title: Evaluation of five selective media for isolation of catalase-negative gram-positive cocci from bulk tank milk.
abstract: 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 streptococci-like 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 (5 mg/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, Enterococcus, 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.
authors.affiliation: Department of Veterinary Science, The Pennsylvania State University, University Park 16802, USA.
authors.forename: Ashish A
authors.initials: AA
authors.lastname: Sawant
authors.forename: Shreekumar R
authors.initials: SR
authors.lastname: Pillai
authors.forename: Bhushan M
authors.initials: BM
authors.lastname: Jayarao
mesh_terms: D000818:Animals; D002374:Catalase / Q000032:analysis*; D003091:Colistin; D003470:Culture Media; D016983:Enterococcus / Q000254:growth & development / Q000302:isolation & purification; D005290:Ferric Compounds; D006095:Gram-Positive Cocci / Q000201:enzymology* / Q000254:growth & development / Q000302:isolation & purification*; D017045:Lactococcus / Q000254:growth & development / Q000302:isolation & purification; D008892:Milk / Q000382:microbiology*; D010093:Oxolinic Acid; D040141:Staphylococcaceae / Q000254:growth & development / Q000302:isolation & purification; D013289:Streptococcaceae / Q000254:growth & development / Q000302:isolation & purification; D013291:Streptococcus / Q000254:growth & development / Q000302:isolation & purification; D013793:Thallium
chemical_list: D003470:Culture Media; D005290:Ferric Compounds; C025314:ferric citrate; D013793:Thallium; D002374:Catalase; D010093:Oxolinic Acid; D003091:Colistin