Development of a Phage-based Typing System to Use in Combination with Multi-locus Variable Number of Tandem Repeat Analysis (MLVA) to Differentiate Escherichia coli O157:H7 Isolates

Introduction: The identification of the dissemination routes of Escherichia coli O157:H7 on farms and in different food production systems is an important step in controlling this pathogen. It is often difficult to differentiate E. coli O157:H7 isolates, however, due to clonal similarity within the strain.

Purpose: Here we combined phage-typing with multi-locus variable number of tandem repeat analysis (MLVA) in attempt to develop a more powerful protocol for identifying specific O157:H7 isolates.

Methods: Forty-eight E. coli O157:H7 isolates from various sources (e.g., cattle manure, ground beef, human fecal samples, etc.) were analyzed by phage-typing and MLVA. MLVA was performed by comparing the number of tandem repeats at seven loci. For phage typing, 16 phages were isolated from wastewater treatment facilities throughout the State of Indiana. Five levels of lytic activity were determined based on plaque formation characteristics.

Results: MLVA was able to differentiate the isolates into 26 MLVA types. Phage-typing was able to differentiate the test isolates into 27 phage types. Importantly, phage-typing was able to differentiate isolates with the same MLVA-type from one another based on a combined MLVA- and phage-type.

Significance: Thus, these data indicate that combining these two techniques, both of which are rapid and require little labor, could result in a higher capacity to differentiate very similar E. coli O157:H7 isolates. Thus, they could be powerful tools in identifying critical sources of E. coli O157:H7 transmission in different food systems.