Role of Williamsia and Segniliparus in human infections with the approach taxonomy, cultivation, and identification methods

The genera Williamsia and Segniliparus are of aerobic actinomycetes and at the time of writing, they have 12 and 2 species, respectively. These genera cause various infections in humans. In this review, we surveyed their taxonomy, isolation, identification, as well as their role to cause human infections.


Isolation methods for Segniliparus spp.
For the genus Segniliparus, the use of Middlebrook 7H10 and 7H11 media [3], Lowenstein-Jensen (LJ) medium [54], LJ with 5% sodium chloride [54] and American Trudeau Society (ATS) media [54] have been suggested for isolation, good growth, and examination of morphological characteristics. Also, Segniliparus rugosus can grow on MacConkey agar [3] and it has been reported that S. rugosus is resistant to decontamination methods such as NaOH and N-acetyl-l-cysteine in clinical specimens [55].

Phenotypic identification of Williamsia
Phenotypic characterizations are the first step for these bacteria identification at the genus and species levels.

Phenotypic identification of Segniliparus
Two species of Segniliparus rotundus and S. rugosus belong to the Segniliparus genus [3]. Its species are distributed in different environments; however, they have also been isolated from clinical specimens [57]. Pigment  colors in Segniliparus spp. is white to beige [3]. An aerial hyphae are not seen in the Segniliparus [4]; and the genus is acid-fast [56]. Some of the phenotypic characterization of the Segniliparus spp. are shown in

Molecular identification of the Williamsia
The most common molecular method for Williamsia accurate identification and assessment of taxonomic characteristics is sequence-based identification. 16S rRNA gene sequencing is an effective standard method for accurate identification of the novel bacteria and emerging pathogens at the genus and species levels [58]. Primers to amplify 16S rRNA gene for Williamsia identification include 27F (5′-AGA GTT TGA TCC TGG CTC AG-3′)/1492R (5′-GGT TAC CTT GTT ACG ACT T-3′) and 27f (5′-GAGTM′GAT CCT GGC TCA G-3′)/1525r (5′-AGA AAG GAG GTG ATC CAG CC-3′) [40,44]. Montoya-Porras et al. [59] identified the genus Williamsia with 454 pyrosequencing for the variable region of the 16S rRNA gene. The phylogenetic tree of the 16S rRNA gene for Williamsia standard species is shown in Fig. 1. The gold standard method to discern bacterial species is DNA-DNA hybridization (DDH) [30]; however, this method is not used in clinical laboratories for bacterial identification. Another molecular method is the whole-genome sequencing (WGS), which has been deposited for five Williamsia species in the National Center for Biotechnology Information (NCBI). Data are provided in Additional file 1.

Pathogenesis in Williamsia and Segniliparus
Our knowledge about pathogenesis and virulence factors in two genera is limited. Cell wall components, such as mycolic acid, phagolysosome inhibition, immune response promotes, and the production of enzymes, such as catalase, may play a role in their pathogenesis.

Clinical disease, antibiogram and treatment associated with Williamsia
Physicians need to pay attention to these symptoms such as bilateral alveolar infiltrates [61], fever [62], having an underlying disease such as diabetes mellitus for detection of this rare infection [52]. Infections in humans caused by Williamsia have been reported. Infection occurs as a result of exposure to the environment; however, there is no evidence of an environmental source for Williamsia and Segniliparus infections. For antimicrobial susceptibility testing (AST), breakpoints have not been established for these genera, and researchers use recommended AST (the gold standard for antibiogram is micro broth dilution) for Nocardia and related aerobic actinomycetes by the Clinical and Laboratory Standards Institute (CLSI) [63]. Tomas et al. [61] first reported W. muralis as the cause of lung infection in an old woman.
In their study, this bacterium was isolated from a brush sample and results of AST showed that this bacterium was susceptible to amoxicillin-clavulanate, cephalosporin (cefotaxime), carbapenem (imipenem), Quinolone (ciprofloxacin), aminoglycoside (tobramycin, gentamicin), sulfonamide (cotrimoxazole) and resistant to beta-lactam (ampicillin) and macrolide (erythromycin) family. In another study by Yassin et al. [40] reported W. deligens of human blood in 2006. Also, W. serinedens has been isolated of perinatal sepsis from a pregnant woman in 2010 and this bacterium was susceptible to amikacin, ampicillin, doxycycline, imipenem, linezolid, meropenem, penicillin G, tobramycin, vancomycin and was resistant to oxacillin and trimethoprim-sulfamethoxazole with E-test method [62]. The case reports published regarding Williamsia spp. are provided in Table 2.

Clinical disease, antibiogram and treatment associated with Segniliparus
Physicians should pay more attention to symptoms such as chronic cough and sputum more than 3 months, fever, multiple small nodules in lung [55,60] and radiologic finding similar to other genera in actinomycete family such as M.    20:10 broth dilution showed that this isolate was susceptible to ciprofloxacin, gatifloxacin, imipenem and resistant to amikacin, cefoxitin, ceftriaxone, tobramycin. Koh et al. [55] isolated S. rotundus from sputum in 2011 in a patient treated with clarithromycin and ciprofloxacin. In another study, S. rugosus was isolated from sputum in 2014 [60]. S. rugosus possibly is an emerging pathogen in cystic fibrosis patients. Antibiotic resistance genes have not been reported in the genera Williamsia and Segniliparus [65]. The case reports published on Segniliparus spp. are provided in Table 3. On the basis of the clinical reports, the pulmonary infection of Segniliparus spp. is associated with chronic cough, fever and hypoventilation, as well as the presence of multiple small nodules, with symptoms of acid-fast bacilli in sputum and radiologic results similar to M. tuberculosis, NTM, Nocardia and so on. Therefore, pulmonary infection should be identified in microbiology laboratories.

Conclusion
In this review, we surveyed taxonomy and the role of the genera Williamsia and Segniliparus in human infections. The identification of pathogenic factors in these bacteria requires more investigations. A few studies have been conducted on Williams and Segniliparus infections because of the lack of attention and insufficient experience in medical laboratory personnel as well as the lack of optimization of the phenotypic and molecular methods to identify these bacteria in hospitals. The use of novel molecular methods is necessary for accurate identification of Williamsia and Segniliparus species.