All in the family: Plant and animal pathogens share virulence gene

Research Areas:

Gene regulation, Exopolysaccharides, Virulence, Host-pathogen interaction, B. melitensis

Imaging Needs:

Fluorescence microscopy, DIC microscopy

Imaging System:
  • Nikon i80 fluorescence microscope at room temperature
  • 100x differential interference contrast (DIC) (Nomarski) or phase-contrast objective
  • Hamamatsu ORCA-ER camera
  • Nikon NIS element software
Imaging Bacteria

The ORCA camera line provided Mirabella et al. with the resolution and sensitivity needed to visualize bacterial cells. Find out how Ashley Cadby and colleagues use the enhanced low-light capability and precise localization of Hamamatsu’s ImageEM camera to elucidate the mechanism of bacterial cell wall growth. Read now.


How do parasitic bacteria establish a presence within live host cells?

The intracellular bacterial parasite Brucella melitensis causes the widespread zoonotic illness brucellosis, or “undulant fever,” that affects both humans and livestock. As with most bacteria, Brucella virulence is strongly influenced by its cell surface characteristics. The pathogen alters its cell membranes, cell wall, proteins, and exopolysaccharide coating in response to environmental cues, including oxidative or enzymatic attacks by the host cell.

The closely related plant symbiont Sinorhizobium meliloti causes a similar chronic intracellular infection, but in plants rather than mammals. As a model organism for studies of bacterial surface properties, S. meliloti can provide clues to the molecular workings of B. melitensis, and potential medical interventions for brucellosis.


The two bacteria follow a similar path when establishing their presence within host cells. Both S. meliloti and B. melitensis take up residence within a host-generated, membrane-bound compartment, with the exopolysaccharide coating playing a key role in protection from host cell defense mechanisms, such as lysis and apoptosis.

The genomes of both S. meliloti and B. melitensis code for a highly conserved gene—mucR—involved in exopolysaccheride production and, in S. rhizobium, important for symbiosis. The B. melitensis mucR gene shares 61% identity with the S. meliloti mucR sequence and codes for a zinc finger transcription factor.

In addition to affecting exopolysaccharide production, the mucR gene product of the plant symbiont alters resistance to environmental stressors such as oxidation or detergent. In B. melitensis, the mucR mutation had been shown to reduce virulence, but the gene remained otherwise uncharacterized in the animal pathogen.


Brucella melitensis MucR, an Orthologue of Sinorhizobium meliloti MucR, Is Involved in Resistance to Oxidative, Detergent, and Saline Stresses and Cell Envelope Modifications
A. Mirabella, M. Terwagne, M. S. Zygmunt, A. Cloeckaert, X. De Bolle, J. J. Letesson
J Bacteriol. 2013 Feb 195(3): 453–465. PMCID: PMC3554010.

Mirabella, et al,1 demonstrate similar roles of the S. meliloti and B. melitensis mucR genes in exopolysaccharide production, inhibition of flagellar genes, and cell aggregation in response to ionic challenge. Using fluorescence microscopy and a Hamamatsu ORCA-ER CCD camera, the team confirmed that cells grown in hypersaline 2YT broth display a significant, aggregation-associated growth defect, bulging at the middle, accompanied by enhanced mucR promoter activity as revealed by flow cytometry.

The authors conclude that mucR is functionally equivalent in the plant and animal pathogens, and an important regulator of cell surface polysaccharides with effects on cellular response to environmental threats.


Mirabella, et al, used the Hamamatsu ORCA-ER camera to view morphological changes in bacterial cells. Find out how other microbiologists are observing bacterial features down to 35–42 nm detail—read Exciting Insights into Cell Growth.


  1. Mirabella, et al. Brucella melitensis MucR, an Orthologue of Sinorhizobium meliloti MucR, Is Involved in Resistance to Oxidative, Detergent, and Saline Stresses and Cell Envelope Modifications. J Bacteriol. 2013 Feb 195(3): 453–465. PMCID: PMC3554010.
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