Table 3.

The impact of subtherapeutic levels of antibiotics on conjugation frequency [log₁₀(CF¹)] of an E. coli donor and a Salmonella Typhimurium recipient, calculated per donor, recipient and total bacterial count corrected for the stress impact factor.

Antibiotic 2 (µg/mL)	CF(D)	CF(R)	CF(T)	CF(D) (SIF)	CF(R) (SIF)	CF(T) (SIF)
CTX
0	−4.9 ± 0.5	−4.3 ± 0.6 a	−5.0 ± 0.5 a	−4.9 ± 0.5 a	−4.3 ± 0.6	−5.0 ± 0.7 a
0.4	−4.4 ± 0.7	−3.0 ± 0.6 b	−4.4 ± 0.7 b	−5.6 ± 0.5 b	−4. 9 ± 0.6	−5.7 ± 0.4 b
0.5	−4.1 ± 0.7	−2.9 ± 1.0 b	−4.1 ± 0.7 b	−5.0 ± 0.6 a	−5.0 ± 0.7	−5.3 ± 0.6 ab
0.6	−4.2 ± 0.5	−2.6 ± 0.9 b	−4.2 ± 0.5 b	−4.7 ± 0.5 a	−4.7 ± 0.6	−5.0 ± 0.5 a
p-value	0.060	0.002	0.002	0.011	0.163	0.021
SXT
0	−4.9 ± 0.5 ab	−4.3 ± 0.6 a	−5.0 ± 0.5 a	−4.9 ± 0.5 a	−4.3 ± 0.6 a	−5.0 ± 0.5 a
1.0	−5.6 ± 0.9 ac	−5.9 ± 0.7 b	−6.1 ± 0.7 b	−6.9 ± 0. 9 b	−5.9 ± 0.7 b	−6.6 ± 0.8 b
2.5	−4.3 ± 0.3 b	−5.3 ± 0.4 b	−5.3 ± 0.4 ab	−5.7 ± 0.4 ab	−5.3 ± 0.4 b	−5.9 ± 0.4 b
5.0	−4.1 ± 0.4 c	−5.6 ± 0.5 b	−5.6 ± 0.5 b	−6.2 ± 0.4 b	−5.6 ± 0.5 b	−6.2 ± 0.5 b
p-value	0.001	0.001	0.017	<0.001	0.001	0.001
F
0	−4.9 ± 0.5 a	−4.3 ± 0.6 a	−5.0 ± 0.7 a	−4.9 ± 0.5 a	−4.3 ± 0.6 a	−5.0 ± 0.5 a
2.0	−5.2 ± 0.5 a	−4.6 ± 0.5 a	−5.3 ± 0.4 a	−4.8 ± 0.7 a	−4.5 ± 0.5 a	−5.0 ± 0.6 a
4.0	−5.2 ± 0.5 a	−4.6 ± 0.3 a	−5.3 ± 0.5 a	−4.9 ± 0.3 a	−4.5 ± 0.4 a	−5.1 ± 0.3 a
6.0	−6.2 ± 0.7 b	−5.8 ± 0.8 b	−6.4 ± 0.7 b	−6.3 ± 0.6 b	−5.8 ± 0.8 b	−6.4 ± 0.7 b
p-value	0.002	0.001	0.001	0.001	0.001	0.001

¹ Conjugation frequency (CF) calculated as CF/D = transconjugants/donor, CF/R = transconjugants/recipients or CF/T = transconjugants/total bacterial count; SIF = stress impact factor (Supplementary Data, Table S3); 2CTX = cefotaxime; SXT = sulfamethoxazole/trimethoprim; F = nitrofurantoin; p-values were calculated comparing CF of different levels of exposure to the stressor using the nonparametric Kruskal–Wallis test and Mann–Whitney test. Significant differences (p ≤ 0.05) between values are indicated by different superscript letters. Raw data are provided in supplementary data, Table S1.