Bacterial Flora and Antimicrobial Resistance in Raw Frozen Cultured Seafood Imported to Denmark

Intensified aquaculture includes the use of antimicrobials for disease control. In contrast to the situation in livestock, Escherichia coli and enterococci are not part of the normal gastrointestinal flora of fish and shrimp and therefore not suitable indicators of antimicrobial resistance in seafood. In this study, the diversity and phenotypic characteristics of the bacterial flora in raw frozen cultured and wild-caught shrimp and fish were evaluated to identify potential indicators of antimicrobial resistance. The bacterial flora cultured on various agar media at different temperatures yielded total viable counts of 4.0 x 10(4) to 3.0 x 10(5) CFU g(-1). Bacterial diversity was indicated by 16S rRNA sequence analysis of 84 isolates representing different colony types; 24 genera and 51 species were identified. Pseudomonas spp. (23% of isolates), Psychrobacter spp. (17%), Serratia spp. (13%), Exiguobacterium spp. (7%), Staphylococcus spp. (6%), and Micrococcus spp. (6%) dominated. Disk susceptibility testing of 39 bacterial isolates to 11 antimicrobials revealed resistance to ampicillin, amoxicillin-clavulanic acid, erythromycin, and third generation cephalosporins. Resistance to third generation cephalosporins was found in Pseudomonas, a genus naturally resistant to most beta-lactam antibiotics, and in Staphylococcus hominis. Half of the isolates were susceptible to all antimicrobials tested. Results indicate that identification of a single bacterial resistance indicator naturally present in seafood at point of harvest is unlikely. The bacterial flora found likely represents a processing rather than a raw fish flora because of repeated exposure of raw material to water during processing. Methods and appropriate indicators, such as quantitative PCR of resistance genes, are needed to determine how antimicrobials used in aquaculture affect resistance of bacteria in retailed products.