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MICROBIOLOGY LEGEND
CYCLE 37 ORGANISM 2
Serratia marcescens
Serratia marcescens are opportunistic rod-shaped Gram-negative bacteria in the family Enterobacteriaceae. A human pathogen, S. marcescens is involved in hospital-acquired infections (HAIs), particularly catheter-associated bacteraemia, urinary tract infections and wound infections, and is responsible for 1.4% of HAI cases in the United States. It is commonly found in the respiratory and urinary tracts of hospitalized adults and in the gastrointestinal system of children. Due to its abundant presence in the environment, and its preference for damp conditions, S. marcescens is commonly found growing in bathrooms (especially on tile grout, shower corners, toilet water line, and basin), where it manifests as a pink, pink-orange, or orange discoloration and slimy film feeding off phosphorus-containing materials or fatty substances such as soap and shampoo residue. Serratia marcescens is the primary pathogenic species of Serratia. Some strains of S marcescens are capable of producing a pigment called prodigiosin, which ranges in color from dark red to pale pink, depending on the age of the colonies. S marcescens has a predilection for growth on starchy foodstuffs, where the pigmented colonies are easily mistaken for drops of blood.
In 1819, Bartolomeo Bizio, a pharmacist from Padua, Italy, discovered and named S. marcescens when he identified the bacterium as the cause of a miraculous bloody discoloration in a cornmeal mush called polenta. Bizio named Serratia in honour of an Italian physicist named Serrati, who invented the steamboat, and Bizio chose marcescens (from the Latin word for decaying) because the bloody pigment was found to deteriorate quickly. Since 1906, physicians have used S. marcescens as a biological marker for studying the transmission of microorganisms because, until the 1950s, this bacterium was generally considered a harmless saprophyte. Only since the 1960s has S. marcescens been recognized as an opportunistic pathogen in humans. Derivatives of prodigiosin have recently been found to have immunosuppressive properties and antitumor activity in vivo and are also currently being considered as a candidate treatment for Chagas disease.
Identification 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. S. marcescens also exhibits tryptophan and citrate degradation. One of the end products of tryptophan degradation is pyruvic acid, which is then incorporated into different metabolic processes of S. marcescens. A final product of citrate degradation is carbon. Thus, S. marcescens can rely on citrate as a carbon source. 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
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P.O. Box 131375, Bryanston, 2074
Ground Floor, Block 5 Bryanston Gate, 170 Curzon Road Bryanston, Johannesburg, South Africa
804 Flatrock, Buiten Street, Cape Town, 8001
www.thistle.co.za Tel: +27 (011) 463 3260 Fax: +27 (011) 463 3036 Fax to Email: + 27 (0) 86-557-2232 e-mail : [email protected] determination of S. marcescens is its capability to produce lactic acid by oxidative and fermentative metabolism. Therefore, it is said that S. marcescens is lactic acid O/F+.
Test Result Test Result Gram stain - Gelatin hydrolysis, 22°C + Oxidase - Acid from lactose -
Indole production - Acid from glucose + Methyl Red >70% - Acid from maltose + Voges-Proskaeur + Acid from mannitol +
Citrate (Simmons) + Acid from sucrose + Hydrogen sulfide production - Nitrate reduction + (to nitrite) Urea hydrolysis >70% - Deoxyribonuclease, 25°C + Phenylalanine deaminase - Lipase + some biovars produce Lysine decarboxylase + Pigment red
Motility + Catalase production (24h) +
Serratia marcescens colony Serratia marcescens Gram - negative
Pathogenicity In humans, S. marcescens can cause infection in several sites, including the urinary tract, respiratory tract, wounds and the eye, where it may cause conjunctivitis, keratitis, endophthalmitis, and tear duct infections. It is also a rare cause of endocarditis and osteomyelitis (particularly in people who use intravenous drugs recreationally), pneumonia, and meningitis. Most S. marcescens strains are resistant to several antibiotics because of the presence of R-factors, which are a type of plasmid that carry one or more genes that encode resistance; all are considered intrinsically resistant to ampicillin, macrolides, and first-generation cephalosporins (such as cephalexin).
Epidemiology - International The yearly incidence of Serratia bacteraemia is 1.03 per 100 000 population, with 47% of episodes having their onset in the community. The prevalence of Serratia species as a cause of nosocomial infections is diminishing, but these bacteria are still able to cause hospital outbreaks, especially in intensive care units.
Mortality/Morbidity In a population-based study of Serratia bacteraemia, the 7-day and 6-month mortality rates were 5% and 37%, respectively. Serratia meningitis and Serratia endocarditis carry a high mortality rate. Serratia species cause less
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P.O. Box 131375, Bryanston, 2074
Ground Floor, Block 5 Bryanston Gate, 170 Curzon Road Bryanston, Johannesburg, South Africa
804 Flatrock, Buiten Street, Cape Town, 8001
www.thistle.co.za Tel: +27 (011) 463 3260 Fax: +27 (011) 463 3036 Fax to Email: + 27 (0) 86-557-2232 e-mail : [email protected] than 6% of cases of hospital-acquired bacterial pneumonia. S. marcescens causes 11% of burn-related surgical wound infections.
Sex and Age Most (68%) episodes of Serratia bacteraemia occur in males. Outbreaks of Serratia infection occur in neonates and infants. In adults, most Serratia infections are isolated, but occasional nosocomial outbreaks occur.
Causes Sepsis or bacteremia Urinary tract infection Respiratory tract infection Meningitis and cerebral abscess Osteomyelitis and arthritis Endocarditis Ocular infections Serratia infection frequently causes nonulcerating bacterial keratitis, which is associated with wearing rigid or soft contact lenses. Serratia endophthalmitis usually occurs after eye surgery. Parotitis Cutaneous infections
Medical Care Antibiotic therapy is the primary treatment in most patients with Serratia infection. Home therapy is an option in patients who are clinically stable.
Surgical Care Purulent collections (abscesses) may require drainage.
Medication Summary S. marcescens is naturally resistant to ampicillin, macrolides, and first-generation cephalosporins. Extended spectrum beta-lactamases are produced by most S marcescens strains. Serratia infections should be treated with an aminoglycoside plus an antipseudomonal beta-lactam, as the single use of a beta-lactam can select for resistant strains. Most strains are susceptible to amikacin, but reports indicate increasing resistance to gentamicin and tobramycin. Quinolones also are highly active against most strains. Definitive therapy should be based on the results of susceptibility testing because multiresistant strains are common.
References 1. http://emedicine.medscape.com/article/228495-followup#a2648 2. http://en.wikipedia.org/wiki/Serratia_marcescens 3. http://www.antimicrobe.org/b26.asp
Questions 1. Discuss the morphological characteristics of Serratia marcescens. 2. Discuss the role of Serratia marcescens in disease. 3. Discuss the lab identification of Serratia marcescens.
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