Growth, cell division and sporulation in mycobacteria

Profiling Mycobacterium ulcerans: sporulation, survival strategy and response to environmental factors

Mycobacterium ulcerans is the causative agent of Buruli ulcer – a necrotizing skin infection. As an environmental pathogen, it has developed stress response mechanisms for survival. Similar to endospore formation in M. marinum, it is likely that M. ulcerans employs sporulation mechanisms for its survival and transmission. In this review, we modeled possible transmission routes and patterns of M. ulcerans from the environment to its host. We provided insights into the evolution of M. ulcerans and its genomic profiles. We discuss reservoirs of M. ulcerans as an environmental pathogen and its environmental survival. We comprehensively discuss sporulation as a possible stress response mechanism and modelled endospore formation in M. ulcerans. At last, we highlighted sporulation associated markers, which upon expression trigger endospore formation.

Mycobacterial Populations Partly Change the Proportions of the Cells Undergoing Asymmetric/Symmetric Divisions in Response to Glycerol Levels in Growth Medium

Twenty to thirty percent of the septating mycobacterial cells of the mid-log phase population showed highly deviated asymmetric constriction during division (ACD), while the remaining underwent symmetric constriction during division (SCD). The ACD produced short-sized cells (SCs) and normal/long-sized cells (NCs) as the sister-daughter cells, but with significant differential susceptibility to antibiotic/oxidative/nitrite stress. Here we report that, at 0.2% glycerol, formulated in the Middlebrook 7H9 medium, a significantly high proportion of the cells were divided by SCD. When the glycerol concentration decreased to 0.1% due to cell-growth/division, the ACD proportion gradually increased until the ACD:SCD ratio reached ~50:50. With further decrease in the glycerol levels, the SCD proportion increased with concomitant decrease in the ACD proportion. Maintenance of glycerol at 0.1%, through replenishment, held the ACD:SCD proportion at ~50:50. Transfer of the cells from one culture with a specific glycerol level to the supernatant from another culture, with a different glycerol level, made the cells change the ACD:SCD proportion to that of the culture from which the supernatant was taken. RT-qPCR data showed the possibility of diadenosine tetraphosphate phosphorylase (MSMEG_2932), phosphatidylinositol synthase (MSMEG_2933), and a Nudix family hydrolase (MSMEG_2936) involved in the ACD:SCD proportion-change in response to glycerol levels. We also discussed its physiological significance.

The selective advantage of facultative anaerobes relies on their unique ability to cope with changing oxygen levels during infection

Bacteria, including those that are pathogenic, have been generally classified according to their ability to survive and grow in the presence or absence of oxygen: aerobic and anaerobic bacteria, respectively. Strict aerobes require oxygen to grow (e.g., Neisseria), and strict anaerobes grow exclusively without, and do not survive oxygen exposure (e.g., Clostridia); aerotolerant bacteria (e.g., Lactobacilli) are insensitive to oxygen exposure. Facultative anaerobes (e.g., E. coli) have the unique ability to grow in the presence or in the absence of oxygen and are thus well-adapted to these changing conditions, which may constitute an underestimated selective advantage for infection. In the WHO antibiotic-resistant 'priority pathogens' list, facultative anaerobes are overrepresented (8 among 12 listed pathogens), consistent with clinical studies performed in populations particularly susceptible to infectious diseases. Bacteria aerobic respiratory chain plays a central role in oxygen consumption, leading to the formation of hypoxic infectious sites (infectious hypoxia). Facultative anaerobes have developed a wide diversity of aerotolerance and anaerotolerance strategies in vivo. However, at a single cell level, the modulation of the intracellular oxygen level in host infected cells remains elusive and will be discussed in this review. In conclusion, the ability of facultative bacteria to evolve in the presence or the absence of oxygen is essential for their virulence strategy and constitute a selective advantage. TAKE AWAY: Most life-threatening pathogenic bacteria are facultative anaerobes. Only facultative anaerobes are aerotolerant, anaerotolerant and capable of consuming O₂ . Facultative anaerobes induce and are well adapted to cellular hypoxia.

Arrayed CRISPRi and quantitative imaging describe the morphotypic landscape of essential mycobacterial genes

Mycobacterium tuberculosis possesses a large number of genes of unknown or predicted function, undermining fundamental understanding of pathogenicity and drug susceptibility. To address this challenge, we developed a high-throughput functional genomics approach combining inducible CRISPR-interference and image-based analyses of morphological features and sub-cellular chromosomal localizations in the related non-pathogen, M. smegmatis. Applying automated imaging and analysis to 263 essential gene knockdown mutants in an arrayed library, we derive robust, quantitative descriptions of bacillary morphologies consequent on gene silencing. Leveraging statistical-learning, we demonstrate that functionally related genes cluster by morphotypic similarity and that this information can be used to inform investigations of gene function. Exploiting this observation, we infer the existence of a mycobacterial restriction-modification system, and identify filamentation as a defining mycobacterial response to histidine starvation. Our results support the application of large-scale image-based analyses for mycobacterial functional genomics, simultaneously establishing the utility of this approach for drug mechanism-of-action studies.

