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The Hindu
25-06-2025
- Health
- The Hindu
Widely used fungicide found to be driving C. tropicalis infections
Candida tropicalis is an important fungal pathogen in India and many parts of the world. Its infections are associated with a mortality rate of 55-60%. Anti-fungal drugs called azoles, such as fluconazole and voriconazole, are used to treat its infections. But there is a growing concern in medical circles that clinics are seeing an increasing number of infections by strains of C. tropicalis that exhibit high resistance to these drugs. What is driving this alarming increase in drug resistance? A paper published recently in PLoS Biology by researchers from Fudan University in Shanghai has provided the answer. They found that an azole-related fungicide called tebuconazole widely used by farmers and gardeners, and which can accumulate and persist in the environment has driven the increase in azole-resistant C. tropicalis infections seen in clinics. The team also found that tebuconazole-resistant strains exhibited aneuploidy — meaning their chromosome number showed differences from the normal chromosome count for the organism. Such deviation from the normal chromosome complement is known as ploidy plasticity. Deviating from normal ploidy In the human body, most cells have two sets of the genome: thus they are diploid. One set of 23 chromosomes comes via the father's sperm and the other set of 23 from the mother's egg. When we, in turn, make eggs or sperm, a process called meiosis ensures only one set of chromosomes each of the 23 pairs is transmitted to them. Eggs or sperm are the only haploid cells in us — meaning they each have one copy of the genome. When a sperm fertilises an egg to make the zygote, diploidy is restored. The zygote then develops into the baby. On rare occasions, meiosis isn't as efficient and produces sperm or eggs that contain one copy more or one copy less of one or more chromosomes. Such eggs and sperm are said to be aneuploid. Aneuploidy can have serious consequences. A zygote with three copies instead of two of the smallest chromosome leads to the development of Down syndrome. These individuals have delayed development, characteristically aberrant physical features, and mild to moderate intellectual impairment. Aneuploidy for any of the other chromosomes almost invariably ends in prenatal death. In short, humans don't tolerate ploidy plasticity well — nor do most other animals, plants, and fungi, For a long time, C. tropicalis was also thought to be a diploid organism. Thus, finding that in most tebuconazole-resistant strains the ploidy was altered surprised the researchers. Enhanced resistance to anti-fungals The team started with five different C. tropicalis strains that were susceptible to tebuconazole and also to clinically-used fluconazole and voriconazole. They exposed the strains to incrementally more tebuconazole concentrations from 0.125 to 16 micrograms/millilitre in liquid medium, then grew them on tebuconazole-containing semisolid media in Petri plates. Finally, they picked 35 tebuconazole-resistant colonies. All these colonies exhibited cross-resistance to fluconazole and voriconazole. The tebuconazole-resistant strains showed slower growth than their progenitor strains in the absence of antifungals. But in the presence of antifungals, they grew much better. It seems the resistant strains had traded cell growth for antifungal resistance. The researchers found that the ploidy of tebuconazole-resistant strains ranged from haploid to triploid (that is, three copies of the genome). Those identified as diploid or close to diploid were found by more detailed analyses to, in fact, be segmental aneuploids: they carried duplications or deletions of some chromosome segments. The duplicated chromosome segments carried genes whose overexpression was known from other studies to increase resistance to azoles. For example, several TBZ-resistant strains had duplications of a chromosomal segment carrying a gene named TAC1, which encodes a protein that helps the cell to produce more of another protein named the ABC-transporter. The ABC-transporter pumps toxic compounds such as the azoles out of the cell. Conversely, other segmental aneuploids showed haploidisation, that is, deletion of one copy of a segment of another chromosome that carried the HMG1 gene. The overexpression of HMG1 decreased the biosynthesis of a chemical in cell membranes named ergosterol. Previous studies had shown that in budding yeast, HMG1 overexpression led to lower synthesis of ergosterol and a lower resistance to fluconazole — whereas reduced expression of HMG1 stimulated ergosterol synthesis and elevated resistance to fluconazole. Thus, although the aneuploidies created imbalances in the C. tropicalis genome that reduced their growth rate, they enabled the strains to better resist antifungals. The researchers also verified that the strains with altered ploidy were more virulent than the progenitor strains in mice treated with fluconazole. An unanticipated haploid Another unexpected bonus from the new study was the discovery that tebuconazole-resistant strains included stable haploid strains of C. tropicalis. The haploid cells were able to undergo mating. These serendipitous findings now provide researchers a useful tool for future genetic analyses. The researchers recovered a haploid cell from among the tebuconazole-resistant strains generated in the laboratory. They wondered whether any of the 868 C. tropicalis strains recorded in clinical visits around the world might include any haploids (that is, naturally haploid rather than as an abnormality). They examined publicly available genomic sequences of these strains and found that two of them, isolated from Spain, were indeed haploid. In conclusion, the research showed that the reckless use of triazole antifungals in agriculture can unwittingly promote the emergence of pathogenic strains showing cross-resistance to azoles of clinical importance. Further, some of the resistant strains were haploid, like our sperm and egg cells, and could likewise mate and hence be capable of introducing their resistance mechanisms into new genetic backgrounds. This exemplifies the prophetic warning: 'sow the wind, reap the whirlwind'. D.P. Kasbekar is a retired scientist.
