Graduate Thesis Or Dissertation
 

Lactic streptococci: growth characteristics and plasmid profiles of drug- and bacteriophage-insensitive mutants

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  • Fast acid-producing streptomycin-resistant mutants for several strains of lactic streptococci currently used in commercial Cheddar cheese manufacture were isolated. Their isolation involved use of a recently developed differential plating medium that allows identification of fast acid-producing colonies. Both parents and mutants were characterized with regard to growth, acid production, proteolytic abilities, optimum temperatures, and phage sensitivities, and comparisons were made. The mutants were found to be very similar to the parent strains and could conceivably be used to overcome streptomycin in the milk supply in the manufacture of Cheddar cheese. These streptomycin-resistant mutants were used to study strain interaction in a commercially used mixed-strain starter culture composed of two Streptococcus lactis and four Streptococcus cremoris strains. Mutants were available for each of the strains in the starter and were grown in various combinations with the parent strains in nonfat milk. Resistance to streptomycin served as a marker to identify a particular strain when grown in mixed culture and was identified by plating onto media containing streptomycin. Both S. lactis strains were found to significantly inhibit two of the four S. cremoris strains and one instance of S. cremoris - S. cremoris inhibition was found. Speculation regarding a dominance pattern in this starter was made. The plasmid profiles of four strains of S. lactis and their phage-insensitive mutants were examined and compared. The mutants of three of the four strains were found to have very similar plasmid patterns to that of their parents. However, the phage-insensitive mutants of the fourth strain which were examined contained one more plasmid band than their parent strain. Removal of this band by curing with ethidium bromide did not result in loss of phage insensitivity. The plasmid profiles of streptomycin-resistant isolates of two of the four strains were also examined and proved to be similar to that of the parent strains. A study was conducted to determine if various types of storage abuse influenced the incidence of slow acid- producing variants in three selected strains of lactic streptococci. This also involved use of.the new differential plating medium mentioned previously. No increase in slow variants was detected when cultures were stored in either the ungrown or mature states in nonfat milk at -20° and -80°C and subjected to occasional thawing and refreezing. Considerable injury and cell death were detected when cultures were stored in the mature state at 4°C. The presence of injured colony types hindered accurate enumeration of slow acid-producing colonies in this instance.
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