Comparison of sandwich-ELISA and GM1-ELISA for the detection of Escherichia coli thermolabile enterotoxin.

Two different microtiter plate ELISA tests were devised for the detection of Escherichia coli thermolabile toxin (LTh) either free or extracted from isolated colonies. Both tests used as detection systems purified anti-LTh rabbit immunoglobulins conjugated to biotin, streptavidin peroxidase and TMB. The tests differed by their capture phase which was the GM1-ganglioside for GM1-ELISA and purified anti-LTh rabbit immunoglobulins for sandwich ELISA. The two methods were rapid since they could be performed in less than 2 hours. The detection limits for purified LT were 50 pg/ml and 1.3 ng/ml for sandwich ELISA and GM1-ELISA respectively. For the detection of toxinogenic isolates the extraction buffer containing Triton X-100 was always superior to polymyxin buffer. Using the polymyxin extraction buffer the sandwich ELISA was again more sensitive than the GM1-ELISA since a lower number of isolated colonies could be used for the detection of positive strains. With the Triton X-100 buffer both ELISAs could detect positive strains using a single colony but the sandwich ELISA gave the highest delta OD. We concluded that our sandwich ELISA can rapidly detect either the free Escherichia coli thermolabile toxin or LTh producing strains and could be applied routinely.