bBiofilms are a common cause of food contamination with undesirable bacteria, such as pathogenic bacteria. Staphylococcus aureus is one of the major bacteria causing food-borne diseases in humans. A study designed to determine the presence of S. aureus on food contact surfaces in dairy, meat, and seafood environments and to identify coexisting microbiota has therefore been carried out. A total of 442 samples were collected, and the presence of S. aureus was confirmed in 6.1% of samples. Sixty-three S. aureus isolates were recovered and typed by random amplification of polymorphic DNA (RAPD). Profiles were clustered into four groups which were related to specific food environments. All isolates harbored some potential virulence factors such as enterotoxin production genes, biofilm formation-associated genes, antibiotic resistance, or lysogeny. PCR-denaturing gradient gel electrophoresis (PCR-DGGE) fingerprints of bacterial communities coexisting with S. aureus revealed the presence of bacteria either involved in food spoilage or of concern for food safety in all food environments. Food industry surfaces could thus be a reservoir for S. aureus forming complex communities with undesirable bacteria in multispecies biofilms. Uneven microbiological conditions were found in each food sector, which indicates the need to improve hygienic conditions in food processing facilities, particularly the removal of bacterial biofilms, to enhance the safety of food products.
M.L. CABO, M.A. MURADO, M.P. GONZÁ L E Z A ND L . P AS T OR IZ A . 1999. Different aspects of the most commonly used assay methods in the study of bacteriocins were examined. The conditions under which extraction and incubation (including exposure time) take place were analysed, and several different formal models that are usually employed to calculate ID 50 were compared. As an alternative designed to overcome the problems which characterize the response of micro-organisms that are sensitive to bacteriocins, an operative procedure in a liquid medium and a modified re-parameterized logistic equation is proposed. When applied to the inhibition of Leuconostoc mesenteroides by nisin, the model allows an optimal experimental procedure to be defined.
Fifty-six dairy bacteria belonging to the genera Lactococcus, Lactobacillus, Pediococcus, Propionibacterium, Streptococcus, Enterococcus, Leuconostoc, and Brevibacterium were screened for antifungal activity against four species of fungi relevant to the cheese industry (Penicillium discolor, Penicillium commune, Penicillium roqueforti, and Aspergillus vesicolor). Most of the active strains belonged to the genus Lactobacillus, whereas Penicillium discolor was found to be the most sensitive of the four fungi investigated. Further studies on P. discolor showed antifungal activity only below pH 5. This effect of pH suggests that organic acids present in the culture could be involved in the detected activity. Determination of acid composition revealed lactic acid production for active dairy strains and the presence of acetic acid in active as well as inactive strains. It was demonstrated that the undissociated acetic acid originates from the bacterial growth medium. The synergistic effect of the acetic acid present and the lactic acid produced was likely the main factor responsible for the antifungal properties of the selected bacteria. These results could explain some discrepancies in reports of the antifungal properties of lactic acid bacteria, since the role of acetic acid has not been considered in previous studies.
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