Virulence-related characteristics of Bordetella pertussis mutants deficient in intracellular survival

Callie Effler, a student at Lee University, worked in the lab of Dr. Eric Harvill.

Abstract: Intracellular survival is a common trait among human pathogenic bacteria that has advantages for the bacteria’s protection from the host immune response, persistence, and dissemination within the host. Bordetella pertussis, the gram-negative bacteria that causes whooping cough in humans, is commonly regarded as an extracellular pathogen. However, it has been recovered from macrophages in in vitro experiments, and reported anecdotally in clinical samples. It is unknown what contribution to pathogenicity the intracellular population has, if any, on the host. In this work, our broad objective was to evaluate the impact of B. pertussis’ intracellular survival and its role in pathogenicity. To do so, we planned to identify mutants similar to the wild type in general measured aspects of virulence, but that failed to survive inside of macrophages. A transposon library of B. pertussis UT25 was screened, resulting in the identification of several putative mutants that were deficient in intracellular survival. These strains were further screened for intracellular deficiency as a confirmatory measure, and went through further in vitro assays screening for cytotoxicity, hemolytic activity, resistance to serum complementation, and general fitness (growth). Based upon these assays, mutant strain G4 was the best candidate among those tested for an intracellularly-deficient mutant with similar virulence-related characteristics to the wild type. Preliminary C57 mouse infection studies suggest that the mutant strain behaves similarly to the wild type in vivo, indicating that intracellular survival may not be contributing to virulence. It is hypothesized that intracellular survival may be a phenotypic remnant of an ancestral strain of B. pertussis that transitioned from the environment to a become a human pathogen using this trait.