Any of the various bacteria (order Actinomycetales) with a branching, filamentous structure; especially any of a family (Actinomycetaceae) of Gram-positive bacteria that are pathogenic to humans and animals.

Good smells exude from crumpled earth.
The rough bark of humus erupts
knots of potatoes (a clean birth)
whose solid feel, whose wet inside
promises taste of ground and root.

Seamus Heaney
From "At a Potato Digging"
Death of a Naturalist

January thaw. The earth softens a bit under a soft rain. You wander outside, trying to shake off the heaviness of winter, to smell the awakening earth.

The mud disappoints your nose. Inert. Lifeless.

You wander back inside, dreaming of May.

Actinomycetes is a class of organisms essential to making good dirt. They are what give compost the sweet, earthy smell that makes gardeners wild with desire.

Actinomycetes give us the smell of rain in the summer. Storms breathe life. Poets and lovers already know this. Leave it to the microbiologists to tell us why.

Actinomycetes form grey strands in rotting piles of vegetation, like strands of fungus. Indeed, these critters were originally classified as fungus, though now are considered a form of bacteria.

Actinomycetes love chitin, cellulose,and lignin, breaking down materials other microorganisms cannot digest. In the process they produce geosmin (trans-1,10-dimethyl-trans-9-decalol), a compound described as smelling like freshly tilled earth. The smell, considered pleasant by (most) humans, has been added to some perfumes to give them an earthiness. (Geosmin, however, has also been blamed for the mustiness occasional found in wine and drinking water.)

Besides adding romance to summer showers, actinomycetes has antibacterial properties. Streptomycin and related antibiotics come directly from actinomycetes; Biaxin and Zithromax are semi-synthetic antibiotics made from this same class of bacteria.

So why does mud smell lifeless in January? Actinomycetes goes dormant in colder climes. While it is possible to grow actinomycetes in a petri dish (and yes, it will smell like the rich, sweet soil that makes gardeners swoon), waiting for the Earth to awaken reminds us of the cycle of life.

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