How does antibiotic resistance spread? Scientists find answers in the nose

Now, University at Buffalo microbiologists studying bacterial colonization in mice have discovered how the very rapid and efficient spread of antibiotic resistance works in the respiratory pathogen, Streptococcus pneumoniae (also known as the pneumococcus). The UB team found that resistance stems from the transfer of DNA between bacterial strains in biofilms in the nasopharynx, the area just behind the nose.

In a paper published in last month's mBio, the authors found that genetic exchange of antibiotic resistance occurs about 10 million times more effectively in the nose than in the blood of animals, an efficiency far higher than expected.

"The high efficiency of genetic transformation that we observed between bacteria in the nose has a direct clinical implication, since this is how antibiotic resistance spreads, and it's increasing in the population," explains lead author Anders P. Hakansson, PhD, assistant professor of microbiology and immunology in the UB School of Medicine and Biomedical Sciences. "The bacteria 'borrow' each others' DNA in order to become more fit in the host environment and more elusive to the actions of antibiotics."

Hakansson, who also is affiliated with the Witebsky Center for Microbial Pathogenesis and Immunology and the New York State Center of Excellence in Bioinformatics and Life Sciences, performed the study with co-authors Laura R. Marks, an MD/PhD candidate, and Ryan M. Reddinger, a PhD candidate, both in the Department of Microbiology and Immunology at UB. Hakansson explains that the work has opened up a novel direction into the mysteries of how bacteria organize during colonization and how this organization promotes antibiotic spread and the evolutionary fitness of Streptcoccus pneumoniae.

Streptococcus pneumoniae is a major colonizer: It's carried in the nasopharynx by essentially everyone by about one year of age. Only occasionally do people get sick from it, but often enough to make it a leading cause of morbidity and mortality from respiratory tract and invasive infections in children and the elderly worldwide.

"It's rampant in daycare centers and the cause of many childrens' ear infections," Hakansson says. "In developing countries, where fresh water, nutrition and antibiotics are lacking, it is a major cause of disseminating pneumonia leading to sepsis and death of about a million children worldwide, often in combination with virus infections, such as the flu."

The research exposes what Hakansson describes as the puzzling history of studies into the transformation of genetic material between bacteria.

He explains that natural transformation or genetic exchange of DNA in infected mice was first described in 1928 by Frederick Griffith who was studying Streptococcus pneumoniae, because of its role in the Spanish flu epidemic of 1918-1919. Genetic transformation also helped identify DNA as the hereditary material and thus figured in the milestone research of James D. Watson and Francis Crick in determining the structure of DNA.

"Since then, all experiments with pneumococcal transformation have been done artificially in test tubes or in blood infection models," says Hakansson, "even though it's known epidemiologically that genetic exchange occurs almost exclusively when the organism exists in the nose.

"For some reason, no one had looked at how resistance spread in the environment where it really happens, in the nasopharynx," he continues." So we decided to do that. When we did, we found that the efficiency with which antibiotics spread in the nasopharynx was way above what we expected."

And last summer, the UB researchers published in Infection & Immunity findings showing that when they colonize the nose, pneumococci form sophisticated, highly structured biofilm communities.

"We found that the bacteria make biofilms in the nose that protect against the action of antibiotics, which have a hard time destroying biofilms," says Hakansson. "In addition, we know that some of the bacteria have to die in order to develop good biofilms. So dead bacteria help create good biofilms and provide DNA that other bacteria can take up and use, which is how bacteria spread antibiotic resistance and become more fit."

The mBio paper shows that the environment in the nasopharynx provides ideal conditions for these phenomena to occur.