Teresa-María López-Díaz

Bacterial microflora of wild brown trout (Salmo trutta), wild pike (Esox lucius), and aquacultured rainbow trout (Oncorhynchus mykiss)

Initial numbers of bacteria associated with wild (brown trout and pike) and cultured (rainbow trout) freshwater fish as well as with the water in which they were caught were determined. Subsequently, a total of 979 randomly selected isolates were characterized and identified to the genus level. For all counts performed (aerobes, psychrotrophs, anaerobes, Enterobacteriaceae, and enterococci), no significant differences were observed in water samples, the highest level corresponding to psychrotrophs in pike environments (4.23 X 10(3) CFU/ml). Overall, the skin and intestinal content of brown trout were the most contaminated, while rainbow trout specimens (gills and gut) yielded the lowest numbers. For all bacterial groups, pike gills had the highest numbers. Counts for all of the sampling sites compare well with findings in other temperate geographical environments. Biological characteristics (feeding and skin properties) and the use of antimicrobials in aquaculture might have influenced these results. Motile and nonmotile aerobic gram-negative bacteria together with Enterobacteriaceae accounted for 50 to 70% of the psychrotrophs isolated from water. Micrococcaceae, lactic acid bacteria, Bacillus, and coryneforms were also found. The groups represented in psychrotrophic isolates from the outer surfaces do not reflected those detected in water, so it was common that those organisms recovered in significant numbers from fish were not detected in surrounding habitat of the fish. Motile aeromonads and Carnobacterium were the dominant psychrotrophs in the guts of pike and brown trout, respectively. The intestinal content of reared fish gave a high incidence of Bacillus and coryneforms, while Enterobacteriaceae was absent. Again, rearing practices could have influenced this finding. Listeria monocytogenes was not detected in any of the examined samples. Two strains of Salmonella, which belonged to the same serovar and lysotype, were recovered from pond-water samples taken from one facility on different sampling days. From the gut of a pike specimen and from the pikes environment, two Plesiomonas shigelloides strains of different serovars were recovered. These latter four strains were resistant to a considerable number of antimicrobial compounds (multiple antibiotic resistance indices > 0.2).

Yeast populations on Spanish fermented sausages

Yeast populations on 24 lots of Spanish fermented sausages, made by four factories (F1, F2 and F4, artisanal; F3, industrial) were investigated throughout manufacture and the influence of different variables evaluated. In addition, 41 yeast strains were identified at the species level using two miniaturised systems: ATB32C (API System) and Vitek Yeast Biochemical Card (Vitek YBC). Levels of yeasts found in the sausage mixture (mean counts around 4 log units/g) were similar to those described by other authors. In sausages from factories F1 and F2, a further increase was noted, reaching 5.5 log units/g after fermentation. Counts subsequently decreased to 3.6 and 5 log units/g, respectively. In sausages from factories F3 and F4, decreasing counts were observed from the beginning, particularly in sausages from F3, where yeasts were almost absent in the finished product. Type of manufacture and sausage diameter, were the variables most influencing yeast counts. Debaryomyces hansenii (teleomorph of C. famata) was the dominant species, being found at all stages of manufacture. Trichosporon ovoides (formerly T. beigelii), Yarrowia lipolytica (perfect form of C. lipolytica), C. intermedia/curvata, C. parapsilosis, C. zeylanoides and Citeromyces matritensis (teleomorph of C. globosa) were also present. Direct identification was possible only with 50% of the total of strains investigated, although a higher number of strains was identified using the API than the Vitek YBC system.

Mycotoxins in two Spanish cheese varieties

Samples of cheeses naturally contaminated with moulds (12 samples of mouldy Manchego cheese and 10 of a naturally ripened blue cheese) were analysed for the presence of mycotoxins (aflatoxins BI and MI, sterigmatocystin, patulin, penicillic acid and mycophenolic acid in Manchego cheese, and mycophenolic acid and roquefortine in blue cheese). In addition, 24 Penicillium and Aspergillus strains isolated from the samples were assessed for their mycotoxigenicity. Four of Manchego cheese samples were positive to mycophenolic acid and one sample of blue cheese contained roquefortine. The rest of mycotoxins investigated were not found. One Aspergillus strain isolated from Manchego cheese showed the ability to produce aflatoxin MI. The rest of strains from these samples being no producers. In contrast, 7 out of 9 Penicillium (P. roqueforti) strains isolated from blue cheese were able to produce roquefortine, with one strain also producing mycophenolic acid.

Some technological properties of Penicillium roqueforti strains isolated from a home-made blue cheese

Nine strains of Penicillium roqueforti isolated from a traditional Spanish blue cheese (Valdeón cheese) along with two commercial strains were investigated for their ability to grow at different concentrations of salt and at different temperatures as well as for their proteolytic and lipolytic activities. Low concentrations of salt (1‐3%) were stimulating for all the strains, with 1% salt being the concentration with the highest stimulating effect in nearly all. The rate of growth at 10°C was 2‐3 times lower than at 25°C, the optimum temperature for the species. None of the strains, including the commercial cultures, showed proteolytic activity on casein agar, while all of them were lipolytic on tributyrin agar.

Minimum water activity for the growth of Aeromonas hydrophila as affected by strain, temperature and humectant

The influence of water activity (adjusted with three humectants: sodium chloride, glycerol and polyethylene glycol) on the growth of three strains of Aeromonas hydrophila at 28, 10 and 3.8°C was studied. Minimum water activity for growth (MWAG) of Aer. hydrophila varied with strain, temperature and type of humectant. MWAG ranged from 0.940 to 0.973 (28°C), 0.959 to 0.980 (10°C) and 0.975 to 0.980 (3.8°C).