Inquilinus limosus, a multi-drug-resistant Gram-negative rod belonging to the alpha-Proteobacteria, has been increasingly reported as an emerging pathogen in cystic fibrosis (CF) patients. Isolation and accurate identification in routine laboratory settings has traditionally been difficult due to relative slower growth than the coexisting complex microbial flora in CF sputum specimens and misidentification by routine commercial identification methods.
An 11 year-old boy with CF (homozygous F508del) and recurrent episodes of Allergic Bronchopulmonary Aspergillosis requiring corticosteroids and itraconazole had Inquilinus limosus isolated from sputum. Previous specimens had grown Staphylococcus aureus, Stenotrophomonas maltophilia and Aspergillus fumigatus. At the time of isolation, the patient was not on antibiotics or steroids. Lung function was excellent – FEV1 2.41L (104% predicted), FVC 2.69L (97% predicted) – but deteriorated slightly thereafter.
Routine culture plates grew scanty amounts of S. aureus and normal respiratory flora. Burkholderia cepacia selective agar demonstrated no growth after 72hrs. Sabouraud agar with chloramphenicol and gentamicin incubated at 30oC demonstrated moderate growth of short Gram-negative rods at 10 days. This strict aerobe was oxidase-positive, catalase-positive, indole-negative and C390-resistant. Growth occurred at 25oC, 30oC, 35oC and 42oC, but not at 4oC or 60oC. It grew slightly mucoid, non-pigmented, 0.5mm-1.0mm colonies on most commonly used agar media after 48-72 hours. Both Vitek2 and API20NE identified the organism as Sphingomonas paucimobilis. Biolog gave an identification of Inquilinus limosus (SIM only 0.193). Bruker MALDI-TOF identified it as Inquilinus limosus (score 2.490), which was confirmed by 16S rRNA gene PCR and sequencing. It was resistant to ceftriaxone, ceftazidime, chloramphenicol, colistin, aztreonam, ticarcillin/clavulanate, piperacillin/tazobactam and tobramycin, but susceptible to meropenem, ciprofloxacin, gentamicin and amikacin.
The overall impact of Inquilinus limosus remains unclear since difficulty in isolation and correct identification contributes to under-reporting. In individual cases, its role as ‘colonizer’ versus ‘pathogen’ may be uncertain.