Mathematical analysis-based feasibility study of pre-emptive medicine for Staphylococcus aureus infectious disease: Early detection and antibiotic-free maintenance therapy
Introduction
In the past half-century, the clinical implementation of various indwelling medical devices, such as the needle, cannula, catheter and ventricular assist device, has been achieved. These devices have contributed to an improvement in the survival rate. However, an inability to prevent infectious diseases has remained as one of the most important problems in this field. Some bacterial strains including Staphylococcus and Streptococcus form a resident flora around these devices. As such, the unsanitary use of these devices frequently causes an exit-site infection. Staphylococcus aureus (S. aureus) shows the most extensive virulence within this group (Archer, 1998).
S. aureus is a gram-positive bacterium that inhabits soft tissues such as the epidermis and nasal cavity (Miller and Findon, 1997). Hence, the contact contamination of indwelling medical devices is one of the major factors inducing S. aureus infectious disease (Miller and Findon, 1997). Moreover, S. aureus exhibits various intercellular community effects on pathogenic expression and causes opportunistic infections and deadly infections such as skin abscess (Mayville et al., 1999; Wright et al., 2005) and endocarditis (Cheung et al., 1994). Patients intermittently cleans the exit-site of medical devices in order to decline of the number of cells and cell density. It is difficult to continuously clean the exit-site of medical devices. The intermittent clean of the exit-site has no fully suppression of the infectious disease, thus medical staff typically uses antibiotics as the therapeutic strategy for S. aureus infectious disease. However, long-term antibiotic therapies have detrimental effects on living systems. For instance, S. aureus easily acquires drug resistance, resulting in intractable cases (Wang et al., 2009; Cho et al., 2012). In other cases, the resident flora causes dysfunction and disturbs the immune system (Gyires et al., 2014). These findings imply that (1) the prolonged and heavy use of antibiotics complicates the S. aureus infection control and (2) treatments need to maintain resident flora in a bacteriostatic condition. Nonetheless, the prolonged use of ointment including antibiotics is clinically practiced to prevent from the progress of S. aureus infectious disease. Under present circumstances, it may be difficult to alleviate a severe infectious disease without employing antibiotics. To decline antibiotics dose effectively in S. aureus infectious disease, we have to develop a novel therapeutic strategy realising the early detection and treatment of such diseases, that is called pre-emptive therapy.
The pathogenic expression of S. aureus is regulated by the Accessory gene regulator (Agr) system (Miller and Bassler, 2001; Novick and Geisinger, 2008). This system mainly consists of four proteins, AgrB, AgrD, AgrC and AgrA, which are encoded by the agrBDCA operon. AgrB modifies AgrD into auto-inducing peptide (AIP) and transports the AIP to extracellular space (Fig. 1a). When the extracellular AIP concentration sufficiently increases, the AgrC receptor on the cellular membrane binds with the extracellular AIP and phosphorylates intracellular AgrA (AgrAp). AgrAp upregulates agrBDCA as well as the Staphylococcal accessory element (Sae) gene expression (Fig. 1b). Sae is an important positive regulator of many virulence factors, such as sea (S. aureus-specific staphylococcal enterotoxin A gene), hla (alpha-hemolysin gene), hld (delta-hemolysin gene) and coa (coagulase gene), and a negative regulator of spa (S. aureus-specific staphylococcal protein A gene) and sspA (glutamyl endopeptidase gene) (Giraudo et al., 1997, 1999). Thus, both Agr and Sae systems are enhanced by a positive feedback mechanism. Moreover, an increase in S. aureus cell density promotes the concentration of extracellular AIP, which is one of the crucial factors behind pathogenic expression. This intercellular community effect, which is called quorum sensing, makes it possible for S. aureus to determine a suitable time for pathogenic gene expression as a function of cell density (Engebrecht et al., 1983; Fuqua et al., 1994). Many researchers have engaged in novel therapeutic development for the prevention of S. aureus infectious disease and have proposed several antibiotic-free therapies. For example, Park et al. isolated AIP antibody and suppressed an increase in extracellular AIP concentration by using it (Park et al., 2007). In other cases, Lyon et al. and Scott et al. synthesised AIP analogues that inhibit quorum sensing (Lyon et al., 2000, 2002; Scott et al., 2003). Moreover, Nakayama et al. identified a cyclic depsipeptide with high constitutional similarity to AIP, and demonstrated that it acts as an antagonist of the AgrC receptor (Shojima and Nakayama, 2014; Desouky et al., 2015). Their therapeutic strategy involves inhibiting upregulation of the Agr system positive feedback mechanism by the interruption of extracellular AIP signal transduction, and disturbing activation of the Sae system. Such antibiotic-free therapies could be potential candidates of pre-emptive medicine for S. aureus. However, there is currently no method for early pre-symptom detection. Furthermore, we have no universal method for identifying therapeutic targets at the pre-symptom stage. Resolution of these issues is essential for the validation of above-mentioned candidates, and should contribute to the establishment of pre-emptive medicine for S. aureus infectious disease.
