mBiosphere

Grace under fire: when stressed, Candida mutates and adapts

When Candida albicans encounters stressful conditions, does it curl up and die? No! According to a paper just published in mBio, this crafty pathogen gets to work on its inventory of genes, slashing away until it finds a winning combination that can get it through the tough times. (I’ve written up other mBio papers about Candida here and here. I think I am gaining a grudging respect for this feisty fungus.)

In laboratory experiments, Forche et al. put Candida through the wringer by subjecting it to different physiologically stressful conditions, including increases in temperature (mimicking a moderate fever in a human), oxidative stress (mimicking the production of reactive oxygen species by immune cells) and antifungal stress (fluconazole, the most widely and commonly used antifungal drug). Like us, Candida is in possession of two sets of chromosomes, so it is heterozygous for many genes (it carries two different copies of them). If Candida shuffles its genes around, it inevitably loses some of these genes and finds itself with two identical copies of these genes where it once had two different copies. The authors call this “Loss of Heterozygosity”, or LOH.

Under stress, Candida shuffled its genes, and its LOH rates increased with increasing levels of stress. What’s more, the stressor determined what kind of shuffling went on: different stress conditions caused different types of LOH events. Many of the LOH events are identical to what has been seen in clinical settings or in Candida passaged through another mammalian host. The authors propose that the different types of LOH are related to the type of DNA damage inflicted by the stressors and that losing heterozygosity due to stress may be a way to ramp up adaptation to changing conditions. The feisty fungus wins again.

One from many: single 'species' biofilms differentiate into multiple roles

The thick, mucilaginous biofilms that coat the insides of carbonate chimneys in the Lost City hydrothermal field are dominated by a single type of archaeon, according to their 16S genes. But this is a case where 16S doesn’t tell the whole story – not by a long shot. A study in mBio this week takes a close look at these biofilms and finds these archaea, which are practically identical at first blush, are probably highly differentiated.

The chemical engine behind these archaea and the rest of the rich microbial life of the Lost City is the process of serpentinization: the mineral olivine in the rocks reacts with seawater to make (among other things) hydrogen, methane, and larger hydrocarbons. This process, and locations like the Lost City may also have been the cradle for the earliest life on the planet, so teasing apart the different ecological roles of the biofilm members and their cooperative interactions could lend insights into how multicellular life developed.

In the Lost City, one result of serpentinization is the tall carbonate chimneys lined with thick biofilms that live off the bounty. Prior work on these communities revealed they are dominated by one uncultivated type of archaea known as Lost City Methanosarcinales (LCMS). Brazelton et al. thought it curious that a single species should dominate this habitat, considering that hydrothermal activity at the vents has been ongoing in the area for at least 100,000 years.

Transmission electron microscopy of the biofilm revealed multiple cell morphologies within the biofilm, which is not too surprising, given the fact that bacteria comprise as many as 20% of biofilm cells. But some of the cells contained intracellular membranes that could facilitate methane oxidation, and the most abundant bacteria in the chimneys are a group of sulfur-oxidizers, which are not likely to be methane oxidizers.

Experiments in the lab showed the biofilm both oxidizes and produces methane – and both processes were stimulated by hydrogen, indicating the two reactions are part of a syntrophic interaction. Metagenomic sequences from the biofilm yielded sequence that indicate acetate may play a role in the methane-cycling syntrophy, and a broad range of nitrogen fixation genes were identified, many of which were probably acquired via lateral gene transfer.

No vacation from tuberculosis: pausing drug therapy encourages resistance

Tuberculosis is no picnic, but taking the required medicines to treat the disease is a long slog: it   
takes six months, at least, to treat TB. To prevent patients from quitting treatment too soon, many doctors insist on “directly observed therapy”, where patients take their antibiotics under the watchful eye of a medical professional. However, they may also give their patients a weekly 2-day vacation from treatment. Taking a “drug holiday” from TB antibiotics not only offers patients a break from seeing their doctor every day, it can also boost the effectiveness of the drugs and increase patient adherence to the regimen.

But a holiday isn’t always such a good thing, say the authors of a new study. If the antibiotic drugs aren’t well-matched, drug holidays can encourage Mycobacterium tuberculosis to develop resistance to one of the drugs and undermine therapy.

Drusano et al. noted that two antibiotics commonly used together in TB therapy, moxifloxacin and rifampin, have very different half-lives in the body, so when a patient taking both drugs suddenly stops therapy, rifampin is quickly cleared but moxifloxacin hangs around. What effect could that residual moxifloxacin have on M. tuberculosis? Moxifloxacin is a fluoroquinolone, which are known to induce error-prone replication in M. tuberculosis, so the authors were concerned that the low, residual concentrations of moxifloxacin that result from drug holidays could give M. tuberculosis a weekly chance to regrow under conditions that could actually enhance the development of drug resistance. And they were right.

Using a hollow fiber infection model (an arrangement of flasks, nutrients, drugs, and bacteria that simulate the interactions in a human body), the authors showed that administering low doses of moxifloxacin and rifampin on a five-out-of-seven-day dosing schedule enabled M. tuberculosis to develop resistance to moxifloxacin. Maintaining daily drug treatments did not.

The authors strongly suggest doctors pay close attention to which antibiotics they prescribe if their patients are going to be on a drug holiday schedule, being careful to avoid drugs that induce error-prone replication and drug pairs that are cleared from the body at widely different rates. In the case of moxifloxacin plus a rifamycin, it may be that rifapentine is a better choice for a 5 days/7 schedule, since rifapentine has a serum half-life that is closer to that of moxifloxacin.