Chlamydial infections in animals

Chlamydophila abortus

Typing

C. abortus strains are typical of the C. psittaci biotype 1 / immunotype 1 abortion strains previously defined (Spears & Storz, 1979; Perez-Martinez & Storz, 1985). The causative agents of ovine and bovine abortion have a serotype that is distinct from strains associated with encephalomyelitis and polyarthritis (biotype 2 / immunotype 2) or normal intestinal infection (biotype 3 / immunotype 3). These latter strains have been reclassified as C. pecorum.

C. abortus strains demonstrate almost 100% conservation of ompA gene and ribosomal gene sequences. DNA sequence analyses carried out so far on C. abortus strains indicate that the species is still evolving from Chlamydophila psittaci. One variation is illustrated in differences in the pathogenic characteristics of the two species (Everett, 2000). In addition, C. abortus strains do not appear to carry an extra-chromosomal plasmid. The type strain for C. abortus is B577. However, the ribosomal gene sequence of C. abortus B577 differs slightly from that of other C. abortus strains (Everett et al., 1999).

Traditionally, the differentiation of C. abortus strains from C. psittaci, C. felis or C. pecorum involved comparison of inclusion morphologies, invasiveness in mouse and sheep models, antigen profiles, immunoblotting and reactivities, and restriction enzyme pattern differences. However advances in chlamydial typing have ensued from studies of chlamydial gene sequences. Kaltenboeck et al., 1993 compared the ompA gene sequences of some 40 chlamydial strains. They identified four main genetic groups corresponding to the former four chlamydial species [old Page taxonomy]. Tests have been devized to differentiate between C. abortus, C. pecorum, C. pneumoniae and C. trachomatis using specific primer pairs (PCR) or by detection of restriction fragment length polymorphisms (RFLPs) within PCR products.

Denamur et al., 1991 showed that 24 C. abortus abortion strains (the former C. psittaci serotype 1) from sheep and goats, classified as invasive in a mouse model of virulence, produced a specific RFLP pattern when amplified ompA fragments were digested with Alu_I. In contrast, C. pecorum (serotype 2) strains demonstrated greater genetic heterogeneity, with12 non-invasive faecal isolates producing 8 different patterns. Using a different approach, Picard et al., 1992 found variations in the electrophoretic mobilities of the enzymes glucose-6-phosphate dehydrogenase and L-malate dehydrogenase between ruminant abortion strains and C. pecorum strains.The majority of PCR tests developed have been based on amplification of the ompA gene encoding the major outer membrane protein. However, assays for the detection of other genes have been reported: ompB (Sheehy et al., 1996), 16S rRNA (Pudjiatmoko et al., 1997; Takahashi et al., 1997; Petersson et al., 1997) and the 16S to 23S gene locus (Everett & Andersen, 1997; Meijer et al., 1997).

Another approach involved analysis of the 16S ribosomal RNA sequences (Pudjiatmoko et al., 1997; (Sheehy et al., 1996 ) and the 16S to 23S rRNA intergenic spacer region sequences (Everett & Andersen, 1997). Ribosomal RNA sequences may be used to assess bacterial evolution and show phylogenetic relationships between organisms. The usefulness of 16S rRNA genes in the diagnosis and differentiation of chlamydia species and strains is limited, however, as these genes are 93-97% identical (Everett & Andersen, 1997). However the 16S-23S intergenic spacer and flanking ribosomal segments are potentially useful for the systematic identification and differentiation of chlamydiae. This region shows substantial sequence diversity (Engel & Ganem, 1987; Fukushi & Hirai, 1993; Scieux et al., 1992). Everett & Andersen, 1997 suggested two genetic clusters for chlamydiae at the genus level, one containing the three biovars of C. trachomatis, and the other containing six groups including C. psittaci, C. pecorum and C. pneumoniae. These studies formed an important basis for their classification of the Chlamydiales (Everett et al., 1999) [see: [classification]]]. Further sequence comparisons of chlamydial strains and species at the genomic level, should further improve understanding of the evolutionary relationships of these organism and may contribute to the design of experiments leading to a greater understanding of their biology and the diseases they cause.

Amplified fragment length polymorphism (AFLP) analysis of C. abortus strains from sheep has revealed polymorphisms between strains of this highly conserved group of chlamydiae (Boumedine & Rodolakis, 1998; Siarkou et al., 2002). AFLP represents a convenient and highly discriminatory method for the molecular typing of chlamydial isolates, and could lead to the rapid identification of strains for epidemiological purposes.

[PG] Updated [MEW] July 2003

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