Shiken premium Upgrade Banner

Bacterial morphology:

Bacterial morphology is an important factor in determining the type of bacteria being studied. The three main categories include Cocci, Bacilli, and Spirochetes. Cocci bacteria, which are sphere shaped, can form diplococci, clusters, or chains. Bacilli bacteria, which are rod shaped, can form diplobacilli or chains. Vibrio bacteria on the other hand, are curved rods. As for Spirochetes, they appear as spirals.

Gram Staining:

Gram staining is a valuable tool used to differentiate between different types of bacteria. Bacteria that stain pink are referred to as Gram negative, while those that stain purple are Gram positive, as Gram positive bacteria have a thicker peptidoglycan layer in their cell wall and therefore retain the purple stain.

Gram positive:

  • Staphylococci: aureus, epidermidis, pneumoniae
  • Streptococci: pyogenes, viridans, agalactiae
  • Enterococci: faecalis, faecium

Gram negative:

  • Corynebacterium: diphtheria
  • Listeria: monocytogenes
  • Mycobacterium: tuberculosis, avium, bovis, leprae
  • Clostridium: difficile, botulinum, perfringens
  • Propionibacterium: acnes


  • Neisseria: meningitidis, gonorrhoeae


  • Salmonella: typhi, paratyphi, enterica
  • Pseudomonas: aeruginosa
  • Klebsiella: pneumoniae
  • Shigella: dysenteriae, flexneri
  • Yersinia: pestis
  • Escherichia: coli


  • Campylobacter: jejuni
  • Vibrio: cholerae


  • Helicobacter: pylori
  • Treponema: Pallidum

Key: Aerobic/ anaerobic/ facultatively anaerobic

Other tests:

Coagulase test detects if a sample is positive or negative for Staphylococcus aureus, which is usually positive, while other staphylococci are negative. The Catalase test detects if staphylococci, streptococci, and enterococci are positive or negative. The Optochin test detects if the sample is positive or negative for Strepococcus pneumoniae, which is usually positive.

Optochin Sensitivity and Haemolysis

In terms of Streptococcus pneumoniae sensitivity, Optochin has been proven to be effective. However, other streptococci have been identified as resistant.

Haemolysis: In experiments using bacterial growth on blood agar, distinct alpha and beta haemolysis patterns have been observed. Alpha haemolysis appears in the form of a green coloring when the bacteria in question are S. pneumoniae or S. viridans. On the other hand, Beta haemolysis is characterized by a yellow/clear tint, most commonly observed when examining S. pyogenes. Other streptococci and staphylococci typically are non-haemolytic.

Join Shiken For FREE

Gumbo Study Buddy

Explore More Subject Explanations

Try Shiken Premium
for Free

14-day free trial. Cancel anytime.
Get Started
Join 10,000+ learners worldwide.
The first 14 days are on us
96% of learners report x2 faster learning
Free hands-on onboarding & support
Cancel Anytime