Exopolyphosphatase of Mycobacterium Tuberculosis Might Limit the Growth of Bacteria Which Thrive in Inflamed and Injured Lung
Kupat Cholim Leumit, Beit Shemesh, Israel
*Corresponding Author: Janice Block, Kupat Cholim Leumit, Beit Shemesh, Israel
Received: 19 August 2019; Accepted: 26 August 2019; Published: 29 August 2019
Citation: Janice Block. Exopolyphosphatase of Mycobacterium Tuberculosis Might Limit the Growth of Bacteria Which Thrive in Inflamed and Injured Lung. Archives of Microbiology & Immunology 3 (2019): 102-106.View / Download Pdf
Pulmonary tuberculosis resembles cystic fibrosis and other chronic inflammatory lung diseases in its ability to cause chronic tissue destruction, hypoxia, tissue acidosis, and leakage of blood into the surrounding milieu. Destructive and inflammatory changes of the lung in diseases such as cystic fibrosis support the increased growth of the commensal Prevotella, a bacterial commensal which requires polyphosphate and thrives under inflammatory conditions. Since the nutritional needs of Prevotella are in some ways mirrored by those of the pathogens Pseudomonas aeruginosa and Berkholderia cepacia, it is possible that Prevotella may serve as a marker for these destructive lung pathogens. The biochemical milieu of pulmonary tuberculosis resembles that of other chronic inflammatory lung diseases in its ability to cause inflammation, destruction, tissue acidosis, and bleeding, yet in pulmonary tuberculosis, contrary to expectation, Prevotella species are decreased rather than increased. It is hypothesized that the M. tuberculosis exopolyphosphatase may serve to reduce polyphosphates required by Prevotella species. Since the nutritional needs of P. aeruginosa and B. cepacia resemble those of Prevotella, it is reasonable to hypothesize that addition of exopolyphosphatase might also hamper the growth of these dangerous lung pathogens, as well.
Tuberculosis; Cystic fibrosis; Prevotella; Pseudomonas; Berkholderia; Polyphosphate; Exopolyphosphatase
In some respects, the process of decline in pulmonary tuberculosis resembles that of cystic fibrosis and other chronic inflammatory conditions of the lung. In all such conditions there may be lung tissue destruction, hypoxia, local tissue acidosis, leakage of blood into the airway, or loss of weight. Bacterial pathogens involved pulmonary inflammatory processes - such as Pseudomonas aeruginosa and Berkholderia cepacia in cystic fibrosis and various causes of inflammation and bronchiectasis; Mycobacterium tuberculosis in pulmonary tuberculosis - often have similar nutritional requirements for iron and polyphosphate. Acquisition of polyphosphate depends on the availability of ATP and bacterial polyphosphate kinase, and leakage of blood and heme into the airway supplies the requirement for iron in ample measure. In the bacterial pathogens above - P. aeruginosa, B. cepacia, and M. tuberculosis - all utilize heme for iron, and all depend on polyphosphate kinase to synthesize polyphosphate [1-8].
Since the composition of pulmonary microbiota depends on local nutritional, inflammatory, and infectious factors, it should come as no surprise that inflammatory disease processes such as cystic fibrosis, bronchiectasis, and toxic lung exposure select for pulmonary commensals which require iron and polyphosphate for survival and growth and which are capable of thriving in an acidic environment. Prevotella species conform to expectations in both respects: they depend on polyphosphate and iron, and they are capable of growing at lower pH than are many other commensals. Accordingly, commensal Prevotella species are increased in cystic fibrosis, in toxic insult to the lung, and in other pulmonary infectious/inflammatory conditions [9-16].
Given the proclivities of Prevotella, the presence of this commensal may serve as a marker for certain environmental conditions, as well as for bacterial pathogens which are associated with those conditions. Since Prevotella requires heme iron and polyphosphate for growth and is capable of thriving at low pH, its increased dominance as a commensal in BAL fluid might suggest the increased presence of iron, blood, polyphosphate, and acid within the lung. Such conditions, as heralded by Prevotella, might also predispose to infection by P. aeruginosa or B. cepacia, lung pathogens whose nutritional needs resemble those of Prevotella, and which may promote acidosis of local tissue.
