Rapid detection by a coagglutination test of heat-labile enterotoxin in cell lysates from blood agar-grown Escherichia coli.

Colonies of Escherichia coli from blood agar plates were suspended and lysed in saline containing polymyxin B and a detergent (Triton X-100). The lysates were assayed for heat-labile enterotoxin (LT) by a coagglutination test (coa-test). The coa-test reagent consisted of Formalin-treated and heat-killed cells of Staphylococcus aureus, strain Cowan 1, sensitized with a high-titer rabbit anti-LT serum. Purified LT was detected in the coa-test at the nanogram level (2 to 5 ng), whereas larger amounts of cholera toxin (50 ng) were required to give a positive test. The coa-test was compared with the CHO cell test for detection of LT among E. coli strains isolated from human and animal stool cultures. The results of the coa-test and the CHO cell test correlated with 63 of 67 strains of human origin. Six of nine animal strains, defined as LT positive by the CHO cell test, gave positive results in the coa-test. We conclude that the coa-test is probably accurate and sensitive enough to be used in routine diagnosis of LT-producing E. coli strains isolated from human stool cultures. No special laboratory equipment is required, which makes the coa-test suited for diagnosis of enterotoxigenic E. coli in small hospital laboratories and in developing countries.