Comparison of solid-phase cytometry and the plate count method for the evaluation of the survival of bacteria in pharmaceutical oils


Hans J. Nelis, Laboratory for Pharmaceutical Microbiology Ghent University Harelbekestraat 72, B-9000 Ghent, Belgium. E-mail:


Aim:  To compare the survival of four bacterial strains (Escherichia coli, Proteus mirabilis, Staphylococcus aureus, Pseudomonas aeruginosa) in pharmaceutical oils, including jojoba oil/tea tree oil, carbol oil, jojoba oil and sesame oil.

Methods and Results:  Oils were spiked with the test bacteria in a concentration of 104 CFU ml−1. Bacteria were extracted from oils with phosphate-buffered saline containing 0·5% Tween 20. Aliquots of the pooled water layers were analysed by solid-phase cytometry and plate counting. Plate counts dropped to zero for all test strains exposed for 24 h to three of the four oils. In contrast, significant numbers of viable cells were still detected by SPC, except in the jojoba oil/tea tree oil mixture and partly in sesame oil.

Conclusions:  Exposure of bacteria for 24 h to the two oils containing an antimicrobial led to a loss of their culturability but not necessarily of their viability. The antibacterial activity of the jojoba oil/tea tree oil mixture supersedes that of carbol oil.

Significance and Impact of the Study:  These in vitro data suggest that the jojoba oil/tea tree oil mixture more than carbol oil inhibits bacterial proliferation when used for intermittent self-catherization.