Resuscitation Promotion Factor: A Pronounced Bacterial Cytokine in Propelling Bacterial Resuscitation

While confronted with unfavorable growth conditions, bacteria may transform into the dormant state, such as viable but nonculturable (VBNC) state, which is a reversible state characterized by low metabolic activity and lack of division. These dormant cells can be reactivated through the influence of the resuscitation promoting factor (Rpf) family, which are classified as autocrine growth factors and possess peptidoglycan hydrolase activities. To date, with the significant resuscitation or growth promotion ability of Rpf, it has been extensively applied to increasing bacterial diversity and isolating functional microbial species. This review provides a comprehensive analysis of the distribution, mode of action, and functional mechanisms of Rpf proteins in various bacterial species. The aim is to create opportunities for decoding microbial communities and extracting microbial resources from real samples across different research fields.

Reaching unreachables: Obstacles and successes of microbial cultivation and their reasons

In terms of the number and diversity of living units, the prokaryotic empire is the most represented form of life on Earth, and yet it is still to a significant degree shrouded in darkness. This microbial "dark matter" hides a great deal of potential in terms of phylogenetically or metabolically diverse microorganisms, and thus it is important to acquire them in pure culture. However, do we know what microorganisms really need for their growth, and what the obstacles are to the cultivation of previously unidentified taxa? Here we review common and sometimes unexpected requirements of environmental microorganisms, especially soil-harbored bacteria, needed for their replication and cultivation. These requirements include resuscitation stimuli, physical and chemical factors aiding cultivation, growth factors, and co-cultivation in a laboratory and natural microbial neighborhood.

Solicola gregarius gen. nov., sp. nov., a soil actinobacterium isolated after enhanced cultivation with Micrococcus luteus culture supernatant

An actinobacterial strain, designated A5X3R13^T, was isolated from a compost soil suspension supplemented with extracellular material from a Micrococcus luteus-culture supernatant. The strain was cultured on tenfold-diluted reasoner's 2A agar. The cells were ovoid-to-rod shaped, non-motile, Gram-stain-positive, oxidase-negative, catalase-positive and had a width of 0.5 µm and a length of 0.8-1.2 µm. The results of both 16S rRNA-based phylogenetic and whole-genome analyses indicate that A5X3R13^T forms a distinct lineage within the family Nocardioidaceae (order Propionibacteriales). On the basis of the 16S rRNA gene sequence, A5X3R13^T was closely related to Aeromicrobium terrae CC-CFT486^T (96.2 %), Nocardioides iriomotensis IR27-S3^T (96.2 %), Nocardioides guangzhouensis 130^T (95.6 %), Marmoricola caldifontis YIM 730233^T (95.5 %), Aeromicrobium alkaliterrae KSL-107^T (95.4 %), Aeromicrobium choanae 9H-4^T (95.4 %), Aeromicrobium panaciterrae Gsoil 161^T (95.3 %), and Nocardioides jensenii NBRC 14755^T (95.2 %). The genome had a length of 4 915 757 bp, and its DNA G+C content was 68.5 mol %. The main fatty acids were 10-methyl C_17 : 0, C_16 : 0, C_15 : 0, C_18 : 0, C_17 : 0 and iso-C_16 : 0. The main polar lipids were phosphatidylglycerol, diphosphatidylglycerol, phosphatidylinositol and two unidentified phospholipids. MK-9(H₄) was the predominant respiratory quinone. The peptidoglycan type was A3γ (A41.1) and contained alanine, glycine, glutamic acid and ll-diaminopimelic acid in a molar ratio of 1.2 : 0.9 : 1.0 : 0.8. On the basis of the results of the phylogenetic and phenotypic analyses and comparisons with other members of the family Nocardioidaceae, strain A5X3R13^T is proposed to represent a novel species within a novel genus, for which the name Solicola gregarius gen. nov., sp. nov. is proposed. The type strain is A5X3R13^T (=DSM 112953^T=NCCB 100840^T).

