Serratia marcescens is a bacterium found in various environments. It is a member of the Enterobacteriaceae family, a group of Gram-negative bacteria. Some strains produce a distinctive red pigment, which has historically led to its mistaken identification as blood or a mystical phenomenon.
General Characteristics
Serratia marcescens is a Gram-negative bacterium, meaning it does not retain the crystal violet stain due to its thin peptidoglycan cell wall. This bacterium has a rod shape, appearing as short, straight, or slightly curved rods that can be arranged in short chains. Its motility is attributed to flagella, which enable movement through liquid environments.
These bacteria are facultative anaerobes, able to grow both in the presence and absence of oxygen. Serratia marcescens is mesophilic, thriving in moderate temperatures. Its optimal growth temperature is around 37-38°C (98.6-100.4°F), but it can grow from about 5°C to 40°C (41-104°F). The bacterium also tolerates a pH range from 5 to 9.
Distinctive Features and Metabolism
Serratia marcescens produces prodigiosin, a red pigment. This tripyrrole dye gives colonies a distinctive pink-orange to dark red color, depending on the strain, age, and environmental factors. Prodigiosin production is temperature-dependent, with optimal synthesis occurring at room temperature, between 20°C and 30°C (68-86°F). Pigment and flagella production ceases above approximately 28°C (82.4°F).
Beyond pigmentation, Serratia marcescens exhibits metabolic versatility, utilizing various substrates for growth. It performs casein hydrolysis, breaking down casein, a milk protein, through metalloproteinases. The bacterium can also degrade compounds like tryptophan and citrate, using citrate as a carbon source. Prodigiosin production is influenced by carbon and nitrogen sources.
Ecological Niche and Clinical Relevance
Serratia marcescens is widely distributed in various environments, including soil, water, and on plants. It has also been isolated from insects and animals. The bacterium thrives in damp, moist places, often appearing as a pink or red discoloration and slimy film in bathrooms, such as on tile grout, shower corners, and basins, where it consumes phosphorus-containing materials or fatty substances like soap residue.
Despite its common environmental presence, Serratia marcescens is primarily recognized as an opportunistic pathogen, particularly in healthcare settings. It is a significant cause of hospital-acquired infections, including those affecting the urinary tract, respiratory tract, and surgical wounds. The bacterium’s ability to form biofilms on surfaces, including medical equipment, is a key factor in its survival and spread. Biofilms are aggregations of microorganisms encased in a self-produced matrix, providing protection and enabling persistence on inanimate objects. Transmission within hospitals often occurs through hand-to-hand contact by healthcare personnel.
Antimicrobial Resistance Patterns
Serratia marcescens develops resistance to many antimicrobial agents, posing challenges for infection management. This resistance can be intrinsic, naturally present in the bacterium’s genetic makeup, or acquired through genetic transfer from other bacteria. A common mechanism involves the production of enzymes that inactivate antibiotics.
One group of these enzymes are beta-lactamases, which break down the beta-lactam ring structure found in many common antibiotics, such as penicillins and cephalosporins, rendering them ineffective. These include extended-spectrum beta-lactamases (ESBLs) and AmpC-type cephalosporinases, which confer resistance to a wide range of beta-lactam drugs. Serratia marcescens can also exhibit resistance through other mechanisms, such as reduced outer membrane permeability, modifications to antibiotic target sites, and the overexpression of efflux pumps that actively pump antibiotics out of the bacterial cell. Infections caused by this bacterium often require careful selection of antibiotics, with carbapenems or cefepime sometimes used as treatment options, though resistance to these can also emerge.