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MI1.{1-2,10,12-13} | General Microbiology I: History, Morphology, Sterilisation & Staining — Case Study
CLINICAL SCENARIO
Clinical Scenario:
Mrs Lakshmi Devi, a 52-year-old school teacher from Pondicherry, underwent an elective laparoscopic cholecystectomy at a secondary care hospital. On the 5th post-operative day, she developed fever (38.8°C), localised redness, swelling, and purulent discharge from the umbilical port site wound. Her blood investigations showed: TLC 13,400 cells/μL (neutrophils 82%), ESR 68 mm/hr, CRP 42 mg/L. A wound swab was sent to the microbiology laboratory.
The laboratory technician performed a Gram stain of the pus. The report read: 'Gram-positive cocci in clusters, moderate pus cells, no other organisms seen.' A culture was set up on blood agar and MacConkey agar. After 24 hours of incubation at 37°C, golden-yellow β-haemolytic colonies grew on blood agar; no growth on MacConkey agar.
The ward nurse has just noticed that two other patients in the same post-operative ward — both of whom had their dressings changed on the same day as Mrs Lakshmi — are now developing similar wound infections.
Instructions
Read the clinical scenario carefully. Answer all four sections in your own words, using correct microbiological terminology. Support your answers with reasoning grounded in what you have learnt in this module. Marks are awarded for accuracy, clinical reasoning, and clarity of explanation — not for memorised lists.
Word guidance: 700–900 words across all four sections.
Submit as a single document with clearly labelled section headings.
Length: 700–900 words total across all four sections
What to Submit
Section 1: Identify and Justify (25 marks)
Based on the Gram stain result and colony characteristics described, identify the most likely causative organism. Explain how the Gram stain morphology (arrangement of cocci, Gram reaction) and culture findings (colony colour, haemolysis pattern, MacConkey growth) together lead you to this identification.
Guidance: Name the genus AND species. Explain the significance of 'clusters' vs 'chains'; explain β-haemolysis; explain why MacConkey agar is selective and what no growth on it tells you. Do NOT simply list features — connect them to the identification.
Section 2: Staining Technique (25 marks)
Describe the Gram staining procedure step-by-step as it should be performed on this wound swab specimen. At each step, explain WHY it is done — not just WHAT is done. Specifically explain what would happen to the Gram stain result if the decolourisation step were performed for too long.
Guidance: Cover: smear preparation and heat-fixing, crystal violet, Gram's iodine, acetone-alcohol decolourisation, safranin counterstain, microscopy. For the decolourisation error: name the specific change in appearance and the mechanism (what happens to the CV-I complex under over-decolourisation).
Section 3: Sterilisation & Infection Prevention (25 marks)
Two additional patients on the same ward developed similar infections after shared dressing changes. What does this pattern suggest about the infection source (colonisation of equipment/hands vs environmental source)? Identify TWO potential failures in sterilisation or disinfection practice that could explain this cluster. For each failure, recommend the correct practice using the Spaulding classification framework.
Guidance: Consider: dressing instruments (critical/semi-critical), antiseptic choice for skin prep vs surface disinfection, hand hygiene between patients. Apply Spaulding terminology explicitly. Do not suggest that the pathogen is unusually resistant — focus on process failures.
Section 4: Historical and Broader Context (25 marks)
Ignaz Semmelweis introduced hand hygiene in a Vienna obstetric ward in 1847 and dramatically reduced puerperal sepsis mortality. Connect his historical observation to the cluster outbreak described in this case. What does Mrs Lakshmi's case illustrate about the 'role of microbes in health and disease' as defined in MI1.1? Is the organism in this case a commensal, a mutualist, a pathogen, or an opportunist — and how does normal skin flora vs host defence explain why most surgical patients do NOT develop wound infections?
Guidance: Show the connection between Semmelweis's principle and the current ward cluster. Apply the ecological roles table from the SDL (commensal, mutualist, pathogen, opportunist). Discuss the concept of 'host susceptibility' — post-surgical immune suppression, skin barrier breach, contaminated instruments as co-factors.
Grading Rubric — Post-Surgical Wound Infection Case Study Rubric (100 points)
| Criterion | Points | Full-marks descriptor |
|---|---|---|
| Section 1 — Organism Identification & Justification: Correctly names organism with full justification from Gram stain morphology and culture characteristics | 25 pts | Names Staphylococcus aureus (genus + species). Explains Gram-positive cocci in clusters (vs chains for Streptococcus). Correctly interprets β-haemolysis on blood agar. Correctly explains MacConkey selectivity (bile salts/crystal violet inhibit Gram-positives) and infers Gram-positive identity. Golden-yellow pigment connected to S. aureus specifically. All four findings integrated coherently. |
| Section 2 — Gram Staining Procedure & Over-decolourisation Error: Accurate step-by-step procedure with mechanistic explanation for each step; correct analysis of over-decolourisation effect | 25 pts | All 5 steps covered in correct sequence with accurate 'why' for each (heat-fixing: denatures proteins and adheres smear; crystal violet: primary stain; iodine mordant: CV-I complex formation; acetone-alcohol: differentiating step; safranin: counterstains Gram-negatives). Over-decolourisation: correctly states Gram-positive organisms appear Gram-negative (pink), names mechanism (CV-I complex stripped from thick peptidoglycan by prolonged acetone-alcohol), and names clinical consequence (misidentification). |
| Section 3 — Sterilisation Failures & Spaulding Classification: Identifies two plausible process failures, applies Spaulding framework correctly, recommends appropriate corrective practice | 25 pts | Correctly identifies the cross-patient cluster as suggesting a shared transmission vehicle (dressing instruments or healthcare worker hands). Names two specific, plausible sterilisation/disinfection failures (e.g., reuse of non-sterile dressing forceps without re-sterilisation; failure of hand hygiene between patients). Applies Spaulding classification accurately for both (e.g., dressing forceps = critical items requiring sterilisation; intact skin surface = non-critical, low-level disinfection with 70% alcohol). Recommends correct corrective measures with evidence-based rationale. |
| Section 4 — Historical Context, Ecological Role & Host Susceptibility: Integrates Semmelweis principle, ecological role classification, and host susceptibility concept | 25 pts | Semmelweis connection explicit and accurate (handwashing between patients prevents cross-transmission of pathogen; the 1847 principle directly maps to the ward cluster). Correctly classifies S. aureus as an opportunistic pathogen (commensal on skin in ~30% carriers; causes disease when host barriers are breached). Explains why most surgical patients do not develop infections: intact skin barrier, normal neutrophil function, surgical asepsis, post-op prophylaxis. Identifies specific co-factors in this case: wound breach, immunosuppression from surgery, contaminated instrument. |
PEER REVIEW
Your peer reviewer will assess your work against the same rubric your faculty will use. For each section: (1) identify one strength — something that is accurate, well-reasoned, or clearly explained; (2) identify one area for improvement — something factually incomplete, poorly explained, or missing; (3) provide a suggested mark from the rubric descriptors. Be specific: quote or paraphrase the exact phrase that influenced your rating. Avoid vague praise such as 'good work' without evidence. Your peer review will itself be assessed for quality — reviewers who provide substantive, accurate feedback receive full credit.