You observe a Gram-negative rod isolated on Crystal Violet agar from a pure sample of unknown origin. Based on content covered in your microbiology practicals, which is the BEST next medium to streak to help with identification, and why?

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Question restatement: You observe a Gram-negative rod on Crystal Violet agar from a pure culture. Which is the BEST next medium to streak to aid identification, and why?

Option 1: Salt agar – because it selects for Gram-positive skin bacteria.
- This is incorrect for several reasons. Crystal Violet agar already indicates Gram reaction, and salt-containing media are typically used to select for halotolerant organisms or specific skin flora, not Gram-negative rods. The assertion about selecting for Gram-positive skin bacteria is inconsistent with the observed Gram-negative rod and with the typical use of salt-containing media.

Option 2: Nutrient agar – because it enriches for spore formers.
- This is not appropriate here. Nutrient agar is a general-purpose medium that supports growth of many bacteria but does not differentiate or specifically enrich for spore-formers. Spore-formers are more relevant to Gram-positive organisms (e.g., Bacillus, Clostridium) and require special selective/enrichment steps, making this choice ineffective for narrowing down identification of a Gram-negative rod.

Option 3: Heat treatment – because spores survive heat.
- This statement conflates spore formation with heat resistance. Gram-negative rods are typically non-spore-formers, and heat treatment is not a nutritional or differential medium. The rationale given (spores surviving heat) does not apply to the organism type described and does not advance identification in this context.

Option 4: MacConkey agar – because it differentiates lactose fermenters among Gram-negative bacteria.
- This option is aligned with standard microbiology practice. MacConkey agar is selective for Gram-negative bacteria due to bile salts and crystal violet, and it is differential for lactose fermentation, which helps distinguish among Gram-negative rods (e.g., Enterobacteriaceae vs. non-lactose fermenters). Since you already have a Gram-negative rod, streaking onto MacConkey agar can provide information on lactose fermentation, aiding narrowing of possibilities and guiding further tests.

Overall reasoning: The goal after a Gram stain observation is to choose a medium that both supports growth of Gram-negative bacteria and provides a differential feature relevant to common clinically important groups. The MacConkey medium uniquely offers lactose fermentation-based differentiation for Gram-negative rods, whereas the other options either do not select for Gram-negative organisms, do not differentiate relevant traits, or apply to organisms (e.g., spore-formers) not indicated by the Gram-negative rod. Therefore, MacConkey agar serves as the most informative next step for this isolate.

MIMI2002 (ND) Microbiology Practical Quiz

You observe a Gram-negative rod isolated on Crystal Violet agar from a pure sample of unknown origin. Based on content covered in your microbiology practicals, which is the BEST next medium to streak to help with identification, and why?

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