Mycobacteria Bovis osteomyelitis following intravesical BCG for bladder cancer

Mycobacteria Bovis osteomyelitis is a rare adverse effect after Bacillus Calmette-Guerin (BCG) intravesical therapy. A 62-year-old male presented with acute spinal cord compression three months after completing his second course of therapy for bladder cancer. The first course with intravesical BCG was complicated with an episode of hematuria. He reported intermittent subjective fever for 3 weeks thereafter which resolved with Tylenol. Interferon-α2 B was added to the second cycle of intravesical BCG with the indication here being residual tumor, and was tolerated well. His complete blood count and liver function tests were unremarkable on admission. MRI showed features of osteomyelitis with cord compression at T4/T5. Biopsy of the affected bone showed caseating granuloma which was positive for acid fact bacilli, later confirmed to be Mycobacterium Bovis by PCR and pyrazinamide resistance. He was started on intravenous steroids and underwent spinal cord decompression. Rifampin, Isoniazid, and Ethambutol were then commenced. His weakness improved and after two months of therapy he was asymptomatic and back to his baseline function. Osteomyelitis is a rare but serious complication. Early diagnosis and treatment is important as the outcomes are good.

Pleiomorphism in Mycobacterium

Morphological variants in mycobacterial cultures under different growth conditions, including aging of the culture, have been shown to include fibrous aggregates, biofilms, coccoids, and spores. Here we discuss the diversity in shape and size changes demonstrated by bacterial cells with special reference to pleiomorphism observed in Mycobacterium spp. in response to nutritional and other environmental stresses. Inherent asymmetry in cell division and compartmentalization of cell interior under different growth conditions might contribute toward the observed pleiomorphism in mycobacteria. The regulatory genes comprising the bacterial signaling pathway responsible for initiating morphogenesis are speculated upon from bioinformatic identifications of genes for known sensors, kinases, and phosphatases existing in mycobacterial genomes as well as on the basis of what is known in other bacteria.

Complete genome sequence and cell structure of Limnochorda pilosa, a Gram-negative spore-former within the phylum Firmicutes

Limnochorda pilosa is a pleomorphic facultative anaerobe and the sole species in the class Limnochordia, which has tentatively been placed in the phylum Firmicutes. In the present study, the complete genome sequence of L. pilosa HC45^T was obtained and analysed. The genome size was 3.82 Mbp and the DNA G+C content was 69.73 %. Phylogenetic analyses based on the 30S-50S ribosomal proteins and 23S rRNA gene consistently indicated that L. pilosa is phylogenetically isolated from the other members of the phylum Firmicutes. Ultrastructural observation revealed that L. pilosa possesses a Gram-negative-type cell wall and the capacity to form endospores. Accordingly, the L. pilosa genome has characteristics that are specific to Gram-negative bacteria and contains many genes that are involved in sporulation. On the other hand, several sporulation genes were absent from the L. pilosa genome although they have been regarded as essential for the endospore-forming system of members of the phylum Firmicutes. The gyrB gene of L. pilosa possesses an intein sequence. The genome has a high percentage of GTG start codons and lacks several conserved genes related to cell division.

Comparative Sigma Factor-mRNA Levels in Mycobacterium marinum under Stress Conditions and during Host Infection

We have used RNASeq and qRT-PCR to study mRNA levels for all σ-factors in different Mycobacterium marinum strains under various growth and stress conditions. We also studied their levels in M. marinum from infected fish and mosquito larvae. The annotated σ-factors were expressed and transcripts varied in relation to growth and stress conditions. Some were highly abundant such as sigA, sigB, sigC, sigD, sigE and sigH while others were not. The σ-factor mRNA profiles were similar after heat stress, during infection of fish and mosquito larvae. The similarity also applies to some of the known heat shock genes such as the α-crystallin gene. Therefore, it seems probable that the physiological state of M. marinum is similar when exposed to these different conditions. Moreover, the mosquito larvae data suggest that this is the state that the fish encounter when infected, at least with respect to σ-factor mRNA levels. Comparative genomic analysis of σ-factor gene localizations in three M. marinum strains and Mycobacterium tuberculosis H37Rv revealed chromosomal rearrangements that changed the localization of especially sigA, sigB, sigD, sigE, sigF and sigJ after the divergence of these two species. This may explain the variation in species-specific expression upon exposure to different growth conditions.

Mycobacterial Growth

In this work, we review progress made in understanding the molecular underpinnings of growth and division in mycobacteria, concentrating on work published since the last comprehensive review ( Hett and Rubin 2008). We have focused on exciting work making use of new time-lapse imaging technologies coupled with reporter-gene fusions and antimicrobial treatment to generate insights into how mycobacteria grow and divide in a heterogeneous manner. We try to reconcile the different observations reported, providing a model of how they might fit together. We also review the topic of mycobacterial spores, which has generated considerable discussion during the last few years. Resuscitation promoting factors, and regulation of growth and division, have also been actively researched, and we summarize progress in these areas.