The Hindu
05-06-2025
- Health
- The Hindu
When you want to move, does your brain know before you've decided?
It is the end of a long, hard work day and all you feel like doing is flop on the sofa and watch TV. Your eyes move to something on the screen and watch it for a few minutes, then you think to yourself: 'I wonder what's on elsewhere…'. So you reach for the TV remote and switch the channel. At this precise moment, let's freeze frame and ask: how did this simple decision unfold? Which happened first: the conscious recognition of the intention to move your arm or the brain activity required for the movement? For a long time, people grappled with this as a 'chicken or egg' question and arrived at only philosophical answers, not scientific ones. Indeed, for many years the question was actually believed to be outside the purview of science. The international chain In the early 1980s, American neuroscientist Benjamin Libet published his pioneering work exploring what scientists now call the intentional chain. In its entirety, the intentional chain entails an intent (the desire to change the channel in the example above), an action (reaching for the remote), and an effect (e.g. sounds/sights from a different channel). Due to the technical challenges involved, it wasn't possible for scientists to study the intentional chain from beginning to end — until now. In a study published recently in PLoS Biology, Jean-Paul Noel from the University of Minnesota in the US and collaborators from the US, the UK, and Switzerland, reported an experiment in which they selectively targeted each element of the intentional chain, one by one. They found that conscious recognition of the intent to move coincides with activation in the M1 cortical area, the part of the brain controlling voluntary limb movements. One surprise was a difference in the timing of conscious recognition: the perception of movement and the brain activity corresponding to this intent. First study of its kind The study's participant was a tetraplegic person outfitted with a brain implant in his M1 area (a.k.a. the primary motor cortex). Electrical impulses from the implant stimulated the area. This setup, called a brain-machine interface, used with a device called neuromuscular electrical stimulator (NMES), which activated forearm muscles to cause hand movements, made it possible for the researchers to activate or inactivate individual components of the intentional chain in the study. A particular hand movement was of interest in this setup. The participant held a ball in his hand. When he squeezed it, a sound was emitted exactly 300 mslater. This was the environmental effect, the last piece of the intentional chain. During the experiment, the participant was asked to watch a clock on a computer screen. Depending on the specific trial, he had to report the reading on the clock — at the time he felt the urge to move his hand, the time he moved his hand or the time he heard an audio tone. This was the first study to look in the M1 area in the context of subjective intention of voluntary actions. The researchers found that the timeline of activity in this area was somewhat different than that reported for other brain areas in previous research. Specifically, all the other areas had been activated prior to intention and action — whereas M1 showed activity before but also during a voluntary action. This makes sense given that M1 is the final stop in the brain, before the signal moves to the spinal cord and finally to muscles of the hand. Rearing up Normally, when you intend to move your right hand to pick up an object or lift your foot up to kick a ball, the desire for voluntary movement is reflected as electrical activity in specific parts of the brain. Even before Libet conducted his foundational work, German scientist Hans Helmut Kornhuber placed electrodes along the heads of participants in a study who each made a voluntary decision — to press a button any time they felt like it. He conducted this study in the 1960s. Kornhuber found that in the moments leading up to an individual pressing the button, the electrodes recorded a gradual increase in the strength of an electric signal, which he called the readiness potential. Think of it as the brain gearing up to act. This meant that if these same brain parts were stimulated with electric signals, one could manufacture in the individual an urge to move the hand or the foot. Kornhuber's work, later confirmed by others, proved there was electrical activity in the brain before the individual performed a voluntary action. Subsequent research showed that certain brain circuits are activated before an individual is even aware of their intention to perform a voluntary movement. In the new study, Noel & co. explored the question: when do we become aware of a decision we are about to make? Interesting patterns In the first round with their setup, the researchers studied the full intentional chain. They