Before conducting these growth experiments, Braks spent several weeks at the University of Leeds in the United Kingdom working with experts who specialize in skin microbiology. Subsequent studies of the skin microflora indicate that aerobic coryneforms and staphylococci grow rapidly at first in sweat, before leveling off by the second day, whereas anaerobic propionibacteria grow rather poorly in this medium. Older, putrefying sweat attracts more mosquitoes than does freshly collected sweat, she and her collaborators note. Some 300 to 400 volatile compounds are responsible for human odor. During a two-day incubation of skin microflora mixed with fresh sweat, the pH of this mixture shifts from acidic to alkaline, offering a clue about olfactory signals that lure such mosquitoes, according to Braks.
Gas chromatography reveals that the major alkaline component in the fermented sweat is ammonia.
In past studies of mosquito attractants, ammonia was a neglected component that typically was not tracked in volatility measurements. Tempting as it may appear to lace traps with ammonia as a way of misdirecting mosquitoes in malaria-endemic areas, this strategy probably will not lure enough mosquitoes to reduce the spread of that disease, according to Braks. However, ammonia-spiked traps may help public health experts monitor mosquito populations. By placing these traps in villages, reliable estimates of the percentage of infected mosquitoes can be obtained to learn more about the intricate dynamics of malarial epidemics, she suggests.