Figures
Abstract
Background
The role of vitamins in the combat of disease is usually conceptualized as acting by modulating the immune response of an infected, eukaryotic host. We hypothesized that some vitamins may directly influence the growth of prokaryotes, particularly mycobacteria.
Methods
The effect of four fat-soluble vitamins was studied in radiometric Bactec® culture. The vitamins were A (including a precursor and three metabolites,) D, E and K. We evaluated eight strains of three mycobacterial species (four of M. avium subspecies paratuberculosis (MAP), two of M. avium and two of M. tb. complex).
Principal Findings
Vitamins A and D cause dose-dependent inhibition of all three mycobacterial species studied. Vitamin A is consistently more inhibitory than vitamin D. The vitamin A precursor, β-carotene, is not inhibitory, whereas three vitamin A metabolites cause inhibition. Vitamin K has no effect. Vitamin E causes negligible inhibition in a single strain.
Significance
We show that vitamin A, its metabolites Retinyl acetate, Retinoic acid and 13-cis Retinoic acid and vitamin D directly inhibit mycobacterial growth in culture. These data are compatible with the hypothesis that complementing the immune response of multicellular organisms, vitamins A and D may have heretofore unproven, unrecognized, independent and probable synergistic, direct antimycobacterial inhibitory activity.
Citation: Greenstein RJ, Su L, Brown ST (2012) Vitamins A & D Inhibit the Growth of Mycobacteria in Radiometric Culture. PLoS ONE 7(1): e29631. https://doi.org/10.1371/journal.pone.0029631
Editor: Leonardo A. Sechi, Universita di Sassari, Italy
Received: July 27, 2011; Accepted: December 1, 2011; Published: January 3, 2012
This is an open-access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication.
Funding: This project was extramurally funded by Becton-Dickinson, whose sole contribution was to provide the authors with the culture medium used in these experiments. Intramural funds were from the Bronx Veterans Medical Research Foundation, Inc. a charity at the James J. Peters VAMC Bronx NY. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: Yes, the authors have the following competing interest. This project was partly funded by Becton-Dickinson. RJG has submitted patents based on the hypotheses tested in prior publications. Patents Issued: US Patent # 7,846,420: Issue Date Dec 7, 2010. Mycobacterium Avium Subspecies Paratuberculosis Vaccines and Methods for Using the Same. US Patent # 7,902,350: Issue Date March 8, 2011. Method for Monitoring the Efficacy of a Mycobacterium Avium Subspecies Paratuberculosis Therapy. Patents Submitted: Serial # 12/119,657: Filing Date: May 13, 2008: Methods for Diagnosing and Treating a Mycobacterium Avium Subspecies Paratuberculosis Infection. Serial # 12/892,039: Filing Date: Sept 28, 2010: Medium and Method for Culturing Mycobacterium Avium Subspecies Paratuberculosis. This does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials, as detailed online in the guide for authors. STB was a member of the National Academy of Sciences of the USA panel that issued the Report “Diagnosis and Control of Johne's Disease.” ISBN 0-309-08611-6.
Introduction
Since early in the last century [1] the role of both vitamin A (see [2] for review) and vitamin D (see [3], [4] for review) in combating infectious diseases has been investigated. It is noteworthy that in the vast majority of studies, the underlying assumption has been that any efficacy of these vitamins in combating disease is consequent to enhancement of the immune response of the infected host [5]–[8]. There is no direct inhibition of bacterial growth by synthetic retinoids [9]. In contrast retinaldehyde (but not Retinoic Acid itself) inhibit Gram positive (but not Gram negative) bacteria in culture [10].
The activities of vitamins A & D have been extensively reported in relation to the host immune response in mycobacterial diseases [4], [8], [11]–[15]. We posit that vitamins will have fundamental and necessary activity in both prokaryotes as well as eukaryotes. We hypothesized that vitamins A and D might directly inhibit prokaryotic growth in general and mycobacterial growth in particular. Any direct inhibitory action of vitamins would be in addition to (and possibly synergistic with) their effect on the immune response of a mycobacterial-infected host [5]–[8].
