General Overview
Serratia species are found mostly in soil and water that means widely distributed in nature and some are found in the intestine.
However, in the general human population, fecal carriage is uncommon.
S. marcescens is the main species which is medically important.
S. liquefaciens and S. odorifera also sometimes isolated from the clinical specimens.
These organisms are usually considered as opportunistic pathogens.
They mainly cause nosocomial infections.
Serratia mainly have been reported as causing pulmonary infections like pneumonia and urinary tract infections.
Moreover, they have been reported as causing cross-infections in hospitals.
Hospital related infections occur during invasive procedures such as intravenous catheterization, respiratory intubation, and urinary tract manipulations.
Furthermore, outbreaks of Serratia pneumoniae have been associated with the contamination of water used in respiratory therapy devices as well as out breaks can occur due to introduction of the organisms directly into the bloodstream in contaminated transfusion fluids.
Serratia also causes endocarditis. Mainly this occur in the users of injection drugs. Endotoxins in their cell walls may cause septicemia.
In addition, they can cause meningitis and wound infections.
Pigmented and non-pigmented strains can be found.
Pigmented strains are found occasionally and may be the reason for turning foods, sputum into red color.
Depending on the site of infection specimens are vary. Urine, sputum and blood are used for the laboratory diagnosis.
Microscopy
On Gram stained slides Serratia can be identified as gram-negative bacilli. However, they vary in size. Some can be seen as small coccobacilli and some can be seen as rods.
They are motile rods. Normally capsules are not found.
Culture
Serratia grow well on agar media at 30–37°C. They are Facultative anaerobes They utilize most carbohydrates and produce acid and gas.
Some strains of S. marcescens produce a pigment which is red in color.
Pigment production occur only when there are oxygen and suitable temperatures.
Caprylate-thallous (CT) mineral salt agar
It is a highly selective, enrichment medium for isolation of Serratia. Produce small slightly bluish colonies on CT agar.
Tween 80 agar
It is also a selective medium for isolating Serratia. Produce large, pinkish colonies.
These colonies are surrounded by a white zone of precipitate.
Blood agar
Grow well on blood agar. Produce red color colonies.
MacConkey agar
Produce non-lactose fermenting pale color colonies.
Some strains of Serratia produce a red pigment in nutrient agar at room temperature.
Further more they grow in CLED agar and give red color colonies.
- Catalase positive
- Citrate positive
- Oxidase negative
- ONPG test positive (beta-galactosidase test)
- KIA test red or yellow slant, yellow butt, H2S negative, Gas production may be variable
- LDC test positive (Lysine decarboxylase test)
- MR negative
- VP test positive
- Indole negative
- Nitrate reduction positive
- OF test Facultative anaerobes (Oxidative-fermentative test)
- Urease test positive or variable in some strains
- DNase positive
- Glucose positive
- Lactose negative
- Mannitol positive
- Xylose negative
- Aesculin hydrolysis positive
Treatment
Choice of drug depends on the results of antibiotic sensitivity testing.
Because, the antibiotic resistance of those organisms can vary widely.
Serratia are usually resistant to cephalosporins.
Organisms isolated from hospital-acquired infections frequently resistant to multiple antibiotics.
Aminoglycoside like gentamicin is usually use as the first line drug of choice.
Fluoroquinolones or carbapenems may be useful for treatment.
Prevention
By changing the site of intravenous catheters, removing urinary catheters when they are no longer needed, and taking proper care of respiratory therapy devices can reduce the risk of infections.
There is no vaccine found for the prevention of the disease caused by Serratia.
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