Presence of Indicator Organisms and Recovery of Salmonella in Fish and Shellfish

Abstract

The efficacy of two methods for detection of Salmonella in 29 fish and 312 shellfish samples was evaluated, using replicate samples (100 g) of food homogenate. In method A, samples were preenriched 3 h in lactose broth, selectively enriched overnight in selenite cystine (35 C) and plated on brilliant green sulfa, xylose lysine desoxycholate and Hektoen enteric agar media. In method B, overnight nutrient broth cultures were enriched in tetrathionate brilliant green (43 C) and selenite cystine (35 C) broths and plated on brilliant green sulfa and bismuth sulfite agar media. Salmonella was recovered from seven (3%) shrimp samples of which six were detected by method B alone; the single positive sample detected by method A was negative by method B. Infected shrimp samples did not harbor coagulase-positive staphylococci, and aerobic plate counts ranged from 105 to 107 cells/g; two of the seven positive samples contained no detectable Escherichia coli. Our results suggest that short preenrichment incubation periods are not reliable and that tetrathionate brilliant green is superior to selenite cystine for effective recovery of Salmonella in shellfish. Coliforms are not reliable as an index of microbiological quality of fish and shellfish.