Comprehensive Microbiology Case Study: Investigation of a Nosocomial Infection Outbreak
Background:
An outbreak of nosocomial (hospital-acquired) infections has been reported in a major urban hospital. Several patients in the Intensive Care Unit (ICU) have developed symptoms of sepsis, and initial investigations suggest a common bacterial source. This case study outlines the steps taken to identify the causative agent, determine its source, and implement control measures.
Case Presentation:
Patient Symptoms:
- High fever
- Chills
- Elevated heart rate
- Hypotension
- Elevated white blood cell count
Initial Steps:
Patient Sample Collection:
- Blood cultures from all symptomatic patients
- Swabs from wound sites and central venous catheter insertion points
- Urine and sputum samples if applicable
Laboratory Techniques and Results:
- Blood Culture and Gram Staining:
- Procedure: Inoculated blood culture bottles were incubated at 37°C. Growth was monitored, and Gram staining was performed.
- Results: Gram-positive cocci in clusters were observed in all positive cultures.
- Culturing on Selective Media:
- Procedure: Blood samples were streaked on Blood Agar (BA), MacConkey Agar (MAC), and Mannitol Salt Agar (MSA) plates and incubated at 37°C for 24-48 hours.
- Results: Yellow colonies grew on MSA, indicating the presence of Staphylococcus aureus. No growth on MAC, confirming the organism is not Gram-negative. Beta-hemolytic colonies on BA.
- Biochemical Testing:
- Procedure: Coagulase test, catalase test, and other relevant biochemical tests were performed on isolated colonies.
- Results: Positive coagulase and catalase tests confirmed the presence of Staphylococcus aureus.
- Molecular Techniques:
- Procedure: DNA was extracted from isolated colonies. PCR targeting the mecA gene and pulsed-field gel electrophoresis (PFGE) for strain typing were performed.
- Results: PCR confirmed the presence of the mecA gene, indicating methicillin resistance (MRSA). PFGE showed identical banding patterns among patient isolates, indicating a common source.
- Antibiotic Sensitivity Testing:
- Procedure: The Kirby-Bauer disk diffusion method was used on Mueller-Hinton Agar.
- Results: The isolates were resistant to methicillin, oxacillin, and several other antibiotics but susceptible to vancomycin and linezolid.
Environmental Sampling:
- Procedure: Swabs from potential reservoirs (sinks, ventilators, hospital staff hands, and equipment) were cultured and identified using the same techniques as patient samples.
- Results: MRSA was isolated from the ventilators used in the ICU, matching the strain from Case Study Questions for Students:
- What are the primary characteristics of Methicillin-resistant Staphylococcus aureus (MRSA) that make it a significant pathogen in nosocomial infections?
- Explain the significance of using selective media such as Blood Agar, MacConkey Agar, and Mannitol Salt Agar in isolating and identifying the pathogen.
- Describe the role of Gram staining in the identification process of the bacteria isolated from the patients. Why is this step crucial?
- How does the presence of the mecA gene detected by PCR confirm methicillin resistance in Staphylococcus aureus?
- Discuss the importance of antibiotic sensitivity testing in the management of bacterial infections, specifically in the context of this outbreak.
