ABSTRACT
Bacterial exposure to antibiotic concentrations below the minimum inhibitory concentration (MIC) may result in a selection window allowing for the rapid evolution of resistance. These sub-MIC concentrations are commonly found in soils and water supplies in the greater environment. This study aimed to evaluate the adaptive genetic changes in Klebsiella pneumoniae 43816 after prolonged but increasing sub-MIC levels of the common antibiotic cephalothin over a fourteen-day period. Over the course of the experiment, antibiotic concentrations increased from 0.5 µg/mL to 7.5 µg/mL. At the end of this extended exposure, the final adapted bacterial culture exhibited clinical resistance to both cephalothin and tetracycline, altered cellular and colony morphology, and a highly mucoid phenotype. Cephalothin resistance exceeded 125 µg/mL without the acquisition of beta-lactamase genes. Whole genome sequencing identified a series of genetic changes that could be mapped over the fourteen-day exposure period to the onset of antibiotic resistance. Specifically, mutations in the rpoB subunit of RNA Polymerase, the tetR/acrR regulator, and the wcaJ sugar transferase each fix at specific timepoints in the exposure regimen where the MIC susceptibility dramatically increased. These mutations indicate that alterations in the secretion of colanic acid and attachment of colonic acid to LPS may contribute to the resistant phenotype. These data demonstrate that very low sub-MIC concentrations of antibiotics can have dramatic impacts on the bacterial evolution of resistance. Additionally, this study demonstrates that beta-lactam resistance can be achieved through sequential accumulation of specific mutations without the acquisition of a beta-lactamase gene.
ABSTRACT
BACKGROUND: Combined cytotoxic T-lymphocyte-associated antigen 4 and programmed death 1 inhibitor blockade is a promising strategy in advanced melanoma and other solid tumors. This pilot study assessed the safety and toxicity of nivolumab plus low-dose ipilimumab in patients with high-risk completely resected melanoma. PATIENTS AND METHODS: Patients received ipilimumab, 1 mg/kg every 6 weeks, and nivolumab, 3 mg/kg every 2 weeks, for a total of 24 weeks (4 cycles). The primary objective was to assess the toxicity of the combined regimen. RESULTS: Twenty-one patients with resected melanoma were enrolled. One patient was stage IIC, 16 patients were stage III and 4 patients had resected stage 4 disease. Ten of 21 (48%) had grade 3 treatment-related toxicities but there was no grade 4 or grade 5 toxicities. The rate of grade 3 nonhematologic toxicities exceeded the toxicity limits defined by the study. Fifteen of 21 patients (71%) completed all 4 cycles of therapy. The median follow-up is 41 months. The 2-year recurrence-free survival is 85.7% and the 2-year overall survival is 90.5%. CONCLUSION: A 6-month course of nivolumab and low-dose ipilimumab may be a promising adjuvant treatment for patients with resected melanoma. Further studies of this regimen are indicated.
