Sterility testing for lipid mixtures using the BACT/ALERT^® system

Introduction / Objective
The Parenteral Nutrition Compounding Unit produces binary mixtures (protein, minerals, vitamins and, electrolytes) in bags, and a mixture of lipids and fat-soluble vitamins in syringes, for the nutrition of premature infants hospitalized in neonatal intensive care. The BACT/ALERT^® system (BAS), an automated reflectometry-based microorganism detection system, is used for sterility testing of binary mixtures, while no sterility testing is performed for lipid mixtures. This study aimed to assess the microorganism detection potential of BAS compared to the European Pharmacopoeia membrane filtration (MF) sterility testing on lipid mixtures and to integrate routine sterility testing for lipid mixtures.

Methods
Sterility testing was conducted on six microorganisms recommended by the European Pharmacopoeia: S. aureus, B. subtilis, P. aeruginosa, C. sporogenes, C. albicans and, A. brasiliensis (Bioball^®). Syringes containing a mixture of Smoflipid^® (2/3) and Vitalipide® (1/3) were contaminated with each microorganism. Four concentrations ranging from 0 to 1 CFU/mL were studied. Five mL of these mixtures were inoculated into aerobic and anaerobic BACT/ALERT^® blood culture bottles, while another 5 mL were filtered using MF and cultured. Three tests were conducted for each concentration, except for the syringe at 0 CFU/mL (n=1).

Results
For S. aureus, P. aeruginosa, and C. sporogenes, BAS showed higher sensitivity and shorter detection times compared to MF. For C. albicans, the sensitivity was the same for both methods and detection times were shorter with BAS. For B. subtilis, the sensitivity and detection times of BAS were lower. Regarding A. brasiliensis, the sensitivity of BAS was lower and detection times higher compared to MF. No detection by MF was noted for S. aureus and C. sporogenes.

Conclusion
BAS consistently provided repeatable results at the concentration of 1 CFU/mL, except for A. brasiliensis. Despite the lower sensitivity for B. subtilis, BAS detected it at a concentration of 1 CFU/mL in all three tests. For A. brasiliensis, Sabouraud agar was used after MF, which explains the better sensitivity compared to BAS, as the latter is a non-specific culture medium. BAS also significantly reduced the turnaround time for test results. Consequently, this method will be integrated into routine practices. Production will be pooled to achieve a minimum final volume of 5 mL, which will be inoculated into both aerobic and anaerobic bottles.

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