The Non-Essential Mycolic Acid Biosynthesis Genes hadA and hadC Contribute to the Physiology and Fitness of Mycobacterium smegmatis

Intercellular communication and social behaviors in mycobacteria

Cell-to-cell communication is a fundamental process of bacteria to exert communal behaviors. Sputum samples of patients with cystic fibrosis have often been observed with extensive mycobacterial genetic diversity. The emergence of heterogenic mycobacterial populations is observed due to subtle changes in their morphology, gene expression level, and distributive conjugal transfer (DCT). Since each subgroup of mycobacteria has different hetero-resistance, they are refractory against several antibiotics. Such genetically diverse mycobacteria have to communicate with each other to subvert the host immune system. However, it is still a mystery how such heterogeneous strains exhibit synchronous behaviors for the production of quorum sensing (QS) traits, such as biofilms, siderophores, and virulence proteins. Mycobacteria are characterized by division of labor, where distinct sub-clonal populations contribute to the production of QS traits while exchanging complimentary products at the community level. Thus, active mycobacterial cells ensure the persistence of other heterogenic clonal populations through cooperative behaviors. Additionally, mycobacteria are likely to establish communication with neighboring cells in a contact-independent manner through QS signals. Hence, this review is intended to discuss our current knowledge of mycobacterial communication. Understanding mycobacterial communication could provide a promising opportunity to develop drugs to target key pathways of mycobacteria.

Mycobacterium fortuitum fabG4 knockdown studies: Implication as pellicle and biofilm specific drug target

The fatty acid biosynthesis pathway is crucial for the formation of the mycobacterial cell envelope. The fatty acid synthase type-II (FAS-II) components are attractive targets for designing anti-biofilm inhibitors. Literature review, bioinformatics analysis, cloning, and sequencing led to the identification of a novel Mycobacterium fortuitum FAS-II gene MFfabG4 which interacts with mycobacterial proteins involved in biofilm formation. A manually curated M. fortuitum fatty acid biosynthesis pathway has been proposed exploiting functional studies from the Kyoto Encyclopedia of Genes and Genomes and Mycobrowser databases for MFFabG4. M. fortuitum MFfabG4 knockdown strain (FA) was constructed and validated by quantitative polymerase chain reaction. The FA strain displayed unstructured smooth colony architecture, correlating with decreased pathogenicity and virulence. MFfabG4 knockdown resulted in diminished pellicle and attenuated biofilm formation, along with impaired sliding motility, and reduced cell sedimentation. The FA strain showed lowered cell surface hydrophobicity, indicating attenuation in M. fortuitum intracellular infection-causing ability. Stress survival studies showed the requirement of MFfabG4 for survival in a nutrient-starved environment. The results indicate that MFfabG4 maintains the physiology of the cell envelope and is required for the formation of M. fortuitum pellicle and biofilm. The study corroborates the role of MFfabG4 as a pellicle- and biofilm-specific drug target and a potential diagnostic marker for M. fortuitum and related pathogenic mycobacteria.

Knockdown of the Type-II Fatty acid synthase gene hadC in mycobacterium fortuitum does not affect its growth, biofilm formation, and survival under stress

Background

Mycobacterial fatty acid synthase Type-II (FAS-II) components are major virulence factors exploited as potential targets for developing novel antimycobacterial drugs. The FAS-II enzyme 3-hydroxyacyl-ACP dehydratase (HadC) is important for biofilm development and pathogenesis of Mycobacterium tuberculosis and other mycobacterial species.

Methods

Literature review and homology search led to the identification of Mycobacterium fortuitum MFhadC gene. Functional interaction study of MFHadC protein was done using STRING. M. fortuitum MFhadC over-expressing (HS) and knockdown (HA) strains were constructed and validated by expression analysis using quantitative polymerase chain reaction. The strains were analyzed for growth behavior and surface spreading ability. Biofilm formation was assayed through crystal violet assay, viability count, and basic fuchsin staining. In addition, survival of the strains was studied under in vitro nutrient starvation and detergent stress.

Results

STRING analysis showed the interaction of HadC with proteins involved in biofilm formation. The strains HS and HA showed spreading ability on the agarose surface, exhibiting translocation patterns similar to the vector control strain. All three strains showed a similar amount of biofilm formation when analyzed using crystal violet assay, viability count, and basic fuchsin staining. The strains showed no deviation in survival when incubated under nutrient starvation and detergent stress.

