We describe the discovery of a new human pathogen with clinical features similar to M. abscessus that is implicated as a cause of infection for patients with chronic lung diseases, intravascular catheters, and chronic sinusitis. On the basis of our investigations, we propose CV isolates become a new member of the M. chelonae-abscessus complex and be named Mycobacterium franklinii sp. nov. M. franklinii (frank li′ ni I, N.L. masc. gen. n. franklinii of Franklin, pertaining to Benjamin Franklin, statesman, founder of the University of Pennsylvania, inventor, and scientist who helped create the nation’s first public hospital in Philadelphia, Pennsylvania, USA, the origin of the isolates).
The microorganisms are acid-fast, gram-positive bacilli, and colony morphology alone is not sufficient for differentiation from other rapidly growing Mycobacterium
spp. Colonies are nonpigmented appearing on 5% sheep blood agar, Middlebrook 7H10 agar and egg-based Lowenstein-Jensen slants in 2–5 days at temperatures between 24 and 37°C (optimally at 30°C). Even with molecular techniques, underrecognition of this new pathogen is not surprising because it shares 100% full 16S rRNA gene identity with M.
and has sequence variation in the hsp65
gene that results in inconsistent amplification with typical diagnostic primers for sequence or PCR restriction fragment length polymorphism analysis (10,11
). However, diagnosis of M. franklinii
infection is essential because it is more susceptible to antimicrobial drugs than other members of the M. chelonae-abscessus
complex, and its susceptibility pattern with cefoxitin was a distinguishing characteristic leading to its discovery.
Multilocus sequencing on a population of M. chelonae-abscessus complex isolates using 5 DNA regions enabled us to examine a population of closely related isolates to accurately assess species variability. M. franklinii shared complete 16S rRNA gene sequence identity with the type strain of M. chelonae, but was differentiated from M. chelonae and other members of the M. chelonae-abscessus complex by partial sequencing of rpoB, hsp65, sodA, and ITS DNA targets. Concatenated analysis of 43 genes (≈40,000 bp) from deeper sequencing of M. franklinii demonstrated that this novel species shares <90.5% identity with any other M. chelonae-abscessus group member. DNA-DNA hybridization analysis also supports the novel classification with its low relative binding ratios and higher percent divergence from all other M. chelonae-abscessus complex type strains. Cefoxitin susceptibility or intermediate susceptibility is another distinguishing feature. Preliminary testing on 6 M. franklinii isolates revealed inducible resistance (data not shown) in 50% of the isolates upon prolonged clarithromycin incubation (14 days). This finding suggests that similar to M. abscessus, isolates of this species may have an inducible erm gene.
The pathophysiology of diseases associated with M. franklinii
is largely unknown. Most M. franklinii
isolates were from respiratory sources and from patients with underlying lung conditions. Three of these disorders (cystic fibrosis, primary ciliary dyskinesia, and recurrent pneumonia) had associated bronchiectasis, which is a known risk factor for M. abscessus
and M. massiliense
nodular lung disease but not for M. chelonae
). It is unclear whether this microorganism causes respiratory tract disease, or simply colonizes damaged airways and sinuses. Similar to patients with M. abscessus
infections, we found 2 cases each of sinusitis and catheter-associated infection from M. franklinii
). The association with chronic sinusitis presumably relates to sinus washes using tap water rinses in previously diseased sinuses. Although the exact reservoir of M. franklinii
is unknown, a recent study in the Netherlands by Van Ingen et al. reported 2 isolates from tap and shower water based on rpoB
gene sequence (27
), and these 2 isolates shared 99.6% identity to our M. franklinii
sequevar A rpoB
gene sequence. The observation in the Netherlands suggests an environmental source for this organism, and it is likely that the novel species is regionally specific and can survive in municipal water sources. This hypothesis would be supported by the large number of cases in a focused region in Pennsylvania.
Our population analyses of clinical isolates of M. chelonae
demonstrate a lack of taxonomic clarity. Additionally, our investigations lend further evidence that species distinctions for M.
and M. massiliense
may be inappropriate and support the recent proposal to modify their classifications (5,28
). Taxonomic uncertainty likely arises as our understanding of microbial phylogeny expands with rapid advances in technologies and often results in inconsistent standards being applied for species designations. For example, DNA-DNA hybridization is a relatively standard technique, but upon review of 14 species descriptions in 2009 only 5 were supported by using DNA-DNA hybridization (29–36
The discovery of an emerging pathogen should be taken in the context of microbial ecology and evolution, the interaction between host and microbe, and factors of virulence. This investigation underscores the need for accurate identification of Mycobacterium spp. for detection of a new pathogen. The interplay between colonization and disease is not clearly defined, but we demonstrate its role in central line infections and for patients with sinopulmonary disease. M. franklinii may have newly emerged as a human pathogen over the past 5 years, or it has been involved in human disease previously and was unrecognized. In order to further our understanding of this pathogen and its role in disease, greater surveillance and awareness is necessary. At this time, clinical laboratories can identify M. franklinii by sequencing based assays that target either the ITS region (between 16S and 23S rRNA genes), hsp65, rpoB, and sodA genes, or by complete 16S rRNA gene sequence analysis in conjunction with cefoxitin and minocycline susceptibility patterns. The type strain, CV002 (ATCC [pending] and DSMZ 45524), was isolated from a skin lesion.