Mostly harmless

Serratia marcescens is one of the gamma-proteobacteria; a subgroup of bacteria which includes our own Escherichia coli as well as Yersinia pestis and Vibrio cholerae.  The last, as it says on the tin, causes cholera, Yersina is responsible for plague and and some varieties of Escherichia (O104:H4 for example, which killed 50 Germans in 2011) will make you run at both ends until you die.  Other "gammas" are less deadly but cause what we call opportunitistic infections: if the situation is right (overwhelming numbers, compromised immune system, peculiar genetic make-up) these boys move in and cause trouble.  Pseudomonas aeruginosa, for example is commonly associated with pneumonia in young people with Cystic Fibrosis,  It's not associated with old people who have CF, because until very recently, there were no such people: pneumonia would kill them off early.  Even so, the average life-expectancy for CF sufferers, even with the best, most interventionist, medical care, is about 40 years.

But we hear all about these man-killing microbes and much less about the many species and varieties that live in and around us in very much a live-and-let-live manner.  One of the latter is Serratia marcescens which a) grows in bright red colonies and b) was believed to be entirely innocent of causing disease.  So much so that, when I was very young, modern science experiments were designed around it for class-room settings.

A cotton-bud/Q-tip was dipped in one of the red colonies on a petri-dish and reamed up the nose of some of the students, who were then encouraged to sneeze in class and out in the corridors.  Later, the rest of the class would use another sterile cotton-bud to swab the bench-tops, door-handles, notice-boards, windows and stationery cupboards.  These potentially contaminated samples were then wooffled out [techical term L] on more clean petri-dishes and checked to see if they would grow red colonies.  So far, so instructive: it revealed that a sneeze would effectively transmit a fine aerosol of wet snot 6 or 7 meters.

All this was years after the fiasco of Operation Sea-Spray in which the US Navy used S.marcescens as a marker in an experiment to trace the environmental distribution of a potential bio-warfare agent that they imagined that Them Russkies might use to attack America. Is it possible that they chose Serratia because it also is red? The test involved bursting balloons full of the bacteria in the atmosphere over San Francisco in late September 1950.  Because they knew that S.marcescens was harmless, they didn't see any need to inform their own citizenry, let alone ask their permission. A few days after this assault, 11 people reported to hospital with rare and life-threatening urinary-tract infections, and one of them, Edward Nevin, subsequently died.  But the US Government and its agents managed to evade a conviction in the courts.

Serratia is able to survive for a comparatively long time outside the body in quite inhospitable conditions and has been implicated in a number of cases this century where it has been allowed to contaminate a) batches of influenza vaccine b) medicinal fluids and c) medical equipment d) generally in neonatal intensive care units.  Because it is everywhere, we have all been exposed to Serratia at some stage and out immune system mops it up whenever it is met. Newborns have not met the Red Peril yet and, if for other reasons they are in an ICU, they are in their weakened condition susceptible to attack.

If you see a reddish growth round the plug of your wash-hand-basin or in the grouting of your bathroom tiles, don't scrape it off and pop it up your nose.  We don't do that anymore.

Note: I find I'm accumulating a number of microbial tales on The Blob: Acetone -
Archaea - Augmentin - Campylobacter - C.diff - Cryptosporidium - Escherichia - Flora - Helicobacter - Impetigo - Lactobacillus - Not very Neisseria - Swimming water