In the generalization that chronic inflammatory conditions of the lung tend to lead to increased growth of Prevotella, pulmonary tuberculosis is an exception to the rule. As a pathogen, M. tuberculosis meets all of the presumed criteria for increased growth of Prevotella species: M. tuberculosis requires both heme iron and polyphosphate, induces leakage of blood into the airway, and is capable of producing an acidic, inflammatory environment within the lung. In such an environment, one might expect that Prevotella species, and perhaps also Pseudomonas or Berkholderia species, would thrive. Instead, the opposite is true. In pulmonary tuberculosis, Prevotella species are decreased rather than increased, and superinfection by organisms such as Pseudomonas or Berkholderia is much rarer in pulmonary tuberculosis than in cystic fibrosis and other causes of bronchiectasis [6,17-20] Why might this be?
Structural differences aside, one answer is suggested by the unique ability of M. tuberculosis to adjust its metabolism in order to adapt to starvation conditions, such as that of the oxygen and inorganic phosphate-limited environment of the pulmonary granuloma. This metabolic adaptation, a type of stringent response, is controlled by the enzyme RelMtb. Inorganic phosphate starvation, hypoxia, and polyphosphate accumulation activate transcription of RelMtb, which upregulates synthesis of the small molecules involved in the stringent response: guanosine 5’-diphosphate 3’diphosphate (ppGpp) and guanosine 5’-triphosphate 3’-diphosphate (pppGpp), together denoted (p)ppGpp. (p)ppGpp, in turn, serves as a second messenger for the stress response . The result is decreased growth and decreased biofilm formation.
An important aspect of the stringent stress response in M. tuberculosis is that it is triggered both by inorganic phosphate deficit and also by polyphosphate excess. In M. tuberculosis, polyphosphate is synthesized via the action of polyphosphate kinase and hydrolysed via the action of exopolyphosphatase 2 (ppx2). When inorganic phosphate is scarce in the local environment, the stringent response regulates growth. On the other hand, when polyphosphate reaches a critical threshold, once again, the stringent response regulates growth [22,23].
Chuang et. al. (2015) found that, in M. tuberculosis, in a ppx2 knockdown strain, excess accumulation of polyphosphate resulted in growth restriction and reduced biofilm formation [22,23]. The resultant phenotype is one of growth regulation on both ends of the spectrum. M. tuberculosis differs from pathogens such as P. aeruginosa and B. cepacia in its ability to persist for many years in a latent or semi-latent state within the lung. It is possible that the regulation of growth by M. tuberculosis during times of plenty might contribute to the ability of M. tuberculosis to evade host immunity, thereby persisting over extended periods of time in a latent or semi-latent state .
In pulmonary infection by M. tuberculosis, polyphosphate overproduction triggers the stringent response to restrict bacterial growth, which in turn reduces subsequent polyphosphate production by the organism. Ongoing bacterial exopolyphosphatase activity hydrolyzes polyphosphate, which enables growth, but growth, in turn, requires further production of polyphosphate. The effect is a tightly regulated system in which less polyphosphate may be available in the surrounding milieu.
This polyphosphate regulating behavior of M. tuberculosis could have exciting implications for predisposition to superinfection, as well as for growth of associated microbiota such as Prevotella. In pulmonary tuberculosis, a commensal such as Prevotella would not be expected to lack for heme iron and would not be hampered by the inflammatory milieu. But thanks to the ongoing activity of mycobacterial exopolyphosphatase and to ongoing growth restriction during times of excess polyphosphate production, what Prevotella might lack is a source of readily available polyphosphate.
This condition differs from the situation in cystic fibrosis or the situation in traumatic or toxic lung damage. In these cases, the underlying cause of lung damage is genetic/metabolic or traumatic rather than infectious, and the bacterial regulatory processes and exopolyphosphases of M. tuberculosis do not play a role in limiting environmental polyphosphate. Perhaps it is for that reason that Prevotella often thrives under such conditions.
What can be said for Prevotella might also be said for pathogens such as P. aeruginosa and B. cepacia, pathogens which appear to have similar nutritional needs and characteristics to those of Prevotella. For if M. tuberculosis exopolyphosphatase can control the growth of Prevotella, perhaps an exopolyphosphatase could limit the growth of Pseudomonas and Berkholderia species, as well. This trait may hold potential for control of pulmonary infection in predisposing conditions such as cystic fibrosis.
The author acknowledges no source of external funding.
Conflicts of interest
The author declares no conflicts of interest.
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