Conclusion

Our results suggest that MFhadC may not be important for the formation and maintenance of biofilm, a factor critically important in M. fortuitum pathogenicity. However, not essential for survival and growth, MFhadC maintains the viability of M. fortuitum under a nutrient-starved environment. Collectively, MFhadC may not be used as a biofilm-specific marker for M. fortuitum.

Mycobacterial biofilms as players in human infections: a review

The role of biofilms in pathogenicity and treatment strategies is often neglected in mycobacterial infections. In recent years, the emergence of nontuberculous mycobacterial infections has necessitated the development of novel prophylactic strategies and elucidation of the mechanisms underlying the establishment of chronic infections. More importantly, the question arises whether members of the Mycobacterium tuberculosis complex can form biofilms and contribute to latent tuberculosis and drug resistance because of the long-lasting and recalcitrant nature of its infections. This review discusses some of the molecular mechanisms by which biofilms could play a role in infection or pathological events in humans.

The aceE involves in mycolic acid synthesis and biofilm formation in Mycobacterium smegmatis

Background

The integrity of cell wall structure is highly significant for the in vivo survival of mycobacteria. We hypothesized that changes in morphology may indicate changes in cell wall metabolism and identified an aceE gene mutant (aceE-mut) which presented a deficient colony morphology on 7H10 agar by screening transposon mutagenesis in Mycolicibacterium smegmatis, basonym Mycobacterium smegmatis (M. smegmatis). This study aimed to identify the functional role of aceE gene in cell wall biosynthesis in M. smegmatis.

Results

We observed that the colony morphology of aceE-mut was quite different, smaller and smoother on the solid culture medium than the wild-type (WT) strain during the transposon library screening of M. smegmatis. Notably, in contrast with the WT, which aggregates and forms biofilm, the aceE-mut lost its ability of growing aggregately and biofilm formation, which are two very important features of mycobacteria. The morphological changes in the aceE-mut strain were further confirmed by electron microscopy which indicated smoother and thinner cell envelope images in contrast with the rough morphology of WT strains. Additionally, the aceE-mut was more fragile to acidic stress and exhibited a pronounced defects in entering the macrophages as compared to the WT. The analysis of mycolic acid (MA) using LC-MS indicated deficiency of alpha-MA and epoxy-MA in aceE-mut strain whereas complementation of the aceE-mut with a wild-type aceE gene restored the composition of MA.

Conclusions

Over all, this study indicates that aceE gene plays a significant role in the mycolic acid synthesis and affects the colony morphology, biofilm formation of M. smegmatis and bacteria invasion of macrophage.

The protein kinase PknB negatively regulates biosynthesis and trafficking of mycolic acids in mycobacteria

Mycobacterium tuberculosis is the causative agent of tuberculosis and remains one of the most widespread and deadliest bacterial pathogens in the world. A distinguishing feature of mycobacteria that sets them apart from other bacteria is the unique architecture of their cell wall, characterized by various species-specific lipids, most notably mycolic acids (MAs). Therefore, targeted inhibition of enzymes involved in MA biosynthesis, transport, and assembly has been extensively explored in drug discovery. Additionally, more recent evidence suggests that many enzymes in the MA biosynthesis pathway are regulated by kinase-mediated phosphorylation, thus opening additional drug-development opportunities. However, how phosphorylation regulates MA production remains unclear. Here, we used genetic strategies combined with lipidomics and phosphoproteomics approaches to investigate the role of protein phosphorylation in Mycobacterium The results of this analysis revealed that the Ser/Thr protein kinase PknB regulates the export of MAs and promotes the remodeling of the mycobacterial cell envelope. In particular, we identified the essential MmpL3 as a substrate negatively regulated by PknB. Taken together, our findings add to the understanding of how PknB activity affects the mycobacterial MA biosynthesis pathway and reveal the essential role of protein phosphorylation/dephosphorylation in governing lipid metabolism, paving the way for novel antimycobacterial strategies.

