The bacterial ydcI gene encodes a highly conserved transcriptional regulatory protein found in a wide range of Gram-negative bacteria and is involved in a number of Salmonella enterica serovar Typhimurium phenotypes. Given its high conservation, the YdcI protein has the potential for studies and applications across bacterial genera. However, no studies have been performed with YdcI outside of S. Typhimurium. Here we report that different Gram-negative genera display dramatically different tolerances for YdcI expression. In non-tolerant genera, YdcI expression results in rapid loss of cell viability several log-fold in magnitude, and the viability loss is observed at YdcI levels that are physiologically relevant. The N-terminal and C-terminal halves can be exchanged between the S. Typhimurium and Escherichia coli YdcI proteins with the resulting proteins still displaying the differential tolerance phenotype. Comparison of YdcI expression from the respective chromosomal gene in S. Typhimurium and E. coli revealed much lower levels in E. coli suggesting that this species has evolved a lower endogenous YdcI expression level and does not tolerate increases above this level. Expression of YdcI resulted in increased sensitivity to a range of antibiotics indicating the possibility that this protein could augment antibacterial strategies in non-tolerant genera. Overall, the results indicate vastly different outcomes for YdcI expression depending on bacterial genus and unmask differences in YdcI expression, regulation, target interactions, and/or YdcI regulon activity in different bacteria. The results also impact future work on YdcI when the protein is being studied/expressed in different Gram-negative genera.