In patients with diabetes mellitus, TB treatment outcomes are poorer. Most parameters, when measured, reflect the slower bacteriological conversion from positivity to negativity and higher risks of disease relapse and mortality, as well as a greater propensity to develop drug-resistant TB. Aside from the well-known immunological dysfunction inherent to patients with diabetes mellitus, oxidative stress is likely a major underlying mechanism adversely impacting their TB treatment outcomes. Mycobacterium tuberculosis persisters, formed as a result of the core dormancy response to stress, possibly play a central role in this hypothesis. This hypothetical model also underscores the paramount importance of programmatic management of TB and diabetes mellitus, in collaboration, to improve the outcomes of patients with both diseases. The validity of these ideas could be further ascertained by laboratory and clinical research.
About 15% of patients with TB globally are estimated to have diabetes mellitus and this proportion will likely increase in the coming decades.1 Diabetes mellitus increases the risk of TB and worsens TB treatment outcomes, including slower bacteriological conversion, lower cure, higher relapse and mortality and possibly escalated drug resistance. Reciprocally, TB also hampers the optimal management of diabetes mellitus.1
It has been shown that the function of neutrophils, macrophages, dendritic cells and natural killer cells, as well as some other components of innate immunity and adaptive/acquired immunity, are significantly compromised by metabolic alterations in diabetes mellitus.2 Thus, immune dysfunction as a result of diabetes may play an important role in allowing the reactivation of TB from endogenous latent infection and increasing the host’s susceptibility to exogenous reinfection.2
In recent years, diseases that involve chronic inflammation and altered metabolism have been shown to be linked pathogenetically to oxidative stress.3 Complications of these diseases often result in significant morbidity and mortality. The higher mortality of TB in patients with diabetes mellitus may also be closely associated with complications of the metabolic disease.4 In diabetes mellitus, endogenous predisposition and exogenous factors may lead to development of inappropriate oxidative stress (Figure 1). Endogenous oxidative stress is likely related to hyperglycaemia and advanced glycation end-products.5 Exogenous oxidative stress factors principally include tobacco smoke, indoor air pollution, chemicals and radiation. We hypothesize that oxidative stress is directly responsible for the poorer TB treatment outcomes in patients with diabetes mellitus, largely through inducing formation of Mycobacterium tuberculosis persisters (Figure 1).
Persister phenotype in bacteria is well known as a population phenomenon. The characteristics of bacterial persistence generally include dormancy, multidrug tolerance and stochasticism.6 Environmental cues that induce formation of bacterial persisters are diverse, with antibiotic use, starvation, extreme changes in pH, temperature and oxygen status, as well as host immunity, being commonly listed as stress conditions.7
Recently there have been a number of reports regarding stronger evidence of oxidative stress in stimulating the formation of bacterial persisters. Paraquat-generated oxidative stress was found to induce the SoxRS and OxyR regulons and to markedly increase persister cell formation in Escherichia coli, especially after fluoroquinolone challenge.8 Oxidative stress generated by pyocyanin was shown to increase the production of catalase and superoxide dismutase in Acinetobacter baumannii, together with a significant rise in the number of bacterial persisters.9 Similarly, oxidative stress, through bile administration, was shown to induce persister formation in Salmonella Typhi, after amikacin/carbenicillin exposure.10 Furthermore, the role of the quorum-sensing mechanism(s) related to oxidative stress-induced persister formation was shown.9,10,A. baumannii has one quorum-sensing system, involving the transcriptional regulator AbaR, which forms a complex with the lactone encoded by the auto-inducer synthase gene (abaI). AbaI and AbaR have significant homology with LuxI and LuxR, a family of cell density-responsive transcriptional regulators first reported in Vibrio fischeri. By using A. baumannii M2 and its quorum-sensing mutant M2 (abaI::Km), a lower production of anti-oxidative enzymes in the mutant alongside its greater pyocyanin sensitivity, as compared with the WT strain, was observed.9 Similar results were obtained in another report regarding Salmonella Typhi.10
The toxin–antitoxin systems are of great importance in controlling the survival of bacteria. More than 30 years ago, the hipA7 allele in E. coli K-12 was found to be associated with a greater propensity to induce persister formation. Subsequently, mutagenesis studies have shown a marked effect on persistence, with deletion of tisB, mqsR or hipA operons.11 Association of MazEF and RelBE with bacterial persistence has also been found.12 Among the toxin–antitoxin type II system families, HipA-HipB is highly recognized for its role in bacterial persistence.11 In M. tuberculosis, there are many more toxin–antitoxin systems than in other bacteria, suggesting the probably greater capacity of tubercle bacillus to enter into dormancy and persistence under conditions of stress. Using Mycobacterium smegmatis, the toxins of the largest family of the systems, VapBC, were found to act by inhibiting translation via mRNA cleavage. The toxin–antitoxin systems, Rv2009-2010 