Actinomycetes from the Red Sea Sponge Coscinoderma mathewsi: Isolation, Diversity, and Potential for Bioactive Compounds Discovery

The diversity of actinomycetes associated with the marine sponge Coscinoderma mathewsi collected from Hurghada (Egypt) was studied. Twenty-three actinomycetes were separated and identified based on the 16S rDNA gene sequence analysis. Out of them, three isolates were classified as novel species of the genera Micromonospora, Nocardia, and Gordonia. Genome sequencing of actinomycete strains has revealed many silent biosynthetic gene clusters and has shown their exceptional capacity for the production of secondary metabolites, not observed under classical cultivation conditions. Therefore, the effect of mycolic-acid-containing bacteria or mycolic acid on the biosynthesis of cryptic natural products was investigated. Sponge-derived actinomycete Micromonospora sp. UA17 was co-cultured using liquid fermentation with two mycolic acid-containing actinomycetes (Gordonia sp. UA19 and Nocardia sp. UA 23), or supplemented with pure mycolic acid. LC-HRESIMS data were analyzed to compare natural production across all crude extracts. Micromonospora sp. UA17 was rich with isotetracenone, indolocarbazole, and anthracycline analogs. Some co-culture extracts showed metabolites such as a chlorocardicin, neocopiamycin A, and chicamycin B that were not found in the respective monocultures, suggesting a mycolic acid effect on the induction of cryptic natural product biosynthetic pathways. The antibacterial, antifungal, and antiparasitic activities for the different cultures extracts were also tested.

An improved Xer-cise technology for the generation of multiple unmarked mutants in Mycobacteria

Xer-cise is a technique using antibiotic resistance cassettes flanked by dif sites allowing spontaneous and accurate excision from bacterial chromosomes with a high frequency through the action of the cellular recombinase XerCD. Here, we report a significant improvement of Xer-cise in Mycobacteria. Zeocin resistance cassettes flanked by variants of the natural Mycobacterium tuberculosis dif site were constructed and shown to be effective tools to construct multiple unmarked mutations in M. tuberculosis and in the model species Mycobacterium smegmatis. The dif site variants harbor mutations in the central region and can therefore not recombine with the wild-type or other variants, resulting in mutants of increased genetic stability. The herein described method should be generalizable to virtually any transformable bacterial species.

Monoterpenoid Geraniol Improves Anti-mycobacterial Drug Efficiency by Interfering with Lipidome and Virulence of Mycobacteria

BACKGROUND

Tuberculosis (TB) remains a global infectious disorder for which efficient therapeutics are elusive. Nature is a source of novel pharmacologically active compounds with many potential drugs being derived directly or indirectly from plants, microorganisms and marine organisms.

OBJECTIVE

The present study aimed to elucidate the antimycobacterial potential of Geraniol (Ger), monoterpene alcohol, against Mycobacterium smegmatis.

METHODS

Disrupted membrane integrity was studied by membrane permeability assay and PI uptake. Cell surface phenotypes were studied by colony morphology, sliding motility and cell sedimentation rate. Lipidome profile was demonstrated by thin-layer chromatography and liquid chromatography-electrospray ionization mass spectrometry. Amendment in iron homeostasis was assessed by using iron chelator ferrozine and ferroxidase assay while genotoxicity was estimated with EtBr and DAPI staining. Biofilm formation was measured by staining, dry mass and metabolic activity using crystal violet. Cell adherence was examined microscopically and spectrophotometrically.

RESULTS

We found the antimycobacterial activity of Ger to be 500 μg/ml against M. smegmatis. Underlying mechanisms revealed impaired cell surface phenotypes. Lipidomics analysis exposed profound decrement of mycolic acids, phosphatidylinositol mannosides and triacylglycerides which are crucial for MTB pathogenicity. We further explored that Ger impairs iron homeostasis and leads to genotoxic stress. Moreover, Ger inhibited the potential virulence attributes such as biofilm formation and cell adherence to both polystyrene surface and epithelial cells. Finally, we have validated all the disrupted phenotypes by RT-PCR which showed good correlation with the biochemical assays.

CONCLUSION

Taken together, the current study demonstrates the antimycobacterial mechanisms of Ger, which may be exploited as an effective candidate of pharmacological interest.

The extracellular matrix of mycobacterial biofilms: could we shorten the treatment of mycobacterial infections?

