Aims: To analyse and compare the effect of selection power for antimicrobial resistance (AMR) in coliforms of two kinds of β-lactams—aminopenicillins; ampicillin (Amp) and cephalosporins; cephalexin (Cpn) and ceftiofur (Cef)—and tetracycline (Tet) using an approach based on a swine faecal microcosmos. Methods and Results: Sixteen faecal samples from 32 pigs (mixed two by two) were treated with Amp, Cpn, Cef and Tet for 6 h (T6h) at concentrations expected to reach the animals gut when using in vivo standard doses. Controls (no drug added) were also tested. Next, samples were 1 : 100 diluted and left under the same conditions (no antimicrobial added) for further 20 h (T20h). The proportion of resistant coliform bacteria (R coliforms) to each antimicrobial was analysed just before starting the treatment (T0), at T6h and at T20h. Coselection was also studied by replica plating. Treatment for 6 h yielded significant increase in proportion of R coliforms, regardless of the drug and lack of selection pressure showed different effects at T20h depending on the antimicrobial used. Selective pressure was associated with the type of the β-lactam with Amp selecting for significantly higher numbers of R coliforms than cephalosporins. Conclusions: AMR development was observed following short treatment, and for Amp and Tet treatment, resistance persisted 20 h beyond the interruption of treatment. An association between kind of β-lactam and power of selection was found. Significance and Impact of the Study: AMR represents a threat to human health globally and antimicrobial treatment of livestock has a direct impact on this problem. Through our approach based on a swine faecal microcosmos, we demonstrated the effect on AMR development of several drugs commonly used in livestock. Cephalosporins, representing last-line antimicrobials in human medicine, exerted lower selective pressure than Amp under the conditions used and yielded higher proportion of multidrug-R strains.