What are the possible sources of Serratia marcescens bacteremia in a patient with a complex medical history, including recent hospitalization, invasive medical procedures, or indwelling catheters?

Sources of Serratia marcescens Bacteremia

Serratia marcescens bacteremia most commonly originates from the urinary tract in catheterized patients, contaminated infusate or medical equipment, and nosocomial transmission through healthcare workers' hands or environmental reservoirs.

Primary Sources and Risk Factors

Urinary Tract as Primary Portal

• The urinary tract is the most frequent source, accounting for the majority of cases, particularly in patients with indwelling urinary catheters 1, 2, 3
• Serratia spp. are among the most common species found in complicated urinary tract infections, alongside E. coli, Proteus, Klebsiella, Pseudomonas, and Enterococcus 1
• Catheter-associated UTIs are the leading cause of secondary healthcare-associated bacteremia, with approximately 20% of hospital-acquired bacteremias arising from the urinary tract 1
• The incidence of bacteriuria with indwelling catheterization is 3-8% per day, with catheterization duration being the most important risk factor 1

Intravascular Catheter-Related Sources

• Central venous catheters represent a major source, though Serratia is less common than coagulase-negative staphylococci, Candida, S. aureus, enterococcus, pseudomonas, and acinetobacter in catheter-related bloodstream infections 1
• Primary bacteremia (without identifiable source) occurred in 68% of cases in one series, with 12 patients having central venous catheters in place, though only one met strict criteria for catheter-related infection 2
• The microbes colonizing catheter hubs and skin surrounding the insertion site are embedded in biofilm matrix and serve as the source of most catheter-related bloodstream infections 1

Contaminated Infusate and Medical Equipment

• Contaminated intravenous fluids and medical equipment are critical sources during outbreaks 1
• Gram-negative bacilli capable of reproducing at room temperature, including Serratia species, Klebsiella, Enterobacter, Burkholderia cepacia, and Ralstonia pickettii, are most often implicated in contaminated infusate 1
• Urine measuring containers and urinometers have been identified as inanimate reservoirs, with the epidemic organism recovered from 4 of 4 urinometers and 3 of 7 urine measuring containers during one outbreak 3
• Environmental surveillance during outbreaks has shown handling of urine jugs to be a point source of contamination 4

Healthcare-Associated Transmission

• Cross-infection from hospital reservoirs through healthcare workers' hands is a significant transmission route 3, 4, 5
• The resistant organism was recovered from pooled handwashings of nursing personnel, suggesting hands serve as vectors for transmission 3
• Faecal excretion of Serratia was found in 11 cases and may be a significant portal of dissemination 5

Clinical Syndromes and Associated Conditions

Infection Sites Beyond Bacteremia

• Clinical presentations include primary bacteremia (68%), pneumonia (14%), urinary tract infection (9%), suppurative thrombophlebitis (5%), and surgical wound infection (5%) 2
• Fatal complications can include meningitis, brain abscesses, and myocarditis, particularly in elderly patients with chronic conditions 6

High-Risk Patient Populations

• All patients with Serratia bacteremia have severe underlying disease, most commonly diabetes mellitus 2
• Eighty-two percent of cases are nosocomial infections 2
• Risk factors include prolonged catheterization, female sex, diabetes, longer hospital and ICU stays, and debilitated state 1, 5
• In 64% of patients, the portal of entry remains unknown despite thorough investigation 2

Common Pitfalls to Avoid

Dismissing Catheter Urine Isolates

• Do not dismiss Serratia marcescens isolated from catheter urine as a contaminant, even in asymptomatic patients with recognized risk factors 6
• Consider old age, previous antibiotic treatment, and underlying chronic or debilitating disease when assessing significance 6

Overlooking Environmental Reservoirs

• During outbreaks, extensive microbiologic evaluation should include urinometers, urine measuring containers, and other medical equipment that may serve as reservoirs 3, 4
• The epidemic strain may be found in hospital areas where no infected patients are currently located, indicating persistent environmental contamination 3

Underestimating Antimicrobial Resistance

• Serratia marcescens exhibits increasing multidrug resistance, with all isolates resistant to ampicillin and cephalothin 2
• Only imipenem showed universal susceptibility, with ciprofloxacin demonstrating good activity (MIC90 of 0.19 mcg/mL) 2
• Resistance patterns show 95% susceptibility to moxalactam, but only 68% to amikacin, 55% to ceftazidime, and as low as 9% to piperacillin 2