Human serum lipids affect Staphylococcus aureus sensitivity to phage infection

To explore phage therapy as an alternative for targeting multidrug-resistant bacterial strains, it is crucial to study phage-bacteria interaction under conditions that mimic in vivo environments. Recent studies have demonstrated that staphylococcal phage activity can be significantly hindered by human blood components, including plasma and serum. This study aimed to identify serum components responsible for phage inhibition and assess whether this effect occurred across multiple Staphylococcus aureus strains. Phage Sb-1 activity against S. aureus ATCC 43300 was tested by pre-incubating or co-incubating bacteria and phages with 30% (v/v) heat-inactivated (56°C and 80°C) human, bovine, or foetal calf serum, IgG-depleted or delipidated serum, and BSA. Bacteria were incubated with serum before phage exposure, followed by washing with 0.1% Triton X-100. Additionally, growth kinetics of ten S. aureus strains incubated with or without Sb-1 were assessed over 24 hours in the presence of human serum. We found that adult human serum completely impairs phage infectivity due to interactions between serum components and bacterial cells rather than direct phage neutralisation. Albumin, IgG, and thermolabile components were demonstrated not to significantly contribute to the inhibitory effect, whereas lipids were identified as playing a key role. Furthermore, the sensitivity of different bacterial strains to phages was shown to be differentially affected by the presence of serum, as two of the strains tested remained susceptible to lysis despite serum exposure. Taken together, our findings suggest that serum lipid fraction impacts phage infectivity in a strain-specific manner, highlighting the need for tailored approaches for phage therapy.