Serratia marcescens is a Gram-negative bacterium belonging to the Enterobacteriaceae family. While historically dismissed as a harmless environmental microbe, it is now recognized as a significant opportunistic pathogen. This organism has emerged as an important cause of infection, particularly in healthcare settings, where its ability to survive in diverse environments presents challenges. The bacterium is concerning because of its inherent and acquired resistance to many common antimicrobial drugs.
Defining Characteristics and Common Habitats
This organism is defined as a motile, rod-shaped bacterium that can thrive in a wide range of environments. A notable feature of many S. marcescens strains is the production of prodigiosin, a non-diffusible, brick-red pigment. The brilliant color of this pigment has been linked to historical accounts of “bleeding bread” and other miraculous appearances.
Prodigiosin production is dependent on specific conditions, such as temperature, with optimal pigment formation occurring below human body temperature, around 25 to 30°C. Strains isolated from clinical infections are frequently non-pigmented because growth at 37°C inhibits the pigment’s biosynthesis. S. marcescens is ubiquitous in nature, commonly residing in soil, water, plants, and insects. It readily forms biofilms and is often found in damp household environments, such as shower grout, sink drains, and toilet bowls, where its presence manifests as a pink or reddish slime.
Clinical Relevance and Associated Infections
S. marcescens is primarily an opportunistic pathogen, meaning it rarely causes disease in healthy individuals but poses a serious threat to those with weakened defenses. It is a common cause of nosocomial, or hospital-acquired, infections, which are often difficult to manage. The bacterium’s ability to survive on surfaces and medical equipment contributes to its spread within clinical environments.
Patients at high risk include the elderly, those with compromised immune systems, and neonates, especially premature infants. A primary risk factor is the presence of indwelling medical devices, such as urinary catheters, intravenous lines, and mechanical ventilators. These devices provide a surface for the bacteria to colonize and a direct route into the body.
The infections caused by S. marcescens are diverse and depend on the site of entry. Common manifestations include urinary tract infections (UTIs) associated with catheter use and respiratory tract infections like pneumonia, often linked to contaminated respiratory equipment. It also causes wound infections and severe bloodstream infections, known as bacteremia or sepsis. If the infection spreads to the bloodstream, it can progress rapidly, potentially leading to septic shock and organ failure.
Understanding Antibiotic Resistance
Treating S. marcescens infections is challenging due to its complex profile of antibiotic resistance. The organism possesses intrinsic resistance to several commonly used antibiotics, including polymyxins and certain penicillin-based drugs. This natural resistance is often due to the presence of chromosomally encoded enzymes and unique cell membrane structures.
The bacterium harbors a chromosomal \(\beta\)-lactamase enzyme called AmpC, which can be expressed at high levels. This confers resistance to penicillins and first- and second-generation cephalosporins. This enzymatic activity can be induced by exposure to certain antibiotics, complicating treatment selection.
Beyond its intrinsic defenses, S. marcescens can acquire resistance genes from other bacteria, greatly expanding its resistance spectrum. These acquired genes often code for Extended-Spectrum \(\beta\)-Lactamases (ESBLs) or carbapenemases. The presence of carbapenemase enzymes is concerning because carbapenems are often reserved for multidrug-resistant Gram-negative bacteria. Other mechanisms contributing to resistance include modifications to drug target sites, the synthesis of enzymes that inactivate aminoglycosides, and the action of efflux pumps that actively push antibiotic molecules out of the bacterial cell. This combination of natural and acquired resistance mechanisms requires careful laboratory testing to determine the most effective treatment.
Prevention and Control Measures
Effective control of S. marcescens relies heavily on rigorous infection prevention practices, especially in healthcare environments. The hands of healthcare workers are a major vehicle for transmission, making stringent hand hygiene the most important control measure. This includes frequent and thorough handwashing and the use of alcohol-based hand sanitizers before and after patient contact.
Disinfection protocols are also paramount, focusing on surfaces and equipment that the bacteria can colonize and form biofilms on. Proper sterilization and maintenance of medical devices, such as ventilators and catheters, are necessary to prevent the organism from gaining access to the patient’s body. In the event of an outbreak, prompt identification and isolation of colonized or infected patients, along with contact precautions, are implemented to curb the spread of the organism.