The medical importance of Chlamydiae

With this special issue on chlamydial infections, our intention is that non-specialist readers will be exposed to the many aspects of chlamydiology, including epidemiology, taxonomy, biodiversity, evolution, diagnosis and treatment, and hopefully that even specialists will appreciate this overview of the pathogenesis of chlamydial infections. Chlamydia trachomatis is the most common bacterial cause of urogenital infections. As described in the review by Bebear and de Barbeyrac, this strictly intracellular pathogenic bacterium frequently causes an asymptomatic or paucisymptomatic infection that remains undetected and thus untreated for a prolonged period. From an evolutionary perspective, this is favourable for the pathogen, as untreated chronic infections may spread. The chronic inflammation then results in long-term complications such as tubal infertility and increased risk of extra-uterine pregnancy. To reduce this morbidity and to reduce transmission, large routine screening programmes have been initiated. However, some strains of C. trachomatis have lost a DNA region that is targeted by frequently used PCRs. This example of rapid counter-evolution, called ‘the Swedish mutant paradigm’, occurred incredibly early after the introduction of these molecular tests into screening programmes. It shows that Chlamydia spp. may evolve rapidly, despite being intracellularly located and despite exhibiting a highly conserved core genome. It also shows the importance of such a strict intracellular bacterium remaining unrecognized. This example of rapid counter-evolution should encourage clinical microbiologists to use a panel of diagnostic tools and/or to develop tests that include several simultaneous targets for this pathogen. Like C. trachomatis, Chlamydia psittaci may also remain undetected in clinical practice, as this strictly intracellular bacterium does not grow on the media used routinely in clinical diagnostic laboratories. However, in contrast to the frequently chronic and paucisymptomatic C. trachomatis infections, pneumonia caused by C. psittaci may be acute, severe and life-threatening. The identification of C. psittaci infections is important in order to guide public health prevention measures and to guarantee an adequate and targeted antibiotic therapy. In their review, Beeckman et al. highlight the zoonotic potential of this pathogen and propose clues for diagnosis and therapy of C. psittaci infections. Parachlamydia acanthamoebae is another strictly intracellular bacterium, belonging to the order Chlamydiales. It easily grows within free-living amoebae (Fig. 1). It represents an emerging agent of community-acquired pneumonia and bronchiolitis. My own contribution to this special issue on chlamydial infections reviews the current knowledge accumulated during the last 10 years concerning this Chlamydiarelated bacterium. Like that of C. psittaci, the pathogenic role of P. acanthamoebae in humans was initially suspected following an outbreak. Moreover, like C. psittaci, P. acanthamoebae might be zoonotically transmitted. It may also, like Legionella pneumophila, be transmitted through exposure to aerosols, as free-living amoebae—its natural hosts—are largely present in water. This review on P. acanthamoebae is not only addressed to microbiologists interested in this evolutionarily distant member of the Chlamydiales, but also proposes a strategy to determine the pathogenic role of any new bacterial species. FIG. 1. Parachlamydia acanthamoebae strain Hall’s coccus within Acanthamoebae castellanii. The cell wall of P. acanthamoebae (in green) was stained with mouse anti-Parachlamydia antibodies, whereas its DNA was stained with 4¢,6-diamidino-2-phenylindole (DAPI). The amoeba was stained with concavalin A. Confocal microscopy, 1000· magnification.