We herein report on radiometric culture studies of the four fat-soluble vitamins (A, D, E & K) as well at the vitamin A precursor β–carotene and three vitamin A metabolites (retinyl acetate, retinoic acid and 13-cis retinoic acid) on three mycobacterial species. They are the acknowledged human pathogen M. Tuberculosis (M. tb.) complex, M. avium subspecies avium (M. avium) pathogenic in immuno-compromised humans and the possibly zoonotic M. avium subspecies paratuberculosis (MAP) [16].
Methods
This study was approved by the Research & Development Committee at the VAMC Bronx NY (0720-06-038) and was conducted under the Institutional Radioactive Materials Permit (#31-00636-07).
Bacterial Culture
Our Bactec® 460 (Becton-Dickinson Franklin Lakes NJ) 14C radiometric culture inhibition methods have previously been published in detail [17]–[22]. This system quantifies bacterial growth, or lack thereof, by providing 14C in palmitate, an energy source for mycobacterial growth [23]. Vials are assayed on a daily basis, quantifying the amount of 14C released as 14CO2, by the integral detector in the Bactec 460. The data are obtained as a manufacturer determined, arbitrary Growth Units (GU) of 0-999. Because the Bactec 460 is only semi-automatic, and the onerous regulatory requirements of using radionucleotides, this exquisitely sensitive [18] system is being phased out. It is being replaced by the fully automatic, oxygen consumption detecting fluorescent probe MIGT system (Becton-Dickerson NJ.) [24], [25]
The detergent Tween 80 (recommended to minimize mycobacterial clumping [23]) is not used in culture, because of interference with the assay [21], [26]. Strains with the least spontaneous clumping are studied instead. Except for the amount of test agent, every vial has the identical concentration of all constituents (including identical 3.2% concentration of the dissolving agent, DMSO.) In this study, performed in singlicate, eight strains of mycobacteria, four of which are MAP, are evaluated. Two MAP strains had been isolated from humans with Crohn disease “Dominic” (ATCC 43545; Originally isolated by R. Chiodini [27]) and UCF 4 (gift of Saleh Naser, Burnett College of Biomedical Sciences, University of Central Florida, Orlando FL.) [28]. The other two MAP strains were from ruminants with Johne disease, ATCC 19698 and 303 (gift of Michael Collins Madison WI.) The M. avium subspecies avium strains (hereinafter called M. avium) were ATCC 25291 (veterinary source) and M. avium 101 (Human isolate from a patient with AIDS; Gift of Clark Inderlied PhD. UC Los Angles CA.) [29]. To study the M. tuberculosis complex, we used two BioSafety level 2 strains; Bacillus Calmette Guerin (BCG) M. bovis Karlson & Lessel (ATCC 19015) and an avirulent M. tb strain; ATCC 25177 (all ATCC from ATCC Rockville MD).
The fat soluble vitamins studied were: vitamin A (Retinol; Axerophthol, -3,7-Dimethyl-9-(2,6,6-trimethyl-1-cyclohexen-1-yl)-2,4,6,8-nonatetraen-1-ol: Sigma Cat # R7632.) The vitamin A precursor studied was β–Carotene (β,β-Carotene, Provitamin A: Sigma Cat # 22040.) We studied three vitamin A metabolites; Retinyl acetate (Retinol acetate, vitamin A acetate: Sigma Cat # R3250) and Retinoic acid (ATRA, Tretinoin, vitamin A acid, all-trans-Retinoic acid: Sigma Cat # R2625). Additionally we evaluated 13 cis-Retinoic acid (Isotretinoin, Acutane® Sigma Cat # R3255), a medication used to treat intractable acne and occasionally associated with the manifestation of both Crohn's disease and ulcerative colitis. A commercial source of another structural analog of retinoic acid; 9-cis-Retinoic acid could not be identified.
The other three fat-soluble vitamins studied are: vitamin D (Cholecalciferol; (+)-Vitamin D3, 7-Dehydrocholesterol activated, Activated 7-dehydrocholesterol: Sigma Cat # C9576). Vitamin E ((±)-α-Tocopherol DL-all-rac-α-Tocopherol: Sigma Cat # T3251). Vitamin K1 (2-Methyl-3-phytyl-1,4-naphthoquinone, 3-Phytylmenadione, Phylloquinone: Sigma Cat # 95271). Our inhibitory antibiotic control is monensin [20] and the non-inhibitory control is the gluterimide antibiotic, phthalimide [21]. (All Sigma, St Louis. MO.) Chemicals are dissolved in DMSO, aliquoted, stored at −80°C, thawed, used once and discarded. Agents are studied at concentrations ranging from 0.1 to 64 µg/ml.
