A SHORT NOTE ON Gardnerella vaginalis

 Serratia

Compiled by : Zenisha Acharya 

                                                                          Christina Khadka

                                                                 Central Department of Microbiology

                                                                    Tribhuvan University, Kirtipur

 

•       Serratia is a genus of Gram-negative, facultatively anaerobic, rod-shaped bacteria of the family Enterobacteriaceae.

•       The genus contains 10 species namely S. marcescens, S. liquefaciens, S. rubidaea, S. ficaria, S. fonticola, S. odorifera, S. entomophilia, S. plymuthica, S. grimesii and S. proteomaculans.

•       Of these, S. marcescens, S. liquefaciens and S. rubidaea are most often associated with human infection.

•       Certain strains of Serratia produces red-pigmented colonies on agar.

•       Serratia spp. are ranked the twelfth most commonly isolated organism from pediatric patients in North America, Latin America, and Europe.

THE BLOOD OF HISTORY

•       Because of its characteristic blood-red pigment and propensity for contaminating bread, S. marcescens has had a notable place in history.

•       For example, the dark, damp environment of medieval churches provided optimal conditions for the growth on sacramental wafers used in Holy Communion. At times, the appearance of blood was construed to be a miracle.

The history of seeing red on food originates in 6^th century BC, when Pythagoras reported on the blood substance that sometimes appeared on bread. Then, in 332 BC, soldiers of the Macedonian army of Alexander the Great found that their bread sometimes appeared to have blood on it. The Macedonian soldiers interpreted these bizarre phenomena as evidence that blood would soon flow in the city of Tyre and that Alexander would win.

•       In early July 1819, in Italy, the polenta, a dish of cornmeal mush made by many families, turned red. Superstitious peasants were fearful of the “bloody polenta,” which was believed to be diabolical in origin. Families refused to stay in homes where the discolored polenta was kept, and one farmer asked for a priest to free his home from “evil spirits” .

•       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. 

                                    Fig: "Bloody bread": S. marcescens growing on bread

•       The bacterium was named Serratia in honor of an Italian physicist named Serrati, who invented the steamboat. The species name marcescens is derived from the Latin word meaning decaying due to fast-deteriorating nature of the bloody pigment produced by the bacteria.

 CLASSIFICATION

Domain:	Bacteria
Phylum:	Proteobacteria
Class:	Gammaproteobacteria
Order:	Enterobacterales
Family:	Yersiniaceae
Genus:	Serratia

 

HABITAT:

•       Widely distributed in the environment (soil, water, plants), digestive tracts of rodents and insects, and are opportunistic pathogens for hospitalized humans.

•       S. marcescens also grows well in damp basements, on food stored in damp places, and is a frequent contaminant in the laboratory. 

•       Serratia marcescens has a predilection for growth on foodstuffs, especially of the starchy variety, where the pigmented colonies were easily mistaken for drops of blood. 

MORPHOLOGY

•       Gram-negative, straight rods; 05-0.8 µm x 0.9-2.0 µm

•       Non-sporulating

•       Most are motile by means of peritrichous flagella. The flagella are usually best seen in cultures grown at temperatures below 37°C.

•       Capsules are not normally formed, except on a well-aerated medium poor in nitrogen and phosphate.

•       Some species produce red, water insoluble pigment (prodigiosin). Pigment production is variable and not all strains produce it. Production is influenced by culture condition and medium composition.

•       Most species produce fishy urinary odour. S. odorifera and S. ficari smell musty.

•       Serratia marcescens can also form a biofilm (complex structure made of secreted mucilaginous matrix to form a protective coating in which they are encased).

CULTURE

•       Facultative anaerobe   

            

                                            Fig: S. marcescens on agar plate        

•       Optimum temperature 37°C (range may vary from 10-40°C)

•       pH range 5-9

•       Serratia spp. are readily recovered in the laboratory on enriched (BA, CA), nonselective agars as well as selective enteric agar such as MA and SS agar.

•       A highly selective media for the isolation of all Serratia spp. is Caprylate-thallous(CT) mineral salts agar. It contains 0.01% yeast extract, 0.1% caprylic (n-octanoic) acid as a carbon source, and 0.025% thallous sulfate for inhibition of other organisms.

•       CT agar: colonies of Serratia spp. are small and slightly bluish-white.

•       Maconkey Agar: Serratia spp. are late lactose fermenter. Colonies are colorless after 24 hours of incubation; colonies of most strains are still colorless after 48 hours. Some strains of S. marcescens produces a red pigment, as do most strains of S. rubideae and S. plymuthica; colonies may be entirely red or may have only a red centre.  The red pigment is called prodigiosin. On MA, the red pigment can be mistaken for lactose fermentation.

•       Hektoen enteric agar: colorless colonies

•       XLD: yellow colonies

•       Blood agar: red colonies

PRODIGIOSIN

•       Some strains of S. marcescens are capable of producing prodigiosin (2-methyl-3-amyl-6methoxyprodigiosene), which ranges in color from dark red to pale pink, depending on the temperature, substrate and age of colonies.

•       The pigment, prodigiosin, is formed only in the presence of Oxygen and at a suitable temperature, which is not necessarily the same as that for optimal growth. Most strains of S. marcescens are red under 27°C and white above 28°C (Pigment and flagella production stops approximately at 28°C).

•       Certain organisms unrelated to S. marcescens, including an actinomycetes and certain Gram-negative rods isolated from seawater, also form prodigiosin.

•       Prodigiosin, a linear tripyrrole, is synthesized in a bifurcated pathway  in which mono- and bipyrrole precursors are synthesized separately and then couple to form the red pigment.

•       Prodigiosin is a secondary metabolite, which is constructed from several amino acids that accumulate in the cells as a result of primary metabolism.

