Sunday, July 6, 2014

Answer to Case 310

Answer:  No Strongyloides stercoralis or other larvae detected.  A motile Bacillus sp. was present, accounting for the motility observed on the Strongyloides agar culture plate.

This was an unusual case because the bacteria produced an interesting motion which superficially resembled larval movement.  However, the observed motility was not the expected sinuous movement of larvae through bacteria, but instead, consisted of regular and continuous clockwise or counterclockwise swirling within a confined region of the plate.

Multiple Gram stained preparations from these areas showed large Gram-variable to Gram positive rods with spores consistent with a Bacillus species.  Smaller Gram negative bacilli (likely coliforms) were also seen in the background.

It's important to remember that things other than Strongyloides stercoralis larvae can demonstrate motility on Strongyloides agar cultures. In this case, the unusual motility pattern was due to fecal bacteria. However, we've also seen mites, hookworm larvae, and even larvae from free-living nematodes demonstrate visible motility.  That is why we work up all positive cultures with microscopic identification of the motile objects.

Thanks for the great discussion on this one!


Anonymous said...

I like to say that Baccillus sp. is way smaller then Strongyloïdes larvae.
So size matters :)

Nice case,


Bobbi Pritt said...

Very good point Hans!

MicrobeMan said...

Dr. Pritt,

To me, this is a fascinating and beautiful phenomenon exhibited by many bacteria, spanning the various taxa. The process you've so nicely documented is swarming, a form of surface-dependent motility that requires the coordinated effort of tightly-bound, differentiated bacterial cells that move away from the central mass of the colony from which they arose.

Differentiation into "swarmer cells" appears to be driven by various stimuli. However, unlike with other forms of bacterial cell differentiation, swarming does not appear to be a response to starvation, but rather a means for expansion of the area occupied by the organisms. I can imagine that in Nature the ability to rapidly colonize a surface may have several advantages that these organisms can capitalize upon.

Your photomicrographs very nicely demonstrate a mixed population of cells. The elongate cells, with their numerous copies of the chromosome, are hyperflagellated and result from the abstention of cytokinesis; in other words, chromosomal replication occurs, but the membrane(s) and wall are not divided to partition the progeny cells. Interestingly, upon cessation of swarming, swarm cells dedifferentiate in unison, a process called consolidation, and revert to the small, oligoflagellated, and actively growing cells. These cells and their progeny may then differentiate into swarm cells, swarm, and then consolidate. Cycles of this are evident macroscopically as terraces, such as the concentric rings formed by Proteus spp.

Aren't bacteria cool! Have a great night!

Your buddy,
Ryan (MicrobeMan)

Bobbi Pritt said...

what a great description Ryan! Thank you for the very educational comment.