For clarity and ease of understanding the same data are presented in two ways. For individual mycobacteria we present data from a single experiment graphically (Figures 1–9). These data are presented as the cumulative Growth Index (cGI.) In contrast, for each individual chemical agent studied, data are presented in Tables as the “percent change from control cGI” (Inhibition as “%–ΔcGI”; See [18] for calculation: Tables 1–10).
Strains from the two upper panels (UCF-4 & Dominic) were isolated from humans with Crohn disease. Strains in the two lower panels were isolated from ruminants with Johne disease. The inhibitory control is Monensin, and the non-inhibitory control is Phthalimide. cGI = cumulative Growth Index.
The inhibitory control is Monensin, and the non-inhibitory control is Phthalimide. Note that as previously [20], [21], [30], Monensin does not inhibit M avium 101. cGI = cumulative Growth Index.
Vitamin D is less effective against M. tb ATCC 25177. The inhibitory control is Monensin, and the non-inhibitory control is Phthalimide. cGI = cumulative Growth Index.
The inhibitory control is Monensin, and the non-inhibitory control is Phthalimide. cGI = cumulative Growth Index.
The inhibitory control is Monensin, and the non-inhibitory control is Phthalimide. Note that as previously [20], [21], [30], Monensin does not inhibit M avium 101. cGI = cumulative Growth Index.
The inhibitory control is Monensin, and the non-inhibitory control is Phthalimide. cGI = cumulative Growth Index.
ß-carotene, the precursor to vitamin A, exhibits no inhibition at the doses studied. Maximal inhibitory activity against all MAP strains is observed with Retinyl acetate (solid black triangles.) Both Retinoic acid and 13-cis Retinoic acid exhibit intermediate inhibition.
ß-carotene, the precursor to vitamin A, exhibits no inhibition at the doses studied. Retinyl acetate and 13-cis Retinoic acid have some inhibition.
ß-carotene, the precursor to vitamin A, exhibits no inhibition at the doses studied. Both Retinoic acid and 13-cis Retinoic acid result in dose dependent inhibition on our avirulent strain of M. tb. There is no comparable inhibition against Bacillus Calmette-Guerin (BCG).
For simplicity and comprehensibility the data in each of Figues 1–6 are for only two of the four agents tested. For ease of comparison the inhibitory (Monensin) and non-inhibitory control (Phthalimide) are repetitively presented. Data for vitamins A & D are presented in Figure 1 (MAP), Figure 2 (M. avium) & Figure 3 (M. tb. complex) and vitamins E & K are presented in Figure 4 (MAP), Figure 5 (M. avium) & Figure 6 (M. tb. complex.) The vitamin A precursor and structural analogs are presented in tabular form. (β-Carotene; Table 7: Retinol acetate; Table 8: Retinoic acid; Table 9: and 13-cis Retinoic acid; Table 10.) Data for vitamin A precursors and analogs on mycobacterial species and subspecies are presented as Figures (MAP = Figure 7: M. avium = Figure 8 & M. tb. complex = Figure 9.).
Results
In this study we show that all MAP and both M. tb complex strains are inhibited by Monensin (Table 1 and Figures 1 & 4 & Table 1 & Figures 3 & 6.) This corroborates our previous findings with Monensin [20], [21], [30]. As previously [20], [21], [30], Monensin does not inhibit one of our two M. avium control strains (M. avium 101: Table 1 and Figures 2 & 5), attesting to the reliability and reproducibility of our assay.
The non-inhibitory control that we use is Phthalimide, a gluterimide antibiotic that has no mycobacterial inhibition [21]. In this study, as previously [21], [30], [31], Phthalimide has no dose-dependent inhibition against any of the mycobacterial strains tested (Table 2 and Figures 1–6.)
