Table 5.

Final multilevel logistic regression model of the susceptibility^a of 192 environmental streptococci isolates from bovine milk tested against 3 antimicrobials.

	β	SE(β)	OR	95%CI(OR)		P value	Overall P value
Intercept	1.43	0.55	–	–	–	–
Antimicrobial
Ampicillin	0.66	0.41	1.93	0.86	4.32	0.111
Penicillin	−2.08	0.46	0.12	0.05	0.31	<0.001
Tetracycline	Ref.	–	–	–	–	–
Species
Streptococcus uberis	Ref.	–	–	–	–	–
Streptococcus dysgalactiae	−2.41	0.68	0.09	0.02	0.34	<0.001
Streptococcus spp.b	−0.89	0.54	0.41	0.14	1.17	0.095
Lactococcus spp.c	−0.05	0.58	0.95	0.31	2.94	0.925
Enterococcus spp.d	−1.42	0.61	0.24	0.07	0.81	0.021
Antimicrobial × speciese							<0.001
Herd somatic cell countf							0.033
≤150,000 cells/mL	Ref.	–	–	–	–	–
151,000–250,000 cells/mL	−1.01	0.52	0.36	0.13	1.01	0.052
251,000–400,000 cells/mL	−1.52	0.59	0.22	0.07	0.69	0.010
Days in milkg							0.058
0–100	Ref.	–	–	–	–	–
101–200	0.68	0.38	1.97	0.94	4.14	0.072
201+	−0.39	0.37	0.68	0.33	1.39	0.290
Variance	Estimate	SE
Herd level	0.27	0.25
Isolate level	0.90	0.58

^aSusceptible according to CLSI (20, 23).

^bStreptococcus spp. other than S. uberis, S. dysgalactiae, and S. agalactiae.

^cL. lactis and L. garvieae.

^dE. faecalis, E. saccharolyticus, E. faecium, E. pseudoavium, E. avium, E. casseliflavus, and E. hirae.

^eInteraction term between antimicrobial and species.

^fHerd somatic cell count = average within herd somatic cell count from 2007 to 2008.

^gDays in milk at the time of milk sample collection.

OR, odds ratio.