Even if recognizing that microorganisms cause bad breath and "most" other bad odors, microbiologists who study odor makers have "never gotten together, and we're not sharing information on how to sample and characterize [these phenomena]," says Mel Rosenberg of Tel-Aviv University in Tel-Aviv, Israel.
However, he and other similarly minded experts are ready to delve into the intricacies of which microorganisms are responsible for what odors and the tough consequences that may be associated with those strong smells-judging from their formal and informal odor-information swap during the symposium, "Odors in Microbiology: Human and Animals," that was convened at the 109th ASM General Meeting, held last May in Philadelphia, Pa. Thus, the microbially generated odor outlook is changing not only in terms of bad breath from the mouth and other anatomic sites, but also in terms of strong smells from farms.
Breath odors have "social ramifications," Rosenberg says. The same is said for odors released from the other end of the gastrointestinal (GI) tract. Further, they can signal metabolic imbalances with important health consequences, according to symposium participant Glenn Gibson of Reading University in Reading, United Kingdom, who studies microorganisms of the human GI tract. "The gut is the most metabolically active organ of the body," he says. Typically, it holds 1015 microbial cells of perhaps 1,000 different species, and produces as much as 5 liters of gas per day.
About half the human population carries gut microbes that produce predominantly methane, whereas the other half emits mainly hydrogen sulfide, which can be toxic, Gibson continues. For example, high levels of that gas cause localized damage that is typical for individuals with ulcerative colitis. A recent clinical trial to determine whether a "prebiotic" could shift the microbial balance within the GI tract and thus reduce hydrogen sulfide gas levels there was "not successful," he says. "We may need to starve [such individuals] for sulfates," which some gut bacteria metabolize to hydrogen sulfide.
Determining how the complex mix of gut microorganisms contributes to such clinical conditions is a goal of the human microbiome project, according to Ruth Ley, formerly of Washington University in St. Louis but now at Cornell University in Ithaca, N.Y., who spoke during another symposium, "Integrating and Interpreting the Human Microbiome." Genomic sampling experiments involving identical and fraternal human twins and similar experiments using mouse littermates indicate that even slight changes in host genotype may affect the GI microbiota, she says. However, the microbiota of littermates among genetically identical mice is closer than for the same kinds of mice raised separately. Thus, early environmental exposures also influence microbiota composition. With such complexity being the rule for microbiota in compartments such as the GI tract or the oral cavity, it is no wonder that teasing out the precise microbial sources of odor differences looms as a challenging undertaking.
Thus, insights about microbial odor sources tend to be cast in far less granular terms. For instance, although gram-negative bacteria from the oral cavity were thought to be the main source of bad breath odors, recent evidence points to gram-positive bacteria at least sharing in that responsibility, Rosenberg says. The latter bacteria release carbohydrates from glycoproteins, making the underlying protein more accessible for gram-negative bacteria to digest, producing metabolites with strong odors. Despite decades of research, however, the "jury is out as to which gases contribute most to bad breath," he says. "There is still a lot to learn." Similarly, for skin, there is also little known about precisely which microorganisms are critical for producing bad odors, he adds.
Large-scale swine operations have social ramifications, but on a much different scale from bad breath of individuals. Such farming operations are quickly recognized for their pungency by whole neighborhoods that are downwind of them, points out symposium participant Terence Whitehead of the U.S. Department of Agriculture (USDA) research facility in Peoria, Ill. Manure is the main source of those odors, and its value as a fertilizer leads it typically to be stored in on-site lagoons pending the few occasions when fields are ready to be dressed. While stored, the manure is subject to a rich and complicated series of poorly understood fermentative processes, during which more than 300 odoriferous chemicals may be generated. They travel by wind on dust particles, reminding neighbors about the volatile nature of large-scale swine operations.
"Which microorganisms are present, and [whether we] can lower their activity and amend the manure" to better control such odors are a central part of his research program, Whitehead says. The pigs' diet is critical to the microbial and consequential odor mix, and soybeans are a "comparatively cheap" staple for swine. However, that diet leads to a high-volume output of manure, replete with odors. It also is relatively inefficient because what is lost as manure makes for reduced animal weight gains. Further, that manure is an ecosystem that is particularly rich in gram-positive anaerobic bacteria, which are present at 1010 cells per ml and whose metabolic activities offer another point where odors might be controlled, he says.
A key challenge is to control microbially derived odors without jeopardizing the fertilizing qualities of the manure itself, according to Whitehead. One simple but promising approach is to apply polyphenolic tannins that can curb the metabolic activities of gram-positive bacteria within the manure-for example, reducing gas output from manure slurries, at least in lab simulations of much larger scale, swine-manure lagoons. "We don't expect tannins to affect manure quality, but we will test them," he says.
Jeffrey L. Fox
Jeffrey L. Fox is the Microbe Current Topics and Features Editor.