Re-growth of Mycobacterium tuberculosis populations exposed to antibiotic combinations is due to the presence of isoniazid and not bacterial growth rate.

Modulation of growth rate in Mycobacterium tuberculosis is key to its survival in the host; particularly with regard to its adaptation during chronic infection when the growth rate is very slow. The resulting physiological changes will influence the way this pathogen interacts with the host and responds to antibiotics. Therefore, it is important that we understand how growth rate impacts antibiotic efficacy, particularly with respect to recovery/relapse. This is the first study that has asked how growth rates influence the mycobacterial responses to combinations of frontline antimycobacterials, isoniazid (INH), rifampicin (RIF), and pyrazinamide (PZA), using continuous cultures. Time-course profiles of log-transformed total viable counts for cultures, controlled at either a fast growth rate (23.1. mean generation time (MGT)) or slow growth rate (69.3h MGT), were analysed with the fitting of a mathematical model by nonlinear regression that accounted for the dilution rate in the chemostat, and profiled kill rates and recovery in culture. Using this approach, we show that populations growing more slowly were generally less susceptible to all treatments. We observed a higher kill rate associated with INH (compared to RIF or PZA) and the appearance of re-growth. In line with this observation, re-growth was not observed with RIF-exposure, which provided a slower bactericidal response. The sequential additions of RIF and PZA did not eliminate re-growth. We consider here that faster, early bactericidal activity is not what is required for successful sterilisation of M. tuberculosis, but instead slower elimination of bacilli followed by reduced recovery of the bacterial population.

Esterification of geraniol as a strategy for increasing product titre and specificity in engineered Escherichia coli.

BACKGROUND: Geraniol, an acyclic monoterpene alcohol, is found as a primary constituent in the essential oils of plants such as geranium, lemongrass and rose. The floral-like scent of geraniol has made it a popular constituent of flavour and fragrance products. Over recent decades biotechnology has made significant progress towards the development of industrial platforms for the production of commercially valuable monoterpenoids, such as geraniol, through expression of recombinant terpene biosynthetic pathways in microbial hosts. Titres, however, have been hindered due to the inherent toxicity of these compounds-which are often utilised for anti-microbial and anti-fungal functions in their host plant. RESULTS: In this study we modified an Escherichia coli strain, engineered to express a heterologous mevalonate pathway, by replacement of the terpene synthase with a geraniol synthase from Ocimum basilicum for the production of geraniol, and co-expressed an alcohol acyltransferase (AAT) from Rosa hybrida for the specific acetylation of geraniol. The low water solubility of geranyl acetate facilitated its partition into the organic phase of a two-phase system, relieving the cellular toxicity attributed to the build-up of geraniol in the aqueous phase. In a partially optimised system this strain produced 4.8 g/L geranyl acetate (based on the aqueous volume) which, on a molar equivalent basis, represents the highest monoterpene titre achieved from microbial culture to date. It was also found that esterification of geraniol prevented bioconversion into other monoterpenoids, leading to a significant improvement in product specificity, with geranyl acetate being the sole product observed. CONCLUSION: In this study we have shown that it is possible to both overcome the toxicity limit impeding the production of the monoterpene alcohol geraniol and mitigate product loss in culture through endogenous metabolism by using an in vivo esterification strategy. This strategy has resulted in the highest geraniol (equivalent) titres achieved from a microbial host, and presents esterification as a viable approach to increasing the titres obtained in microbial monoterpenoid production.

A Flow Cytometry Method for Rapidly Assessing Mycobacterium tuberculosis Responses to Antibiotics with Different Modes of Action.

Current methods for assessing the drug susceptibility of Mycobacterium tuberculosis are lengthy and do not capture information about viable organisms that are not immediately culturable under standard laboratory conditions as a result of antibiotic exposure. We have developed a rapid dual-fluorescence flow cytometry method using markers for cell viability and death. We show that the fluorescent marker calcein violet with an acetoxy-methyl ester group (CV-AM) can differentiate between populations of M. tuberculosis growing at different rates, while Sytox green (SG) can differentiate between live and dead mycobacteria. M. tuberculosis was exposed to isoniazid or rifampin at different concentrations over time and either dual stained with CV-AM and SG and analyzed by flow cytometry or plated to determine the viability of the cells. Although similar trends in the loss of viability were observed when the results of flow cytometry and the plate counting methods were compared, there was a lack of correlation between these two approaches, as the flow cytometry analysis potentially captured information about cell populations that were unable to grow under standard conditions. The flow cytometry approach had an additional advantage in that it could provide insights into the mode of action of the drug: antibiotics targeting the cell wall gave a flow cytometry profile distinct from those inhibiting intracellular processes. This rapid drug susceptibility testing method could identify more effective antimycobacterials, provide information about their potential mode of action, and accelerate their progress to the clinic.

