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Comparison of the effects of different nutrient media on production of heat-stable enterotoxin-b by Escherichia coli

Erume, J., Berberov, E.M., Moxley, R.A.
Veterinary microbiology 2010 v.144 no.1-2 pp. 160-165
swine, swine diseases, Escherichia infections, enterotoxigenic Escherichia coli, toxigenic strains, bacterial toxins, enterotoxins, biosynthesis, cell culture, culture media, microbial growth, animal proteins, brain, heart, amino acids, soybean products, nutrient availability, lincomycin, enzyme-linked immunosorbent assay
Trypticase soy broth (TSB) has been commonly used to culture Escherichia coli strains for detection or harvest of heat-stable enterotoxin-b (STb); however, in our experience, the yields have been low. In this study, we compared STb yields resulting from growth in brain heart infusion broth (BHI) supplemented with 2% casamino acids (BHI-CA) with that from TSB. Since strains may concurrently express heat-labile enterotoxin (LT) and lincomycin has been reported to cause increased expression of LT, we also compared its effects on STb production. STb⁺ clones had significantly higher production of STb when grown in BHI-CA compared to TSB, based on enzyme-linked immunosorbent assay (ELISA) conducted on cell-free supernatants. The superiority of BHI-CA to TSB was further tested on 17 porcine enterotoxigenic E. coli (ETEC) isolates, all positive for the STb gene (estB) by PCR. Cell-free supernatants of 100% of the ETEC isolates were detectably positive for STb by ELISA when grown in BHI-CA, in contrast to only 10 strains positive (59%) when TSB supernatants were analyzed. For all the samples, the amounts of STb expressed in BHI-CA based on ELISA were significantly higher than those from TSB (p ≤0.03). Incorporation of lincomycin at a concentration optimal for LT production into these media caused a significant decrease in STb levels, and use of both lincomycin and polymyxin B at these concentrations abolished detectable STb production altogether. We conclude that BHI-CA is a better medium for supporting STb production than is TSB. Culture in BHI-CA followed by testing by STb-ELISA is a useful combination for detecting STb production.