Which organism is sometimes mistaken for Salmonella and agglutinates in Salmonella polyvalent antiserum?

• A. C. freundii strains
• B. Proteus mirabilis strains
• C. S. sonnei strains
• D. E. coli

Answer: (A)

The organism that is sometimes mistaken for Salmonella and agglutinates in Salmonella polyvalent antiserum is Citrobacter freundii. This is due to the biochemical similarities and the ability of certain strains to share similar surface antigens with Salmonella.

When performing serological tests using Salmonella antiserum, C. freundii can give false-positive reactions. This can lead to misidentification in laboratory diagnostics. The presence of certain serogroups and the similar reactions observed during agglutination tests contribute to this confusion.

In contrast, while Proteus mirabilis might exhibit some serological similarities, it does not agglutinate in Salmonella polyvalent antiserum. Similarly, S. sonnei, a strain of Shigella, and E. coli do not share the same serological characteristics that would cause them to agglutinate in Salmonella antiserum. Hence, the specificity of the agglutination reaction is why Citrobacter freundii is the organism that fits the criteria in the question.