In this work, we mathematically explored a pre-symptom associated with the pathogenic expression process of S. aureus, and discussed the feasibility of the above-mentioned antibiotic-free therapy from the perspective of pre-emptive medicine. Firstly, we constructed a novel mathematical model for the pathogenic expression of an S. aureus multicellular system based on a conventional model that simulates pathogenic expression of an S. aureus single cellular system (Jabbari et al., 2010). Secondly, using our proposed models, we analysed the dynamic characteristics of the Agr system in the pathogenic expression process, and identified a pre-symptom of S. aureus infectious disease seen early in the incubation period. Thirdly, we explored several therapeutic targets that delay or suppress the appearance of the pre-symptom by employing sensitivity analysis. Finally, we evaluated the availability of the above-mentioned antibiotic-free therapy for S. aureus infectious disease.
Section snippets
Single cellular system
A mathematical model that represents the pathogenic expression process of a single S. aureus cell was constructed in order to evaluate the stochasticity of biochemical reaction processes (the single cellular system). Tables S1–S3 show the details of the mathematical model. The pathogenic expression mechanism was divided into the Agr system and the Sae system (Fig. 1). The Agr system and the Sae system were modelled based on Jabbari's model (Jabbari et al., 2010) and Gustafsson's model (
Mathematical modelling
Jabbari et al. constructed a deterministic mathematical model that represents the activation process of the Agr system (Jabbari et al., 2010). This was useful for analysing the activation process under specific conditions in which agrBDCA gene expression was enhanced, as seen in some single cells in S. aureus colonies that show fully upregulated pathogenic gene expression. However, the current study focused on elucidation of the dynamic characteristics of the Agr system in the early incubation
Discussion
Current primary care for S. aureus infectious disease is antibiotic medication. This therapeutic strategy results in progressively more severe disease if there is a delay in treatment initiation. Moreover, heavy use of antibiotics has been contraindicated given the emergence of drug-resistant organisms, and is among the factors causing both intractable cases and the spread of infection. Against this background, efforts are now being made to achieve the clinical implementation of pre-emptive
Conclusions
We identified a pre-symptom of S. aureus pathogenic gene expression by employing stochastic mathematical analysis. The pre-symptom was a peak of fluctuation in the intracellular AgrD concentration seen in the time course data. Next, employing sensitivity analysis, we explored several therapeutic targets that make it possible to delay or suppress pre-symptom development. Inhibition of binding between the AgrC receptor and extracellular AIP was identified as a target for antibiotic-free
Acknowledgments
This work was supported by JSPS KAKENHI Grant Number JP16K12912. We thank members from the committee for application of new technologies to blood purification therapy in Japanese Society of Dialysis Therapy who provided insight and expertise that greatly assisted the research.
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