Pseudomonas aeruginosacan diversify after host cell invasion to establish multiple intracellular niches
Authors: Kumar Naren G. , Nieto Vincent , Kroken Abby R. , Jedel Eric , Grosser Melinda R. , Hallsten Mary E. , Mettrucio Matteo M. E. , Yahr Timothy L. , Evans David J. , Fleiszig Suzanne M. J. 
Published: October 2022

Within epithelial cells,Pseudomonas aeruginosadepends on its type three secretion system (T3SS) to escape vacuoles and replicate rapidly in the cytosol. Previously, it was assumed that intracellular subpopulations remaining T3SS-negative (and therefore in vacuoles) were destined for degradation in lysosomes, supported by data showing vacuole acidification. Here, we report in both corneal and bronchial human epithelial cells that vacuole associated-bacteria can persist, sometimes in the same cells as cytosolic bacteria. Using a combination of phase-contrast, confocal, and correlative light and electron microscopy, we also found they can demonstrate biofilm-associated markers:cdrAand cyclic-di-GMP (c-di-GMP). Vacuolar-associated bacteria, but not cytosolic counterparts, tolerated the cell-permeable antibiotic ofloxacin. Surprisingly, use of mutants showed that both persistence in vacuoles and ofloxacin tolerance were independent of the biofilm-associated protein CdrA or exopolysaccharides (Psl, Pel, alginate). A T3SS mutant (ΔexsA) unable to escape vacuoles phenocopied vacuolar-associated sub-populations in wild-type PAO1-infected cells, results revealing that epithelial cell death depended upon bacterial viability. Intra-vital confocal imaging of infected mouse corneas confirmed thatP. aeruginosaformed similar intracellular sub-populations within epithelial cellsin vivo. Together, these results show thatP. aeruginosadiffers from other pathogens by diversifying intracellularly into vacuolar and cytosolic sub-populations that both contribute to pathogenesis. Their different gene expression and behavior (e.g., rapid replication versus slow replication/persistence) suggest cooperation favoring both short- and long-term interests and another potential pathway to treatment failure. How this intracellular diversification relates to previously described “acute versus chronic” virulence gene-expression phenotypes ofP. aeruginosaremains to be determined.

Importance

Pseudomonas aeruginosacan cause sight- and life-threatening opportunistic infections, and its evolving antibiotic resistance is a growing concern. MostP. aeruginosastrains can invade host cells, presenting a challenge to therapies that do not penetrate host cell membranes. Previously, we showed that theP. aeruginosatype III secretion system (T3SS) plays a pivotal role in survival within epithelial cells, allowing escape from vacuoles, rapid replication in the cytoplasm, and suppression of host cell death. Here, we report the discovery of a novel T3SS-negative sub-population of intracellularP. aeruginosawithin epithelial cells that persist in vacuoles rather than the cytoplasm, and that tolerate a cell-permeable antibiotic (ofloxacin) that is able to kill cytosolic bacteria. Classical biofilm-associated markers, although demonstrated by this sub-population, are not required for vacuolar persistence or antibiotic tolerance. These findings advance our understanding of howP. aeruginosahijacks host cells, showing it diversifies into multiple populations with T3SS-negative members enabling persistence whilst rapid replication is accomplished by more vulnerable T3SS-positive siblings. IntracellularP. aeruginosapersisting and tolerating antibiotics independently of the T3SS or biofilm-associated factors could present additional challenges to development of more effective therapeutics.