Resistance Of Lactic Acid Bacteria Research Articles

Bacteriophage and bacteriophage resistance in lactic acid bacteria

The study of bacteriophage—host interactions has been instrumental in the development of genetic systems in many genera, and laid many of the foundations of modern molecular genetics. Research into bacteriophage and bacteriophage resistance in the lactic acid bacteria has moved into a new and exciting dimension in recent years. Mechanisms such as adsorption inhibition, restriction and modification, and abortive infection which have been detected and described phenotypically over the past decade are now being subjected to molecular analysis, and this has led to a better understanding of the nature and variety of resistance systems employed by lactic acid bacteria to combat phage attack. In addition, analysis of different bacteriophage has increased our knowledge of these ubiquitous particles to the point where it is possible to construct novel phage resistances based on the phage genome itself. This review outlines the recent progress in the molecular analysis of bacteriophage, bacteriophage resistance and counter resistance, and the construction of novel resistance mechanisms.

Bacteriophages of Lactic Acid Bacteria

Bacteriophages of mesophilic and thermophilic lactic streptococci and Lactobacilli have been the subject of numerous investigations. The most extensive studies have been carried out on phages of mesophilic lactic streptococci. Phages have been differentiated on the basis of morphology, serology, and DNA characterization. Morphologically distinct phages belong to different DNA homology groups, related phages being found in such widely separated locations as Ireland, Germany, and New Zealand. Surveys of thermophilic lactic streptococcal phages to date have indicated that the majority of phages belong to one morphological type. Lactobacillus phages belong to several morphological groups, including phages with contractile and noncontractile tails.The role of lysogenic strains of lactic acid bacteria as a source of lytic phages is unclear. In general, lytic and temperate phages of lactic streptococci are not closely related by DNA homology. However, some DNA homology groups contain both temperate and lytic phages, suggesting that these phages may have a common ancestor. Heteroduplex DNA studies indicated that in some instances, lytic phages evolve from one another, but it is possible that DNA from temperate phages contributes to the evolution of phage. DNA studies on Lactobacillus phages have shown a closer relationship between temperate and lytic phages.A number of recent studies have provided restriction endonuclease maps of phage genomes. Several genes, such as structural genes, and sites for attachment and packaging initiation in temperate phage DNA, have been located. Extensive genetic studies of lactic phages are required to elucidate relationships between phage and host, particularly as they relate to phage resistance in lactic acid bacteria.

Phage resistance in lactic acid bacteria

The interactions between the lactic acid bacteria and their phages are commercially significant. Current research has focused on the elucidation of the mechanisms and genetics of phage resistance. Phage resistance genes have been linked to plasmid DNA for Streptococcus lactis and Streptococcus cremoris, and preliminary studies suggest the operation of mechanisms such as the prevention of phage adsorption, restriction/modification, and abortive infection. Some phage resistance plasmids can be conjugally transfered, providing a means of dissemination among phage-sensitive strains for the construction of phage-resistant starter cultures.

The Heat Resistance of Streptococcus Thermophilus Grown in Mixed Culture with Caseolytic Bacteria

The increased heat resistance of Streptococcus thermophilus and Lactobacillus helveticus grown in mixed culture with Candida krusei, a film-forming yeast, has been observed by Peppler and Frazier (1). The beneficial effect of the film yeast on the bacteria was greatest near the surface of the medium where the yeast grew abundantly. The influence of the yeast was attributable to the combined effect of reduced oxygen tension, reduced acidity, and accessory substances formed during the association with the lactic acid bacteria. It was suggested that other microorganism grown with acid-forming bacteria may increase the activity of the lactic acid bacteria after severe temperature exposures. The present study is a preliminary report of data showing the effect of different caseolytic bacteria on the heat resistance of lactic acid organisms grown in mixed culture in milk. The addition of peptones and malt extract to a reconstituted skimmilk medium is known to increase the rate of acid development and heat resistance of Lad,tobacillus helveticus (2) and Streptococcus thermophilus (3). It is probable that certain proteolytic bacteria grown in whey can produce protein derivatives more available to pure cultures of the acid-forming bacteria than the proteins of milk. Therefore the enrichment of ordinary skimmilk media with small amounts of heat-killed whey cultures of caseolytic bacteria should increase the heat resistance of S. thermophilus and L. helveticus. The following experiments indicate the extent to which such enrichments of milk are a useful method of preparing heat resistant lactic acid bacteria.