1. IDENTIFICATION
The picture above was taken by an electron microscope of typical Serratia cells.
2. TAXONOMIC DESCRIPTION
The genus Serratia, of the family Enterobacteriaceae, in the tribe Klebsielleae, contain within it approximately 13 described DNA related species. Among the species most widely studied and known is the species Serratia marcescens. Serratia, named after an Italian Physicist who invented the steamboat, has been widely considered a red-pigmented organism. For many years the genus Serratia contained only one species, S. marcescens, and was differentiated from other enteric bacteria due to its characteristic red pigmentation. However many species of Serratia are non-pigmented or are vary widely in pigmentation. Due many similarities to the generas Klebsiella and Enterobacter, Serratia was often misidentified. Since 1972, DNA homologies and many intense biochemical comparison studies with other groups and cultures have made possible the proliferation of other species within the genus. Other well known species include S. odorifera, S. liquifaciens, S. rubidaea, S. ficaria, and S. pymuthica, S. fonticola. S. marcescens is the type species.
The photo above shows the basic cell morphology of typical Serratia sp.
| Tests Useful in Differentiating Serratia from Enterobacter (Percent Positive) | ||||
| Test | S. marcescens | S. liquifaciens | S. rubidaea | Enterobacter |
| Chitinase | 95 | 37 | 0 | 0 |
| DNAse | 97 | 88 | 100 | 0 |
| Esterase | 100 | 100 | 91 | 0 |
| Gelatinase | 87 | 96 | 88 | 0 |
| Lecithinase | 92 | 71 | 86 | 0 |
| Lipase | 99 | 86 | 98 | 0 |
| Argenine dihydrolase | 0 | 0 | 0 | Variable |
| Lactose | 2 | 73 | 100 | Variable |
| Malonate utilization | 1 | 0 | 86 | 75 |
| Red pigment | 21 | 0 | 61 | 0 |
3. ISOLATION AND ECOLOGY
Serratia, like other Enterobacteriaceae, grow well on ordinary media under anaerobic and aerobic conditions. They grow well on synthetic media using various compounds as a single carbon source. Optimum growth of all strains of Serratia has been observed at pH 9 and at temperatures from 20-37 C. Growth of all strains of Serratia is inhibited at a pH of < 4.5 or at > 45 C. Some strains can grow well at pH 5, while others at this same pH require longer incubation periods to exhibit growth. Variable growth has been observed at 5 C and 40 C.
Colonies of Serratia have long been known for their red pigment. However, pigmentation is only present in just a small percentage of isolated cultures. Pigment production is highly variable among species and is dependent on many factors such as species type and incubation time. Non-pigmented colonies resemble other members of Enterobacteriaceae.
Many types of differential and selective media have been developed for the isolation and presumptive testing of Serratia. CT agar contains caprylate as a carbon source for Serratia and thallous salts as inhibitors for other organisms. DCT agar contains cephalothin that inhibits most organisms except for Psuedomonas aeruginosa and production of DNAse helps detect Serratia. SMA was developed and contains cephalothin and colistin. Colistin was added to inhibit P. aeruginosa. Of the three types of media described, CT is the best at selecting for Serratia. DTC is the simplest of the three types of media to prepare.
Most species of Serratia are of human origin (fecal). Any strains of Serratia in the environment will eventually occur in humans. The species of the most concern is S. marcescens, a human pathogen responsible for a large percentage of nosocomial infections involving the urinary and respiratory tract, in cystic fibrosis, and in burns. Cultures of S. marcescens have been isolated from soil. From a health safety standpoint, S. marcescens is of great concern due to its increasing number of cases, virulence and its increasing resistance to antibiotics.
The photo above is a streak plate of Serratia marcescens. Notice the characteristic red pigmentation produced by the organism.
4. ADDITIONAL SOURCES OF INFORMATION
W.H. Ewing, Ph.D. 1986. Edwards and Ewing's Identification of Enterobacteriaceae. 4th Edition. Elsevier Science Publishing Co., Inc. New York, New York. 423 pp.
A. von Graevenitz, S. J. Rubin. 1980. The Genus Serratia. CRC Press Inc. Boca Raton, Florida. 3 pp.
R.S. Breed, E.G.D. Murray, N.R. Smith. 1957. Bergey's Manual of Determinative Bacteriology. Seventh Edition. Williams and Wilkins. Baltimore, Maryland. 359 pp.
5. LINKS TO OTHER SITES ON SERRATIA SP.
UNIVERSITY OF TEXAS HOUSTON HEALTH SCIENCE CENTER
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