Vitamin A causes dose dependent inhibition of all MAP, M. avium and M. tb complex strains studied (Table 3 & Figures 1, 2 & 3) with 98%-ΔcGI at 16 µg/ml for MAP ATCC 19698. The precursor to vitamin A, β-Carotene has no dose dependent inhibition on six of eight mycobacterial strains and negligible inhibition on two MAP strains (Table 7 & Figures 7, 8 & 9.) In contrast, the three vitamin A metabolites (Figures 7, 8 & 9); retinyl acetate (Table 8), retinoic acid (Table 9), and 13-cis retinoic acid (Table 10) result in dose dependent inhibition of all three species studied, but with intriguing interspecies variations. Retinyl Acetate is most active against MAP (Figure 7 & Table 8; Dominic 96%-ΔcGI at 64 µg/ml.) Retinoic acid and 13-cis retinoic acid are most active against M. tb (98%-ΔcGI at 16 µg/ml), but have no inhibition against BCG (Figure 9 & Tables 9 & 10.) M. avium is the least susceptible to these vitamin A metabolites (Figure 8 & Tables 8–10).
Vitamin D causes dose dependent inhibition of all MAP strains studied (Table 3 & Figure 1). However, vitamin D is not as potent an inhibitor against the two MAP human isolates (UCF-4; 56%-ΔcGI at 64 µg/ml and Dominic) as it is on the two MAP bovine isolates (Table 4 and Figure 1.) Likewise, vitamin D inhibits all M. avium (Table 4 & Figure 2) and M. tb. complex (Table 4 & Figure 3) strains studied. Finally, vitamin D is less effective than vitamin A against all four M. avium and M. tb complex strains studied (Tables 3 & 4 and Figures 1–3.).
In contrast, vitamin E (Table 5; Figures 4, 5, & 6) results in inhibition of only one of the eight mycobacterial strains studied, MAP Dominic (Table 5 & Figure 4.) Even then, the maximal inhibition of vitamin E on Dominic (44%-ΔcGI at 64 µg/ml; (Table 5 & Figure 4) is far less than is observed with either vitamin A or D.
Vitamin K has no effect on the growth on any of the three mycobacterial species studied (Table 6; Figures 4, 5, & 6.).
Discussion
To our knowledge this is the first study showing dose-dependent inhibition, in radiometric culture, of three mycobacterial species (the M. tb. complex, M. avium and MAP) by two of four fat-soluble vitamins; vitamins A & D. In contrast, vitamin K has no, and vitamin E negligible effect. These therefore provide appropriate non-inhibitory experimental controls. Our observations cannot be ascribed to the acidic nature of vitamin A or its analogs as the pH remains within the manufacturer's recommended range of pH 6.6±2 in the final 5 ml incubation volume (data not presented.) The mechanism(s) by which vitamins A & D inhibit mycobacterial growth, and whether they have similar inhibition on virulent and/or multi-drug resistant M. tb., remains to be determined.
Our finding are directly contradictory to those of Flemetakis et. al. who concluded that there was no direct retinoid effect on bacteria in vivo [9]. Others find that vitamin A and retinoic acid have no antibacterial activity, whereas retinaldehyde does [10]. Neither study evaluated mycobacteria in radiometric culture. We, and others [32], conclude that when evaluating mycobacterial growth kinetics, liquid radiometric [23] data provide exquisitely sensitive data of bacteriostatic in addition to bactericidal effects.
Inhibition of mycobacterial growth by vitamins A & D has been ascribed to down regulation of the tryptophan-aspartate-containing coat protein (TACO) gene in the human macrophage [8], [33]. Our data are compatible with an additional hypothesis. It is that vitamins D, A and vitamin A metabolites have a heretofore unproven, independent and probable synergistic antimycobacterial inhibitory action that complements the immune response of multicellular organisms.
The vitamin A precursor, β-Carotene, does not inhibit mycobacterial growth. This indicates that mycobacterial mechanisms to convert β-Carotene to vitamin A are inadequate to produce sufficient vitamin A levels to inhibit mycobacterial growth. We conclude that the subspecies specific, idiosyncratic, inhibition of the three vitamin A metabolites merit further study, as do structural analogs of vitamin D.
We posit that multiple agents have underappreciated activity against prokaryotes in addition to well-documented eukaryotic activity. For example, we [17], [18], [20]–[22], [30], [31], and others [34], [35], have shown inhibition of MAP growth with medications used to treat “autoimmune” and “inflammatory” diseases. In the present study we show direct inhibition of mycobacterial growth by vitamins A and D in culture. We conclude that the scientific community has neglected the potential direct prokaryotic effects of vitamins, emphasizing instead the indirect role that vitamins have in enhancing the immune response of an infected host.
