Graduate Thesis Or Dissertation
 

An ecological study of the lactic streptococci

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  • In an attempt to determine the natural habitat of Streptococcus lactis, Streptococcus cremoris, and Streptococcus diacetilactis 27 different species of vegetables, 18 species of fruits and many individual cow raw milk samples were examined. S. lactis was found to occur on potatoes, corn, cucumbers, peas, beans, and cantaloupe. In each case the organism was isolated in low numbers. The remaining 21 vegetables failed to yield any of the organisms sought. No lactic streptococci were isolated from any of the fruits examined. Almost all of the organisms isolated from fruits were able to grow at 45°C., thus precluding their identification as lactic streptococci. Those which did not grow at 45°C. had a cell morphology which excluded them from the lactic streptococcus group. Individual cow milk samples provided S. lactis, S. cremoris, and S. diacetilactis isolates. If the milk samples were kept at 21° C. for two to three days before plating, good yields of streptococci could be obtained, especially S. lactis. However, if the milk samples were immediately plated upon receiving them, lower numbers were found. The biochemical, cultural, and morphological characteristics of both the plant and milk isolates agreed in every detail with the standard descriptions of the lactic streptococci available in the recent literature; growth was found to take place at 10°C. and at 40°C, but not at 45°C. Litmus milk was acidified and completely reduced before curdling. No gas production was observed in the coagulated milk. Acid producing abilities of those cultures isolated from plants were somewhat higher than those cultures isolated from milk. Also, there was a wider range in acid producing abilities of the milk isolates than those cultures obtained from plant material. Most of the cultures which were isolated from plants produced concentrations of less than 50 ppm of total carbonyl compounds expressed as pyruvic acid. This also was true for those cultures obtained from milk. However a wide range (16 to 76 ppm) in the ability to produce carbonyl compounds was seen in both cases. The isolation of S. lactis from several different plant sources confirmed the belief that plants are the natural habitat for this bacterium. Failure to isolate S. cremoris from plant material suggests that this species may be a variant of S. lactis with milk as its natural habitat.
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