Comparative evaluation of nutrient media quality control using LYOSHTAMM lyophilised test strains: Results of interlaboratory trials

INTRODUCTION. Microbiological quality control of medicinal products relies on the use of reference test strains with stable cultural and morphological properties. Use of lyophilised test strains standardised by number of viable cells reduces the labour intensity of testing compared to the traditional method of preparing microbial suspensions. AIM. This study aimed to evaluate reproducibility, homogeneity, and stability of results when assessing growth-promoting properties of nutrient media using traditional preparation method of microbial suspensions and the method based on LYOSHTAMM lyophilised test strains standardised by the number of viable cells. MATERIALS AND METHODS . Two test strains were used: Salmonella enterica subsp. enterica serovar Abony IHE 103/39 and Staphylococcus aureus ATCC 6538. Compared was the method using standardised lyophilised LYOSHTAMM samples (one batch per strain, standardised by the number of viable cells) and the reference method. The latter uses microbial suspensions prepared in compliance with the Russian State Pharmacopoeia (pharmacopoeial monograph “Microbiological purity”). The study was conducted across seven independent laboratories over three days. A total of 378 individual results were obtained for each LYOSHTAMM batch and 126 results — for the reference method. RESULTS. When evaluating growth-promoting properties of the media, both test strains based on LYOSHTAMM samples met all acceptability criteria: the number of colony-forming units per plate ranged from 10 to 100; typical cultural and morphological properties and growth time was confirmed. Intralaboratory precision (RSD) ranged from 7% to 21% for LYOSHTAMM method and from 2% to 36% for reference method. The interlaboratory precision for LYOSHTAMM samples was 15–17%, while for reference method, it was 38–45%. Time to visible colonies (≈18 hours) and their cultural and morphological properties showed no significant differences between the methods. Analysis of inter-vial homogeneity showed consistent mean values and relative standard deviations at intralaboratory production control ( n =5) and interlaboratory trials ( n =63), confirming the stability of LYOSHTAMM batch (relative standard deviation ≤17%). CONCLUSIONS. The control method for nutrient media using standardised lyophilised LYOSHTAMM test strains yields results comparable to the traditional method in terms of growth and morphology. A key advantage of this method is a significantly improved interlaboratory reproducibility, with a more than two-fold reduction in variability. LYOSHTAMM test strains can be recommended as reference standards for routine microbiological testing and analytical validation.