Wake Up! Resuscitation of Viable but Nonculturable Bacteria: Mechanism and Potential Application

The viable but nonculturable (VBNC) state is a survival strategy for bacteria when encountered with unfavorable conditions. Under favorable environments such as nutrient supplementation, external stress elimination, or supplementation with resuscitation-promoting substances, bacteria will recover from the VBNC state, which is termed "resuscitation". The resuscitation phenomenon is necessary for proof of VBNC existence, which has been confirmed in different ways to exclude the possibility of culturable-cell regrowth. The resuscitation of VBNC cells has been widely studied for the purpose of risk control of recovered pathogenic or spoilage bacteria. From another aspect, the resuscitation of functional bacteria can also be considered a promising field to explore. To support this point, the resuscitation mechanisms were comprehensively reviewed, which could provide the theoretical foundations for the application of resuscitated VBNC cells. In addition, the proposed applications, as well as the prospects for further applications of resuscitated VBNC bacteria in the food industry are discussed in this review.

Single-cell-level microfluidics assisted with resuscitation-promoting factor technology (SMART) to isolate novel biphenyl-degrading bacteria from typical soils in eastern China

Soil microorganisms represent one of the largest biodiversity reservoirs. However, most low-abundance, slow-growing or dormant microorganisms in soils are difficult to capture with traditional enrichment culture methods. These types of microorganisms represent a valuable "microbial seed bank". To better exploit and utilize this "microbial dark matter", we developed a novel strategy that integrates single-cell-level isolation with microfluidics technology and culture with resuscitation-promoting factor (Rpf) to isolate biphenyl-degrading bacteria from four typical soils (paddy soil, red soil, alluvial soil and black soil) in eastern China. Multitudinous bacteria were successfully isolated and cultured; some of the identified clades have not been previously linked to biphenyl biodegradation, such as Actinotalea, Curtobacterium and Rothia. Soil microcosmic experiments validated that some bacteria are responsible for biphenyl degradation in soil. In addition, genomic sequencing and Illumina MiSeq sequencing of 16S rRNA genes indicated that exogenous Rpf mainly promotes the recovery and growth of bacteria containing endogenous Rpf-encoding genes. In summary, this study provides a novel strategy for capturing target functional microorganisms in soils, indicates potential bioresources for the bioremediation of contaminated soils, and enhances our current understanding of the mechanisms involved in the response to exogenous Rpf.

Streptomyces: Still the Biggest Producer of New Natural Secondary Metabolites, a Current Perspective

There is a real consensus that new antibiotics are urgently needed and are the best chance for combating antibiotic resistance. The phylum Actinobacteria is one of the main producers of new antibiotics, with a recent paradigm shift whereby rare actinomycetes have been increasingly targeted as a source of new secondary metabolites for the discovery of new antibiotics. However, this review shows that the genus Streptomyces is still the largest current producer of new and innovative secondary metabolites. Between January 2015 and December 2020, a significantly high number of novel Streptomyces spp. have been isolated from different environments, including extreme environments, symbionts, terrestrial soils, sediments and also from marine environments, mainly from marine invertebrates and marine sediments. This review highlights 135 new species of Streptomyces during this 6-year period with 108 new species of Streptomyces from the terrestrial environment and 27 new species from marine sources. A brief summary of the different pre-treatment methods used for the successful isolation of some of the new species of Streptomyces is also discussed, as well as the biological activities of the isolated secondary metabolites. A total of 279 new secondary metabolites have been recorded from 121 species of Streptomyces which exhibit diverse biological activity. The greatest number of new secondary metabolites originated from the terrestrial-sourced Streptomyces spp.

Generation of Distinct Differentially Culturable Forms of Burkholderia following Starvation at Low Temperature

Bacteria have developed unique mechanisms to adapt to environmental stresses and challenges of the immune system. Here, we report that Burkholderia pseudomallei, the causative agent of melioidosis, and its laboratory surrogate, Burkholderia thailandensis, utilize distinct mechanisms for surviving starvation at different incubation temperatures. At 21°C, Burkholderia are present as short rods which can rapidly reactivate and form colonies on solid media. At 4°C, Burkholderia convert into coccoid forms that cannot be cultured on solid agar but can be resuscitated in liquid media supplemented with supernatant obtained from logarithmic phase cultures of B. thailandensis, or catalase and Tween 80, thus displaying characteristics of differentially culturable bacteria (DCB). These DCB have low intensity fluorescence when stained with SYTO 9, have an intact cell membrane (propidium iodide negative), and contain 16S rRNA at levels comparable with growing cells. We also present evidence that lytic transglycosylases, a family of peptidoglycan-remodeling enzymes, are involved in the generation of coccoid forms and their resuscitation to actively growing cells. A B. pseudomallei ΔltgGCFD mutant with four ltg genes deleted did not produce coccoid forms at 4°C and could not be resuscitated in the liquid media evaluated. Our findings provide insights into the adaptation of Burkholderia to nutrient limitation and the generation of differentially culturable bacteria. IMPORTANCE Bacterial pathogens exhibit physiologically distinct forms that enable their survival in an infected host, the environment and following exposure to antimicrobial agents. B. pseudomallei causes the disease melioidosis, which has a high mortality rate and is difficult to treat with antibiotics. The bacterium is endemic to several countries and detected in high abundance in the environment. Here, we report that during starvation at low temperature, B. pseudomallei produces coccoid forms that cannot grow in standard media and which, therefore, can be challenging to detect using common tools. We provide evidence that the formation of these cocci is mediated by cell wall-specialized enzymes and lytic transglycosylases, and that resuscitation of these forms occurs following the addition of catalase and Tween 80. Our findings have important implications for the disease control and detection of B. pseudomallei, an agent of both public health and defense interest.

