1. Bacteria

•    Gram stain classification

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.

•    Bacterial shapes

o    Cocci (spherical): Staphylococcus (clusters), Streptococcus (chains).

o    Bacilli (rod-shaped): E. coli, Clostridium.

o    Spirochetes (spiral): Treponema pallidum, Leptospira.

•    Spore formation

o    Some Gram-positive bacteria form spores (e.g., Clostridium, Bacillus).

o    Spores: resistant to heat, desiccation; important in sterilisation.

2. Viruses

•    Genetic material

o    DNA viruses: e.g., herpesviruses (HSV, VZV, CMV), adenovirus.

o    RNA viruses: e.g., influenza, HIV, hepatitis C, SARS-CoV-2.

•    Enveloped vs non-enveloped

o    Enveloped: lipid membrane; less stable in environment (e.g., HIV, influenza).

o    Non-enveloped: more resistant (e.g., adenovirus, norovirus).

3. Fungi

•    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).

4. Parasites

•    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.

5. Mechanisms of Host Defence

•    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.

6. Sterilisation and Disinfection

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).