William E. Sandine

Nisin as a model food preservative.

Nisin is a ribosomally synthesized peptide that has broad-spectrum antibacterial activity, including activity against many bacteria that are food-spoilage pathogens. Nisin is produced as a fermentation product of a food-grade bacterium, and the safety and efficacy of nisin as a food preservative have resulted in its widespread use throughout the world, including the U.S. Nisin is a member of the class of antimicrobial substances known as lantibiotics, so called because they contain the unusual amino acid lanthionine. Lantibiotics, in general, have considerable promise as food preservatives, although only nisin has been sufficiently well characterized to be used for this purpose. As the number of known natural lantibiotics has increased and their useful characteristics have been explored, it has become desirable to synthesize structural analogs of nisin and other lantibiotics that do not occur naturally. The fact that lantibiotics are gene-encoded peptides synthesized by transcription and translation allows structural variants to be generated by mutagenesis. This review focuses on the progress that has been made in the construction and biological expression of genetically engineered nisin structural analogs. For example, a host-vector pair has been engineered that permits the construction of mutants of the structural gene for subtilin, which is a naturally occurring structural analog of nisin. The vector is designed in such a way that the mutant gene can be substituted for the natural subtilin gene in the chromosome of Bacillus subtilis, which in turn directs the transcription, translation, posttranslational modifications, and secretion of the mature form of the structural analog. Several structural analogs have been constructed, and their properties have provided insight into some of the structure-function relationships in lantibiotics, as well as their mechanism of antimicrobial action. These advances are assessed together with potential problems in the future development of nisin analogs as valuable new food preservatives.

Associative Growth and Differential Enumeration of Streptococcus thermophilus and Lactobacillus bulgaricus: A Review

The literature which reports on the associative growth properties of Streptococcus thermophilus and Lactobacillus bulgaricus is reviewed. Also discussed are articles which report on methods to differentially enumerate these two bacteria when they are together either in bulk starters or manufactured products.

New nomenclature of the non-rod-shaped lactic acid bacteria

Bacteria of the Lactococcus (formerly lactic Streptococcus) genus are described to emphasize certain less well-known facts of importance in their successful use as dairy starters. Other streptococci (non-rod forms) are briefly characterized to emphasize their usefulness in industrial milk fermentations.

Standardized method for determining the effect of various antibiotics on lactococcal cultures

Antibiotic resistance of 44 lactic starter culture strains currently used by the dairy industry has been examined in Mueller- Hinton medium, supplemented with glucose (5 g/L) and yeast extract (5 g/L), by the agar disk diffusion assay. Results with various classes of antibiotics used to treat cattle mastitis indicated that lactococcal cultures should be diluted 20% and incubated at 30°C for 16 to 18 h in this medium for reliable results. The degree of antibiotic susceptibility was compared to standards established by the National Committee for Clinical Laboratory Standards. The procedure outlined in the present study is recommended as a routine method for testing antibiotic resistance of lactococci. This represents a more logical drug sensitivity approach than the use of noncheese starter bacteria to test for antibiotics in milk.

Cheese industry development and research in Argentina

Research and development projects concerning cheese industry in Argentina are described in this study. Regional strains of lactic acid bacteria were isolated from different ecological pockets and their taxonomic profiles were determined. Proteolytic and acid activity as well as diacetyl production were analyzed. Results obtained depended on the species and strains under consideration. The cell permeabilization using 20 to 40% ethanol improved the acid production by lactic acid bacteria. Freeze-drying was used for culture preservation. The optimal conditions for obtaining the highest survival rate were determined. Best results were obtained by using 0.75 M adonitol as a cryoprotectant. The rehydration conditions to be used depended on the bacterial species. Freeze-dried cultures showed good viability and activity up to 1 year of storage at 4 degrees C.