A strain uncoloured by Gram staining in a pleural fluid

Abstract

A 23-year-old man with a pyothorax was admitted to the pulmonology unit 3 months after pulmonary re-transplantation for cystic fibrosis. After 3 of the 5 days of incubation, culture of pleural fluid using sheep blood agar and chocolate agar plates (Becton Dickinson, Franklin Lakes, NJ, USA) revealed a few Achromobacter xylosoxidans colonies and many small translucent colonies, not seen on Gram stain and not identified by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (Fig. 1). The colonies were identified as Mycoplasma hominis using 16S rDNA sequencing and species-specific PCR. One pleural fluid sample taken a week later revealed A. xylosoxidans and M. hominis, and another revealed M. hominis alone. Minocycline was added to cefiderocol (initially prescribed for the multidrug-resistant A. xylosoxidans strain) during 6 weeks and pleural drainage was continued, leading to clinical and radiographic improvement. Detection of urogenital mycoplasmas in urine using selective media remained negative. The M. hominis isolate was susceptible to clindamycin, levofloxacin and tetracycline. We describe here a rare co-infection with A. xylosoxidans and M. hominis, two pathogens involved in post-transplant infections in individuals with cystic fibrosis. Achromobacter xylosoxidans came from the previously colonized recipient. Mycoplasma hominis may come from the recipient's or donor's respiratory tree [[1]Smibert O.C. Wilson H.L. Sohail A. Narayanasamy S. Schultz M.B. Ballard S.A. et al.Donor-derived Mycoplasma hominis and an apparent cluster of M. hominis cases in solid organ transplant recipients.Clin Infect Dis. 2017; 65: 1504-1508Crossref PubMed Scopus (13) Google Scholar]. In lung transplant patients, surgical wound and immunosuppression seem to be the main risk factors of M. hominis infection. Mycoplasma hominis may rarely grow slowly on blood agar and produces tiny colonies, not coloured by Gram stain and sometimes not identified by MALDI-TOF. Microbiologists should subsequently consider M. hominis and perform a specific PCR assay to identify the colonies. We have no conflict of interest to declare.