Virulence factors, such as staphylococcal enterotoxin A (SEA), are contained within membrane vesicles (MVs) in the cell membrane of Staphylococcus aureus. In this study, the effects of the growth stage on quantitative and qualitative changes in the components contained in the MVs of S. aureus SEA-producing strains were examined. Changes in the expression levels of S. aureus genes were examined at each growth stage; phenol-soluble modulin (PSM) gene reached a maximum after 8 h, and the expression of cell membrane-related genes was decreased after 6 h. Based on these gene expression patterns, MVs were prepared at 6, 17, and 24 h. The particle size of MVs did not change depending on the growth stage. MVs prepared after culture for 17 h maintained their particle size when stored at 23 °C. The amount of SEA in the culture supernatant and MVs were not correlated. Bifunctional autolysin, a protein involved in cell wall biosynthesis/degradation, was increased in MVs at 17 h. The expression pattern of inflammation-related genes in human adult low calcium high temperature (HaCaT) cells induced by MVs was different for each growth stage. The inclusion components of S. aureus-derived MVs are selective, depend on the stage of growth, and may play an important role in toxicity.

Anaplasma phagocytophilum, an agent of human granulocytic anaplasmosis, is an obligatory intracellular bacterium that dominantly produces P44 outer membrane proteins encoded by the p44/ msp2 multigene family, which are major antigens for serodiagnosis. However, A. phagocytophilum antigens from cultures with different cell lines seem to have varying reactivities with sera. In this study, we performed RNA-seq to investigate the P44 expression of A. phagocytophilum propagated in 4 cell lines. In infected HL-60 cells, the P44-2b transcript was predominant in the first RNA-seq analysis (HL-60.1). However, the P44-23 transcript was predominant in the second RNA-seq analysis at 1 month after additional passages (HL-60.2). We further analyzed the P44 expression of A. phagocytophilum cultured in THP-1, NB4, and RF/6A cells through consecutive passages in the same cell lines for 1 year after transferring A. phagocytophilum from infected HL-60 cells to the respective cell lines. In the long-term cultures, P44-18, P44-78, and P44-51 were predominantly transcribed in infected THP-1, NB4, and RF/6A cells, respectively. Therefore, the predominant shifts of different P44-expressing transcripts of A. phagocytophilum might occur during cell culture even in the same cell line at different time points of sample harvest (HL-60.1 and HL-60.2), which may be attributed to host cell adaptation/selection/interaction.