Bacteriology Biochemicals just got EASIER 🦠😉

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Greetings Gang 😈😜

I gotta be honest... biochemicals are the WORST. Even now after 10 years of being in the field and over a year of teaching them... Catalase, coagulase, oxidase, bile esculin, PYR, CAMP, novobiocin... it starts to feel like a lost cause.

The problem isn’t your memory. It’s how most of us study them: as long lists. Your brain doesn’t store lists well. It stores stories, pictures and decisions. Here’s how to switch.

1. Every biochemical answers ONE question.

Stop memorizing “S. aureus: catalase +, coagulase +, DNase +, mannitol +...” Instead, ask: “𝗪𝗵𝗮𝘁 𝗶𝘀 𝘁𝗵𝗶𝘀 𝘁𝗲𝘀𝘁 𝘀𝗲𝗽𝗮𝗿𝗮𝘁𝗶𝗻𝗴?”

Catalase: Staph (+) vs. Strep/Enterococcus (−)
Coagulase: S. aureus (+) vs. all other Staph (−)
Novobiocin: S. epidermidis (sensitive) vs. S. saprophyticus (resistant)
Bacitracin / PYR: Group A Strep, S. pyogenes (sensitive / +)
CAMP / hippurate: Group B Strep, S. agalactiae (+)
Optochin / bile solubility: S. pneumoniae (sensitive / soluble) vs. viridans strep
Bile esculin + 6.5% NaCl: Enterococcus (+ / +) vs. group D strep (+ / −)
Oxidase: Pseudomonas, Neisseria, Vibrio, Campylobacter, Helicobacter (+) vs. Enterobacterales (−)

If you know what each test splits, you can work out the answer even when you blank on the details.

2. Learn flowcharts, not facts.

The exam gives you clues one step at a time, like a flowchart: Gram stain → morphology → arrangement → first test → second test. Study the same way.

Example: Gram-positive cocci in clusters → catalase + → coagulase − → novobiocin resistant → UTI in a young woman = S. saprophyticus.

Action step: Draw the gram-positive cocci flowchart from memory on a blank page every day this week. Day 1 will be hella messy, but by Day 5 you’ll be doing it in 2 minutes.

3. Give every organism a “personality card," and a gender LOL. Make one card per organism with:

Gram stain + shape: what it looks like under the scope
Plate: what the colony looks like (color, hemolysis, smell)
1–2 key biochemicals: only the ones that separate it from look-alikes
The disease: the clinical story

The senses are the memorable part. Some smells exam writers love:

Pseudomonas aeruginosa: grapes or corn tortillas, plus a blue-green pigment
Proteus: burnt chocolate and swarming across the plate
Alcaligenes faecalis: green apples
Eikenella corrodens: bleach, and it pits the agar
Streptococcus anginosus group: butterscotch / caramel

When you can picture and almost smell the plate, the biochemicals stick to that image.

4. Know what each plate is FOR.

Every plate is either selective (who’s allowed to grow) or differential (how it shows a difference), and often both.

Blood agar (BAP): shows hemolysis (beta = clear, alpha = green, gamma = none)
Chocolate (CHOC): for fussy organisms like Haemophilus and Neisseria
MacConkey (MAC): gram-negatives only; pink = lactose fermenter, colorless = non-fermenter
HE / XLD: stool pathogens. Salmonella has black centers (H2S); Shigella has no black.
Thayer-Martin: N. gonorrhoeae
TCBS: Vibrio (V. cholerae is yellow)
CIN: Yersinia (“bull’s-eye” colonies)
Sorbitol MacConkey: E. coli O157:H7 is colorless (doesn’t ferment sorbitol)
BCYE: Legionella

Action step: For each plate, say out loud: “This plate grows ___, stops ___, and shows me ___.”

5. Play “Unknown.”

Instead of reading notes, quiz yourself backwards: “Lactose non-fermenter, oxidase +, grape smell, burn patient... who is it?” Working forward from clues is exactly what the exam asks you to do.

🧐 Try this one (answer in the comments!):

Beta-hemolytic Gram-positive cocci in chains, catalase negative, bacitracin resistant, CAMP positive, hippurate hydrolysis positive. Who is it, and who’s the patient population we worry about?

Drop your answer below, and I’ll reply to every one.

Want more micro like this? Inside my paid community, we have 7 days of live study sessions starting November 1st (one subject a day), plus my charts, cheat sheets and guides.

👉🏽 Join here: https://www.skool.com/microscopeviews

See you there!

— Marilyn

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Bacteriology Biochemicals just got EASIER 🦠😉