In limiting life span, study finds booming bacteria innocent
Aging flies are simply crawling with bacteria鈥攂oth inside and out鈥攂ut their microbial infestations don鈥檛 seem to hasten the insects toward death, according to a new study in the August issue of the journal Cell Metabolism, a publication of Cell Press. The findings suggest that the energy the flies expend to fight their burgeoning bugs comes without a longevity trade-off, the researchers said.
鈥淚t鈥檚 a surprise,鈥� said John Tower of the University of Southern California. 鈥淓ven though the flies were accumulating so much bacteria and a robust immune response to that bacteria, it鈥檚 not limiting how long the flies live. The question is, if it鈥檚 not bacteria that limit life span, then what is it" We鈥檝e reduced the number of possibilities.鈥�
Tower鈥檚 interest in the problem stemmed in part from the fact that humans and some bacteria are known to have mutually beneficial relationships. People gain nutrients and energy with the aid of bacteria, and the microbes are provided with a buffered environment, carbohydrates, and other nutrients, the researchers noted. Since immune function is impaired with age, it might be expected that 鈥渂acterial load鈥� would increase or be otherwise altered as people grow older. Indeed, studies have found shifts in humans鈥� intestinal bacteria with age and evidence that bacteria may blossom in the prostate and other organs of the elderly.
To explore the consequences of such changes in bacteria with age in greater detail, the researchers looked to flies. Drosophila flies are emerging as an ideal model system in which to study most aspects of immunity, including innate immune pathways鈥攚hich represent the body鈥檚 first line of defense鈥攃ellular immunity, and the metabolic effects of infection, Tower鈥檚 group said.
The researchers showed that the insects exhibited dramatic increases in many types of bacteria during aging, both inside the body and on the surface. 鈥淚t was gradual across their life span,鈥� Tower said. 鈥淚t went from virtually undetectable to a million or more per fly.鈥�
Using a scanning electron microscope to examine the insects鈥� outer surfaces, the researchers showed that some of the bacteria were even banding together, forming continuous 鈥渂iofilms.鈥� Yet treatments that reduced the number of bacteria had no effect on life span, they found. That discovery led the researchers to conclude that, at least under optimal laboratory conditions, Drosophila can tolerate a significant number of bacteria and mount a strong innate immune response against them without cutting their 65-day lives any shorter.
鈥淭here has been some thinking in general that life span is regulated according to trade-offs鈥攖hat the body has only so much energy to devote to different processes,鈥� Tower said. 鈥淚f you devote a lot [of energy] to controlling bacteria, preventing them from growing to a toxic level, then that energy isn鈥檛 available to maintain tissues and organs. However, we found no effect on the animals鈥� longevity even when their bacteria are taken from one extreme to the other.鈥�
Nevertheless, he added, the bacterial infestations seen in the aged flies 鈥渉ave to be a quality of life issue.鈥�
Source: Cell Press
