Investigation of a viable but non-culturable state in Porphyromonas gingivalis and host cell invasion

Porphyromonas gingivalis ( P. gingivalis ) is a gram-negative, black-pigmented, anaerobic pathogen known for its biofilm formation and its central role in periodontal disease. More recently, P. gingivalis has been implicated in various systemic conditions, including atherosclerosis, Alzheimer’s disease, and certain types of cancer, such as pancreatic and oral cancer. This bacterium employs several mechanisms to evade environmental stress, thereby contributing to its pathogenicity. The viable but non-culturable (VBNC) state is characterized by bacteria that remain viable but have reduced metabolic activity and are unable to form colonies on conventional culture media. To induce the VBNC state in P. gingivalis , we subjected the bacterium to oxidative stress using H 2 O 2 and subsequently resuscitated it from this state with sodium pyruvate. We utilized viability staining, confocal microscopy, and flow cytometry (FC) to count live and dead bacteria, confirming the presence of significant numbers of viable P. gingivalis cells both before and after stress induction. Despite being viable, the stressed P. gingivalis failed to form colonies on blood agar plates after seven days of incubation, indicating it had entered the VBNC state. We were then able to resuscitate the VBNC P. gingivalis by adding sodium pyruvate, and the growth of the resuscitated bacteria on plates was comparable to that of control P. gingivalis . Investigation into the invasiveness of P. gingivalis in the VBNC state was conducted using human coronary artery endothelial cells (HCAECs). P. gingivalis in the VBNC state demonstrated the ability to invade and based on live/dead staining, showed that a substantial proportion of the VBNC P. gingivalis remained viable within the host cells for extended periods. In this study, we explore the VBNC survival strategy previously described in many aerobic bacteria but not previously reported in anaerobes such as P. gingivalis . The objectives of this study are to verify the VBNC state in P. gingivalis, determine whether this state can be reversed and assess the extent to which it impacts the ability of P. gingivalis to invade host cells. Understanding the VBNC and resuscitation states will be instrumental in guiding the development of more effective therapies for periodontitis and other diseases associated with P. gingivalis infection.