By turning off the lights, setting up an oatmeal-based bed and slipping some extra vitamins into their food, researchers have persuaded the supposedly asexual mold that makes penicillin to have sex. The fungi's ability to switch it up sexually could help industrial scientists breed more efficient antibiotic-producing strains or even lead to the discovery of new, useful compounds.
Penicillium chrysogenum is the original and still-used source of penicillin. It creates a nitrogen and carbon ring structure called beta-lactam, which prevents bacteria from building cell walls. This antibiotic helps the microscopic fungi kill any bacteria that might try to live where the fungi grows; it is also what doctors have used to combat bacterial illness since the 1940s.
Despite decades of study, researchers still believed that the fungi only reproduced asexually. That misunderstanding limited how scientists could genetically manipulate the fungi for industrial antibiotic production. The idea that P. chrysogenum was asexual persisted because fungal sex is complicated. Fungi—a group that includes large mushrooms growing on the forest floor and microscopic molds colonizing stale bread—have many different strategies for reproducing: Some clone themselves asexually. Others navigate a mating scene populated not by one member of a different sex but thousands of sexes. Some yeast can switch gender completely. The number of sexes in the fungal world depends on the species.
Paul Dyer, a fungal biologist at the University of Nottingham in England, suspected that P. chrysogenum would reproduce sexually if given the right encouragement. A complete sequencing of the fungi's genome revealed that P. chyrosogenum still carried the genes needed for mating. "That told us that there was perhaps sexual compatibility there," he says. So Dyer and researchers at several other European institutions tried to find the ideal conditions that would encourage P.chrysogenum to have sex.
First, Dyer and his colleagues paired strains with compatible mating genes (P. chyrosogenum has two different sexes) and grew them with different food and light conditions. The winning combination was an oatmeal-base supplemented with a vitamin called biotin. After five weeks in the dark, the fungi produced special structures called cleistothecia and ascospores, which only occur after sexual reproduction. Genetic analysis confirmed that genes had been sexually recombined. "We've now revealed its secret sexual side," Dyer says.
Furthermore, the researchers discovered that the genes that regulate the fungi's sexual ability also control the amount of penicillin it produces; the fungi that are having sex make more penicillin. The team published their findings online in January in Proceedings of the National Academy of Sciences. "I've believed for a long time that these guys were having sex but they were just doing it in secret," says Joan W. Bennett, a professor of plant biology and pathology at Rutgers University, who was not involved in the work.
P. chrysogenum isn't the only previously asexual fungi recently reclassified as a reluctant sexual reproducer. Dyer's lab group has also discovered that Aspergillus fumigatus, a mold commonly found in compost heaps and leaf litter, can also reproduce sexually.
The fact that sex in fungi is more widespread than many scientists had thought is one reason to get excited about the findings, says Bennett, but there are also practical reasons.
Just as humans have bred cows to produce more milk than their wild cousins; sexually reproducing P. chrysogenum could be bred to produce more penicillin. Industrial strains with high antibiotic concentrations are already in use, says study co-author and Ulrich Kück, a professor of molecular botany at Ruhr University Bochum in Germany. Those strains, however, are "very genetically unstable," he explains, and penicillin production tends to decrease as the fungal culture ages. "In industry you have to improve your strains every month or every week," he says. "Crossing a high-producing strain with a healthy partner strain would be very valuable."
A 2004 research article estimated that more than $8 billion worth of penicillin and its derivatives are sold yearly. At this scale, the batches are so large that small tweaks to the system can produce large cost savings, Bennett says. Current methods of increasing antibiotic production by selecting high-yielding strains are laborious, and sexual reproduction offers an alternative.
In addition, simply inducing sex in P. chrysogenum could boost penicillin production, Kück says. The sexually reproducing P. chrysogenum grow in pellets rather than filaments. This morphological change would give the fungi easier access to nutrients and oxygen when they are in the large industrial fermenter vats used to grow cultures. A pelleted culture also requires less energy to mix than do sticky filaments.
Besides making penicillin manufacturing more efficient, the discovery could lead to new antibiotics altogether. "You could speculate that maybe among all the different crosses there could conceivably be production of novel metabolites that have antibiotic activity," Dyer says. Developments like that would require just the right conditions.