Physiological and morphological responses of the soil bacterium Rhodococcus opacus strain PD630 to water stress

Formation of specialized aerial architectures by Rhodococcus during utilization of vaporized p-cresol

When grown with vaporized alkylphenols such as p-cresol as the sole carbon and energy source, several isolated Rhodococcus strains formed growth structures like miniature mushrooms, termed here specialized aerial architectures (SAA), that reached sizes of up to 0.8 mm in height. Microscopic examination allowed us to view the distinct developmental stages during the formation of SAA from a selected strain, Rhodococcus sp. KL96. Initially, mounds consisting of long rod cells arose from a lawn of cells, and then highly branched structures were formed from the mounds. During the secondary stage of development, branching began after long rod cells grew outward and twisted longitudinally, serving as growth points, and the cells at the base of the mound became short rods that supported upward growth. Cells in the highly fluffy structures were eventually converted, via reductive division, into structures that resembled cocci, with a diameter of approximately 0.5 μm, that were arranged in chains. Most cells inside the SAA underwent a phase variation in order to form wrinkled colonies from cells that originally formed smooth colonies. Approximately 2 months was needed for complete development of the SAA, and viable cells were recovered from SAA that were incubated for more than a year. An extracellular polymeric matrix layer and lipid bodies appeared to play an important role in structural integrity and as a metabolic energy source, respectively. To our knowledge, similar formation of aerial structures for the purpose of substrate utilization has not been reported previously for Gram-positive bacteria.

Activity of two catabolic enzymes of the phosphogluconate pathway in mesquite roots inoculated with Azospirillum brasilense Cd

The mesquite amargo (Prosopis articulate), one of the main nurse trees of the Sonoran Desert in Mexico, is responsible for major, natural re-vegetation processes. It exudes gluconic acid in root exudates, a favorite carbon source for the plant growth-promoting bacterium Azospirillum brasilense. Two enzymes, gluconokinase (EC 2.7.1.12) and 6-phosphogluconate dehydrogenase (EC 1.1.1.44), participating in the phosphogluconate pathway, are active in the bacteria. Bacterial 6-phosphogluconate dehydrogenase is a constitutive enzyme, while gluconokinase is induced upon exposure to gluconic acid. Both enzymes are active in young, non-inoculated mesquite seedlings growing under hydroponic conditions. When A. brasilense Cd bacteria are inoculated on the root system, the roots exhibit much higher activity of gluconokinase, but not 6-phosphogluconate dehydrogenase. Mesquite roots exhibit high levels of root colonization by the inoculating bacteria. At the same time, and also for plants growing under sand culture conditions, the seedlings grew taller, greener, had longer leaves, and were heavier.

Cytological and transcript analyses reveal fat and lazy persister-like bacilli in tuberculous sputum

BACKGROUND

Tuberculous sputum provides a sample of bacilli that must be eliminated by chemotherapy and that may go on to transmit infection. A preliminary observation that Mycobacterium tuberculosis cells contain triacylglycerol lipid bodies in sputum, but not when growing in vitro, led us to investigate the extent of this phenomenon and its physiological basis.

METHODS AND FINDINGS

Microscopy-positive sputum samples from the UK and The Gambia were investigated for their content of lipid body-positive mycobacteria by combined Nile red and auramine staining. All samples contained a lipid body-positive population varying from 3% to 86% of the acid-fast bacilli present. The recent finding that triacylglycerol synthase is expressed by mycobacteria when they enter in vitro nonreplicating persistence led us to investigate whether this state was also associated with lipid body formation. We found that, when placed in laboratory conditions inducing nonreplicating persistence, two M. tuberculosis strains had lipid body levels comparable to those found in sputum. We investigated these physiological findings further by comparing the M. tuberculosis transcriptome of growing and nonreplicating persistence cultures with that obtained directly from sputum samples. Although sputum has traditionally been thought to contain actively growing tubercle bacilli, our transcript analyses refute the hypothesis that these cells predominate. Rather, they reinforce the results of the lipid body analyses by revealing transcriptional signatures that can be clearly attributed to slowly replicating or nonreplicating mycobacteria. Finally, the lipid body count was highly correlated (R(2) = 0.64, p < 0.03) with time to positivity in diagnostic liquid cultures, thereby establishing a direct link between this cytological feature and the size of a potential nonreplicating population.

CONCLUSION

As nonreplicating tubercle bacilli are tolerant to the cidal action of antibiotics and resistant to multiple stresses, identification of this persister-like population of tubercle bacilli in sputum presents exciting and tractable new opportunities to investigate both responses to chemotherapy and the transmission of tuberculosis.

Global response to desiccation stress in the soil actinomycete Rhodococcus jostii RHA1

Rhodococcus jostii RHA1 is a soil-residing actinomycete with many favorable metabolic capabilities that make it an ideal candidate for the bioremediation of contaminated soils. Arguably the most basic requirement for life is water, yet some nonsporulating bacteria, like RHA1, can survive lengthy droughts. Here we report the first transcriptomic analysis of a gram-positive bacterium during desiccation. Filtered RHA1 cells incubated at either low relative humidity (20%), as an air-drying treatment, or high relative humidity (100%), as a control, were transcriptionally profiled over a comprehensive time series. Also, the morphology of RHA1 cells was characterized by cryofixation scanning electron microscopy during each treatment. Desiccation resulted in a transcriptional response of approximately 8 times more differentially regulated genes than in the control (819 versus 106 genes, respectively). Genes that were differentially expressed during only the desiccation treatment primarily had expression profiles that were maximally up-regulated upon complete drying of the cells. The microarray expression ratios for some of the highly up-regulated genes were verified by reverse transcriptase quantitative PCR. These genes included dps1, encoding an oxidative stress protection protein which has not previously been directly associated with desiccation, and the two genes encoding sigma factors SigF1 and SigF3, possibly involved in the regulatory response to desiccation. RHA1 cells also induced the biosynthetic pathway for the compatible solute ectoine. These desiccation-specific responses represent the best candidates for important mechanisms of desiccation resistance in RHA1.

Biodegradation of phytane (2,6,10,14-tetramethylhexadecane) and accumulation of related isoprenoid wax esters byMycobacterium ratisbonensestrain SD4 under nitrogen-starved conditions

The accumulation of storage lipids during the biodegradation of 2,6,10,14-tetramethylhexadecane (phytane) by Mycobacterium ratisbonense strain SD4 grown under nitrogen-starved conditions was investigated. Detailed chemical analysis of intracellular metabolites revealed the existence of (at least) three different pathways for the catabolism of phytane, and the accumulation of significant proportions (39% of the total lipids) of several isoprenoid wax esters formed by condensation of oxidation products of the hydrocarbon. In contrast, triacylglycerols but no wax esters were accumulated by strain SD4 grown on hexadecane, the unbranched homologue of phytane.