recorded electrical activity in the participant's M1 area caused by the intent to move his hand using functional MRI. They recorded any subsequent movement of that hand with NMES. Finally, they recorded the sound of the participant squeezing the ball in his hand. Thus, they had an objective way to measure each step of the intentional chain — a significant departure from previous studies in which researchers depended on participants' responses themselves. When the researchers compared the objective measurements to the participant's subjective perceptions, some interesting patterns emerged. For example, when the team asked the participant to report the time at which he developed a conscious awareness of his intention, his answer suggested his perception preceded actual electrical activity recorded by the MRI. Similarly, when asked to report the time at which he perceived his hand began to move, the researcher found his perception preceded the signal recorded by NMES. In the next round, the researchers used NMES to move the participant's hand, thus bypassing the subjective intent and therefore electrical activity in the brain. This time, the participant perceived that his hand moved at a time well after the measured electric signal. When the researchers blocked the hand movement signal from NMES, while keeping the intent and effect parts of the chain intact, the participant perceived his intention to occur much earlier — more so than the full intentional chain. In either case the difference was only in the order of milliseconds, but for the brain this is an eternity. The role of M1 The work of Patrick Haggard at University College London may help understand these results better. Haggard & co. asked participants in a study to report the timing of an action (pressing a keyboard button, say) and the timing of an effect of their action (a colour changing on the computer monitor). The team's results showed that participants perceived a shorter time interval between a voluntary action and its effect — called the intentional binding — than what was objectively recorded. In this context, Noel's team have discovered a new form of intentional binding: between intention and action. Since the work of Kornhuber and Libet, as more scientists examined the time between an individual perceiving a voluntary decision and that decision turning into action, it has been becoming clearer that the timing of brain activity in relation to a voluntary decision depends on where in the brain one looks. Through multiple attempts to understand the brain's goings-on in the moments leading up to a voluntary action, scientists have mapped the parts that light up with electrical activity as an individual consciously develops an urge to take some voluntary action as well as areas that light up with the conscious perception of having taken the action. In the new study, Noel et al. have added to this knowledge by revealing the role the M1 area plays with the start of a conscious decision to take some action and during the execution. Where are you looking? In the last few decades, cognitive neuroscientists have found that a single voluntary decision for an individual involves multiple different slices in their brain. There's the slice of 'what' decision to make, 'when' to make it, 'whether or not' to translate that decision to action. Activities in various parts of the brain correspond to different slices and the timing of brain activity in relation to a voluntary decision depends on which slice is examined. So if we look in the premotor or parietal cortical areas, we find them activated before a voluntary movement has occurred. The new study shows that the M1 area integrates signals from premotor-parietal areas, which explains its activity in the moments leading up to the voluntary action. The specific way the tests were set up made it possible for the researchers to separate M1 activity due to intention from its activity due to action. In a situation where a decision is converted to action, that of reaching for the remote in the example earlier, M1 activity relays that decision down to the spinal cord and to muscles of the arm. The fact that the study was conducted with a single tetraplegic participant raises obvious questions about whether its findings can be generalised. In another recent study in Nature Communications, Noel collaborated with Italian scientist Tommaso Bertoni to examine the same question in 30 healthy participants. They aimed to study the participants' brain activity using electrodes placed on their scalps (in contrast to electrodes implanted inside the M1 area of the brain). The results have supported the role of the M1 area of the brain in translating voluntary decisions to actions, adding further credence to the findings by Noel and team in their paper. Dr. Reeteka Sud is a neuroscientist by training and senior scientist at the Center for Brain and Mind, Department of Psychiatry, NIMHANS, Bengaluru.
