Metabolic Profiling of Non-O157:H7 Shiga Toxin-producing Escherichia coli Isolated from Spinach

Introduction: Non-O157:H7 Shiga toxin producing Escherichia coli are a fast rising food safety threat resulting in the contamination of beef and produce.

Purpose: Sugar metabolism, amino acid metabolism and substrate utilization of 6 isolates of non-O157:H7 Shiga toxin producing E. coli isolated from spinach were assayed using a biochemical profiling strip and an enterobacterial profiling test strip.

Methods: Shiga toxin producing E. coli strains O181:H49, O91:H21, O113:H21, O98:H36, O116:H21, O11:H15 were tested for their indole, methyl red, Voges-Proskauer and citrate profiles. Carbohydrate metabolism traits were assayed for the following sugars: lactose, xylose, maltose, fructose, dextrose, galactose, raffinose, trehalose, melibiose, sucrose, L-arabinose, mannose, inulin, sodium gluconate, glycerol, salicin, dulcitol, inositol, sorbitol, mannitol, adonitol, arabitol, erythritol, alpha-methyl-D-glucoside, rhamnose, cellobiose, melezitose, alpha-methyl-D-mannoside, xylitol, ONPG, esculin, D-arabinose, citrate, malonate and sorbose. Lysine, ornithine, urease, nitrate and H₂S reactions were also assayed. These were performed by streaking test organisms on Tryptic Soy Agar plates and incubating at 37°C for 24 h. Suspension of isolates were prepared in Phosphate Buffer Saline (PBS) and inoculated into test ampules as per manufacturer’s recommendation. Biochemical tests were conducted after 24 h incubation using appropriate reagents and protocol suggested by kit manufacturer.

Results: Differences in biochemical and amino acid metabolic profiles were observed between the isolates. E. coli strains O11:H15 and O91:H21 were negative for ornithine metabolism while all other isolates were positive. E. coli strain O116:H21 was positive for phenylalanine metabolism while all other isolates were negative. All isolates showed typical positive reactions for indole and methyl red and Voges-Proskauer tests. Differences in citrate and saccharose metabolism was observed between the isolates. E. coli O98:H36 was negative for raffinose metabolism while all other isolates were positive.

Significance: Difference in sugar, amino acid and citrate metabolism traits could confer differential abilities to persist in environmental and food matrices.