MICROBIOLOGY:

SUMMARY: Biofilm assembly provides insight into the cellular and biochemical mechanisms underlying archaeal adaptation to extreme conditions.

Bacteria defeat antibiotics, evade the immune system, and survive other harsh conditions in part by assembling into biofilms (adhered microbial aggregates). This protective technique is a part of typical bacterial physiology.

It's also being investigated as a phenomenon that can be exploited commercially, e.g. for chemical production. One example is the conversion of benzaldehyde to benzyl alcohol under conditions that would kill most bacteria.

Less well studied, but potentially of great commercial interest, is archaeal biofilm assembly. Archaea are similar to bacteria in that they are both microbes, but the two groups are evolutionarily distinct.

Particularly noteworthy is the ability of some archaea to withstand extreme conditions, e.g. high temperatures, sometimes above the normal boiling point of water. This ability has been exploited commercially for some time now, and continues to be extremely useful, e.g. possibly towards producing hydrogen in an environmentally-friendly manner.

If scientists can figure out how some archaea survive (and even thrive) under extreme conditions, one can easily envision further commercial benefits from these rugged microbes. Sonja-Verena Albers (Max Planck Institute for Terrestrial Microbiology, Germany) and coworkers are working towards uncovering such insights, by studying the fundamental microbiology and biochemistry underlying archaeal biofilm assembly.

Why do Sulfolobus archaea assemble into biofilms?

The scientists studied three archaeal species in their research, each from the genus Sulfolobus. This is an archaeal group that grows at high temperatures and acidic conditions.

Response to temperature and pH depended upon the microbial strain. Growth was optimal between pH 3-4 for all strains.

Maximal extent of biofilm assembly was at pH 6 and (in separate experiments) reduced yet still high temperatures (e.g. 60°C), in the case of Sulfolobus acidocaldrius archaea. In other words, this archaeal strain grows well in acidic and hot (80°C) conditions, and institutes protective measures when the conditions begin to approach those more hospitable to typical microbial life.

The effect of iron concentration was also studied, since acidic geothermal springs (a natural habitat of these archaea) are rich in iron (20 milligrams per liter), and thus may be important to biofilm assembly. Sulfolobus solfataricus seemed to be the most sensitive to iron, with maximal extent of biofilm assembly at a concentration of 45 milligrams per liter; resistance to iron in this strain was greatly diminished when the pH was increased (e.g. pH 6) in combination with additional iron.

Cellular and biomolecular structure of Sulfolobus biofilms.

The scientists grew the three archaeal stains at 76°C, without shaking, to determine the structure of the microbial biofilms. They found that the structure depended upon the particular strain.

Sulfolobus solfataricus formed a loose carpet structure, Sulfolobus tokodaii formed a dense carpet structure, and Sulfolobus acidocaldarius formed dense cellular towers. Over 90% of the cells appeared to be alive.

Extracellular biomolecules are often important to biofilm assembly. Consequently, this feature of the biofilms was also investigated.

Extracellular DNA was present, but does not appear to play a structural role in these assemblies. At least two of the archaeal strains seem to secrete sugar polymers (rich in glucose, mannose, galactose, and N-acetylglucosamine), most likely to assist in attachment to a surface, since certain sugars were produced to either a greater or lesser extent depending upon the time point in biofilm assembly.

Implications.

Sulfolobus acidocaldarius archaea assemble into biofilms to a larger extent than the other strains tested. This suggests that the former is more resistant to changing (harsh as well as benign) environmental conditions, and consequently may be the most promising candidate for commercial investigation in this particular group of archaea.

NOTE: The scientists' research was funded by the Max Planck Society.

Koerdt, A., Gödeke, J., Berger, J., Thormann, K. M., & Albers, S.-V. (2010). Crenarchaeal Biofilm Formation under Extreme Conditions PLoS ONE, 5 (11) DOI: 10.1371/journal.pone.0014104