A lentiviral vector expressing a dendritic cell-targeting multimer induces mucosal anti-mycobacterial CD4+ T-cell immunity

Most viral vectors, including the potently immunogenic lentiviral vectors (LVs), only poorly direct antigens to the MHC-II endosomal pathway and elicit CD4⁺ T cells. We developed a new generation of LVs encoding antigen-bearing monomers of collectins substituted at their C-terminal domain with the CD40 ligand ectodomain to target and activate antigen-presenting cells. Host cells transduced with such optimized LVs secreted soluble collectin-antigen polymers with the potential to be endocytosed in vivo and reach the MHC-II pathway. In the murine tuberculosis model, such LVs induced efficient MHC-II antigenic presentation and triggered both CD8⁺ and CD4⁺ T cells at the systemic and mucosal levels. They also conferred a significant booster effect, consistent with the importance of CD4⁺ T cells for protection against Mycobacterium tuberculosis. Given the pivotal role of CD4⁺ T cells in orchestrating innate and adaptive immunity, this strategy could have a broad range of applications in the vaccinology field.

Promoting effect of the recombinant resuscitation promoting factors-2 of Rhodococcus erythropolis on petroleum degradation and cultivable bacterial diversities of the oil-contaminated soils

Resuscitation-promoting factors (Rpfs) belong to peptidoglycan hydrolases, which participate in recovery of dormant cells and promoting bacteria growth. In this study, the resuscitation promoting factor rpf2 gene of Rhodococcus erythropolis KB1 was expressed in Escherichia coli and purified by Ni²⁺ affinity chromatography. The purified recombinant fusion protein Rpf2 showed a closely 50 kDa band on sodium dodecyl sulphate polyacrylamide gel electrophoresis. The protein showed muralytic activity, with a specific activity of 1503 ± 123 U mg^-1 when determined with 4-methylumbelliferyl-β-d-N, N',N″-triacetotri-ylchitoside as substrate. It also showed protease activity when measured with azocasein as substrate, with a specific activity of 1528 ± 411 U mg^-1 . The addition of the recombinant Rpf2 protein significantly increased petroleum degradation efficiency of the indigenous micro-organisms and the petroleum degradation rates increased from 30·86 to 43·45%, 45·20 and 49·23% in the treatment groups. The recombinant protein also increased the petroleum-degrading bacterial diversities enriched from the contaminated soils. The cultivable bacterial flora of the treatment groups supplemented with different concentrations of Rpf2 increased from 82 genera in 9 phyla to 116 genera in 16 phyla and 138 genera in 16 phyla respectively. Thirteen extra petroleum-degrading bacteria strains were isolated from the petroleum-contaminated soils in the groups containing the recombinant Rpf2.

Evaluation of Rpf protein of Micrococcus luteus for cultivation of soil actinobacteria

