Graduate Thesis Or Dissertation
 

Studies on host range, fine structure, and nucleic acids of lactic streptococcus bacteriophages

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  • The most frequent cause of insufficient acid production during fermentation of dairy products is bacteriophage infection of lactic starter cultures. Previously, efforts to minimize this problem have dealt primarily with the host range relationships of the viruses and susceptibility patterns of the lactic streptococci. This study was undertaken to characterize the host range, morphology, and nucleic acid types of 35 lactic streptococcus bacteriophages; and to investigate the interrelationship between these characters. Preliminary experiments revealed a simple procedure for obtaining high titer phage preparations. In addition, a simple procedure for storage of stock lysates was found to be effective in maintaining phage viability for more than two years. Host range determinations showed that considerably more hosts were lysed at high than at low titer. Groupings of phage were more apparent with low titer host range patterns. Several hosts that plated heterologous phages at low efficiency of pla.ting (E. O. P. ) were further investigated. A single cycle of growth through these hosts produced progeny phage that plated with low E. 0. P. on the original hosts, due either to host controlled modification or host range mutation. Electron microscopy studies revealed phages of the same general shape, differing only in size of the component structures. All of the phages examined had non-contractile tails with a tail end plate structure, typical of the double stranded DNA phages of Bradleys Group B. Head size ranged from 320-850 A in diameter. Tail lengths ranged from 850-5200 X. Six types of ta.11 end plate, structures were observed. The presence of a collar was observed for nine of the lactic phages. Ten separate morphological groups were characterized on the basis of similarity in head size and shape, tail length, tail plate structure, and the presence or absence of a collar. Two phage species, w401x1 and w407x1, were unique from the others with respect to head size and tail length. Head diameters ranged from 700-850 X and tail lengths from 2000-5200 X . The most striking feature of these two phages was the long winding tail of variable length. In addition, these two phages did not form plaques on lawns of the strains which gave rise to the lysate. An extensive search for an indicator strain was not made. The morphology groups showed no correlation with groups based on host range pattern. The buoyant density of these phages covers a wide range, from 1. 428 to 1. 531 g /cc. On the basis of the DNA melting profile, all of the phages in this study proved to be double stranded. The GC base composition ranged from 32. 7 to 40.0 percent. Genome sizes of the lactic phages were determined on the basis of renaturation kinetics. Genome sizes ranged from 6 to 24 x 10⁶ daltons. Renaturation kinetics were also used to assess polynucleotide similarity among the lactic phages. DNA homology ranged from zero to 100 percent. An orderly relationship between percent homology, GC base composition, morphology, and host range pattern was not revealed.
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