and Rv1955-1956, were induced during hypoxia. These loci share the same genomic island with dosT and fdxA, belonging to the dormancy regulon. During macrophage infection, Rv1560-1561 and Rv0549c-0550c were found as induced systems.13 The RelBE module in the toxin–antitoxin systems has the notable role of inducing M. tuberculosis persistence when confronted with antibiotic stress.14 There has been a similar report regarding the up-regulation of three important relMtb toxin genes, and the corresponding antitoxins under states of oxidative stress.15 Alongside the Dos regulon, the definitive roles of mycobacterial genes pertaining to stringent response, SOS, DNA repair and protection, energy production, efflux activity and trans-translation, in the context of oxidative stress and other forms of stress, would benefit from further data accumulation.16–18 A recent study of high-persister mutants of M. tuberculosis has revealed the involvement of a wide array of candidate genes related to lipid biosynthesis and carbon metabolism, as well as transcriptional regulators, similar to the findings in some other bacteria.12,19
In addition to a high overall load of M. tuberculosis, an elevated proportion of persisters in the bacillary population may delay sputum bacteriological conversion and lower the probability of cure of TB. In diabetes mellitus a higher reactivation (relapse) rate of TB, especially in elderly subjects, has been observed,1,20 probably in association with increased formation of mycobacterial persisters (Figure 1). Immune dysfunction and oxidative stress are significant issues in ageing.21,22
Bacterial persisters are noted for their phenotypic tolerance to antimicrobials.8,12 Increased activity of efflux pumps is probably largely responsible for the phenotypic tolerance of M. tuberculosis persisters to anti-TB drugs.23 In diabetes mellitus, reduced exposure to rifampicin could occur in a proportion of patients.24,25 The underlying mechanisms are not fully known, but lowered anti-TB drug bioavailability, when present, may constitute a pharmacokinetic–pharmacodynamic scenario to further induce drug efflux pumps of M. tuberculosis persisters.26 It is possible that even a small effect may have dramatic consequences, as the currently recommended dosing of rifampicin in standard TB treatment regimens may be suboptimal.27
Phenotypic tolerance is conceivably not dichotomous with genetic resistance.28,29 The former may permit or even facilitate the development of the latter, especially with a high mycobacterial burden and host immune dysfunction. As clinical drug resistance results from the selection of spontaneously emerging drug-resistant genetic mutants (Figure 1), largely in the face of poor physician prescription, poor patient adherence and poor drug quality/supply, phenotypic tolerance possibly provides the necessary but not sufficient condition for evolution towards clinically relevant drug resistance. Well-functioning TB control programmes can probably address most of these issues. Disparate findings regarding drug resistance in M. tuberculosis strains in diabetes mellitus cohort studies in different countries and geographical areas might partly reflect the diversity in performance of their TB control programmes.4,30
Regarding adverse reactions due to anti-TB drugs, isoniazid-induced neurotoxicity and hepatotoxicity31,32 and clofazimine-induced cardiotoxicity33 were recently shown in animal studies to be associated with oxidative stress. Complications of diabetes mellitus, putatively also driven by oxidative stress,3 may aggravate these therapeutically induced adverse reactions.
The oxidative stress mechanism may appear too simple to fully explain the impact of diabetes mellitus on the outcomes of TB treatment. It is likely that other molecular mechanisms, especially nitrosative stress and reductive stress, play additional and essential roles. Consequently, participation of the redox homeostasis in a broad perspective is biologically plausible.34 Furthermore, oxidative stress can interact with immunological responses.2 These caveats notwithstanding, the present hypothesis can serve as a platform to explore the interaction of TB, diabetes mellitus, smoking and ageing, and underscores the paramount importance of programmatic management of both TB and diabetes mellitus in collaboration.1 The putative mechanisms impacting TB treatment outcomes, depicted in the hypothesis, may help to shed light on the directions and strategies worthy of exploration in the research agenda for currently rampant epidemics of TB and diabetes mellitus. Oxidative stress induces a number of derangements in cell physiology, including lipid peroxidation, protein oxidation and change in levels of antioxidant substrates and enzymes.3 These alterations potentially constitute a basis for testing the hypothesis in both experimental and clinical settings. Furthermore, studies on the association of oxidative stress and poor glycaemic control in diabetic patients are informative,3 as would be also those regarding the association of oxidative stress and TB pathogenesis, cure, relapse and mortality in animal models and patient populations. M. tuberculosis persister assays have undergone substantial advancement.35 Well-designed epidemiological and clinical cohort studies to better inform the impact of performance of TB control programmes, with respect to drug-resistant TB scenarios in diabetes mellitus, would further complement testing the validity of the current hypothesis.
Li Ka Shing Institute of Health Sciences is gratefully acknowledged for providing technical support to this research.
None to declare.