Coxiellaceae

Abstract: The genus Rickettsiella (class Gammaproteobacteria; order Legionellales; family Coxiellaceae) comprises intracellular bacterial pathogens of a wide range of arthropods that are currently classified in the three recognized species, Rickettsiella popilliae, Rickettsiella grylli, and Rickettsiella chironomi. Rickettsiella bacteria contain a type IVB secretion system (T4SS) known to be a key virulence factor of the related genus Legionella. Providing the first respective sequence information for the nomenclatural type species, R. popilliae, the three T4SS components DotA, IcmB, and IcmQ were used as phylogenetic markers to test hypotheses implicit in the currently accepted taxonomic organization of Rickettsiella at the species, genus, and family level. These results, firstly, firmly corroborate the previous 16S rRNA gene-based coassignment of the species R. grylli and R. popilliae to the γ-proteobacterial order Legionellales and, secondly, support the current classification of the investigated R. grylli and R. popilliae strains in different species of the same genus. In contrast, the analysis of intergeneric sequence distances does not lend support to the current taxonomic classification of the genus Rickettsiella in the family Coxiellaceae, but is consistent with a hierarchically neutral family-level assignment within the order Legionellales.

Abstract: The order Rickettsiales consists of the family Rickettsiaceae and the family Anaplasmataceae. The family Rickettsiaceae contains the genus Rickettsia and the genus Orientia. Members of the family Rickettsiaceae are short rods or coccobacilli, and members of the family Anaplasmataceae are small pleomorphic cocci. The family Bartonellaceae consists of two genera, one of which, Bartonella, includes human pathogens belonging to the α2-group proteobacteria. Currently, the principal techniques used for the serodiagnosis of rickettsial diseases are probe-based immunoassays. Immunoprobe-based tests include the indirect fluorescence assay (IFA) and its micromodification, the micro-IF (MIF) test. The MIF test is still considered as the gold standard. Other immunoprobe tests include immunoperoxidase assays (IPAs); enzyme-linked immunosorbent assays (ELISAs), in which the antigens are adsorbed either onto the well of a microtiter plate or onto nitrocellulose in a dot blot or slot blot configuration; and immunoblot assays (IBAs). IFAs and IPAs require the whole bacterium as the antigen. The remaining immunoprobe assays use either highly purified rickettsiae or an extract. The development of real-time PCR-based diagnosis and rapid PCR-based diagnostic methods has been the focus of much attention in many research laboratories. Recently, a protein-based method for the detection of the small-cell variant protein A (ScvA) plus other protein markers comprising a Coxiella fingerprint via matrix-assisted laser desorption-time of flight (MALDI-TOF) mass spectrometry has been developed.

Abstract: Rickettsiella grylli is an intracellular bacterial pathogen of aquatic and terrestrial arthropods. Previous determination of its 16S rRNA-encoding sequence has led to the taxonomic classification of the genus Rickettsiella in the class Gammaproteobacteria, order Legionellales, family Coxiellaceae, i.e. in close vicinity to vertebrate pathogenic bacteria of the genera Coxiella and Legionella. Here we use the additional information available from the recently published first whole genome sequence from this genus to evaluate critically the taxonomic classification of R. grylli beyond the 16S rRNA gene level. Using phylogenetic reconstruction, together with significance testing on a data basis defined by a core set of 211 previously identified families of protein-encoding genes, together with a reanalysis of 16S rRNA gene data, the present study firmly corroborates the assignment of this species to both the class Gammaproteobacteria and the order Legionellales. However, the results obtained from concatenated and single protein, single protein-encoding gene, and 16S rRNA gene data demonstrate a similar phylogenetic distance of R. grylli to both the Coxiellaceae and the Legionellaceae and are, therefore, inconsistent with its current family-level classification. Consequently, a respective reorganization of the order Legionellales is proposed.

Abstract: Members of the genus Rickettsiella are intracellular bacterial pathogens of arthropods (13). They are found in a wide range of hosts including insects, crustaceans, and arachnids, and they exhibit a worldwide geographic distribution (11–13). In naturally infected hosts, the Rickettsiella-mediated disease affects both larvae and adults and develops very slowly (13). In its crustacean hosts, “Rickettsiella armadillidii” induces death, preceded by loss of weight and a white coloration of intersegmentary membranes, and the host general cavity is filled with an iridescent white liquid (12). Rickettsiella bacteria can potentially be very contagious, since they are capable of surviving in soil for years before contaminating new hosts (13). On the basis of ultrastructural observations, Rickettsiella bacteria have been classified among the Alphaproteobacteria, within the order Rickettsiales, the family Rickettsiaceae, and the tribe Wolbachieae (13). However, the 16S rRNA gene sequence of R. grylli isolated from the cricket suggested that this strain is a Gammaproteobacterium related to the genus Coxiella (10). Therefore, if the genus Rickettsiella is monophyletic (i.e., all Rickettsiella species share an exclusive, common ancestor), then the taxonomic position of the genus Rickettsiella as a whole needs to be reassessed. Otherwise, if only R. grylli has been misclassified among the Gammaproteobacteria, the genus Rickettsiella is polyphyletic (i.e., different Rickettsiella species have different evolutionary origins). To obtain new insight into the evolution of Rickettsiella bacteria, we characterized Rickettsiella molecular genetic variation by analyzing the 16S rRNA gene sequences of three strains of the crustacean pathogen “R. armadillidii” (12) along with a data set of Rickettsiella-like 16S rRNA gene sequences encompassing their entire known host spectrum, gathered through database searches. Wild-caught individuals belonging to the three isopod crus