Aztreonam-induced filamentation in Pseudomonas aeruginosa

Objectives: The proliferation of metallo-beta-lactamase-producing
Pseudomonas aeruginosa represents a significant public health threat. P.
aeruginosa can undergo significant phenotypic changes that can drastically
impair antibiotic efficacy. This study’s objectives were (1) to quantify
the time course of killing of VIM-2-producing P. aeruginosa in response to
aztreonam-based therapies, and (2) to document the capacity of P.
aeruginosa to undergo morphological transformations that facilitate
persistence. Methods: A well-characterized, clinical
VIM-2-producing P. aeruginosa was studied in the Hollow Fiber
Infection Model (HFIM) over 9 days (7 days of active antibiotic therapy, 2
days treatment withdrawal) at a 107.5 CFU/mL starting inoculum. HFIM
treatment arms included: growth control, aztreonam, ceftazidime/avibactam,
aztreonam/‌ceftazidime/‌avibactam, polymyxin B, and
aztreonam/‌ceftazidime/‌avibactam/‌polymyxin B. In addition, real-time
imaging studies were conducted under static conditions to determine the
time course of the reversion of persister cells.  Results: A
pronounced discrepancy was observed between OD620 and bacterial counts
obtained from plating methods (hereafter referred to as ‘OD-count
discrepancy’). For aztreonam monotherapy, observed counts were 0 CFU/mL by
120 h. Despite this, there was a significant OD-count discrepancy as
compared to the pre-treatment 0h. Between therapy withdrawal at 168h and
216h, all arms with suppressed counts had re-grown to the system carrying
capacity. Real-time imaging of the P. aeruginosa filaments after drug
removal showed rapid reversion from a long, filamentous phenotype to many
individual rods within 2 h. Conclusion: Managing MBL-producing P.
aeruginosa will require a multi-faceted approach, focused on maximizing
killing and minimizing proliferation of resistant and persistent
subpopulations, which will involve eliminating drug-induced phenotypic
transformers.