Rpf protein, a kind of resuscitation promoting factor, was first found in the culture supernatant of Micrococcus luteus. It can resuscitate the growth of M. luteus in "viable but non-culture, VBNC" state and promote the growth of Gram-positive bacteria with high G + C content. This paper investigates the resuscitating activity of M. luteus ACCC 41016^T Rpf protein, which was heterologously expressed in E. coli, to cells of M. luteus ACCC 41016^T and Rhodococcus marinonascens HBUM200062 in VBNC state, and examines the effect on the cultivation of actinobacteria in soil. The results showed that the recombinant Rpf protein had resuscitation effect on M. luteus ACCC 41016^T and R. marinonascens HBUM200062 in VBNC state. 83 strains of actinobacteria, which were distributed in 9 families and 12 genera, were isolated from the experimental group with recombinant Rpf protein in the culture medium. A total of 41 strains of bacteria, which were distributed in 8 families and 9 genera, were isolated from the control group without Rpf protein. The experimental group showed richer species diversity than the control group. Two rare actinobacteria, namely HBUM206391^T and HBUM206404^T, were obtained in the experimental group supplemented with Rpf protein. Both may be potential new species of Actinomadura and Actinokineospora, indicating that the recombinant expression of M. luteus ACCC 41016^T Rpf protein can effectively promote the isolation and culture of actinobacteria in soil.

Exploring the Potential of Micrococcus luteus Culture Supernatant With Resuscitation-Promoting Factor for Enhancing the Culturability of Soil Bacteria

A bacterial species is best characterized after its isolation in a pure culture. This is an arduous endeavor for many soil microorganisms, but it can be simplified by several techniques for improving culturability: for example, by using growth-promoting factors. We investigated the potential of a Micrococcus luteus culture supernatant containing resuscitation-promoting factor (SRpf) to increase the number and diversity of cultured bacterial taxa from a nutrient-rich compost soil. Phosphate-buffered saline and inactivated SRpf were included as controls. After agitation with SRpf at 28°C for 1 day, the soil suspension was diluted and plated on two different solid, oligotrophic media: tenfold diluted Reasoner’s 2A agar (R2A) and soil extract-based agar (SA). Colonies were collected from the plates to assess the differences in diversity between different treatments and cultivation media. The diversity on both R2A and SA was higher in the SRpf-amended extracts than the controls, but the differences on R2A were higher. Importantly, 51 potentially novel bacterial species were isolated on R2A and SA after SRpf treatment. Diversity in the soil extracts was also determined by high-throughput 16S rRNA amplicon sequencing, which showed an increase in the abundance of specific taxa before their successful cultivation. Conclusively, SRpf can effectively enhance the growth of soil bacterial species, including those hitherto uncultured.

Recombinant resuscitation-promoting factor protein of Nocardia seriolae, a promissing vaccine candidate for largemouth bass (Micropterus salmoides)

Nocardia seriolae is an important pathogenic bacterium that causes nocardiosis in various fish species and leads to economic losses in the fish industry. To develop an effective subunit vaccine against nocardial infection, the truncated resuscitation-promoting factor (tRPF) of N. seriolae was selected and recombinantly produced using the Escherichia coli expression system. Western blotting results indicated that the recombinant protein could be strongly recognised by largemouth bass anti-N. seriolae antibodies. The protective efficacy of tRPF recombinant protein was assessed in combination with the commercial adjuvant Montanide™ ISA 763 A VG. The results showed that emulsified tRPF + ISA significantly induced high serum antibody response and serum lysozyme activity in the vaccinated fish. Quantitative reverse transcription polymerase chain reaction analysis indicated that tRPF + ISA could notably enhance the expression of immune-related genes in both the head kidney and spleen of the vaccinated fish. Finally, vaccinated largemouth bass displayed higher immuno-protection with a relative percent survival of 69.23% compared to the control groups. Taken together, the combination of tRPF + ISA is an ideal vaccine candidate against N. seriolae infection.

Flow cytometry as a rapid test for detection of tetracycline resistance directly in bacterial cells in Micrococcus luteus

Correct effective doses of antibiotics are important in the treatment of infectious diseases. The most frequently used methods for determination of the antibiotic susceptibility of bacterial pathogens are slow. The detection of multidrug-resistant bacteria currently relies on primary isolation followed by phenotypic detection of antibiotic resistance by measuring bacterial growth in the presence of the antibiotic being tested. The basic requirements for methods of detection of resistance to antibiotics include speed and accuracy. We studied the speed and accuracy of flow cytometry for the detection of tetracycline resistance in the Gram-positive bacteria Micrococcus luteus. Detection of cell viability and reliability of antibiotic resistance was carried out on the Guava EasyCyte flow cytometer (Merck Millipore, Germany) with SYBR Green and PI dyes. M. luteus was exposed to tetracycline (at 30, 90, 180 and 270 μg.mL-1) over 24 hours. Concentrations of live and dead cells were measured after 4 and 24 hours of incubation. The results revealed that the use of mixed dyes PI and SYBR Green allowed the division of cells into large subpopulations of live and dead cells and the DNA of destroyed cells. After 4 h exposure to tetracycline 30 μg.mL-1, the subpopulation of live cells decreased by 47% compared to the positive control. Tetracycline at 90 μg.mL-1 decreased the subpopulation of live cells by 59% compared to the positive control. A continued increase in concentration caused a shift in the population and an increase in dead cells, indicating damage to the cells of the microorganism. Incubation of M. luteus with 180 and 270 μg.mL-1 tetracycline decreased the subpopulation of live cells by 82% and 94%, respectively, in comparison with the positive control. After incubation with 30 μg of tetracycline over 24 h the number of living cells decreased by 70% in comparison with the positive control. Tetracycline treatment (90 μg.mL-1 for 24 h) killed 71% of cells. After exposure to 90 μg.mL-1 tetracycline 29% cells were viable. The viability of living cells was confirmed by a microbiological test.

