Serratia marcescens

Serratia marcescens once performed miracles—or so medieval observers believed when communion bread mysteriously bled red. The 'blood' was actually prodigiosin, a brilliant red pigment produced by this bacterium growing on starch-rich hosts. Today, S. marcescens is better known as a hospital pathogen that forms pink biofilms in bathrooms, ICUs, and on medical devices. Like Pseudomonas aeruginosa, it has transitioned from environmental curiosity to serious healthcare threat.

Prodigiosin production is quorum sensing-controlled, occurring only when S. marcescens populations reach sufficient density. This coordination ensures the metabolically expensive pigment is produced only when the population can benefit—prodigiosin has antimicrobial properties that may protect established colonies from competitors. The same regulatory logic governs S. marcescens biofilm formation and virulence factor production. Low-density cells focus on growth and colonization; high-density populations invest in community defense and expansion.

S. marcescens biofilms present particular challenges because they tolerate the chlorine levels typical in water systems. The bacteria can colonize sinks, drains, and humidifiers, creating reservoirs for patient infection. Pink bathroom biofilms are often S. marcescens, growing on soap residue and skin cells. The organism's environmental resilience, combined with quorum sensing-coordinated community behaviors, makes it a persistent problem wherever water and surfaces meet. Understanding S. marcescens requires understanding how organisms optimize their strategies based on local population density and resource availability.