Mycobacterium tuberculosis Is Resistant to Isoniazid at a Slow Growth Rate by Single Nucleotide Polymorphisms in katG Codon Ser315.

An important aim for improving TB treatment is to shorten the period of antibiotic therapy without increasing relapse rates or encouraging the development of antibiotic-resistant strains. In any M. tuberculosis population there is a proportion of bacteria that are drug-tolerant; this might be because of pre-existing populations of slow growing/non replicating bacteria that are protected from antibiotic action due to the expression of a phenotype that limits drug activity. We addressed this question by observing populations of either slow growing (constant 69.3h mean generation time) or fast growing bacilli (constant 23.1h mean generation time) in their response to the effects of isoniazid exposure, using controlled and defined growth in chemostats. Phenotypic differences were detected between the populations at the two growth rates including expression of efflux mechanisms and the involvement of antisense RNA/small RNA in the regulation of a drug-tolerant phenotype, which has not been explored previously for M. tuberculosis. Genotypic analyses showed that slow growing bacilli develop resistance to isoniazid through mutations specifically in katG codon Ser315 which are present in approximately 50-90% of all isoniazid-resistant clinical isolates. The fast growing bacilli persisted as a mixed population with katG mutations distributed throughout the gene. Mutations in katG codon Ser315 appear to have a fitness cost in vitro and particularly in fast growing cultures. Our results suggest a requirement for functional katG-encoded catalase-peroxide in the slow growers but not the fast-growing bacteria, which may explain why katG codon Ser315 mutations are favoured in the slow growing cultures.

Non-replicating Mycobacterium tuberculosis elicits a reduced infectivity profile with corresponding modifications to the cell wall and extracellular matrix.

A key feature of Mycobacterium tuberculosis is its ability to become dormant in the host. Little is known of the mechanisms by which these bacilli are able to persist in this state. Therefore, the focus of this study was to emulate environmental conditions encountered by M. tuberculosis in the granuloma, and determine the effect of such conditions on the physiology and infectivity of the organism. Non-replicating persistent (NRP) M. tuberculosis was established by the gradual depletion of nutrients in an oxygen-replete and controlled environment. In contrast to rapidly dividing bacilli, NRP bacteria exhibited a distinct phenotype by accumulating an extracellular matrix rich in free mycolate and lipoglycans, with increased arabinosylation. Microarray studies demonstrated a substantial down-regulation of genes involved in energy metabolism in NRP bacteria. Despite this reduction in metabolic activity, cells were still able to infect guinea pigs, but with a delay in the development of disease when compared to exponential phase bacilli. Using these approaches to investigate the interplay between the changing environment of the host and altered physiology of NRP bacteria, this study sheds new light on the conditions that are pertinent to M. tuberculosis dormancy and how this organism could be establishing latent disease.

ELISPOT Refinement Using Spot Morphology for Assessing Host Responses to Tuberculosis.

Tuberculosis is a global health problem. The Mycobacterium bovis Bacille Calmette Guerin (BCG) vaccine has variable efficacy (0-80%) so there is a drive to develop novel vaccines. The cytokine, interferon gamma (IFNγ), is an essential component of the protective response to M. tuberculosis (M. tb) infection and is also produced in response to BCG vaccination. Induction of an IFNγ response is used as a biomarker of successful vaccination in the assessment of new tuberculosis (TB) vaccines. The IFNγ ELISPOT assay provides an important tool for TB research. It is used for both the diagnosis of infection (T.Spot assay), and for the evaluation of the immunogenicity of new TB vaccine candidates in human clinical trials, in the non-human primate (NHP) model of TB infection studies. The ELISPOT assay captures IFNγ produced by peripheral blood mononuclear cells (PBMCs) following specific stimulation, onto a membrane so individual cells can be enumerated and the frequency of responding cells determined. Hence spot forming units (SFU) per 106 cells provide the traditional measure for ELISPOT assays. The discriminatory power of SFU is limited. In some situations, the number of SFU in BCG vaccinated, and unvaccinated, subjects was found to be similar, although the spots were observed to be larger in vaccinated subjects. Spot size potentially provides a measure of the quantity of cytokine produced by individual cells. The AID ELISPOT plate reader software used to determine frequency of spots also has the capability to determine the size of each spot. Consideration of spot size in combination with spot forming units was investigated in our studies of BCG immunogenicity. This additional readout was found to enhance the discriminatory power of the ELISPOT assay, and provide more information on the immune response to BCG vaccination and infection with M.tb.