o Gram-positive: thick peptidoglycan wall, retain crystal violet stain → purple/blue.
Examples: Staphylococcus, Streptococcus, Enterococcus, Clostridium.
o Gram-negative: thin peptidoglycan, outer membrane with LPS → pink/red.
Examples: Escherichia coli, Neisseria, Pseudomonas, Salmonella.
o Cocci (spherical): Staphylococcus (clusters), Streptococcus (chains).
o Bacilli (rod-shaped): E. coli, Clostridium.
o Spirochetes (spiral): Treponema pallidum, Leptospira.
o Some Gram-positive bacteria form spores (e.g., Clostridium, Bacillus).
o Spores: resistant to heat, desiccation; important in sterilisation.
o DNA viruses: e.g., herpesviruses (HSV, VZV, CMV), adenovirus.
o RNA viruses: e.g., influenza, HIV, hepatitis C, SARS-CoV-2.
o Enveloped: lipid membrane; less stable in environment (e.g., HIV, influenza).
o Non-enveloped: more resistant (e.g., adenovirus, norovirus).
• Yeasts: unicellular; reproduce by budding.
o Example: Candida albicans.
• Moulds: multicellular, filamentous hyphae.
o Examples: Aspergillus, Dermatophytes.
• Dimorphic fungi: yeast form at body temp, mould form at environmental temp (e.g., Histoplasma).
• Protozoa: single-celled eukaryotes.
o Examples: Plasmodium (malaria), Giardia, Entamoeba histolytica.
o Life cycles often involve cyst and trophozoite forms.
• Helminths: multicellular worms.
o Nematodes (roundworms): e.g., Ascaris.
o Cestodes (tapeworms): e.g., Taenia.
o Trematodes (flukes): e.g., Schistosoma.
o Complex life cycles often involve intermediate hosts.
• Physical barriers: skin, mucous membranes, cilia, gastric acid.
• Phagocytosis: by neutrophils, macrophages.
• Cytokines: interleukins, TNF-α, interferons → promote inflammation and antiviral responses.
• Complement system: opsonisation, chemotaxis, cell lysis.
Method Main Use Examples / Notes
Autoclaving Sterilisation (moist heat at 121 °C) Surgical instruments, culture media
Dry heat Glassware Higher temps than moist heat
Chemical disinfectants Surface and equipment Alcohol (70%), chlorhexidine, bleach
Radiation Disposable medical supplies Gamma rays
Filtration Heat-sensitive solutions Vaccine prep
UV light Surface air sterilisation Limited penetration
Extra Revision Pearls
• Gram-negative bacteria have an outer membrane containing endotoxin
(lipid A component of LPS) → septic shock.
• Capsule: major virulence factor (e.g., Streptococcus pneumoniae, Neisseria meningitidis).
• Biofilms: protect bacteria from antibiotics and immune clearance
(e.g., prosthetic infections with Staph epidermidis).
• Acid-fast stain (Ziehl-Neelsen): for mycobacteria (TB, NTM).
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Author & Educational Disclaimer
Author:
Dr Phillip Cockrell BM FRCP DipClinEd
Dr Phillip Cockrell is a UK Consultant Physician in Internal Medicine, currently working at Queen Alexandra Hospital, Portsmouth University Hospitals NHS Trust. He has previously worked as a registrar across Intensive Care Medicine, Gastroenterology, Cardiology, Stroke Medicine, Acute Medicine, and Respiratory Medicine.
He has held senior leadership roles including Associate Clinical Director of the Acute Medical Unit, Clinical Director of Internal Medicine, and Chief of Medicine. Dr Cockrell has over 15 years’ experience in postgraduate medical education, having lectured extensively across the MRCP syllabus and contributed to MRCP revision teaching and course development.
Dr Cockrell holds a Bachelor of Medicine (BM), Fellowship of the Royal College of Physicians (FRCP), and a Diploma in Clinical Education (DipClinEd). His teaching approach is based on structured consolidation of complex medical topics to support efficient and effective revision for postgraduate examinations.
Purpose of this content:
The material on this page is intended solely for educational purposes to support revision for the MRCP (UK) Part 1 examination. It reflects examination-relevant principles of internal medicine and is designed to aid learning and pattern recognition.
Medical disclaimer:
This content is designed for postgraduate medical examination revision and does not constitute medical advice, diagnosis, or treatment guidance and must not be used as a substitute for professional clinical judgement, local guidelines, or specialist consultation. Clinical decisions should always be made in the context of individual patient circumstances and current national guidance.
