Production of haemolysis and its correlation with enterotoxicity in Aeromonas spp

A total of 147 clinical and environmental isolates of Aeromonas that included 14 A. hydrophila, 60 A. sobria and 73 A. caviae strains was tested for haemolysin production and its correlation with enterotoxicity; 108 isolates produced beta-haemolysis. For A. hydrophila and A. sobria, titres of haemolysin were 16-128 HU/ml and for A. caviae, 16-64 HU/ml. In the ileal loop test, 82 (55.8%) strains of Aeromonas spp. produced enterotoxin. Of the beta-haemolytic strains, 72.7% of A. hydrophila, 58.6% of A. sobria and 68.6% of A. caviae isolates caused fluid accumulation in rabbit ileal loops. One strain each of alpha-haemolytic A. sobria and A. caviae, one of non-haemolytic A. sobria and nine of non-haemolytic A. caviae also caused a secretory response. The beta-haemolytic strains caused significantly more (p < 0.05) fluid accumulation than the alpha- and non-haemolytic isolates regardless of their species designation. The remaining 65 (44.2%) isolates belonging to the three species included alpha-, beta- and non-haemolytic strains: they failed to cause fluid accumulation in the initial experiments but did so after one to three consecutive passages through rabbit ileal loops. Two alpha- and 13 non-haemolytic strains switched to production of beta-haemolysis when they showed positive ileal loop reactions. However, on repeated subcultures or on storage in the laboratory, all of them reverted to their original haemolytic character and no longer produced enterotoxic activity.