We have previously described two E.Coli heat stable (ST)-like toxins. The first, produced Citrobacter freundii (Cf), has the same 18 amino acid sequence of E. Coli STIa. 1) The second, produced by Klebsiella pneumoniae (Kp) is similar, but not identical to E.Coli STIa and to Cf ST in that it does not react with monoclonal antibodies raised against pure E.Coli STIa. 2) To see whether transfer of toxigenic ability may occurr between different bacterial species we have performed conjugal bacterial mating experiments. We used ST positive Cf (strain AG55) as the donor and the following ST negative recipients: 1) E.Coli, 2) Kp P89 and 3) Kp strain AL55- (which had spontaneously lost the ability to elaborate the ST-like toxin). Mixed cultures were incubated for 2 hours at 37 C and then plated onto XLD agar containing an antibiotic to which only Cf was sensitive. Grown colonies were individually tested for ST production by an ELISA test with monoclonal antibodies and by the classic suckling mouse assay (SMA).Results: both E.Coli and KpP89 acquired the ability to produce the toxin, as indicated by the positivity of both SMA and ELISA. The estimated transfer frequency for E.Coli was 9×10?2 matings. KpAL55- showed positivity in the SMA but not in the ELISA, thus showing that it had acquired the ability to elaborate a toxin somehow different from that of the donor. To assess whether the acquired toxigenic ability was itself transmissible, the new ST+E.Coli was incubated with an ST-Cf. The latter became ST+.In conclusion: 1) Different enterobacteria may trade ST plasmids by conjugation; 2) the ability to produce ST is acquired together with the ability to infect other bacteria; 3) selected Klebsiella strains acquire the capability of producing ST, which is successively modified through a post-conjungative, yet unknown, mechanism.1) Guarino A et al. J Clin Microbiol 25:110-114, 19872) Guarino A et al. Ped Res 25:514-518, 1989
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