Rapid diagnosis of "Escherichia coli" heat labile enterotoxin (LT) by a coagglutination test

Abstract

Protein A positive Staphylococcus aureus cells, strain Cowan 1, were coated with a high titer anti-LT serum produced in rabbits. Purified E. coli heat labile enterotoxin (LT) was detected in a slide coagglutination (coa-LT) test with anti-LT coated staphylococci at the nanogram level (0.1-1 ng) while cholera toxin (CT) was detected only at much higher concentrations (10-50 ng). This LT immunoassay was evaluated in different modifications on enterotoxigenic E. coli (ETEC) isolated from humans and animal stool cultures. Colonies from primary stool cultures on blood agar were suspended and lysed in saline containing polymyxin B and a detergent (Triton X100), centrifuged and assayed for LT. The coa-LT and CHO-cell tests regularly detected LT among human LT producing ETEC, while ETEC strains from piglets and calves usually gave only weak reactions in the standard coa-LT test. We conclude that this coa-LT test is accurate and sensitive enough to be used in routine diagnosis of LT producing ETEC strains in stools from human patients. This test requires no special laboratory equipment and is well suited for rapid diagnosis of ETEC in small hospital laboratories and in developing countries. Heat stable enterotoxin (ST) giving a positive suckling mouse test, has been purified by hydrophobic interaction chromatography (HIC) on Octyl Sepharose. Purified ST was covalently linked to different proteins and such carrier complexes used for immunization of rabbits. The ST antibody titers obtained by various ST-carriers were compared in a ST coagglutination test (ST-coa) and in a conventional ELISA microtiter assay. The results and detection limits for ST in polymyxin-detergent cell lysates in human ETEC strains producing LT-ST or ST only will be presented.