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Journal of Dairy Science Vol. 85 No. 5 1127-1132
© 2002 by American Dairy Science Association ®
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Evaluation of Five Selective Media for Isolation of Catalase-Negative Gram-Positive Cocci from Bulk Tank Milk

Ashish A. Sawant 1, Shreekumar R. Pillai 1, and Bhushan M. Jayarao 1

1 Department of Veterinary Science, The Pennsylvania State University, University Park 16802

Five selective media including Edwards modified medium, Edwards modified medium supplemented with colistin sulfate (5 mg/L) and oxolinic acid (2.5 mg/L), Streptococcus selective medium, Streptosel agar, and thallium-crystal violet-toxin-ferric citrate medium were evaluated for the isolation of streptococci and streptococci-like organisms from raw milk. The sensitivity and specificity of these selective media for streptococci and streptococci-like organisms were determined by using American Type Culture Collection reference strains. Under experimental conditions Edwards modified medium with colistin sulfate (5 mg/L) and oxolinic acid (2.5 mg/L) showed the highest sensitivity (100%) and specificity (100%) for streptococci and streptococcilike organisms followed by thallium-crystal violettoxin-ferric citrate medium, Edwards modified medium, Streptococcus selective medium, and Streptosel agar. Edwards modified medium supplemented with colistin sulfate (5 mg/L) and oxolinic acid (2.5 mg/L) allowed growth of all streptococci and streptococci-like organisms, while inhibiting growth of the staphylococci and gram-negative reference strains. Bulk tank milk samples from 114 dairy herds were spiral plated onto Edwards modified medium with colistin sulfate (5mg/ L) and oxolinic acid (2.5 mg/L). A total of 344 isolates (at least three isolates from each sample) were randomly selected and identified to their species. This medium permitted growth of 328 streptococci and streptococci-like organisms belonging to genera Aerococcus, Entero-coccus, Gemella, Lactococcus, Streptococcus, and Vagococcus. When Edwards modified medium supplemented with colistin sulfate (5 mg/L) and oxolinic acid (2.5 mg/ L) was evaluated using bulk tank milk samples, the sensitivity and specificity of this medium for streptococci and streptococci-like organisms were observed to be 100 and 87.5%, respectively. The positive predictive value for streptococci and streptococci-like organisms was observed to be 99.4%. The results of the study indicate that Edwards modified medium supplemented with colistin sulfate (5 mg/L) and oxolinic acid (2.5 mg/ L) can be used as a selective medium for the isolation of streptococci and streptococci-like organisms from bulk tank milk.

Key Words: bulk tank milk • streptococci • selective media • sensitivity

Submitted on August 13, 2001
Accepted on September 18, 2001




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B. M. Jayarao, S. R. Pillai, A. A. Sawant, D. R. Wolfgang, and N. V. Hegde
Guidelines for Monitoring Bulk Tank Milk Somatic Cell and Bacterial Counts
J Dairy Sci, October 1, 2004; 87(10): 3561 - 3573.
[Abstract] [Full Text] [PDF]


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