Evaluation of omadacycline against intracellular Mycobacterium abscessus in an infection model in human macrophages.

Background: Omadacycline is an aminomethylcycline antibiotic in the tetracycline class that was approved by the US FDA in 2018 for the treatment of community-acquired bacterial pneumonia and acute bacterial skin and skin structure infections. It is available in both IV and oral formulations. Omadacycline has broad-spectrum in vitro activity and clinical efficacy against infections caused by Gram-positive and Gram-negative pathogens. Omadacycline is being evaluated in a 3 month placebo-controlled Phase 2 clinical trial of oral omadacycline versus placebo in adults with non-tuberculous mycobacteria (NTM) pulmonary disease caused by Mycobacterium abscessus (NCT04922554). Objectives: To determine if omadacycline has intracellular antimicrobial activity against NTM, bacteria that can cause chronic lung disease, in an ex vivo model of intracellular infection. Methods: Two strains of M. abscessus were used to infect THP-1 macrophages. Intracellular M. abscessus was then challenged with omadacycline and control antibiotics at multiples of the MIC over time to evaluate intracellular killing. Results: At 16 ×  the MIC at 72 h, omadacycline treatment of intracellular NTM yielded a log10 reduction in cfu of 1.1 (91.74% reduction in cfu) and 1.6 (97.65% reduction in cfu) consistent with killing observed with tigecycline, whereas amikacin and clarithromycin at 16 ×  the MIC did not show any reduction in cfu against the intracellular M. abscessus. Conclusions: Omadacycline displayed intracellular activity against M. abscessus within macrophages. The activity was similar to that of tigecycline; as expected, intracellular killing was not observed with clarithromycin and amikacin.

Activity of plazomicin in combination with other antibiotics against multidrug-resistant Enterobacteriaceae.

Plazomicin is a next-generation aminoglycoside that was approved by the US FDA in June 2018 for the treatment of complicated urinary tract infections (cUTIs), including pyelonephritis due to Escherichia coli, Klebsiella pneumoniae, Enterobacter cloacae and Proteus mirabilis. Plazomicin is active against multi-drug resistant (MDR) Enterobacteriaceae, where combination therapy is often used to treat infections caused by these pathogens. To determine synergy with other antibiotics, plazomicin was combined with antibiotics in checkerboard assays against MDR Enterobacteriaceae, including isolates with resistance to aminoglycosides and ß-lactams; 10 Escherichia coli isolates, 8 Klebsiella spp. isolates, 10 Enterobacter spp. isolates, and 2 Citrobacter freundii isolates were evaluated. Plazomicin had potent activity against MDR Enterobacteriaceae, including aminoglycoside-resistant strains, with MIC ranges of 0.5 - 2 µg/mL against E. coli isolates, 0.12 - 8 µg/mL against Klebsiella spp. isolates, 0.25 - 2 µg/mL against Enterobacter spp. isolates, and 0.06 - 0.25 µg/mL against C. freundii isolates. Synergy between plazomicin and piperacillin/tazobactam or ceftazidime was observed by checkerboard studies and confirmed by time-kill assays. No combination showed antagonism. These studies indicate that plazomicin has potential as a monotherapy and as combination therapy for treating serious Gram-negative infections caused by MDR Enterobacteriaceae.

Evaluation of the Bactericidal Activity of Plazomicin and Comparators against Multidrug-Resistant Enterobacteriaceae.

The next-generation aminoglycoside plazomicin, in development for infections due to multidrug-resistant (MDR) Enterobacteriaceae, was evaluated alongside comparators for bactericidal activity in minimum bactericidal concentration (MBC) and time-kill (TK) assays against MDR Enterobacteriaceae isolates with characterized aminoglycoside and ß-lactam resistance mechanisms. Overall, plazomicin and colistin were the most potent, with plazomicin demonstrating an MBC50/90 of 0.5/4 µg/ml and sustained 3-log10 kill against MDR Escherichia coli, Klebsiella pneumoniae, and Enterobacter spp.

Metabolic imbalance and sporulation in an isocitrate dehydrogenase mutant of Bacillus subtilis.

A Bacillus subtilis mutant with a deletion in the citC gene, encoding isocitrate dehydrogenase, the third enzyme of the tricarboxylic acid branch of the Krebs cycle, exhibited reduced growth yield in broth medium and had greatly reduced ability to sporulate compared to the wild type due to a block at stage I, i.e., a failure to form the polar division septum. In early stationary phase, mutant cells accumulated intracellular and extracellular concentrations of citrate and isocitrate that were at least 15-fold higher than in wild-type cells. The growth and sporulation defects of the mutant could be partially bypassed by deletion of the major citrate synthase gene (citZ), by raising the pH of the medium, or by supplementation of the medium with certain divalent cations, suggesting that abnormal accumulation of citrate affects survival of stationary-phase cells and sporulation by lowering extracellular pH and chelating metal ions. While these genetic and environmental alterations were not sufficient to allow the majority of the mutant cell population to pass the stage I block (lack of asymmetric septum formation), introduction of the sof-1 mutant form of the Spo0A transcription factor, when coupled with a reduction in citrate synthesis, restored sporulation gene expression and spore formation nearly to wild-type levels. Thus, the primary factor inhibiting sporulation in a citC mutant is abnormally high accumulation of citrate, but relief of this metabolic defect is not by itself sufficient to restore competence for sporulation.