The emergence of multidrug‐resistant bacteria necessitates the identification of unique targets of intervention and compounds that inhibit their function. Gram‐positive bacteria use a well‐conserved tRNA‐responsive transcriptional regulatory element in mRNAs, known as the T‐box, to regulate the transcription of multiple operons that control amino acid metabolism. T‐box regulatory elements are found only in the 5′‐untranslated region (UTR) of mRNAs of Gram‐positive bacteria, not Gram‐negative bacteria or the human host. Using the structure of the 5′UTR sequence of the Bacillus subtilis tyrosyl‐tRNA synthetase mRNA T‐box as a model, in silico docking of 305 000 small compounds initially yielded 700 as potential binders that could inhibit the binding of the tRNA ligand. A single family of compounds inhibited the growth of Gram‐positive bacteria, but not Gram‐negative bacteria, including drug‐resistant clinical isolates at minimum inhibitory concentrations (MIC 16–64 μg mL−1). Resistance developed at an extremely low mutational frequency (1.21×10−10). At 4 μg mL−1, the parent compound PKZ18 significantly inhibited in vivo transcription of glycyl‐tRNA synthetase mRNA. PKZ18 also inhibited in vivo translation of the S. aureus threonyl‐tRNA synthetase protein. PKZ18 bound to the Specifier Loop in vitro (Kd≈24 μm). Its core chemistry necessary for antibacterial activity has been identified. These findings support the T‐box regulatory mechanism as a new target for antibiotic discovery that may impede the emergence of resistance.

The effect of direct-fed microbials (DFM) on fecal shedding of Escherichia coli O157:H7 and Salmonella in naturally infected feedlot cattle was evaluated in a clinical trial involving 138 feedlot steers. Following standard laboratory methods, fecal samples collected from steers were evaluated for change in the detectable levels of E. coli O157:H7 and Salmonella shed in feces after DFM treatment. Sampling of steers was carried out every 3 weeks for 84 days. A significant reduction (32%) in fecal shedding of E. coli O157:H7 (P < 0.001), but not Salmonella (P = 0.24), was observed among the treatment steers compared with the control group during finishing. The probability of recovery of E. coli O157:H7 from the feces of treated and control steers was 34.0 and 66.0%, respectively. Steers placed on DFM supplement were almost three times less likely to shed E. coli O157:H7 (odds ratio, 0.36; 95% confidence interval, 0.25 to 0.53; P < 0.001) in their feces as opposed to their control counterparts. The probability of recovery of Salmonella from the feces of the control (14.0%) and the treated (11.3%) steers was similar. However, the DFM significantly reduced probability of new infections with Salmonella among DFM-treated cattle compared with controls (nontreated ones). It appears that DFM as applied in our study are capable of significantly reducing fecal shedding of E. coli O157:H7 in naturally infected cattle but not Salmonella. The factors responsible for the observed difference in the effects of DFM on E. coli O157:H7 and Salmonella warrants further investigation.