•       They are not essential for survival, but very helpful in establishing bacteria’s domain in soil

•       Although S. marcescens primarily produces bright red pigment called prodigiosin, some Serratia mutants can be white, purple, pink, orange, and even blue. It produces a wide diversity of color morphs, depending on the partial or complete synthesis of prodigiosin.

•       Serratia can “lose” its color both to prolonged sunlight (via UV mutagenesis), or if the temperature rises to our body temperature (37° C), fading away due to decreased prodigiosin production.

•       In order for it to turn a deep, blood-red color, the bacteria “culture” needs to age (be older than 8 days) and the substrate needs to have a form of glycerol (oil) and to be low in phosphate and sugar. 

•       Prodigiosin offers protection for Serratia in the natural environment. The red pigment offers protection against excessive UV in sunlight, serves as an antibiotic, and has cytotoxic qualities. It appears that it is worth the energy investment to synthesize prodigiosin when it serves protection against UV light and when it has to compete with fungi in the soil, using it as an antibiotic against neighboring molds and bacteria.

BIOCHEMICAL REACTIONS

•       The genus Serratia is distinguished from other genera of the family Entrobacteriaceae by several biochemical activities. Serratia spp. produce extracellular deoxyribonuclease; gelatinase at 22°C and lipase and are resistant to colistin and cephalothin. They are:

•       Catalase-positive

•       Oxidase negative

•       iMViC: indole negative (except some S. odorifera strains), Methyl red variable, Voges-Proskauer-positive, except S. fonticola , Citrate positive

•       Nitrate positive

•       A/A reaction on TSI and KIA with or without gas production, H2S negative

•       Orthonitrophenyl galactoside (ONPG)-positive

•       Lysine-Decarboxylase-positive

•       Arginine-Dihydrolyase-negative

•       Ornithine-Dihydrolyase-positive

•       Ferment maltose, mannitol, salicin, sucrose and trehalose  but not dulcitol

PATHOGENECITY

•       Serratia spp. are nosocomial and opportunistic pathogens, S. marcescens being the most important.

•       S. marcescens is generally an opportunistic pathogens causing infection in immunocompromised patients. Among the possible pathogenic factors found in Serratia strains are the formation of fimbriae, the production of  potent siderophores, production of cell wall antigens, the ability to resist to the bactericidalal action of serum and the production of proteases

•       Prodigiosin production seems to be  unrelated to pathogenicity since up to 80% of clinical isolates are non-producer.

CLINICAL SYNDROME

•       The bacterium is an opportunistic human pathogen and have ability to form tight-knit surface communities called biofilms wherever it can.

•       The organism is widespread in the environment but not a common component of the human fecal flora. Thus, most infections are acquired exogenously.

•       The important reservoirs in epidemics are the digestive tract, the respiratory tract, the urinary tracts and the perineum of neonates and the artificial nails of adults and health care workers. Medical equipment, lotions, antiseptics, medications, blood products and sinks have also been described as the sources of epidemics. 

Serratia marcescens 

•       accounts for only 1-2% of the nosocomial infections which are mostly confined to the respiratory tract, the urinary tract, surgical wounds and soft tissues.

•        Meningitis, which is caused by Serratia marcescens, has been reported from the paediatrics wards.

•       In persons who are addicted to heroin, it causes endocarditis and osteomyelitis.

•       In urinary tract infections, 30-35% of the patients are asymptomatic and most of the patients have a history of instrumentation.

•       The mortality rate is very high in the nosocomial blood stream infections and in meningitis and endocarditis which are caused by the infection with Serratia.

•       The main risk factors for bacteraemia/sepsis which is caused by Serratia is hospitalization, placement of intravenous catheters, intraperitoneal catheters and urinary catheters and prior instrumentation of the respiratory tract. Serratia marcescens causes both opportunistic and nosocomial infections. 

•       One particularly striking ‘non-illness’ is the red diaper syndrome due to excretion of S. marcescens in the infant stool.

•       S. liquefaciens has been reported as a cause of mastitis in a dairy herd. In humans, it has rarely been reported as a cause of nosocomial infections, including urinary tract infection, pneumonia, neonatal meningitis and septicaemia resulting from transfusion of contaminated blood products.

•       S. ficaria predominantly may cause biliary infection and sepsis in areas that cultivate fig trees.

•       S. entomophila and S. proteamaculans cause amber coloration in the grass grub Costelytra zealandica.

•       S. rubidaea infection has been associated with the consumption of contaminated coconuts or vegetables.

•       S. plymuthica can be a significant pathogen causing chronic osteomyelitis, wound infection, community acquired and nosocomial bacteremia.

Food spoilage by Serratia

•       Serratia spp. are involved in the spoilage of foods (eggs, butter, milk, coconut and bread) and discoloration of cheeses. It also causes greening and malodor formation in meat.

•       Dairy products could become contaminated by using Serratia- contaminated milk. Contamination of ice cream and cheeses can also occur during handling at the retail market.

•       Serratia spp. may survive in foods unsuitable for the growth of other bacteria, such as smoked and dried fish.

•       In conclusion, Serratia spp. can cause food spoilage and act as both foodborne and opportunistic pathogens.

BIBLIOGRAPHY

•       Winn, W. C., & Koneman, E. W. (2006). Koneman's color atlas and textbook of diagnostic microbiology. Philadelphia: Lippincott Williams & Wilkins.

•       Mackie, T. J., Collee, J. G.McCartney, J. E. (1996). Mackie & McCartney practical medical microbiology. New York ; Churchill Livingstone

•       https://answersingenesis.org/biology/microbiology/serratia-marcescens-the-miracle-bacillus/