Final answer:
The correct organism that fits the description of not growing on CNA but growing "string of pearls" colonies in thioglycolate broth is Clostridium botulinum. This organism is a gram-positive, anaerobic bacterium capable of forming endospores and would not be inhibited by CNA.
Step-by-step explanation:
The gram-positive spore-forming organism in question that did not grow on Columbia nalidixic acid agar (CNA) yet exhibited growth in the form of "string of pearls" colonies in thioglycolate broth is indicative of an anaerobic bacteria. Among the options provided:
- Bacillus anthracis
- Clostridium botulinum
- Staphylococcus aureus
- Clostridium perfringens
The organism's failure to grow on CNA, which inhibits the growth of gram-negative bacteria and favors gram-positive cocci, particularly Staphylococcus spp., eliminates Staphylococcus aureus (B) as the correct choice, as it is a gram-positive coccus that would typically grow on CNA. The correct answer is likely Clostridium botulinum (B), a gram-positive, rod-shaped anaerobic bacterium known to form spores and grow well in anaerobic conditions, like those provided by thioglycolate broth. Bacillus anthracis (A) can grow both aerobically and anaerobically, and would not be inhibited by CNA. Clostridium perfringens (D) is also a gram-positive spore-forming anaerobe, but it is not typically described as forming "string of pearls" colonies in culture.