pH-responsive substrate switching in mycobacterial Type VII ESX secretion.

Area of Science:

• Microbiology
• Pathogenesis
• Host-pathogen interactions

Background:

• Pathogenic mycobacteria, including those causing tuberculosis, reside within host macrophages.
• The ESX-1 secretion system (early secreted antigen 6 kilodaltons system-1) is crucial for mycobacterial virulence, enabling phagosome lysis and host cell invasion.
• The mechanisms by which ESX-1 functions in diverse intracellular environments are not fully understood.

Purpose of the Study:

• To investigate how the ESX-1 secretion system adapts its function in response to environmental cues, specifically pH.
• To elucidate the molecular mechanisms underlying ESX-1 substrate switching.
• To determine the role of ESX-1 substrate switching in mycobacterial infection models.

Main Methods:

• Culturing *Mycobacterium marinum* in acidic conditions *in vitro* to observe substrate switching.
• Analyzing changes in ESX-1 substrate transcript levels via gene expression analysis.
• Quantifying ESX-1 substrate and chaperone protein levels using proteomic techniques.
• Evaluating the requirement of specific ESX-1 substrates in an acidic infection model *in vivo*.

Main Results:

• *Mycobacterium marinum* exhibits ESX-1 substrate switching when grown at acidic pH *in vitro*.
• Substrate switching is associated with significant alterations in ESX-1 substrate transcript and protein levels, as well as chaperone levels.
• Specific ESX-1 transcripts are upregulated *in vivo*, and distinct substrate sets are necessary for infection in an acidic environment.

Conclusions:

• The ESX-1 secretion system dynamically switches its secreted substrates in response to environmental acidity.
• This substrate switching is regulated at both the transcript and protein levels.
• Mycobacterial pathogens likely utilize distinct ESX-1 protein sets to lyse macrophage phagosomes of varying acidity during infection, contributing to pathogenesis.