A number of non-tuberculous mycobacterium species are opportunistic pathogens and ubiquitously form biofilms. These infections are often recalcitrant to treatment and require therapy with multiple drugs for long duration. The biofilm resident bacteria also display phenotypic drug tolerance and thus it has been hypothesized that the drug unresponsiveness in vivo could be due to formation of biofilms inside the host. We have discussed the biofilms of several pathogenic non-tuberculous mycobacterium (NTM) species in context to the in vivo pathologies. Besides pathogenic NTMs, Mycobacterium smegmatis is often used as a model organism for understanding mycobacterial physiology and has been studied extensively for understanding the mycobacterial biofilms. A number of components of the mycobacterial cell wall such as glycopeptidolipids, short chain mycolic acids, monomeromycolyl diacylglycerol, etc. have been shown to play an important role in formation of pellicle biofilms. It shall be noted that these components impart a hydrophobic character to the mycobacterial cell surface that facilitates cell to cell interaction. However, these components are not necessarily the constituents of the extracellular matrix of mycobacterial biofilms. In the end, we have described the biofilms of Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis. Three models of Mtb biofilm formation have been proposed to study the factors regulating biofilm formation, the physiology of the resident bacteria, and the nature of the biomaterial that holds these bacterial masses together. These models include pellicle biofilms formed at the liquid-air interface of cultures, leukocyte lysate-induced biofilms, and thiol reductive stressinduced biofilms. All the three models offer their own advantages in the study of Mtb biofilms. Interestingly, lipids (mainly keto-mycolic acids) are proposed to be the primary component of extracellular polymeric substance (EPS) in the pellicle biofilm, whereas the leukocyte lysate-induced and thiol reductive stress-induced biofilms possess polysaccharides as the primary component of EPS. Both models also contain extracellular DNA in the EPS. Interestingly, thiol reductive stressinduced Mtb biofilms are held together by cellulose and yet unidentified structural proteins. We believe that a better understanding of the EPS of Mtb biofilms and the physiology of the resident bacteria will facilitate the development of shorter regimen for TB treatment.

Cell Walls and Membranes of Actinobacteria

Actinobacteria is a group of diverse bacteria. Most species in this class of bacteria are filamentous aerobes found in soil, including the genus Streptomyces perhaps best known for their fascinating capabilities of producing antibiotics. These bacteria typically have a Gram-positive cell envelope, comprised of a plasma membrane and a thick peptidoglycan layer. However, there is a notable exception of the Corynebacteriales order, which has evolved a unique type of outer membrane likely as a consequence of convergent evolution. In this chapter, we will focus on the unique cell envelope of this order. This cell envelope features the peptidoglycan layer that is covalently modified by an additional layer of arabinogalactan . Furthermore, the arabinogalactan layer provides the platform for the covalent attachment of mycolic acids , some of the longest natural fatty acids that can contain ~100 carbon atoms per molecule. Mycolic acids are thought to be the main component of the outer membrane, which is composed of many additional lipids including trehalose dimycolate, also known as the cord factor. Importantly, a subset of bacteria in the Corynebacteriales order are pathogens of human and domestic animals, including Mycobacterium tuberculosis. The surface coat of these pathogens are the first point of contact with the host immune system, and we now know a number of host receptors specific to molecular patterns exposed on the pathogen's surface, highlighting the importance of understanding how the cell envelope of Actinobacteria is structured and constructed. This chapter describes the main structural and biosynthetic features of major components found in the actinobacterial cell envelopes and highlights the key differences between them.

Deletion of MSMEG_1350 in Mycobacterium smegmatis causes loss of epoxy-mycolic acids, fitness alteration at low temperature and resistance to a set of mycobacteriophages

Mycobacterium smegmatis is intrinsically resistant to thiacetazone, an anti-tubercular thiourea; however we report here that it causes a mild inhibition in growth in liquid medium. Since mycolic acid biosynthesis was affected, we cloned and expressed Mycobacterium smegmatis mycolic acid methyltransferases, postulated as targets for thiacetazone in other mycobacterial species. During this analysis we identified MSMEG_1350 as the methyltransferase involved in epoxy mycolic acid synthesis since its deletion led to their total loss. Phenotypic characterization of the mutant strain showed colony morphology alterations at all temperatures, reduced growth and a slightly increased susceptibility to SDS, lipophilic and large hydrophilic drugs at 20 °C with little effect at 37 °C. No changes were detected between parental and mutant strains in biofilm formation, sliding motility or sedimentation rate. Intriguingly, we found that several mycobacteriophages severely decreased their ability to form plaques in the mutant strain. Taken together our results prove that, in spite of being a minor component of the mycolic acid pool, epoxy-mycolates are required for a proper assembly and functioning of the cell envelope. Further studies are warranted to decipher the role of epoxy-mycolates in the M. smegmatis cell envelope.

