Purpose:

The mechanisms of antibiotic persistence in immunosuppressed populations are not fully understood. Patients with cystic fibrosis (CF), both the pre-transplant and post-transplant phase often have persistent pulmonary infections. This in vitro study evaluated persistence in Pseudomonas aeruginosa (PA) and Staphylococcus aureus (SA) isolates by comparing bacterial killing dynamics upon antibiotic exposure.

Methods:

PA and SA persistent isolates were obtained from CF patients with positive-sputum cultures at a single academic hospital. The non-persistent isolates were laboratory control strains of each organism. Experiments within each organism type were conducted at a high and low inoculum of 10¹⁰ and 10⁶ colony forming units (cfu)/ml, respectively. Meropenem was evaluated against PA at 6.5 mcg/mL. Vancomycin was evaluated against SA at 15 mcg/mL. Bacterial quantification was determined by spot plating samples at 0, 4, 24, 48, and 72 hours post antibiotic introduction. Killing dynamics were compared by area under the curve of organism remaining over 72 h antibiotic exposure.

Results:

PA displayed an inoculum effect: meropenem was rapidly bactericidal in all strains at low inoculum but inactive against high inoculum, despite being susceptible (MIC=2). At high inoculum, both the persistent and non-persistent strains exhibited similar killing dynamics at 72 hours. However, low inoculum resulted in reduced killing in the PA persistent isolates vs non-persistent isolates (AUC_0-72h 90.2±0.3 versus 64.9, respectively, P<0.001). (mean difference 0.35, P=0.069. In SA, persistence isolates had reduced vancomycin activity at low inoculum with AUC_0-72h of 212.0±4.7 versus 81.5 with non-persistent strains, P=0.001. At high inoculum, no persistence was observed.

Conclusion:

Persistent PA and SA strains demonstrated differential killing at low inoculum, but this was not present at a high inoculum. This may be significant clinically at then end of antimicrobial therapy when bacterial burden has been reduced to low inoculum levels, allowing persistence to emerge, and in lung alveolar fluid where bacterial inocula often occur in this range during infection or colonization in CF. Future studies are needed evaluating this effect beyond 72 hours.

CITATION INFORMATION: Fose J., Bruckbauer D., Jorgenson M., Rose W. In Vitro Evaluation of Antibiotic Persistence in Pseudomonas aeruginosa and Staphylococcus aureus Isolates from Cystic Fibrosis Patients Am J Transplant. 2017;17 (suppl 3).

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