TABLE 2.

Conjugation/mobilization frequencies of IncX4, IncHI2, and IncI1-I(α) plasmids of food-borne colistin resistant isolates of the monophasic ST34 variant of Salmonella enterica serovar Typhimurium.

Donor strain / plasmid(s)	Selection for:	Selection witha:	Transfer frequencyb (standard error)
			28°C	37°C
LSP 237/15 / IncX4; IncI2(δ)c	IncX4	RIF + COL	nd	1.5 × 10–6 (0.98)
LSP 295/15 / IncX4	IncX4	RIF + COL	nd	2.6 × 10–6 (0.85)
LSP 298/15 / IncX4; IncI1-I(α)c	IncX4	RIF + COL	nd	0.6 × 10–6 (0.42)
LSP 38/19 / IncHI2; IncI1-I(α)	IncH12	RIF + COL	2.6 × 10–5 (1.2)	1.6 × 10–5 (1.2)
LSP 38/19 / IncHI2; IncI1-I(α)	IncI1-I(α)	RIF + CTX	<1.9 × 10–8	3.6 × 10–5 (1.8)

^aRIF, rifampicin; COL, colistin; CTX, cefotaxime.

^bConjugation frequencies (shown in bold) are reported as the number of transconjugants per donor cell, and result from the average of three independent experiments, nd, not determined.

^cCriptic plasmids for which selection is not available.