Il Duomo di Mi­lano

by Elio

Grime on old build­ings is a mi­cro­bi­o­log­i­cal con­cern – not be­cause mi­crobes cause it, but be­cause they can help get rid of it. The gray pall shroud­ing old build­ings comes from the con­ver­sion of chalky stone such as mar­ble (cal­cium car­bon­ate) to gyp­sum (cal­cium sul­fate), a re­sult of at­mos­pheric pol­lu­tion. Not only is gyp­sum more sol­u­ble (which fa­cil­i­tates de­te­ri­o­ra­tion), but smog and min­eral par­ti­cles stick to it, which gives ven­er­a­ble old build­ings a mourn­ful cast. The con­ven­tional way to clean such faça­des is to douse them with an am­mo­nium car­bon­ate-EDTA-sur­fac­tant mix­ture. Now there is a bet­ter way.

View of the left side of the Mi­lan cathe­dral, taken from the stairs to the roof.

A group of re­searchers from sev­eral Ital­ian in­sti­tu­tions have de­vised a bi­o­log­i­cal clean­ing method that em­ploys sul­fur re­duc­ing bac­te­ria to con­vert the gyp­sum back into cal­cium car­bon­ate. Desul­fovibrio desul­fu­ri­cans is adept at that, and these re­searchers clev­erly de­vised pro­ce­dures for its de­liv­ery to the af­fected ar­eas. Us­ing an au­totrophic or­gan­ism has ad­van­tages too, in that you don't have to feed it with an abun­dance of or­ganic nu­tri­ents. By sev­eral bi­o­log­i­cal, chem­i­cal, and vi­sual cri­te­ria, this bi­o­log­i­cal treat­ment method is clearly su­pe­rior to the chem­i­cal one. The bac­teria are care­ful work­ers, re­mov­ing the gyp­sum but leav­ing a warm patina, and mak­ing the sur­face stronger than when they ar­rived. How­ever, we must be mind­ful that this re­port deals with small scale lab­o­ra­tory ex­per­i­ments. Tack­ling a whole build­ing may re­quire con­sid­er­able engin­eering de­vel­op­ment.

I want to bring up a per­sonal rea­son for men­tion­ing this work. I was born in Mi­lan and lived there un­til al­most teen age. Since we lived only a short walk­ing dis­tance away, I of­ten went to the Pi­azza del Duomo, the Cathe­dral Square. I got to ad­mire the cathe­dral and its hun­dreds of spires. As the fam­ily story goes, when I was a tod­dler my nanny took me into the cathe­dral, most likely to pray. When we got home, my Teddy Bear was miss­ing. It turned out that I had left it in a con­fes­sional where I'm sure that it got freed of all its sins.