Microbiology CHEAT CODES: Flow Charts for Gram-Positives & Gram-Negatives

Greetings my Little Gang of Nasties 😈😜

Today we’re breaking down the basics of Gram-positives vs Gram-negatives in Microbiology.

Attached, you’ll find two flow charts: use these as much as you can when answering your practice questions. The more you use them, the faster it’ll become second nature when you see those bacteria show up on your exam.

🦠 Gram-Positives (Purple!)

• Cell wall: Thick peptidoglycan layer → holds onto crystal violet stain.

• Examples: Staphylococcus, Streptococcus, Bacillus, Clostridium, Listeria.

• Biochemicals you’ll use a lot: Catalase, Coagulase, Hemolysis patterns, Bile esculin, 6.5% NaCl, Optochin & bacitracin sensitivity

👉 TSI Slant for Gram-Positive Rods (like Bacillus or Listeria): usually A/A (acid slant/acid butt), but Clostridium can be variable depending on species.

🦠 Gram-Negatives (Lactose and Non-Lactose Fermenters)

• Cell wall: Thin peptidoglycan layer + outer membrane with LPS.

• Examples: Enterobacteriaceae (E. coli, Salmonella, Shigella, Klebsiella), Pseudomonas, Neisseria, etc.

• Biochemicals to remember: Oxidase, Indole, Urease, Citrate, H2S production.

👉 TSI Slant for Gram-Negative Rods:

• E. coli: A/A (acid slant/acid butt), gas, no H2S

• Salmonella: K/A (alkaline slant/acid butt), H2S positive

• Shigella: K/A, no H2S

• Klebsiella: A/A, gas

• Pseudomonas: K/K (alkaline slant/alkaline butt) = non-fermenter

🔑 Tip: Your flow charts and TSI reactions are your cheat codes. Use them over and over until you don’t even need to think… you’ll just know the answer.

Now go run through some practice questions using the charts. The more you do, the more this sticks, and the faster you’ll dominate your exam.

-Marilyn 🧪✨

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