Metagenomic analysis of buffalo rumen microbiome: Effect of roughage diet on Dormancy and Sporulation genes

Buffalo rumen microbiome experiences a variety of diet stress and represents reservoir of Dormancy and Sporulation genes. However, the information on genomic responses to such conditions is very limited. The Ion Torrent PGM next generation sequencing technology was used to characterize general microbial diversity and the repertoire of microbial genes present, including genes associated with Dormancy and Sporulation in Mehsani buffalo rumen metagenome. The research findings revealed the abundance of bacteria at the domain level and presence of Dormancy and Sporulation genes which were predominantly associated with the Clostridia and Bacilli taxa belonging to the phyla Firmicutes. Genes associated with Sporulation cluster and Sporulation orphans were increased from 50% to 100% roughage treatment, thereby promoting sporulation all along the treatments. The spore germination is observed to be the highest in the 75% roughage treatment both in the liquid and solid rumen fraction samples with respect to the decrease in the values of the genes associated with spore core dehydration, thereby facilitating spore core hydration which is necessary for spore germination.

Cell division site placement and asymmetric growth in mycobacteria

Mycobacteria are members of the actinomycetes that grow by tip extension and lack apparent homologues of the known cell division regulators found in other rod-shaped bacteria. Previous work using static microscopy on dividing mycobacteria led to the hypothesis that these cells can grow and divide asymmetrically, and at a wide range of sizes, in contrast to the cell growth and division patterns observed in the model rod-shaped organisms. In this study, we test this hypothesis using live-cell time-lapse imaging of dividing Mycobacterium smegmatis labelled with fluorescent PBP1a, to probe peptidoglycan synthesis and label the cell septum. We demonstrate that the new septum is placed accurately at mid-cell, and that the asymmetric division observed is a result of differential growth from the cell tips, with a more than 2-fold difference in growth rate between fast and slow growing poles. We also show that the division site is not selected at a characteristic cell length, suggesting this is not an important cue during the mycobacterial cell cycle.

Asymmetry and aging of mycobacterial cells lead to variable growth and antibiotic susceptibility

Cells use both deterministic and stochastic mechanisms to generate cell-to-cell heterogeneity, which enables the population to better withstand environmental stress. Here we show that, within a clonal population of mycobacteria, there is deterministic heterogeneity in elongation rate that arises because mycobacteria grow in an unusual, unipolar fashion. Division of the asymmetrically growing mother cell gives rise to daughter cells that differ in elongation rate and size. Because the mycobacterial cell division cycle is governed by time, not cell size, rapidly elongating cells do not divide more frequently than slowly elongating cells. The physiologically distinct subpopulations of cells that arise through asymmetric growth and division are differentially susceptible to clinically important classes of antibiotics.

Recent progress in Bacillus subtilis sporulation

The Gram-positive bacterium Bacillus subtilis can initiate the process of sporulation under conditions of nutrient limitation. Here, we review some of the last 5 years of work in this area, with a particular focus on the decision to initiate sporulation, DNA translocation, cell-cell communication, protein localization and spore morphogenesis. The progress we describe has implications not only just for the study of sporulation but also for other biological systems where homologs of sporulation-specific proteins are involved in vegetative growth.

Dormant ovoid cells of Mycobacterium tuberculosis are formed in response to gradual external acidification

It is believed that latent tuberculosis is associated with the persistence of Mycobacterium tuberculosis (MTB) in a dormant-like state. Dormant cells of MTB with coccoid morphology were produced in some in vivo studies, but similar forms were not produced in the known in vitro models in sufficient amounts to permit their characterization. This work demonstrates the efficient formation of phase-dark ovoid cells in MTB cultures within 150 days after the onset of stationary phase. During this time the medium underwent gradual acidification (pH 8.5 → 4.7) as a result of cellular metabolism. A rapid change in the external pH resulted in cell degradation and death. In common with the dormant forms found in other organisms, the ovoid cells had thickened cell walls, a low metabolic activity and elevated resistance to antibiotics and heating. The ovoid cells had lost the ability to form colonies on solid medium and were thus regarded as operationally «non-culturable». At an early stage in the acidification process (about 40 days post inoculation), the ovoid cells self-resuscitated when placed in fresh liquid medium. However, ovoid cells, stored for a prolonged time, required supernatant from active MTB cells, or externally added recombinant form of resuscitation promoting factor (Rpf) for successful resuscitation. It is suggested that the adaptation of cellular metabolism leading to gradual acidification of the external medium results in the formation of morphologically distinct dormant MTB cells in vitro. The model of MTB dormancy developed here could be a useful tool for the development of new drugs against latent TB.