Supplementing resuscitation-promoting factor (Rpf) enhanced biodegradation of polychlorinated biphenyls (PCBs) by Rhodococcus biphenylivorans strain TG9T

The biodegradation of polychlorinated biphenyls (PCBs) occurs slowly when the degrading bacteria enter a low activity state, such as a viable but nonculturable (VBNC) state, under unfavorable environmental conditions. The introduction of resuscitation-promoting factor (Rpf) can re-activate VBNC bacteria. This study tested the feasibility of enhancing PCB biodegradation via supplementing Rpf in liquid culture and soil microcosms inoculated with Rhodococcus biphenylivorans strain TG9^T. Exogenous Rpf resuscitated TG9^T cells that had previously entered the VBNC state after 90 d of nutrient starvation, resulting in the significantly enhanced degradation of PCB by 24.3% over 60 h in liquid medium that originally contained 50 mg L^-1 Aroclor 1242. In soil microcosms containing 50 mg kg^-1 Aroclor 1242 and inoculated with VBNC TG9^T cells, after 49 d of supplementation with Rpf, degradation efficiency of PCB reached 34.2%, which was significantly higher than the control. Our results confirmed that exogenous Rpf resuscitated VBNC TG9^T cells by stimulating endogenous expression of rpf gene orthologs. The enhanced PCB-degrading capability was likely due to the increased cell numbers and the strong expression of PCB catabolic genes. This study demonstrated the role of Rpf in enhancing PCB degradation via resuscitating PCB-degrading bacteria, indicating a promising approach for the remediation of PCB contamination.

Extracellular organic matter from Micrococcus luteus containing resuscitation-promoting factor in sequencing batch reactor for effective nutrient and phenol removal

Culture supernatant containing resuscitation-promoting factor (SRpf) from Micrococcus luteus was added to the sequencing batch reactor (SBR) for effective treatment of phenol-containing wastewater. SRpf acclimation significantly improved combined removal of phenol and nutrients. Moreover, the Illumina high-throughput sequencing analysis revealed that the SRpf boosted bacteria diversity, which enhanced the stability of the system under phenol stress. Addition of SRpf increased the abundances of Actinobacteria and Proteobacteria phyla which are involved in nutrient and phenol removal. Specifically, SRpf promoted Nitrosomonas and Nitrospira which participate in nitrification, family Comamonadaceae, genera Dechloromonas and Pseudomonas involved in denitrification, and Acinetobacter, Pseudomonas and Rhodocyclus which remove phosphorus elements. Moreover, the abundances of Bacillus and Klebsiella responsible for phenol removal as well as Pseudomonas and Acinetobacter were significantly increased after SRpf acclimation. These results show that SBR combined with SRpf acclimation provide optimal nutrient and phenol removal.

Structural features of HtpGMtb and HtpG-ESAT6Mtb vaccine antigens against tuberculosis: Molecular determinants of antigenic synergy and cytotoxicity modulation

