Staphylococcus aureus Enriched in Ordered Lipids Present Resistance Towards the Antibacterial Agent sPLA2-IIA: An Unusual Mechanism to Survive

Abstract

Bacterial membranes present solid-ordered/liquid-disordered (so/ld) cooperative melting event close to physiological temperature. The cellular advantage of this thermotropic melting event is yet to be determined. We show that this thermal behavior provides resistance towards a membrane active antibacterial agent. Phospholipase A2 type IIA (sPLA2-IIA) is a hydrolytic enzyme which presents antibacterial properties towards Gram positive bacteria. The enzyme has higher activity in ld phase compared to so phase in anionic membranes. We show that the lipid phase behavior of Staphylococcus aureus (S.aureus) membranes as measured by FTIR modulates sPLA2-IIA by inducing a sharp drop in activity below the melting temperature of the membrane (centered at 15.3°C). The effects of sPLA2-IIA treatment on cell viability are also investigated. While above the main melting event viability drops to 20% of the initial CFU after treatment, below the main melting event cell viability only drops to 60% under the same treatment. This strongly suggests that cells in the solid-ordered phase are better adapted to survive the enzymatic insult. These results led us to explore if a subpopulation of S.aureus enriched in ordered lipids can be selected after repeated treatment with sPLA2-IIA at 37°C. After selecting for resistance at 37°C we measured growth curves, membrane order, and cell viability as a function of treatment temperature. The results suggest that at 37°C there is a bacterial subpopulation with increased membrane rigidity that insures survival of the colony to an insult by sPLA2-IIA. This subpopulation also presents a longer latency period which can be explained by the increased presence in ordered lipids, which are known to inhibit cell division. Even if the growth conditions are not optimal, the presence of this subpopulation ensures survival from the antibacterial insult.