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Abstract

SIR-We read with great interest the report by Raguin et al.[1] on Shigella boydii as a possible causative agent of colonic malacoplakia in an HIV-infected patient. In the absence of positive cultures, S. boydii was identified by means of universal bacterial 16S rDNA-based amplification and sequencing. Since malacoplakia is usually caused by Escherichia coli, a close relative of shigellae and genetically belonging to the same species (!), despite being classified in 2 separate genera for reasons of different clinical and epidemiological significance, the reliable separation of these 2 organisms would be essential to accepting the conclusion made by the authors. On the basis of the data presented (highest relatedness of the sequence detected to S. boydii, with 459 [98%] of 468 nucleotides being identical) and the analysis shown below, this does not seem possible.When the fragment of the S. boydii 16S rRNA gene amplified with primers 91E and 13B [2, 3] is compared with the corresponding sequences deposited in the EMBL/Genebank databases, no differences at all are found between it and several other members of the family Enterobacteriaceae, including E. coli, Escherichia vulneris, Salmonella paratyphi A, Salmonella weltevreden, Shigella sonnei, Shigella flexneri, and Hafnia alvei. Above the 98% identity level, there are also other organisms, such as Klebsiella planticola and Citrobacter diversus. Furthermore, intraspecies variation of 16S sequences has been described for various organisms [4,5].This illustrates that several species and genera within the family Enterobacteriaceae are so closely related to each other that their identification solely on the basis of partial sequences of their 16S rRNA genes is certainly not conclusive, even if the amplified fragments are repeatedly sequenced in both directions, thus resulting in absolutely reliable sequence data.On the basis of the above considerations, we think that it is rather unlikely (though not impossible) that malacoplakia in the patient described was indeed caused by S. boydii.