Date of Award:

1971

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Nutrition, Dietetics, and Food Sciences

Department name when degree awarded

Bacteriology and Public Health

Advisor/Chair:

G. H. Richardson

Abstract

Growth and enterotoxin production by Staphylococcus aureus straings, in the presence of different starter [2:1 (V/V) blend of AM2:ML8 strains of Streptococcus lactis] levels was investigated. Sterile, 10 per cent non-fat dry milk was inoculated with S. aureus strains capable of producing all types of enterotoxins, and reduced levels of starters; and was incubated at 32 C for 24 hours. The pH and S. aureus population were determined at 2 hour intervals until 8 hours and at 24 hours. The inhibitory response of lactic streptococci was studied by spot-tests on a lawn of S. aureus strains.

The drop in pH, from 4 to 8 hours incubation, for all starter levels, was proportional to their inocula. The rate of acid formation, or drop in pH, from 4 to 8 hours, was correlated with the change in staphylococcal population from 6 to 24 hours (Correlation Coefficient = Y = -0.805). Regression analysis indicated that change in pH from 4 to 8 hours could be used to predict the staphylococcal population change from 6 to 24 hours.

All four enterotoxigenic strains showed differential susceptibility to the starter metabolite(s). A 0.1 per cent starter level did not allow the increase of approximately 104 cells per milliliter, of an eterotoxin D producing strain of S. aureus (23235). Approximately 106 cells per milliliter of S. aureus 23235, decreased to about 104 cells in the presence of a 0.1 per cent starter level; while 0.01 per cent starter level did not allow the inocula of approximately 106 cells pe milliliter, of enterotoxin B producing strain of S. aureus, did not increase in the presence of 0.01 per cent starter level. The same inocula of enterotoxin A and C producing straings of S. aureus decreased to about 103 to 104 cells per milliliter in the presence of 0.01 per cent starter. These strains sharply declined in population in the presence of 0.1 per cent starter level.

The lactic organism did not produce inhibitory levels nisin, or over 5 micrograms of hydrogen peroxide per milliliter of broth. When the lactic streptococci were spotted on lawns of enterotoxins B, C, and D producing strains of S. aureus, staphylococcal grothwas inhibited around the spots, on both agar, with and without added calcium carbonate. Enterotoxin A producing strain was not inhibited on agar. The degree of inhibition for B and D enterotoxin producing strains, was greater in agar fortified with calcium carbonate, than that without fortification while the revers was true for enterotoxin C producing strain.

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