Table 3.

Chlortetracycline sensitivities of Megasphaera elsdenii isolates from organically raised and feral swine^a

Swine origin	No. of M. elsdenii isolates growing at chlortetracycline concn (μg/ml):
	0	16	64
Organic	540	355 (66%)	264 (52%)
Feral	91	0	0

^aM. elsdenii population levels averaged 2.4 × 10⁸ CFU/g (range, 1.2 × 10⁸ to 4 × 10⁸) in organically raised swine. M. elsdenii cultivable populations in feral swine were significantly lower than those in organically raised animals, averaging 1.5 × 10⁶ CFU/g (P < .001, CI = 95%, Welch's t test), and also more variable (range, <5 × 10³ to 2 × 10⁷ CFU/g). Chlortetracycline susceptibilities of M. elsdenii cloned isolates were determined by replica-patching colonies sequentially onto PYG agar plates containing 64, 16, and 0 μg chlortetracycline per ml (18a). Values in parentheses indicate resistant isolates expressed as percentages of totals tested.