Bacteria are microscopic, single-celled organisms.
Most are harmless. Some cause disease: pathogens.
What Makes Bacteria Dangerous?
Toxins (e.g. Clostridium botulinum)
Tissue invasion (e.g. Streptococcus pyogenes)
Inflammation-triggering (e.g. post-streptococcal glomerulonephritis)
Cancer risk (e.g. Helicobacter pylori)
How Bacteria Are Classified
1. Genus and Species:
- Standard scientific naming: Genus species (e.g. Escherichia coli)
- Different strains within a species can vary in virulence or drug susceptibility
2. Shape:
- Cocci = Spheres
- Bacilli = Rods
- Spirochetes = Spirals
- Vibrio = Comma-shaped
3. Staining:
- Gram-positive: Stain blue/purple (thick peptidoglycan wall)
- Gram-negative: Stain red/pink (thin wall + outer membrane)
- Other stains:
- Acid-fast (e.g. Mycobacterium)
- Endospore stain (e.g. Clostridium, Bacillus)
- Capsule stain (detects encapsulated bacteria)
4. Oxygen Requirements:
- Obligate aerobes: Need oxygen (e.g. Mycobacterium tuberculosis)
- Obligate anaerobes: Cannot tolerate oxygen (e.g. Clostridium spp.)
- Facultative anaerobes: Can switch (e.g. E. coli)
- Microaerophiles: Like a little oxygen (e.g. Helicobacter pylori)
Common Bacteria You Should Know
| Type | Examples |
| Gram-positive cocci | Staphylococcus aureus, Streptococcus pneumoniae |
| Gram-negative cocci | Neisseria meningitidis, Neisseria gonorrhoeae |
| Gram-positive bacilli | Clostridium tetani, Listeria monocytogenes |
| Gram-negative bacilli | E. coli, Salmonella spp., Legionella spp. |
Oxygen Use – Quick Examples
| Oxygen Requirement | Examples |
| Obligate aerobe | Mycobacterium tuberculosis, Pseudomonas aeruginosa |
| Obligate anaerobe | Clostridium spp., Bacteroides spp. |
| Facultative anaerobe | Staphylococcus spp., E. coli |
| Microaerophile | Campylobacter jejuni, Helicobacter pylori |
Key Exam Tips
Gram-positive = thick wall = purple
Gram-negative = outer membrane = pink
Facultative anaerobes = most medically important bacteria
Always link bacteria to their shape, Gram status, and oxygen needs.
If you get stuck, think: “Shape + Stain + Air” = 90% of exam questions answered.
Thank you to the joint editorial team of MRCEM Exam Prep for this ‘Exam Tips’ blog post.
