Waste from people, pets, pigs and even seagulls may be playing a significant role in the rise of antibiotic-resistant infections, including methicillin-resistant Staphylococcus aureus (MRSA), a number of new studies warn.

Widespread fear of diminishing returns for modern medicine is becoming amplified, scientists say, by the discovery of soils and waterways polluted with both traces of antibiotics and bacteria encoded with antibiotic-resistant genes, the information that tells a microbe how to evade drugs designed to kill it. And even if that fortified microbe isn't capable of causing illness in humans itself, scientists add, its DNA could find its way into the more malignant microbes in the environment.

"Antibiotic resistance is likely the biggest public health challenge that we'll be facing this century," said Amy Pruden, an expert on antibiotic resistance at Virginia Tech. "We're in a state of complacency right now. We count on antibiotics working for us, but they are slowly starting to lose their effectiveness."

While progress has been made in the clinical realm -- limiting unnecessary uses of antibiotics, for example, and encouraging patients to take the full course of their prescribed drugs -- Pruden noted "mounting evidence that the environment is another important piece of the puzzle."

Drug residues and bacteria with drug-resistant genes can pass together through a human's or animal's gut and into the environment, even if the living contaminants take a detour through a wastewater treatment plant.

In a study published on Tuesday, Scottish researchers found that relatively low concentrations of antibiotics in certain environments -- such as river sediments, swine feces lagoons and farmed soil -- may be enough to speed along the proliferation of the drug-resistant genes. It's another survival-of-the-fittest story: Bacteria that can withstand the drugs will survive and reproduce, while their antibiotic-susceptible counterparts die out.

The winning genes then have the potential to infiltrate drinking water or produce, which increases human exposure and raises the likelihood that the genes will spread.

"Antibiotic resistance is such a big global health concern," said Alfredo Tello of the University of Stirling, lead researcher on the study. "We need to consider the effect that antibiotics released into the environment can have on development of this resistance."

Adding to the danger is the fact that bacteria can easily swap genes with each other. A bacterium that passes through the intestines into the local waterway, for example, may not itself be a pathogen that normally threatens human health, but that benign bug can share its drug-tolerating secrets.

"It's not necessarily important what species is holding on to the DNA as long as the DNA is held on to and propagated," explained David Cummings, a biologist at Point Loma Nazarene University in San Diego. "Then it can later be released to cause disease in an animal, plant or human."

Cummings' own research has identified dangerous DNA in the river sediments around San Diego and across the Mexican border into Tijuana.

"These coastal wetland habitats are becoming sinks and ultimately sources for drug-resistant bacteria -- more importantly, sinks for the DNA that provide resistance," said Cummings, who points his finger at pet waste, bird feces, leaky sewer pipes and hospital waste effluent as the likely culprits in the San Diego area, which is home to few livestock operations. "We've tinkered with a lot of resistance genes, and anything we look for, we find."

A separate study published last month also emphasized the importance of oft-overlooked aquatic sources of antibiotic resistance. Canadian researchers analyzed four different bodies of water affected by varying levels of human activity. They found resistance genes at all four sites, although the intensity varied: A harbor hosting sewer overflows suffered from higher levels than a nature preserve.

"Antibiotic resistance is widespread in aquatic environments ranging from heavily impacted urban sites to remote areas," Lesley Warren of McMaster University in Canada, and the lead researcher on the study, said in a statement. "The presence of environmental bacterial communities in aquatic environments represents a significant, largely unknown source of antibiotic resistance."

What's more, antibiotic residue and resistance genes may be spread farther and more widely by wildlife, particularly seabirds. Researchers at the University of Miami recently found a large number of seagulls and pelicans were host to bacteria associated with broad-spectrum resistance to infectious bugs, such as the E. coli that causes urinary tract infections in women.

It is becoming increasingly evident that the world's dire antibiotic-resistance problem involves a lot of players, all acting through a variety of complicated means. So what should be done?

"The solutions need to come from upstream, figuratively and literally," said Cummings. "That can be public education, improving our wastewater management and treatment -- even something as simple, albeit expensive, as separating stormwater from the sewage system." The latter would limit the untreated sewage flowing into waterways.

Of course, excrement from livestock is subject to even looser waste management practices than human waste.

The use of antibiotics in livestock is the subject of ongoing debate. According to the latest estimates from the U.S. Food and Drug Administration, 80 percent of the country's antibiotics are given to food animals, predominantly for the purpose of promoting growth or preventing disease, rather than for treating illness.

Also published this Tuesday was a study implicating the widespread use of antibiotics in swine feed. Not only do antibiotic-resistant genes end up in the soil and wastewater around the feedlots, but researchers suggest the genes are often spread further by the application of the waste on crop lands.

In response to the growing concerns, the FDA released contentious guidelines last month that ask pork, beef and poultry producers to choose to stop using antibiotics for fattening up their livestock. As The Huffington Post reported in March, the agency has also been ordered by a federal court to follow through on a rule proposed in 1977 that would withdraw approvals for most non-therapeutic uses of penicillin and tetracyclines in livestock, drugs particularly crucial in human medicine.

"Every time you use antibiotics, you can select for resistance," said Gail Hansen, senior officer with the Pew Campaign on Human Health and Industrial Farming. "When giving them to healthy animals for no reason other than to get them to grow faster or compensate for unhygienic conditions, you're adding to that."

"The new research," added Hansen, "really points out that antibiotics aren't just affecting the bacteria while they're inside the pig."