Our radiometric assay [23] is sufficiently sensitive to identify mycobacterial growth enhancement in culture [31]. Using it, we have corroborated the classic study of Bernheim in 1940 [36] showing that salicylic acid increased oxygen consumption by the tuberculosis bacillus. Additionally, we showed growth enhancement of mycobacteria by vitamin B3 (nicotinamide), nicotinic acid (a tobacco constituent) and α &ß NAD [31]. In 1940 the possibility that vitamin K enhanced the growth of MAP was considered [37]. (see [38] for review). The identification of the necessary, and potent, iron chelating mycobactins of M. phlei [39], [40] (see [41] for review), left unresolved a possible enhancing role of vitamin K on MAP growth [37]. In this present study we observe no growth enhancement by vitamin K1. It is of interest however, that vitamin K2 (menaquinone), which we did not evaluate, may inhibit mycobacterial growth [42]. We now conclude that vitamin K1 has no effect on the growth of three mycobacterial species, including MAP.
This study does not address how vitamin concentrations that are inhibitory in our culture system, relate to concentrations actually found in multicellular organisms. For example our “normal” laboratory range in humans for circulating vitamin A is 0.3–0.9 µg/ml, a level below those tested in our studies (1–64 µg/ml.) Lipophylic antibiotics, such as azithromycin, may achieve tissue levels 1,000 fold greater than circulating values [43]. Since “normal” laboratory concentrations are “circulating” plasma levels, they may vastly underestimate concentrations that these fat-soluble vitamins achieve in lipid rich regions, such as prokaryotic and eukaryotic cell walls and other lipophylic regions within cells.
Prevailing dogma considers that all of the anti-mycobacterial activity of vitamins A & D is mediated, indirectly, via enhancement of the immune system of the eukaryotic host. Our data are compatible with an alternative hypothesis: In addition to their eukaryotic activity, vitamins A & D may directly inhibit mycobacteria within the eukaryotic host. Similarly whether vitamins may act as naturally occurring “antibiotics” and help prevent a host infected by mycobacteria from progressing to active disease will require extensive and complicated, IRB compliant, additional studies. Nevertheless, it is of considerable interest that low exposure to sunlight, which is associated with diminished vitamin D levels [44], is associated with an increase in the incidence of Crohn disease [45], which may be caused by MAP [46].
Author Contributions
Conceived and designed the experiments: RJG. Performed the experiments: LS RJG. Analyzed the data: RJG LS STB. Contributed reagents/materials/analysis tools: RJG STB. Wrote the paper: RJG.
References
- 1. Mellanby E, Green HN (1929) Vitamin a as an Anti-Infective Agent: Its Use in the Treatment of Puerperal Septigaemia. Br Med J 1: 984–986.
- 2. Semba RD (1999) Vitamin A as “anti-infective” therapy, 1920–1940. J Nutr 129: 783–791.
- 3. Bruce D, Ooi JH, Yu S, Cantorna MT (2010) Vitamin D and host resistance to infection? Putting the cart in front of the horse. Exp Biol Med (Maywood) 235: 921–927.
- 4. Hewison M (2010) Vitamin D and the intracrinology of innate immunity. Mol Cell Endocrinol 321: 103–111.
- 5. Liu PT, Stenger S, Li H, Wenzel L, Tan BH, et al. (2006) Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response. Science 311: 1770–1773.
- 6. Sutherland R (1934) Vitamins A and D: Their Relation to Growth and Resistance to Disease. Br Med J 1: 791–795.
- 7.
Ross A, Hämmerling U (1994) Retinoids and the Immune System. In: Sporn M, Roberts A, Goodman D, editors. The Retinoids: Biology, Chemistry and Medicine. 2 ed. New York: Raven Press. pp. 597–630.
- 8. Anand PK, Kaul D, Sharma M (2008) Synergistic action of vitamin D and retinoic acid restricts invasion of macrophages by pathogenic mycobacteria. J Microbiol Immunol Infect 41: 17–25.
- 9. Flemetakis AC, Tsambaos DG (1989) Effects of synthetic retinoids on the growth of bacteria and their susceptibility to antibiotics. J Chemother 1: 374–376.
