The Enterobacteriaceae family includes many clinically important Gram-negative bacteria with high infectious burden, including numerous Escherichia coli strains and diverse Salmonella enterica subsp. enterica serovars. Antibiotics are the primary tool to treat bacterial infections. Unfortunately, many bacteria, including those in the Enterobacteriaceae family have acquired multi-drug resistance (MDR), i.e., resistance to multiple drug classes. With increasing rates of MDR, the use of last resort antibiotics like colistin and carbapenems, as well as infection prevention have become new important tools in fighting bacterial infections. In this collection of works, we focused on (i) understanding phosphoethanolamine-mediated colistin resistance, (ii) characterizing β-lactamase resistance genes, and (iii) understanding pESI-mediated S. Infantis persistence.

A review of the current literature on phosphoethanolamine-mediated lipopolysaccharides (LPS) was conducted to understand how phosphoethanolamine modifications impact bacterial phenotypes like colistin resistance, virulence, and fitness. Of the identified knowledge gaps, the deviation in the number of studies focusing on mobile colistin resistance (mcr) gene 1, which confers colistin resistance through phosphoethanolamine modification of lipid A (i.e., a component of gram-negative outer membrane component lipopolysaccharide), from the number of studies focusing on the other nine families of mcr genes was most significant. Subsequent works focused on characterizing mcr genes from different families in heterologous expression systems to better understand (i) phenotypic impacts conferred by different mcr genes and (ii) bacterial adaptation to mcr carriage. In that process, a work on the importance of choosing the right heterologous expression host when studying mcr genes was also published. To expand the utility of the expression system created when studying mcr genes, blaOXA-genes which confer resistance to several important β-lactam antibiotics were studied using a similar expression system. Lastly, the persistence of a MDR strain of S. Infantis which causes many illnesses every year was bioinformatically analyzed to reveal that pESI-located genes might be associated with this strain’s persistence in chickens. Overall, the works presented here provide insights that can be used in the study of novel AMR genes and the development of strategies to (i) minimize colistin resistance and (ii) reduce further spread and persistence of MDR S. Infantis.