Porphyromonas gingivalis is a rod-shaped to pleomorphic non-motile, gram-negative, obligate anaerobe, opportunistic pathogenic bacterium that forms black-pigmented colonies on supplemented (hemin, vitamin K1) blood-agar-plates after 3–7 days of incubation. Together with Tannerella forsythia and Treponema denticola it belongs to the red-complex of etiologically important anaerobes associated with periodontitis [1,2]. In the periodontal sulcus P. gingvalis makes use of a plethora of virulence factors such as lipopolysaccharide (LPS)-variants, hemophores and fimbriae to facilitate colonization and biofilm formation [3,4]. Furthermore, Porphyromonas gingivalis secretes exoproteases to circumvent the bactericidal innate host defense mechanisms, whilst further upregulating the cytokine-mediated inflammatory response to subsequently cause bacterial proliferation and dysbiosis [5]. Some of the more virulent P. gingivalis strains form a protective capsule that reduces phagocytosis ultimately increasing bacterial survival [6,7]. Consequently, P. gingivalis is recognized as the key periodontal pathogen [1]. On the systemic level, P. gingivalis strains have been isolated intra-orally from acute apical abscesses, infected root canals [8], periimplantitis-sites [9], as well as intra-abdominally [10], from vaginal samples in cases of vaginosis [11], amniotic fluid [12], occluded arteries of Buerger's disease patients [13] and from synovia of rheumatoid arthritis/psoriatic arthritis patients [14]. Reports of P. gingivalis being linked to age-related macule degeneration [15], Alzheimer's disease [16,17], adverse pregnancy outcomes [18], atherosclerotic disease [19], and cancer [20] need further confirmation.

Apart from the composition of the oral microbiome and host response [21], the incidence and severity of periodontitis are closely linked to several local, systemic and environmental risk factors such as smoking, diabetes mellitus, poor oral hygiene, and stress [22]. With the latter providing direct effects mediated by the systemic release of stress hormones (catecholamines and glucocorticoids) as well as indirect effects such as the deterioration of oral hygiene and unhealthy coping mechanisms including a high-fat diet and smoking, thereby further accentuating the vicious cycle of risk factors in favor of developing periodontitis [[23], [24], [25], [26], [27], [28], [29]]. Since the introduction of the microbial endocrinology concept in 1992, the direct role of (stress)-hormones on bacterial growth has received increasing attention [30]. A few studies attempted to understand the association between elevated hormone levels and the composition of subgingival microbiota [31,32], as well as the mechanism underlying this inter-kingdom signaling [33]. Among the stress hormones, catecholamines (adrenaline syn. epinephrine, noradrenaline syn. norepinephrine, dopamine) and hydrocortisone (cortisol) were studied in this context.

With regard to the possible effects of catecholamines on prokaryotes, the documented results in literature are very controversial. While some studies report a reduction in growth rates of up to 40% when challenged with noradrenaline [34,35], other findings document no effect on P. gingivalis proliferation [32,36]. Interestingly, the expression of virulence factors like gingipains [35,37], especially ArgX-protease levels, and genes related to iron acquisition as well as oxidative stress, were elevated by noradrenalin [35]. Other findings proposed a modulation of virulence genes by autoinducer (AI-2), a novel molecule and LuxS gene product, that is suggested to be involved in the quorum-sensing of this bacterium [38,39]. The production of this heat-stable molecule is modulated by noradrenaline in E. coli via Qsec sensor kinase proteins [[40], [41], [42]]. A similar mechanism could be involved in the inter-kingdom signaling between oral microbiota and the host [43,44]. Nevertheless, the mechanism behind these effects is yet to be fully understood and divergent - even contradictory - results were observed which could be explained by strain (responder)-dependency.

Hydrocortisone (HC, synonym cortisol) is a glucocorticoid hormone secreted by stimulation of the hypothalamus-pituitary-adrenal axis in response to psychological and physiological stress situations [45]. While numerous studies have proven the positive correlation between elevated hydrocortisone levels and the severity of periodontitis [29,[46], [47], [48], [49]], the direct effect of HC on P. gingivalis is understudied. In 2014, one group described a significant growth promotion of P. gingivalis ATCC 33277 within the first 24 h of incubation, peaking at 12 h in a dose-independent manner [50]. In 2016, the association of elevated HC levels in chronic periodontitis patients was examined and a positive correlation with P. gingivalis, but not with T. forsythia or T. denticola, was found [51]. Known from older data, the severity of periodontitis (measured by probing depth, bleeding on probing, attachment-loss) and IL-6 secretion seems to correlate with the presence of P. gingivalis and serum HC levels [52,53]. Furthermore, stress-suppressed recruitment of macrophages and stimulated NO-secretion upon exposure to P. gingivalis LPS [54], as well as high HC concentrations being observed to upregulate matrix-metalloproteinases-1,-2,-7,-11 and TIMP-1 (tissue inhibitor of MMP) in human gingival fibroblasts [55], additionally contribute to the stress related periodontal breakdown. Interestingly, recent findings show that HC can directly induce shifts in gene expression profiles of the oral microbiome to affect periodontal disease and its progression [56].

The current study aimed at providing further information on the strain- and dose-dependent effect of HC on P. gingivalis cultures in vitro, thereby establishing a novel real-time semi-automated method for spectrophotometric measurement of the kinetics of anaerobic cultures. Our findings could thus help to, i) increase the sensitivity when (sub-)cultivating P. gingivalis (or other anaerobes) in low abundance from clinical samples, and ii) increase the understanding of the complex (sometimes inconsistent) link between psychological stress and periodontitis.