HadD, a novel fatty acid synthase type II protein, is essential for alpha- and epoxy-mycolic acid biosynthesis and mycobacterial fitness

Mycolic acids (MAs) have a strategic location within the mycobacterial envelope, deeply influencing its architecture and permeability, and play a determinant role in the pathogenicity of mycobacteria. The fatty acid synthase type II (FAS-II) multienzyme system is involved in their biosynthesis. A combination of pull-downs and proteomics analyses led to the discovery of a mycobacterial protein, HadD, displaying highly specific interactions with the dehydratase HadAB of FAS-II. In vitro activity assays and homology modeling showed that HadD is, like HadAB, a hot dog folded (R)-specific hydratase/dehydratase. A hadD knockout mutant of Mycobacterium smegmatis produced only the medium-size alpha’-MAs. Data strongly suggest that HadD is involved in building the third meromycolic segment during the late FAS-II elongation cycles, leading to the synthesis of the full-size alpha- and epoxy-MAs. The change in the envelope composition induced by hadD inactivation strongly altered the bacterial fitness and capacities to aggregate, assemble into colonies or biofilms and spread by sliding motility, and conferred a hypersensitivity to the firstline antimycobacterial drug rifampicin. This showed that the cell surface properties and the envelope integrity were greatly affected. With the alarmingly increasing case number of nontuberculous mycobacterial diseases, HadD appears as an attractive target for drug development.

ESX-1 and phthiocerol dimycocerosates of Mycobacterium tuberculosis act in concert to cause phagosomal rupture and host cell apoptosis

Although phthiocerol dimycocerosates (DIM) are major virulence factors of Mycobacterium tuberculosis (Mtb), the causative agent of human tuberculosis, little is known about their mechanism of action. Localized in the outer membrane of mycobacterial pathogens, DIM are predicted to interact with host cell membranes. Interaction with eukaryotic membranes is a property shared with another virulence factor of Mtb, the early secretory antigenic target EsxA (also known as ESAT-6). This small protein, which is secreted by the type VII secretion system ESX-1 (T7SS/ESX-1), is involved in phagosomal rupture and cell death induced by virulent mycobacteria inside host phagocytes. In this work, by the use of several knock-out or knock-in mutants of Mtb or Mycobacterium bovis BCG strains and different cell biological assays, we present conclusive evidence that ESX-1 and DIM act in concert to induce phagosomal membrane damage and rupture in infected macrophages, ultimately leading to host cell apoptosis. These results identify an as yet unknown function for DIM in the infection process and open up a new research field for the study of the interaction of lipid and protein virulence factors of Mtb.

Influence of iron deprivation on virulence traits of mycobacteria

Novel strategies to combat the ever increasing burden of drug resistance in Mycobacterium tuberculosis (MTB) causing tuberculosis (TB) remains a global concern. The ability of MTB to sense and adapt to restricted iron conditions in the hostile environment is essential for their survival and confers the basis of their success as dreadful pathogen. The striking and clinically relevant virulence trait of MTB is its ability to form biofilms and adhere to the host cells. The present study elucidated the effect of iron deprivation on biofilm formation and cell adherence of Mycobacterium smegmatis, a non-pathogenic surrogate of MTB. Firstly, we showed that iron deprivation leads to enhanced cell sedimentation rate and altered colony morphology depicting alterations in cell surface envelope properties. We explored that biofilm formation and cell adherence to polystyrene surface as well as human oral epithelial cells were considerably reduced under iron deprivation both in presence of 2,2 BP (iron chelator) and siderophore mutant Δ011-14 strain. We further investigated that the potency of three first line anti-TB drugs (Isoniazid, Ethambutol, Rifampicin) to inhibit both biofilm formation and cell adhesion were enhanced under iron deprivation in contrast to the drugs when tested alone. Taken together, by virtue of the indispensability of iron for functional virulence traits in mycobacteria, iron deprivation strategies could be further exploited against this notorious human pathogen to explore novel drug targets.