMAC.

An interlaboratory study to find an alternative to the MPN technique for enumerating Escherichia coli in shellfish

Nine laboratories in eight countries tested 16 batches of common mussels (Mytilus edulis) over a 32 week period in order to find an alternative to the Most Probable Number (MPN) technique to enumerate E. coli. The alternatives investigated included the 3M Petrifilm system, the Merck Chromocult agar method and a Malthus conductance technique. The Petrifilm was found to be unsuitable and was subsequently dropped from the trial. After 669 analyses, a correlation of 0.83 was observed for log E. coli counts between the MPN and Chromocult methods and there was no significant evidence that either method tended to give higher readings than the other. The MPN was slightly better than the Chromocult method for repeatability but the Chromocult was slightly better for reproducibility. However, the observed differences are probably too small to be of practical importance. On the basis of these data therefore, the two methods appear equally suitable for E. coli enumeration in shellfish. There were poor correlations between these methods and the Malthus technique. A small but significant number of samples tested positive on the Malthus instrument but were recorded negative on the MPN and Chromocult tests. Subsequent analysis positively identified E. coli from these Malthus assays. After statistical analysis, errors were noted in both the MPN and Chromocult methods but it was found that there would be no statistical differences if the Chromocult agar were used as an alternative to the MPN technique.

Detection of Escherichia coli in potable water using direct impedance technology

Direct impedance measurement utilizing a medium previously described as being specific for Escherichia coli and which contains trimethylamine N-oxide (TMAO) and glucuronic acid was used to detect E. coli in water samples. The system was compared with the Colilert presence/absence test and the United Kingdom standard membrane filtration technique using membrane lauryl sulphate broth. The impedance method correlated well with both the traditional membrane method (93%) and the Colilert method (93.95%) for a number of different water types. No interference from Citrobacter spp. (as reported in previous studies) was detected in this study although some Salmonella spp. did give false-positive results. The data presented here suggest that the use of direct impedance may offer an alternative to conventional methods for the detection of E. coli in water.

Evaluation of CPS ID2 medium for detection of urinary tract bacterial isolates in specimens from a rehabilitation center

CPS ID2 medium (bioMérieux) enables the presumptive identification of Escherichia coli and enterococci as well as the detection of indologenous or nonindologenous Proteeae and bacteria belonging to the Klebsiella, Enterobacter, and Serratia group with a specificity ranging from 98 to 100%. When the cultures were polymicrobial, the sensitivity varied from 70 to 97%, and the sensitivity varied from 97 to 100% when they were monomicrobial.

Application of the polymerase chain reaction to the identification of Escherichia coli and coliforms in water

The polymerase chain reaction (PCR) has been applied to the identification of Escherichia coli and coliforms after overnight growth using two sets of primers described previously. The primer set for E. coli, which was derived from the uid A gene, correctly identified all E. coli strains tested. The sequence was also identified in five non-E. coli coliforms. The coliform primer set correctly identified approximately 70% of the coliforms tested. We conclude that PCR can be used for the rapid identification of E. coli using the primers described here but that further work is required accurately to define a new primer set for the coliform group.

Enumeration of Escherichia coli in cooked and raw meats by ion mobility spectrometry

A novel method for enumerating Escherichia coli in foods is described. It is based on the production of o-nitrophenol (ONP) from o-nitrophenyl-beta-D-glucuronide as detected by an ion mobility spectrometer. The time taken for the detection of ONP in a food sample was inversely proportional to the logarithm of the initial E. coli population. Escherichia coli could be detected at a level of 10 g-1 within 9 h.