Slow-growing Salmonella enterica Typhi mis-identified as Salmonella Gallinarum in Ibadan, Nigeria
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Salmonella enterica s erovar Typhi is endemic in Nigeria where S. G allinarum is rarely reported. Probable S . Gallinarum was blood-cultured from three Ibadan patients within 10-days leading to suspicion of an outbreak. All three isolates were re-identified using VITEK-2, whole genome sequenced on Illumina and Oxford Nanopore platforms and confirmed as S . Typhi genotype 3.1.1. Two isolates from the same household had no single nucleotide polymorphisms, suggesting a point-source, but the third was an independent infection event. The isolates formed small colonies, were negative for motility by hanging drop method, and in Sulphide-Indole-Motility medium at 24h, but positive after 48h incubation. Hybrid genome assemblies revealed chromosomal fragment arrangements with imbalances on either side of ori and ter , which has been shown to slow S . Typhi growth. Suspected S . Gallinarum isolates in typhoid endemic areas should be evaluated biochemically and for motility after extended incubation, and verified by serological or molecular methods.
Main contributions of the research
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This study demonstrates that slow-growing Salmonella Typhi can be mis-identified as S . Gallinarum in resource-limited endemic countries.
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Extended motility testing and confirmatory testing can avoid S. Gallinarum miss-calls.
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The genetic basis for slow-growth may be chromosomal rearrangements at rrn operons, which are known to occur in S. Typhi
