Mesophilic actinomycetes [MESO]

Published: July 8th, 2009

Revised: February 23rd, 2023

This test determines the presence of culturable mesophilic actinomycetes in bulk and swab samples. Test results are provided semiquantitatively as 1+, 2+. 3+, etc., where the number preceding the “plus sign” indicates the order of magnitude of colonies observed.

Actinomycetes are a large and diverse group of Gram-positive bacteria that have growth habits similar to fungi in that they produce branching networks of filaments. Mesophilic actinomycetes occur in a wide range of habitats including common materials such as soil and plant matter, as well as highly specialized habitats like marine buoys, ant exoskeletons and paleolithic cave paintings. The term mesophilic refers to the fact that the actinomycetes in question grow optimally at temperatures between 15–40 °C. Other groups of actinomycetes are known to occur at extreme hot or cold temperatures.

Actinomycetes are notable as prolific producers of microbial volatile organic compounds (mVOCs). Indeed in this regard, they tend to be much more active than most fungi. Some of the mVOCs commonly produced by actinomycetes are responsible largely for the mouldy or musty odours associated with soils as well as damp basements. Actinomycetes are also extremely active producers of antimicrobial chemicals. Many of the naturally produced antimicrobial drugs in common use today are derived from actinomycetes (e.g. streptomycin, nystatin, and tetracycline). Inhalation exposure to cells and other materials colonized by actinomycetes has been associated with respiratory diseases such as hypersensitivity pneumonitis. This is particularly evident in agricultural settings where actinomycete exposures are known to play a role in the disease known as Farmer’s Lung.

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