Vaccine development against tuberculosis is an urgent need as the only available vaccine, M. bovis Bacillus Calmette Guerin (BCG), is unable to provide significant protection in adults. Among newly identified antigens, Rv2299c is an excellent candidate for the rational design of an effective multi-antigenic TB vaccine. Also, when fused to the T cell antigen ESAT6, it becomes highly effective in boosting BCG immunization and it adopts low cytotoxicity compared to ESAT6. We here characterize these proteins by coupling various biophysical techniques to cytofluorimetry and computational studies. Altogether, our data provide an experimental evidence of the role of Rv2299c as a dimeric and highly thermostable molecular chaperone, here denoted as HtpGMtb. Molecular dynamics simulations show that ATP rigidly anchors the ATP-binding loop in a conformation incompatible with the structure of the free enzyme. We also show that HtpGMtb dimeric state is an important molecular feature for the improved antigenic and cytotoxic properties of HtpG-ESAT6Mtb. Indeed, structural features of HtpG-ESAT6Mtb show that not only does this molecule combine the antigenic properties of HtpGMtb and ESAT6, but HtpGMtb locks ESAT6 in a dimeric state, thus improving its cytotoxicity properties. The data presented here provide solid basis for the rational design of upgraded antigens.

Enzyme characterization and biological activities of a resuscitation promoting factor from an oil degrading bacterium Rhodococcus erythropolis KB1

Resuscitation-promoting factors (Rpf) are a class of muralytic enzymes, which participate in recovery of dormant cells and promoting bacteria growth in poor media. In the present study the expression vector of the rpf-1 gene from an oil-degrading bacterium Rhodococcus erythropolis KB1 was constructed and expressed in Escherichia coli. The expressed protein was purified by Ni2+-affinity chromatography, and showed muralytic activity when measured with 4-methylumbelliferyl-β-D-N,N',N″-triacetyl chitotrioside as substrate. Addition of purified Rpf-1 to R. erythropolis culture efficiently improved bacterial cell growth. The purified protein also increased resuscitation of viable but nonculturable cells of R. erythropolis to culturable state. The conserved amino acid residues including Asp45, Glu51, Cys50, Thr60, Gln69, Thr74, Trp75 and Cys114 of the Rpf-1 were replaced with different amino acids. The mutant proteins were also expressed and purified with Ni2+-affinity chromatography. The muralytic activities of the mutant proteins decreased to different extents when compared with that of the wild type Rpf-1. Gln69 was found to play the most important role in the enzyme activity, substitution of Gln69 with lysine (Q69K) resulted in the greatest decrease of muralytic activity. The other amino acid residues such as Asp45, Glu51, Cys50 and Cys114 were also found to be very important in maintaining muralytic activity and biological function of the Rpf-1. Our results indicated that Rpf-1 from R. erythropolis showed muralytic activities and weak protease activity, but the muralytic activity was responsible for its growth promotion and resuscitation activity.

Sub-minimum inhibitory concentration of rifampin: a potential risk factor for resuscitation of Mycobacterium tuberculosis

BACKGROUND

Mycobacterium tuberculosis possesses five resuscitation-promoting factors, Rpf A to E, which are required for the resuscitation of dormancy in mycobacteria. This study explores the transcriptional profile of all five rpfs of M. tuberculosis, in response to sub-MIC concentration of rifampin, in multidrug and mono-rifampin resistant clinical isolates.

METHODS

Thirteen multidrug and two rifampin mono resistant clinical isolates were analyzed. Drug susceptibility testing and determination of MIC were performed. The relative expression of rpfs was measured, by real-time quantitative PCR.

RESULTS

A significant upregulation of relative expression (p < 0.05) was observed, as follows: 7/15(46.66%); 5/15(33.33%); 9/15(60%); 10/15(66.66%) and 9/15(60%) in rpfA, rpfB, rpfC, rpfD and rpfE, respectively.

CONCLUSION

Our results showed that the rpfs could be overexpressed in some extent in the presence of sub-MIC concentration of rifampin in multidrug and mono drug resistant M. tuberculosis. These results highlight the potential risk of sub-MIC rifampin concentrations, as a risk factor for tuberculosis reactivation.

Opening Pandora's Box: Mechanisms of Mycobacterium tuberculosis Resuscitation

Mycobacterium tuberculosis (Mtb) characteristically causes an asymptomatic infection. While this latent tuberculosis infection (LTBI) is not contagious, reactivation to active tuberculosis disease (TB) causes the patient to become infectious. A vaccine has existed for TB for a century, while drug treatments have been available for over 70 years; despite this, TB remains a major global health crisis. Understanding the factors which allow the bacillus to control responses to host stress and mechanisms leading to latency are critical for persistence. Similarly, molecular switches which respond to reactivation are important. Recently, research in the field has sought to focus on reactivation, employing system-wide approaches and animal models. Here, we describe the current work that has been done to elucidate the mechanisms of reactivation and stop reactivation in its tracks.