In the Gram stain image, the mixed culture contains Bacillus sp. and Staphylococcus sp. Based on morphology, which interpretation of A and B is most correct?

Question

BlackTom AI · Accepted Answer

To interpret the Gram stain image, we assess the morphology of the two circled areas A and B and describe what each shape suggests about the genus.
Option 1: 'A = Staphylococcus, B = Bacillus, because A shows Gram-positive cocci in clusters while B shows Gram-positive rods.' This aligns with the general morphology: Staphylococcus is typically cocci in irregular clusters, and Bacillus appears as rod-shaped bacilli. However, the option wording uses 'Gram-positive cocci in chains' for A, which is not characteristic of Staphylococcus (that phrase more closely describes some Streptococcus species). The misdescription of A weakens this choice.
Option 2: 'A = Bacillus, B = Staphylococcus, because A shows rod-shaped cells while B shows cocci in clusters.' This reverses the expected morphology for the labeled areas if A is indeed cocci and B is rod-shaped. Since the image labeling indicates A as round/coccus-like and B as elongated/rod-like, this option contradicts the observed forms and would be incorrect.
Option 3: 'A = Staphylococcus, B = Bacillus, because A shows cocci in clusters while B shows rod-shaped cells with possible endospores.' This matches the typical Gram stain morphology: A appears as cocci in clusters (Staphylococcus), and B appears as rod-shaped bacilli (Bacillus), with endospore formation being a hallmark feature of Bacillus species that may be observed as endospore-related structures in some rods. The phrase 'possible endospores' appropriately acknowledges that endospores may or may not be clearly visible in a standard smear.
Option 4: 'A = Bacillus, B = Staphylococcus, because A shows Gram-negative rods while B shows Gram-positive cocci.' This is inconsistent with the observed Gram-positive nature of both organisms in typical Gram stains of Bacillus and Staphylococcus, which are Gram-positive. Additionally, it reverses the expected morphologies (rods vs. cocci) for the two organisms.
In summary, the reasoning aligns with option 3's description: A corresponds to cocci in clusters (Staphylococcus), and B corresponds to rod-shaped cells (Bacillus), with the note that endospore presence may be observed in Bacillus organisms. The other options either misstate the morphology (chains vs clusters, or mislabeling Gram reaction) or reverse the organism identities based on the shapes and typical staining patterns.

MIMI2002 (ND) Microbiology Practical Quiz

In the Gram stain image, the mixed culture contains Bacillus sp. and Staphylococcus sp. Based on morphology, which interpretation of A and B is most correct?

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