Bacterial Snitch: Species competes by telling on another
Christen Brownlee
A bacterial species that typically colonizes people's noses may win out over another bacterium by tattling to the human immune system, a new study suggests.
Both Haemophilus influenzae and Streptococcus pneumoniae can establish residence in the upper respiratory tract either separately or together. The microbes are usually harmless, but in some circumstances, either species can cause ear infections, chronic bronchitis, or pneumonia.
In previous studies, Jeffrey Weiser of the University of Pennsylvania School of Medicine in Philadelphia and his colleagues examined competition between these two microbes. In a series of lab tests, the researchers found that S. pneumoniae always triumphed. The bacterium overpowered H. influenzae by attacking it with hydrogen peroxide and stripping off surface molecules important for H. influenzae's survival.
Elena Lysenko says that she and other researchers in Weiser's lab wondered whether the dynamic would change when both bacteria occupied an animal host instead of a petri dish.
To investigate, Lysenko and her colleagues sprayed solutions containing H. influenzae or S. pneumoniae into the nostrils of mice. In some mice, both nostrils were sprayed with the same bacterial species. In others, each nostril got a different species.
When the scientists examined the mice on the 1st, 3rd, and 14th days after the spray treatment, they found the microbes thriving within the nasal cavities of the animals that had received a single species of bacteria. However, in those mice sprayed with both species, only H. influenzae remained after 2 weeks.
The situation was "quite the opposite" from what the researchers had expected, given their petri dish results, says Lysenko.
Searching for an explanation, the scientists examined slices of upper respiratory tissue from the mice. In tissue exposed to both bacteria, Lysenko's team found an extraordinarily large number of neutrophils, immune system cells that fight bacteria. These cells weren't present in the mice that had received just one species.
Further tests in the lab showed that neutrophils exposed to killed H. influenzae attacked S. pneumoniae more aggressively than did unexposed neutrophils. However, killed H. influenzae had no effect on neutrophils' killing of live H. influenzae.
Taken together, these results suggest two scenarios: Either H. influenzae signals the immune system to attack S. pneumoniae or the two species together set off an immune system alarm that isn't tripped by either species individually. Lysenko says that neither scenario explains why the immune onslaught doesn't affect H. influenzae.
Timothy F. Murphy of the State University of New York at Buffalo says that these findings underscore the need for researchers to investigate bacterial competition when studying infections. "A lot of the time, we stick one bacterium into an animal model and forget about interactions [among bacterial species]," he says.
The new results may also have implications for the use of antibacterial drugs and vaccines, says Peter Hermans of the University Medical Center St. Radboud in the Netherlands. He and his colleagues have found that vaccines designed to eliminate one species often increase infections caused by other species. "When one bacterium is removed, the niche is taken over by another," he says.
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References:
Lysenko, E.S., A.J. Ratner, A.L. Nelson, and J.N. Weiser. In press. The role of innate immune responses in the outcome of interspecies competition for colonization of mucosal surfaces. PLoS Pathogens. Preprint available at http://dx.doi.org/10.1371/journal.ppat.0010001.
Further Readings:
Brogden, K., J. Guthmiller, and C. Taylor. 2005. Human polymicrobial infections. Lancet 365(Jan. 15):253–255. Abstract available at http://dx.doi.org/10.1016/S0140-6736(05)17745-9.
Murphy, T.F. 2003. Respiratory infections caused by non-typeable Haemophilus influenzae. Current Opinion in Infectious Diseases 16(April):129–134. Abstract available at http://www.co-infectiousdiseases.com/pt/re/coinfdis/abstract.00001432-200304000-00009.htm.
Regev-Yochay, G., et al. 2004. Association between carriage of Streptococcus pneumoniae and Staphylococcus aureus in children. Journal of the American Medical Association 292(Aug. 11):716–720. Available at http://dx.doi.org/10.1001/jama.292.6.716.
Sources:
Kim A. Brogden
Department of Periodontics
University of Iowa
Iowa City, IA 52242
Peter W.M. Hermans
Laboratory of Pediatric Infectious Diseases
University Medical Center St. Radbourd
6500 HB Nijmegen
The Netherlands
Elena Lysenko
Department of Microbiology
School of Medicine
University of Pennsylvania
36th & Hamilton Walk
Johnson Pavilion, Room 402A
Philadelphia, PA 19104
Timothy F. Murphy
Department of Infectious Disease
State University of New York, Buffalo
3495 Bailey Avenue
Buffalo, NY 14215
Adam J. Ratner
Department of Microbiology-JP403
University of Pennsylvania
3610 Hamilton Walk
Philadelphia, PA 19104
Jeffrey N. Weiser
University of Pennsylvania
School of Medicine
3610 Hamilton Walk
Philadelphia, PA 19104-6076
From Science News, Volume 168, No. 4, July 23, 2005, p. 51.
