Christoph Cavadini

Hydrogen peroxide production by Lactobacillus johnsonii NCC 533 and its role in anti‐Salmonella activity

The human intestinal isolate Lactobacillus johnsonii NCC 533 (La1) is a probiotic strain with well-documented antimicrobial properties. Previous research has identified the production of lactic acid and bacteriocins as important factors, but that other unidentified factors are also involved. We used the recently published genome sequence of L. johnsonii NCC 533 to search for novel antipathogen factors and identified three potential gene products that may catalyze the synthesis of the known antimicrobial factor hydrogen peroxide, H(2)O(2). In this work, we confirmed the ability of NCC 533 as well as eight different L. johnsonii strains and Lactobacillus gasseri to produce H(2)O(2) when resting cells were incubated in the presence of oxygen, and that culture supernatant containing NCC 533-produced H(2)O(2) was effective in killing the model pathogen Salmonella enterica serovar Typhimurium SL1344 in vitro.

Application of Lysostaphin-Producing Lactobacilli to Control Staphylococcal Food Poisoning in Meat Products

The potential of lysostaphin-producing strains of Lactobacillus curvatus (Lys+) to prevent food-borne illness by Staphylococcus aureus was investigated under practical conditions. A response surface model was developed to estimate the effect of pH, temperature, and salt concentration on the lysostaphin activity. The model was applied to fermenting sausages, and a 90% reduction of lysostaphin activity at ripening was predicted. The residual was sufficiently high to reduce staphylococcal counts by 10(4) to 10(5) CFU/g within 2 to 3 days to below the level of detection. These results were obtained in pilot scale experiments with L. curvatus (Lys+) as a starter culture and S. aureus as well as Staphylococcus carnosus as model contaminants. The applicability of L. curvatus (Lys+) as a protective culture was studied in a mayonnaise-based meat salad. Upon incubation at 25 degrees C the staphylococci were rapidly killed within 24 h, whereas in the presence of the isogenic Lys(-) strain the staphylococci grew up to numbers of 10(7) CFU/g.

Safety assessment of genetically modified microorganisms applied in meat fermentations

Summary The use of genetically modified microorganisms (GMMs) in food processing requires a thorough assessment of potential risks which may arise from their application. To follow a science based vertical approach, a strategy for experimental design of strains and the investigation of their effects on food was developed permitting a safety assessment of GMMs. As a realistic model genetically modified lactic acid bacteria were studied in meat fermentation. Model organisms were constructed by cloning into the competitive meat starter organismus Lactobacillus curvatus LTH1432 the lysostaphin gene lys of Staphylococcus simulans (model I) and the catalase gene katA of Lactobacillus sake LTH677 (model II). Based on the concept of employing a food grade system pJK356 was used as cloning vector. Investigation of the stability of the recombinant plasmids in the host revealed that the constructs were structurally stable but the segregational stability was decreased, however, not beyond those numbers of generations required to ensure maintenance of the new property during the fermentation process. In fermenting sausages the constructs retained their full activity. Horizontal gene transfer was studied in vitro and during fermentation of sausages. No transfer of the recombinant marker genes was observed. On the other hand, conjugal transfer of pAMβ1 between strains of L. curvatus was detected, however, without mobilisation of the recombinant plasmids. It is remarkable that under optimal laboratory conditions no transfer of pAMβ1 between L. curvatus and Staphylococcus carnosus was observed, whereas, the conditions prevailing in the meat matrix permitted an efficient transfer. The models used provide realistic examples of GMMs with the potential to improve food safety and/or quality. Furthermore, they show that a case bv case saferv assessment is the appropriate procedure which is facilitated by applying the concept of constructing food grade organisms.