Specific distribution within the Enterobacter cloacae complex of strains isolated from infected orthopedic implants

Abstract

Bacteria belonging to the Enterobacter genus are frequently isolated from clinical samples but are unusual causative agents of orthopedic implant infections. Twelve genetic clusters (clusters I to XII) and one sequence crowd (sequence crowd xiii) can be distinguished within the Enterobacter cloacae nomenspecies on the basis of hsp60 sequence analysis, and until now, none of these clusters could be specifically associated with a disease. In order to investigate if specific genetic clusters would be involved in infections of orthopedic material, two series of bacterial clinical isolates identified as E. cloacae by routine phenotypic identification methods were collected either from infected orthopedic implants (n = 21) or from randomly selected samples of diverse anatomical origins (control; n = 52). Analysis of the hsp60 gene showed that genetic clusters III, VI, and VIII were the most frequent genetic clusters detected in the control group, whereas cluster III was poorly represented among the orthopedic implant isolates (P = 0.006). On the other hand, E. hormaechei (clusters VI and VIII), but not cluster III, is predominantly associated with infections of orthopedic implants and, more specifically, with infected material in the hip (P = 0.019). These results support the hypothesis that, among the isolates within the E. cloacae complex, E. hormaechei and hsp60 gene sequencing-based cluster III are involved in pathogenesis in different ways and highlight the need for more accurate routine Enterobacter identification methods.

FIG. 1.

Neighbor-joining unrooted tree resulting from analysis of the hsp60 gene sequences of 73 clinical strains and previously reported sequences. Isolation site of clinical strains: P, infected orthopedic implant; B, blood; J, joint or bone, in the absence of implanted material; G, gastrointestinal tract; R, respiratory tract; S, skin and soft tissue; U, urine; K, intravascular catheter. For the previously described strains, type strains are indicated (8, 10-12). Strains labeled EN were reported previously (8) and correspond to sequences with GenBank accession numbers AJ417125, AJ417127, AJ543819, AJ567887, AJ543876, AJ543894, AJ567893, AJ543787, AJ543803, AJ543806, AJ543829, AJ543864, AJ543882, AJ543804, AJ543789, AJ543781, AJ543784, AJ567846, AJ543776, AJ543808, AJ543877, AJ543807, AJ543866, AJ543775, AJ543878, AJ543881, AJ543820, AJ567878, AJ567885, AJ543857, AJ543831, AJ543821, AJ543816, AJ543798, AJ567847, AJ543777, AJ543861, AJ543768, AJ543855, AJ543870, AJ567881, and AJ543837. EH, E. hormaechei; EC, E. cloacae. The bar indicates the number of substitutions per site. Genetic clusters are numbered according to the description provided previously (8).

FIG. 2.

Distribution of clinical strains within the genetic clusters of the E. cloacae complex. All strains could be assigned to one of the previously reported hsp60 gene sequencing-based genetic clusters of the E. cloacae complex. (A) Strains isolated from implanted orthopedic material at different anatomical sites (n = 21). Hip-associated strains exclusively belonged to the E. hormaechei species (clusters VI and VIII). (B) Randomly selected clinical strains of diverse anatomical origins (n = 52). S, skin and soft tissue; R, upper and lower respiratory tract; U, urine; J, joint or bone, in the absence of infected material; K, intravenous catheter; B, blood; G, gastrointestinal tract. Irrespective of the site if isolation, cluster III accounted for 42% of the control isolates.