For the first time, scientists at Umeå University show how bacterial resistance to antimicrobial peptides in the immune defence may develop as the result of treatment with an antibiotic substance.

The rapid emergence antimicrobial resistance among pathogenic is a big medical and scientific challenge of our time. There is a need for development of new treatment strategies and such efforts require that the molecular mechanisms of bacterial resistance are understood.

Umeå University’s scientists professor Sun Nyunt Wai, professor Bernt Eric Uhlin and their colleagues at the Laboratory for Molecular Infection Medicine Sweden (MIMS) study the molecular mechanisms contributing to the survival and spread of Vibrio cholerae, the causative organism of cholera. This Gram-negative bacterium is responsible for many large epidemics of infectious disease that has killed millions of people. Cholera infections are getting more and more life-threatening as several bacterial variants already exist that are resistant against some antibacterial components of the innate human immune system.

During their growth Vibrio cholerae bacteria release small membrane vesicles from the outer membrane. They are filled with substances that may affect and damage the human host cells.

The MIMS researchers now publish in the international journal PLOS Pathogens how Vibrio bacteria can respond to treatment with sublethal amounts of an antibiotic agent by a mechanism making them resistant against antimicrobial peptides of the innate human immune system.

- We grew Vibrio cholerae bacteria in a culture medium containing both the antibiotic polymyxin and the antimicrobial peptide (AMP) LL37. This peptide is normally released from the epithelial cells of the human intestine during immune responses, Sun Nyunt Wai explains the experimental approach.

The infection biologists studied then the formation of outer membrane vesicles (OMV) both with fluorescence and electron microscopy on the cell surface of Vibrio cholerae.

Surprisingly, the scientists observed that the pathogen, treated with a sublethal dose of polymyxin, formed larger vesicles and that there was an increased amount of a Vibrio protein called BAP1.

- We observed that BAP1 binds to structures at the OMV surface and forms molecular traps. These traps bind then very effective to natural antimicrobial substances such as LL37 which are formed by the innate immune system. This means that the innate immune reaction is deactivated and the Cholera bacterium is able to resist the attacks from the natural antimicrobial defence system and its AMP LL37, explains Sun Nyunt Wai.

In a normal innate defence reaction the AMP LL37 released by the intestinal epithelium would act by increasing the permeability of the bacterial cells and enable their destruction before they cause major damage in the organism. Under the influence of the polymyxin the AMP LL37-polypeptides were trapped and degraded instead.

- When we did the same experiment with mutated Vibrio bacteria that do not produce BAP1 we observed that the mutants could not destroy the AMP of the immune system although they were treated with polymyxin at the same time.

The now published study shows very clearly how important it is to investigate the potential cross-resistance effects of antimicrobial agents in connection with the molecular mechanisms of the innate immune system.

(Sun Nyunt Wai/Eva-Maria Diehl)


Swedish version of the press release


Original publication:

Marylise Duperthuy, Annika Sjöström, Dharmesh Sabharwal, Fatemeh Damghani, Bernt Eric Uhlin, Sun Nyunt Wai. 2013. Role of the Vibrio cholerae matrix protein Bap1 in cross-resistance to antimicrobial peptides. PLoS Pathog 9(10): e1003620. doi:10.1371/journal.ppat.1003620

Contact:
Sun Nyunt Wai, professor
The Laboratory for Molecular Infection Medicine Sweden (MIMS)
Department for Molecular Biology, Umeå University
90187 Umeå, Sweden
+46 90 785 67 04
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Picture: Transmission Electron Microscopy picture of Vibrio cholerae. Picture by Sun Nyunt Wai (Umeå University) and Akemi Takade (Kyushu University, Japan). Click on the picture for a enlargement.

 Read the article at the MIMS web site