- 10. Pechere M, Germanier L, Siegenthaler G, Pechere JC, Saurat JH (2002) The antibacterial activity of topical retinoids: the case of retinaldehyde. Dermatology 205: 153–158.
- 11. Pattison CL (1930) Treatment of Bone Tuberculosis by Large Amounts of Vitamins A and D. Br Med J 2: 178–179.
- 12. Martineau AR, Wilkinson RJ, Wilkinson KA, Newton SM, Kampmann B, et al. (2007) A single dose of vitamin D enhances immunity to mycobacteria. Am J Respir Crit Care Med 176: 208–213.
- 13. Martineau AR, Timms PM, Bothamley GH, Hanifa Y, Islam K, et al. (2011) High-dose vitamin D(3) during intensive-phase antimicrobial treatment of pulmonary tuberculosis: a double-blind randomised controlled trial. Lancet 377: 242–250.
- 14. Lalor MK, Floyd S, Gorak-Stolinska P, Weir RE, Blitz R, et al. (2011) BCG Vaccination: A Role for Vitamin D? PLoS ONE 6: e16709.
- 15. Shapira Y, Agmon-Levin N, Shoenfeld Y (2010) Mycobacterium tuberculosis, autoimmunity, and vitamin D. Clin Rev Allergy Immunol 38: 169–177.
- 16.
Greenstein R, Gillis T, Scollard D, Brown S (2009) Mycobacteria: Leprosy, a Battle Turned; Tuberculosis, a Battle Raging; Paratuberculosis, a Battle Ignored. In: Fratamico P, Smith J, Brogden K, editors. Sequelae and Long-Term Consequences of Infectious Diseases. First ed. Washington DC 20036-2904: ASM Press. American Society for Microbiology. pp. 135–168.
- 17. Greenstein RJ, Su L, Haroutunian V, Shahidi A, Brown ST (2007) On the Action of Methotrexate and 6-Mercaptopurine on M. avium subspecies paratuberculosis. PLoS ONE 2: e161.
- 18. Greenstein RJ, Su L, Shahidi A, Brown ST (2007) On the Action of 5-Amino-Salicylic Acid and Sulfapyridine on M. avium including subspecies paratuberculosis. PLoS ONE 2: e516.
- 19. Rastogi N, Goh KS, Labrousse V (1992) Activity of clarithromycin compared with those of other drugs against Mycobacterium paratuberculosis and further enhancement of its extracellular and intracellular activities by ethambutol. AntimicrobAgents Chemother 36: 2843–2846.
- 20. Greenstein RJ, Su L, Whitlock R, Brown ST (2009) Monensin causes dose dependent inhibition of Mycobacterium avium subspecies paratuberculosis in radiometric culture. Gut Pathogens 1: 4.
- 21. Greenstein RJ, Su L, Brown ST (2009) On the effect of thalidomide on Mycobacterium avium subspecies paratuberculosis in culture. Int J Infect Dis 13: e254–263.
- 22. Greenstein RJ, Su L, Juste RA, Brown ST (2008) On the Action of Cyclosporine A, Rapamycin and Tacrolimus on M. avium including subspecies paratuberculosis. PLoS ONE 3: e2496.
- 23. Siddiqi SH, Libonati JP, Middlebrook G (1981) Evaluation of rapid radiometric method for drug susceptibility testing of Mycobacterium tuberculosis. J Clin Microbiol 13: 908–912.
- 24. Rusch-Gerdes S, Pfyffer GE, Casal M, Chadwick M, Siddiqi S (2006) Multicenter Laboratory Validation of the BACTEC MGIT 960 Technique for Testing Susceptibilities of Mycobacterium tuberculosis to Classical Second-Line Drugs and Newer Antimicrobials. J Clin Microbiol 44: 688–692.
- 25. Krishnan MY, Manning EJ, Collins MT (2009) Comparison of three methods for susceptibility testing of Mycobacterium avium subsp. paratuberculosis to 11 antimicrobial drugs. J Antimicrob Chemother 64: 310–316.
- 26. Damato JJ, Collins MT (1990) Growth of Mycobacterium paratuberculosis in radiometric, Middlebrook and egg-based media. Vet Microbiol 22: 31–42.
- 27. Chiodini RJ, Van Kruiningen HJ, Thayer WR, Coutu JA (1986) Spheroplastic phase of mycobacteria isolated from patients with Crohn's disease. J Clin Microbiol 24: 357–363.
