Effect of sampling method on the representative recovery of microorganismsfrom the surfaces of aquacultured finfish

The objective of this study was to determine if a gentle rinse procedure wa s equivalent to the combination of excision and homogenization with a stoma cher for the relative removal of various microorganisms from finfish fillet s. Fillets of hybrid striped bass and rainbow trout were obtained from loca l markets and sampled using three methods: rinse (R), excision followed by homogenization in a stomacher (S), and homogenization of fillets following a rinse (RS). Microorganisms were enumerated on selective and nonselective media, and randomly selected colonies from aerobic plate counts were identi fied using MIDI Sherlock and BIOLOG microbial identification systems. Enric hments and selective media were used for the isolation of Listeria monocyto genes, Salmonella spp., and Yersinia enterocolitica. This study confirms pr evious reports that stomaching is superior to rinsing for enumerating total microbial populations from fish fillets. Rinsing was more effective for ra inbow trout than for striped bass. Sampling method did not affect the relat ive magnitude of plate counts on media selective for aeromonads, pseudomona ds, Shewanella, lactic acid bacteria, enterics, and gram-positive cocci. In the compositional analysis of random isolates, R recovered significantly l ower fractions of aeromonads than did S or RS, but sampling method did not affect the percent recovery of lactic acid bacteria, pseudomonads, Shewanel la, Moraxellaceae, or Cytophaga/Flavobacterium. However, observations sugge st that with increased replication, differences among Moraxellaceae, Pseudo monas, and gram positives might be significant. Only one L. monocytogenes c olony was isolated, and no Salmonella or Y. enterocolitica, so the effect o f sampling method could not be determined for these organisms. Differences in predominant bacterial populations were seen between fish species.