Haemophilus segnis polymicrobial and monomicrobial bacteraemia identified by 16S ribosomal RNA gene sequencing

Summary auto-generated

This case report describes two bacteraemia cases caused by Haemophilus segnis, identified using 16S ribosomal RNA gene sequencing. The first patient was a 32-year-old intravenous drug user with left thoracic empyema who developed polymicrobial bacteraemia with H. segnis, Streptococcus intermedius, and S. sanguis. The second was an 82-year-old woman with Clostridium difficile colitis who developed monomicrobial H. segnis bacteraemia and died of septicaemic shock. Both isolates appeared as gram-negative coccobacilli requiring factor V but not factor X, grew on chocolate agar but barely on blood agar, and were initially misidentified by conventional biochemical systems (Vitek and API). 16S rRNA sequencing definitively identified both as H. segnis, showing only four and two base differences from the reference strain, respectively. The authors review H. segnis infections in literature, noting it is rarely pathogenic despite being an oropharyngeal commensal. Including these cases, only four H. segnis bacteraemia cases were identified, with 50% mortality. The study demonstrates that 16S rRNA sequencing reliably identifies H. segnis when conventional methods produce ambiguous results, and suggests the true prevalence of this organism may be underestimated.

Key findings

16S rRNA sequencing successfully identified two H. segnis bacteraemia cases that were misidentified or inconclusive by conventional biochemical systems (Vitek and API)
H. segnis caused polymicrobial bacteraemia in a drug addict with empyema and monomicrobial bacteraemia in an elderly patient with C. difficile colitis
Among four documented H. segnis bacteraemia cases in the literature, mortality was 50%, whereas non-bacteraemic H. segnis infections showed 100% cure rates with treatment
Both isolates showed identical growth characteristics: factor V-dependent, grew 1 mm colonies on chocolate agar but pinpoint size on blood agar
H. segnis appears to be an underrecognized pathogen; conventional identification methods frequently misreport it as H. parainfluenzae or other Haemophilus species

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Abstract

This paper reports a case of Haemophilus segnis polymicrobial bacteraemia and a case of H. segnis monomicrobial bacteraemia identified by 16S ribosomal RNA gene sequencing. In the first case, a gram-negative aerobic coccobacillus was isolated with Streptococcus intermedius and S. sanguis from the blood culture of a 32-year-old intravenous drug addict with left thoracic empyema. In the second case, a gram-negative aerobic coccobacillus was isolated from the blood culture of an 82-year-old woman with Clostridium difficile colitis and septicaemic shock. Both gram-negative coccobacilli grew on chocolate agar as colonies of 1 mm in diameter after incubation for 24 h at 37°C in air with CO₂ 5%, but only to pinpoint sizes on blood agar under the same incubation conditions. Both strains were factor V-dependent, but not factor X-dependent. For the first isolate, the Vitek system (NHI) showed that it was 56% likely to be Actinobacillus actinomycetemcomitans and 40% Neisseria subflava; whereas the API system (NH) showed that it was 58% likely to be H. aphrophilus/paraphrophilus and 42% H. parainfluenzae. For the second isolate, the Vitek system (NHI) showed that it was 95% likely to be H. influenzae VIII; whereas the API system (NH) showed that it was 58% likely to be H. aphrophilus/paraphrophilus and 42% H. parainfluenzae. 16S rRNA gene sequencing showed that there were four base differences between isolate 1 and H. segnis and two base differences between isolate 2 and H. segnis, indicating that both isolates most closely resembled a strain of H. segnis. Only two cases of H. segnis bacteraemia were found in the English scientific literature, one in a case of infective endocarditis and the other in a case of pancreatic abscess. Including the present two cases, the overall mortality of H. segnis bacteraemia was 50%.