- 28. Naser SA, Ghobrial G, Romero C, Valentine JF (2004) Culture of Mycobacterium avium subspecies paratuberculosis from the blood of patients with Crohn's disease. Lancet 364: 1039–1044.
- 29. Bertram MA, Inderlied CB, Yadegar S, Kolanoski P, Yamada JK, et al. (1986) Confirmation of the beige mouse model for study of disseminated infection with Mycobacterium avium complex. J Infect Dis 154: 194–195.
- 30. Greenstein RJ, Su L, Brown ST (2010) The Thioamides Methimazole and Thiourea Inhibit Growth of M. avium subspecies paratuberculosis in Culture. PLoS ONE 5: e11099.
- 31. Greenstein RJ, Su L, Brown SL (2010) Growth of M. avium subspecies paratuberculosis in Culture Is Enhanced by Nicotinic Acid, Nicotinamide, and α and β Nicotinamide Adenine Dinucleotide. Dig Dis Sci 56: 368–375.
- 32. Springer B, Lucke K, Calligaris-Maibach R, Ritter C, Bottger EC (2009) Quantitative drug susceptibility testing of Mycobacterium tuberculosis by use of MGIT 960 and EpiCenter instrumentation. J Clin Microbiol 47: 1773–1780.
- 33. Anand PK, Kaul D (2003) Vitamin D3-dependent pathway regulates TACO gene transcription. Biochem Biophys Res Commun 310: 876–877.
- 34. Shin SJ, Collins MT (2008) Thiopurine drugs (azathioprine and 6-mercaptopurine) inhibit Mycobacterium paratuberculosis growth in vitro. Antimicrob Agents Chemother 52: 418–426.
- 35. Krishnan MY, Manning EJ, Collins MT (2009) Effects of interactions of antibacterial drugs with each other and with 6-mercaptopurine on in vitro growth of Mycobacterium avium subspecies paratuberculosis. J Antimicrob Chemother 64: 1018–1023.
- 36. Bernheim F (1940) The Effect of Salicylate on the Oxygen Uptake of the Tubercle Bacillus. Science 92: 204.
- 37. Woolley DW, McCarter JR (1940) Antihemorrhagic compounds as growth factors for the Johne's bacillus. Proc Soc Exp Biol Med 45: 357–360.
- 38. Bentley R, Meganathan R (1982) Biosynthesis of vitamin K (menaquinone) in bacteria. Microbiol Rev 46: 241–280.
- 39. Francis J, Madinaveitia J, Macturk HM, Snow GA (1949) Isolation from acid-fast bacteria of a growth-factor for Mycobacterium johnei and of a precursor of phthiocol. Nature 163: 365.
- 40. Francis J, Macturk HM, Madinaveitia J, Snow GA (1953) Mycobactin, a growth factor for Mycobacterium johnei. I. Isolation from Mycobacterium phlei. Biochem J 55: 596–607.
- 41. Snow GA (1970) Mycobactins: iron-chelating growth factors from mycobacteria. Bacteriol Rev 34: 99–125.
- 42. Kurosu M, Narayanasamy P, Biswas K, Dhiman R, Crick DC (2007) Discovery of 1,4-dihydroxy-2-naphthoate [corrected] prenyltransferase inhibitors: new drug leads for multidrug-resistant gram-positive pathogens. J Med Chem 50: 3973–3975.
- 43. Brown ST, Edwards FF, Bernard EM, Tong W, Armstrong D (1993) Azithromycin, rifabutin, and rifapentine for treatment and prophylaxis of Mycobacterium avium complex in rats treated with cyclosporine. Antimicrob Agents Chemother 37: 398–402.
- 44. Holick MF (2011) Vitamin D deficiency in 2010: health benefits of vitamin D and sunlight: a D-bate. Nat Rev Endocrinol 7: 73–75.
- 45. Nerich V, Jantchou P, Boutron-Ruault MC, Monnet E, Weill A, et al. (2011) Low exposure to sunlight is a risk factor for Crohn's disease. Aliment Pharmacol Ther 33: 940–945.
- 46. Greenstein RJ (2003) Is Crohn's disease caused by a mycobacterium? Comparisons with leprosy, tuberculosis, and Johne's disease. Lancet Infect Dis 3: 507–514.