In vitro treatment of Enterococcus faecalis with calcium hydroxide impairs phagocytosis by human macrophages

Area of Science:

• Microbiology
• Immunology
• Endodontics

Background:

• Enterococcus faecalis is a common cause of persistent root canal infections.
• Monocyte-derived macrophages (MDMs) are crucial immune cells involved in clearing bacterial pathogens.
• Alkaline conditions, such as those from calcium hydroxide used in root canal treatment, may affect bacterial virulence and host immune responses.

Purpose of the Study:

• To investigate the impact of alkaline stress on the phagocytic capacity and nitric oxide (NO) production of MDMs against Enterococcus faecalis.
• To compare the responses of MDMs to root canal strains versus urine isolates of Enterococcus faecalis under alkaline stress.

Main Methods:

• Enterococcus faecalis strains were subjected to alkaline stress in vitro.
• MDM-bacteria interactions were assessed using fluorescence microscopy to quantify phagocytosis.
• Nitric oxide (NO) concentration in cell supernatants was measured.

Main Results:

• Alkaline stress significantly reduced MDM phagocytosis of most tested E. faecalis strains.
• One root canal strain (ATCC4083) showed resistance to impaired phagocytosis.
• Nitric oxide production by MDMs was not significantly altered by alkaline stress.
• Urine isolates of E. faecalis induced higher NO levels compared to root canal strains.

Conclusions:

• Alkaline stress may compromise the immune system's ability to clear E. faecalis in root canals by reducing phagocytosis.
• Impaired bacterial recognition and phagocytosis by MDMs, possibly due to cell wall alterations, could contribute to treatment failure.
• Root canal strains of E. faecalis elicit distinct immune responses compared to urinary isolates.
• These findings suggest a mechanism for E. faecalis persistence in alkaline environments within root canal systems.