Serratia pneumonia is a serious lung infection caused by the bacterium Serratia marcescens. This respiratory tract infection can lead to severe illness. Although the bacterium is found widely in the environment, the infection is most frequently identified in healthcare settings. It is typically classified as a nosocomial, or hospital-acquired, infection, which presents significant challenges for patient care.
Understanding the Causative Bacterium and Its Environment
The organism responsible is Serratia marcescens, a rod-shaped, Gram-negative bacterium belonging to the order Enterobacterales. It is considered an opportunistic pathogen, primarily causing disease in individuals with compromised defenses. This bacterium is widespread, thriving in environments including soil, water, plants, and various surfaces.
In the home, Serratia marcescens is recognized as the pink or pink-orange slimy film that grows in moist areas like showers and sinks. It is a mesophilic organism, capable of growing across a wide temperature range, though its optimal growth is near body temperature. Some strains produce a distinctive dark red to pale pink pigment called prodigiosin, responsible for the characteristic coloration seen on damp surfaces.
Community-acquired Serratia pneumonia is rare; most infections occur within hospitals or long-term care facilities. In these healthcare settings, the bacterium colonizes the respiratory and urinary tracts of adults, leading to serious infections. The concentration of vulnerable patients in these facilities makes infection control a constant concern.
Identifying Risk Factors and Clinical Symptoms
Individuals whose natural defenses have been weakened by underlying conditions or medical interventions are the most susceptible. Prolonged hospitalization, particularly in intensive care units, is a major risk factor. The use of invasive medical devices provides a direct route for the bacteria to enter the body.
Patients with chronic respiratory diseases, such as Chronic Obstructive Pulmonary Disease (COPD), face a higher chance of developing Serratia infections. Other factors include advanced age, a weakened immune system due to conditions like cancer or HIV, and chronic illnesses such as diabetes. Frequent or long-term use of broad-spectrum antibiotics can also alter the body’s natural microflora, allowing Serratia marcescens to proliferate.
The clinical presentation of Serratia pneumonia mirrors that of other severe bacterial pneumonias. Symptoms often include fever, chills, and difficulty breathing. A persistent, productive cough is common and may be accompanied by chest pain. A distinguishing feature sometimes associated with this infection is the production of thick sputum that may appear blood-tinged or rust-colored, sometimes referred to as pseudohemoptysis.
Diagnosis and Navigating Treatment Challenges
Diagnosis begins with clinical suspicion based on the patient’s symptoms and risk factors, especially if they are hospitalized. Imaging studies, such as a chest X-ray or CT scan, visualize infiltrates or consolidation in the lungs, confirming pneumonia. Definitive diagnosis requires laboratory confirmation to identify the specific causative organism.
Samples of respiratory secretions, such as sputum or fluid obtained via bronchoscopy, are cultured to isolate Serratia marcescens. Once isolated, antibiotic sensitivity testing (antibiograms) is performed. This testing determines which specific antibiotics will be effective against the isolated strain, directly guiding the treatment plan.
Managing Serratia infections is complicated by the bacterium’s intrinsic resistance to several classes of antibiotics. Serratia marcescens naturally carries genes that confer resistance to common drugs, including ampicillin and first- and second-generation cephalosporins. This intrinsic resistance is often due to the production of the chromosomally encoded enzyme known as AmpC beta-lactamase.
Some strains acquire additional resistance mechanisms, such as extended-spectrum beta-lactamases (ESBLs) or carbapenemases, which broaden their resistance profile. These multi-drug resistant (MDR) strains severely narrow therapeutic options, making antibiotic selection challenging. Treatment often requires powerful, broad-spectrum agents, such as fourth-generation cephalosporins (e.g., cefepime), carbapenems (e.g., meropenem), or a combination of newer-generation drugs based on sensitivity results.
Prevention Strategies and Infection Control
Since Serratia pneumonia is predominantly a healthcare-associated infection, prevention focuses heavily on rigorous infection control protocols within medical facilities. Strict adherence to hand hygiene is the most fundamental and effective measure for all healthcare personnel. This includes thorough washing before and after contact with patients and their environment to minimize the spread of the organism.
Sterile techniques must be consistently employed for all invasive procedures, such as the insertion and maintenance of catheters and breathing tubes. Hospitals implement specific “ventilator bundle” protocols aimed at reducing the risk of ventilator-associated pneumonia, a common gateway for Serratia infection. For patients and visitors, frequent hand washing and ensuring the timely removal of unnecessary medical devices, like urinary catheters, help reduce the risk of colonization and subsequent infection.