Appearance
''S. marcescens'' is a motile organism and can grow in temperatures ranging from 5–40 °C and in pH levels ranging from 5 to 9. It is differentiated from other Gram-negative bacteria by its ability to perform casein hydrolysis, which allows it to produce extracellular metalloproteinases which are believed to function in cell-to-extracellular matrix interactions. Since this bacterium is a facultative anaerobe, meaning that it can grow in either the presence of oxygen or in the absence of oxygen , it is capable of nitrate reduction under anaerobic conditions. Therefore, nitrate tests are positive since nitrate is generally used as the final electron acceptor rather than oxygen. ''S. marcescens'' also exhibits tyrosine hydrolysis and citrate degradation. Citrate is used by ''S. marcescens'' to produce pyruvic acid, thus it can rely on citrate as a carbon source and test positive for citrate utilization. In identifying the organism, one may also perform a methyl red test, which determines if a microorganism performs mixed-acid fermentation. ''S. marcescens'' results in a negative test. Another determination of ''S. marcescens'' is its capability to produce lactic acid by oxidative and fermentative metabolism. Therefore, ''S. marcescens'' is lactic acid O/F+.Defense
Until the 1950s, ''S. marcescens'' was erroneously believed to be a nonpathogenic "saprophyte", and its reddish coloration was used in school experiments to track infections. During the Cold War, it was used as a simulant in biological warfare testing by the U.S. military, which studied it in field tests as a substitute for the tularemia bacterium, which was being weaponized at the time.On 26 and 27 September 1950, the U.S. Navy conducted a secret experiment named "Operation Sea-Spray" in which balloons filled with ''S. marcescens'' were released and burst over urban areas of the San Francisco Bay Area in California. Although the Navy later claimed the bacteria were harmless, beginning on September 29, 11 patients at a local hospital developed very rare, serious urinary tract infections. One of the afflicted patients, Edward J. Nevin, died. Cases of pneumonia in San Francisco also increased after ''S. marcescens'' was released. Nevin's son and grandson lost a lawsuit they brought against the government between 1981 and 1983, on the grounds that the government is immune, and that the chance that the sprayed bacteria caused Nevin's death was minute. The bacterium was also combined with phenol and an anthrax simulant and sprayed across south Dorset by US and UK military scientists as part of the DICE trials which ran from 1971 to 1975.
Since 1950, ''S. marcescens'' has steadily increased as a cause of human infection, with many strains resistant to multiple antibiotics. The first indications of problems with the influenza vaccine produced by Chiron Corporation in 2004 involved ''S. marcescens'' contamination.
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